Use LEFT and RIGHT arrow keys to navigate between flashcards;
Use UP and DOWN arrow keys to flip the card;
H to show hint;
A reads text to speech;
220 Cards in this Set
- Front
- Back
|
Define hydraulic |
Liquid in pipes of various sizes to transfer power |
|
|
How is a specific amount of force expressed |
Commonly expressed in newtons or pounds, imperial unit of force is pounds force |
|
|
Define pressure, pascal, atmospheric pressure |
Pressure- force per unit area P = force/area Pascal -one newton per square meter Atmospheric pressure-the pressure of the atmosphere at sea level (14.7 psi) |
|
|
Define the following terms related to work, power and energy |
Joule- the amount of work is equal to the distance moved in meters or feet times the force exerted 1J = 1N X 1 m 1ft -lbf = 1 lbf x 1 ft = 1 ft/lb 1 watt = is 1 Newton lifted 1 meter in 1 second 1 Hp = 746 watts or 33000 lbs lifted 1 foot in 1 minute or 550 ft/lb per second |
|
|
State the law of conservation of energy |
Energy cannot be converted or destroyed , but can be converted from one form to another |
|
|
State the properties of liquids |
Has no definite form, takes on the shape of its container It is virtually incompressible Shows the same resistance to compression as a solid Extremely flexible but as unyeilding as steel |
|
|
What does pascals law state |
Pressure applied to a confined fluid at rest, force is transmitted equally in every direction and always at right angles to the containing surface |
|
|
How is mechanical advantage gained with hydraulics. |
It’s does this by acting as a force multiplier |
|
|
How is the fluid flow measured |
By the volume of fluid passing a given point in a unit of time |
|
|
How is velocity measured |
Is the distance travelled by the fluid in a unit of time |
|
|
Describe laminar and turbulent flow |
Laminar flow-in ideal flow conditions the fluid moves in layers parallel to the walls of the conductor Turbulent flow-when the fluid moves in a random pattern rather than parallel to the direction of flow |
|
|
List the main causes of excessive friction in hydraulic lines |
High velocity, obstructions or projections in the fluid streams, sharp bends and or roughness in the conductor, a large number of bends in the system or a combo of these |
|
|
What does Bernoulli’s principle state |
That if the flow rate is constant, the sum of the kinetic energy and the potential energy at various points in the system is constant. Therefore, whenever the velocity (kinetic energy) of a fluid increases, the pressure (potential energy) decreases |
|
|
What is the purpose and whatever are the types of hydraulic actuators |
They convert hydraulic energy to mechanical energy (cylinders or motors) |
|
|
Describe Single acting cylinder Rod and ram cylinder Spring return cylinder Diaphragm spring return cylinder A double acting cylinder A double acting cylinder with a double ended piston rod Cylinders with cushions Tandem cylinders Telescopic cylinders |
A single acting cylinder-applies force or is pressurized in one direction only Rod and ram cylinders- rod cylinders the rod is less than half of the piston face area, ram cylinders the cross-sectional area is more than half the piston face area Spring return cylinders-cylinder rod returns by a spring Diaphragm spring-return cylinder- are used to provide short strokes with large forces Double-acting cylinders - have hydraulic fluid delivered under pressure to both sides of the piston Double acting cylinder with a double ended piston rod-when rods are the same diameter the forces on the piston can be the same Cylinders with cushions- slows down the piston as it approaches the end of its travel Tandem cylinders-have two pistons attached to one rod, this design allows for higher forces at the rod end without an increase in fluid pressure or cylinder diameter Telescopic cylinders- have a series of tubular rod segments called sleeves which fit inside each other, permits a working stroke much longer than its retracted length |
|
|
Describe cylinder mounting practices |
Can be mounted in any position and will work successfully as long as they are mounted on a strong and rigid base and aligned with the part they are to move |
|
|
How are motors similar to and how do they differ from pumps |
Pumps drive the hydraulic fluid Motors are driven by the hydraulic fluid |
|
|
What is the advantage of variable capacity motors versus fixed capacity motors for controlling motor speed |
Variable oil supply to the motor allows the motor to be run over a wide range of speeds -fixed-if any variation must be controlled independently of the motor than the motor is considered to be fixed -variable- when the variation of oil is to be done in the motor it is said to be variable |
|
|
Describe the operating principles of the following |
Gear motors-the fluid flow from the system enters the inlet port and travels in wither direction around the casing, forcing the gears to turn opposing each other, both gears are driven by the fluid but only one is connected to the output shaft. It is a fixed capacity motor Screw motors-uses the force of the fluid against the face of the screw threads to generate motion. There are two or more screws in the housing but only one is attached to the drive. The others are idlers which act as a seal between helical chambers. This prevents reverse fluid flow in the housing. Screw motors have fixed capacity and operate quietly and free of vibration Vane motors- the fluid flow from the system enters the inlet port and exerts force against the vanes and the rotor the max force is exerted against the vane with the largest area exposed to the fluid...can be fixed or variable Radial piston pumps- |
|
|
Define the terms related to motor calculations |
Displacement- the amount of liquid handled in one rotation of the motor Pressure- pressure requirements for motors vary with the size of the motors displacement the larger the displacement of the motor the less pressure required to produce a given torque Torque- torque output is a function of the system pressure and the motor displacement Mechanical eff= actual torque/theoretical torque X 100 Motor speed(S) =flow rate or volume per unit of time (V) divided by the motors displacement or area (A) S=V/A |
|
|
How is a hydraulic motors torque increased? |
By increasing displacement |
|
|
How is a hydraulic motors speed decreased |
By decreasing flow rate |
|
|
What would be the result if you replaced a hydraulic motor with a motor of a smaller displacement |
System pressure would increase Speed increases Torque decreases |
|
|
What is the purpose of a direct control valve |
Used to control the direction of hydraulic fluid flow |
|
|
How are the number of position and flow patterns represented by the DCV symbol |
The first figure indicates the number of ways the fluid can flow Second figure indicates the number of distinct positions |
|
|
How are the ports labelled on the symbol |
Top ports A, B Bottom ports P,T |
|
|
How is the transitory condition indicated on a DCV symbolhow |
By dashed lines in between envelopes |
|
|
How are flow paths indicated on DCV symbols |
By an arrow |
|
|
How are DCVs identified numerically |
First figure indicated ways fluid can flow Second figure indicates number of positions |
|
|
List how dcv’s are classed other than numerically |
Spool size, nominal size (usually nominal pipe connection size, Max psi, port connections |
|
|
List two types of spools for dcvs |
Rotary or sliding |
|
|
Compare throttling to non throttling dcvs |
Throttling dcvs allow the spool to pass through on env to another at any given rate (infinite positioning) Non throttling- snap into only 1 env at a time
|
|
|
How are DCVs identified numerically |
The first number indicates the number of ways the fluid can flow. This is also usually the number or ports (excluding pilot ports) The second figure indicates the number of positions |
|
|
Which pressure control valves are normally closed and which are normally open |
Normally closed -relief valves, sequence valves Normally open - pressure reducing valves |
|
|
How many squares (envs) are in a pressure relief valve symbol |
One square or env |
|
|
Describe the following and or operating principles of the following |
Direct acting relief valve - is a normally closed valve. It uses a ball or poppet held on a seat by a spring similar to a check valve. Some valves use a guided piston instead of a poppet. Its spring is either set to a predetermined force or adjusted to meet a range of requirements. Direct acting relief valves are frequently used as safety valves to prevent damage from high surge pressure. Pressure which the valve first starts to open is called cracking pressure, the difference between cracking pressure and full flow pressure is called the pressure override or pressure differential Pilot operated relief valve - normally closed valve, used if a large amount of fluid is to be relieved under a small pressure differential Unloading valve- returns pump output (at low pressure) to the reservoir after the required system pressure has been reached Pressure reducing valve- normally open valve, has a pilot line downstream of the valve , when a secondary circuit operates efficiently at a pressure lower than the relief valve setting Sequence valve- used when you need to operate valves in a specific order. It’s a normally closed two way valve. It has a pilot line that senses the pressure of the inlet port and a line that drains the spring chamber back to the reservoir Counterbalance valve- normally closed pressure control valve, a pilot line senses the pressure of the inlet port and an internal drain, this valve is used to maintain a set pressure in part of a circuit, controls a load from dropping Brake valve- commonly found with a motor, it stops its rotation as the DCV shifts to its center position (8 times less pressure on the external line) |
|
|
What is the purpose for flow control |
Occasionally you need to control the speed of a motor or the rate of travel of a cylinder must be regulated. This is done by controlling the volume of hydraulic fluid entering or leaving the actuator |
|
|
Define non-compensating flow control valves |
Throttle valves are non compensating because they do not compensate for any variation in pressure or flow in the system. As pressure increases, the flow through the valve increases |
|
|
Describe fixed and adjustable non compensating flow control valves |
Fixed (non adjustable) pressure compensating valves - has a preset orifice sized for its specific application. As the pressure increases at the control orifice: the poppet moves against the spring tension reducing the opening at the throttling orifice and therefore the flow. Opening the relief port and allowing the excess fluid to return to the reservoir Adjustable non-compensating valves- have an adjustable amount of flow in one direction and a free flow in he other direction by means of a separate check valve |
|
|
Define pressure compensating flow control valves |
Maintains a constant rate of flow through the valve regardless of downside pressure |
|
|
Describe the following |
Fixed pressure compensating flow control valves-has a preset orifice sized for its specific application as the pressure increases at the control orifice the poppet moves against the spring tension reducing the opening at the throttling orifice and therefore the flow, opening a relief port and allowing the excess fluid to return to the reservoir Adjustable pressure compensating flow control valves - has an orifice that can be adjusted to the required flow rate. This valve acts in a similar manner as the fixed pressure compensating flow control valve |
|
|
Describe the operating principles of pressure and temperature compensating flow control |
As the temp of hydraulic fluid changes so does its viscosity. When the fluid warms up more goes through and orifice. A common method of controlling this the amount of fluid passing through an orifice as the temperature changes is by means of throttle attached to an aluminum alloy or bi-metal rod. The temp compensating rod expands and contracts with the changing temps and moves the throttle to decrease and increase the orifice |
|
|
Describe the operating principles of flow dividing valves |
Flow dividing valves control the amount of fluid to each circuit. They are located between the pump and the direction control valves. The flow divider is shown in the circuit as two fixed or variable restrictions. The valve can deliver equal flow rates or a preset ratio of flow rates into two separate circuits |
|
|
List how DCVs are classified other than numericallys |
Spool type-sliding or rotary Nominal size-usually nominal pipe connection size, the recommended max volume through the valve Maximum allowable pressure Port connection-threaded, NPT or NPS, flanged or flat face |
|
|
What are the three possible stages to convert an electronic signal to activate a main valve |
Electrical signal converting to mechanical movement Mechanical movement to a piloting stage Piloting stage to the main valve |
|
|
Define a solenoid and describe how it is used to open and close valves |
A solenoid is a simple form of an electromagnet. It is a coil of insulated copper wire or other suitable conductor. When the coil is electrically energized a magnetic field is produced which attracts the plunger and draws it up into the core of the solenoid.the electromagnetically induced movement can be used to open or close valve ports. Springs are used to return the plunger to its original position |
|
|
Describe the operating principle of a solenoid controlled pilot operated valve |
The solenoid controlled pilot valve is called the master slave. It directs flow to either end of the pilot operated main valve which is called the slave valve. The pilot valve is normally mounted on top of the larger main valve. |
|
|
Describe the operating principle of a proportional solenoid |
Allows the operator to vary the position of the plunger by varying the amount of current going to the solenoid. This varies the amount of flow. These are used in conjunction with throttling DCVs pressure control valves and flow control valves. With the use of proportional solenoids, valves can be placed near the actuators and controlled by a remote microprocessor control unit |
|
|
How do hydraulic pumps convert mechanical energy into hydraulic energy |
1. The mechanical action of the pump first creates a partial vacuum at its inlet side 2. This vacuum allows atmospheric pressure in the reservoir to force the hydraulic fluid through the inlet line to the pump 3. The pumps mechanical action then forces the hydraulic fluid to the pumps outlet and into the system |
|
|
What is the basic type of pump used in hydraulic systems |
Positive displacement pumps |
|
|
Define fixed capacity and variable capacity |
Fixed capacity - these run at a given speed, delivering a constant flow rate. They may be gear, vane or piston pumps Variable capacity - these run at a given speed, delivering a variety of flow rates from maximum to zero in one or both directions. They may be vane or piston pumps |
|
|
Briefly describe the following |
Gear pumps- External gear pumps Lobe pumps Screw pumps Internal gear pumps Vane pumps Hand pumps Axial and radial piston pumps |
|
|
List two types of spools for DCVS |
Rotary or sliding |
|
|
Draw each of the following |
Lever control Spring control Pneumatic flow Direct acting by release of pressure Indirect acting by application of pressure Controlled by a reversing electric motor Control held in position by a detent |
|
|
Compare throttling to non throttling |
Throttling DCVs allow the spool to pass through from one envelope to another at any given rate. They are also called infinite positioning. A throttling valve is a tracer valve activated by a plunger spring Non throttling DCVS snap into only one of the envelopes at a time. It includes two three and four way valves, closed and open, tandem, float and rengenerative center envs |
|
|
Draw symbols for the following |
Throttling DCV Normally open two way DCV Normally closed three way DCV |
|
|
Describe the functions of the following valves with regard to their center envelope flow patterns |
Closed center - all ports are blocked off in neutral. The actuator is locked so that it cant move out of position Open center- all ports are connected in neutral the actuator moves in the direction of any external forces. The flow from the pump is back to the tank with minimum power demand. There is no pressure and only a low heat rise Tandem center-the actuator ports are blocked off, and the flow from the pump is back to the tank. This center provides a hydraulic lock to hold the actuator in place Float center-ports A and B are connected to the tank, and the pump is blocked off. This centre allows the actuator to coast to a stop or be moved manually without disconnecting the circuit Regenerative center- Ports A and B are connected to the pump. This center maintains constant pressure to both ports of the actuator. This design allows for a very rapid start of the actuator. The fluid from the pump must go through the relief valve at max pressure, generating heat and requiring maximum power. |
|
|
Describe the purpose, application and types of check valves |
Are single position valves which allow the free flow of fluid in one direction only. Some check valves have a restricted flow in the reverse direction. The common styles are ball or poppet. Check valves are available for inline or right angle mounting |
|
|
Draw the symbol for a |
Pilot controlled check valve to close the valve |
|
|
What is the purpose for pressure control valves |
Control the hydraulic pressure in all or part of the circuit. They may be either normally closed or normally open valves. Normally closed valves open when pressure reaches a set limit eg relief or sequence valves Normally open valves close when pressure reaches a set limit eg pressure reducing valves |
|
|
Briefly describe the following |
Gear pumps- is a fixed capacity pump which has two or more rotors External gear pumps-a conventional external gear pump has its gears meshing on their periphery (outer edges), one gear is driven by the other. The gears carry the liquid from the suction port to the discharge port, around the inner walls of the casing Lobe pumps- a lobe pump has larger spaces between its teeth than conventional gear pumps do. Their rotors must be driven by suitable drive gears mounted outside the casing. These pumps deliver a more pulsating flow than conventional gear pumps. Screw pumps - axial flow pump. It may have one, two or three screws which carry the liquid from the suction port to the discharge port. This pump also needs to be driven by external gears. These pumps move the liquid linearly through the pump which eliminates pulsation. There is no metal to metal contact within the pump, which makes its operation very quiet Internal gear pumps - has one external gear rotating within an internal gear. The rotation of the external gear is off-center to that of the internal gear. This arrangement is compact. The crescent seal and the gerotor pump are two commonly used internal gear pumps Vane pumps- may be unbalanced or balanced and have fixed or variable capacity. The variable vane pumps may be able to reverse flow through the system Hand pumps-most basic of piston pumps. Found in equipment such as hydraulic jacks and floor jacks. Design is similar to hydraulic cylinders and are either single or double acting. Check valves are used to maintain correct flow direction. A release valve is used to return the liquid to the reservoir Axial and radial piston pumps - are used in powered hydraulic systems. Both may have fixed or variable capacity, some reverse flow through the system |
|
|
What are the nominal and absolute filter ratings |
Nominal filter rating - indicates that the filter will remove most particles of that size or larger Absolute filter rating - means that a filter will stop all particles of that size or larger |
|
|
How is the filter condition indicated |
Indicators that show condition Gauges with green, yellow and red divisions of the dial Tell tales with green, yellow and red bands |
|
|
What are the advantages and disadvantages of the following |
Inlet side filters - filters all the hydraulic fluid going to the system, risk of cavitation Pressure side filters - protects the valves but does not protect the pump or main main pressure valves Return side filters often considered the best for a fine filters as the fluid is at its highest temp and therefore lowest viscosity. Most common |
|
|
List the purposes of a reservoir |
To provide an adequate supply of fluid to the system Also allows air in the hydraulic fluid to escape, dirt and water to settle out and heat to dissipate |
|
|
How much capacity should a reservoir have |
2-3 times rated pump delivery for 1 min |
|
|
Describe the construction of a reservoir |
Normally made from steel plate with all joints welded. Must meet general industrial specifications. Some have a sloping bottom and a drain plug at the lowest point |
|
|
What is the importance of a vent, a temperature gauge and a sight glass |
Vent- opening must be large enough to let the air move as fast as the fluid is either removed or returned to the reservoir Temperature gauge-is often installed in the reservoir to help monitor any excessive heat buildup in the system Sight glass - important to be able to see the oil level |
|
|
What are the purposes of heat exchangersbrei |
To ensure correct operating temperature of the hydraulic fluid. Heat exchangers are frequently used to cool the oil in a hydraulic system. They can also heat the oil for cold weather startups |
|
|
Briefly describe |
Water coolers - usually consist of a nest of tubes in a shell (cylindrical container) oil flow in one direction and cooling water flow is in the opposite direction Air coolers - the tubes are vertically mounted and have fins for heat removal. Oil flows through the tubes and a fan drives cooling air over the tubes and fins to remove heat Heaters- adding heat to the fluid by use of either electric immersion heaters with thermostat control, by passing steam or hot water through a coil or length of pipe submerged in the tank, by starting up the hydraulic system and pumping oil over the relief valve at maximum pressure to create heat from fluid friction |
|
|
What are the purposes of accumulators |
Store fluid under pressure for future use as a source of potential energy. They also absorb shock waves or dampen pulsations and maintain a constant pressure in the system |
|
|
Describe how pumps are rated |
By their capacity and pressure |
|
|
When using an accumulator, what should be known about it? |
The amount of oil to be added before the gas charge The recommended pressure of the gas charge (precharge is a percentage of the operating systems max pressure |
|
|
What must be done before any work is started on an accumulator loaded system |
Isolate the accumulator with a shut off valve Discharge or drain the accumulator back to tank |
|
|
Describe |
Weight loaded accumulator - a vertical cylinder fitted with a piston. A packing gland or similar device keeps the fluid in the cylinder as the piston moves. A platform on the piston is loaded with scrap iron, concrete blocks or other heavy material. The force of gravity provides the energy to keep the fluid under constant pressure Spring loaded accumulator-use compression springs instead of gravity to supply resistance. Springs must be evenly loaded to allow even travel of the piston through the cylinder. This type of accumulator does not produce constant pressure through the entire stroke. The springs exert minimum pressure when the accumulator is at a low volume Gas charged accumulator-depend on the compressibility of a gas (such as nitrogen or air) to produce the necessary pressure and delivery. Compressed air is used for low pressures Dry Nitrogen is used for med to high pressures **Pure oxygen is never used with petroleum oil because an explosion or fire may result Non separated accumulator -have no physical barrier between the gas and liquid, they are mainly used on low pressure systems. The low pressure limits the amount of gas that dissolves in the liquid Piston accumulator - are much like cylinders without piston rod. A simple piston accumulator has a free floating piston between the liquid and the gas. Has two sets of required packing. These seal the two chambers and centralize the piston to prevent any metal to metal contact. A bleed hole is used to eliminate any buildup of pressure between the seals Bladder accumulator- have a natural or synthetic rubber bag mounted inside a chamber. This bag separates the liquid from the gas. The bag is molded around the gas valve and mounted through the top of the chamber. The liquid connection is mounted through the bottom of the chamber Diaphragm accumulator- are similar in operation to bladder accumulators. They use a natural or synthetic rubber diaphragm mounted in the center of the chamber to separate the liquid from the gas |
|
|
Describe the operating principles of the following |
A pressure intensifier- when extremely high pressure is needed. It is not possible for the system to create such a pressure, pressure intensifiers are used. Intensifiers for one type of fluid use hydraulic fluid in both sides. Those with two types use compressed air or other gas on one side and hydraulic fluid on the other. The cylinder has two different size pistons connected by a piston rod, or one piston and a plunger. It operates as a force multiplier. The lower pressure act on the large piston are and forces the small piston forward, creating much higher pressure A bourdon gauge - consists of a calibrated dial face and pointer attached through linkage to a flexible metal tube, called a bourdon tube. The bourdon tube is connected to the system pressure. As the system pressure increases the Bourdon tube tends to straighten slightly. This is due to the difference in area between its inside and outside surfaces. This action causes the pointer to move around its dial face and indicate the pressure A Schrader gauge- has a calibrated dial face with a pointer attached through linkage to a plunger and bias spring. The system pressure is connected to the gauge and acts on the plunger. As the pressure increases the plunger is forced against the bias spring. This moves and rotates the pointer around the dial face, indicating the pressure A flow meter - portable testing device. They are rarely permanently attached to the equipment. Used to monitor the flow in a line and to determine the efficiency of pumps and motors A pressure activated electric switch- the most common electrical interface device found in a hydraulic system. They protect operators, equipment and work in progress by sounding alarms and shutting off equipment when the pressure gets too high |
|
|
What units are used to measure pressure |
Bar, kpa, psi |
|
|
Why is the measurement of temperature important |
Indicates whether the heaters or coolers are set correctly as to forewarn its malfunction |
|
|
List the hydraulic fluids available today |
Petroleum oil - most common Fluids with high water content Invert emulsion fluids Glycol fluids Synthesized hydrocarbon hydraulic fluids Vegetable or grain oils |
|
|
List the qualities that should be considered when selecting a hydraulic fluid |
Viscosity Viscosity index Pour point Thermal stability Resistance to oxidation Resistance to rusting Resistance to air foaming Fire resistance Lubricating qualities Long life Cost Disposability |
|
|
Briefly describe the following |
HFA fluids- high water content, are a soluble oil or synthetic chemical emulsion, they generally contain 5% to 10% oil. The soluble oil has a milky appearance and the synthetic solutions have a clear appearance. These fluids have excellent cooling ability HFB fluids- water in oil fluids, are invert emulsions which contains more oil then water. They generally contain only 40% water. HFB fluids should be checked regularily to maintain the viscosity and the water- oil ratio. Water or oil can be added to maintain the required ratio HFC fluids - water glycol fluids, consists of 30%-40% water dissolved into glycol. HFC fluids should be checked regularily to maintain the correct water glycol ratio. Glycol is the same family as permanent antifreeze ethylene , therefore the fluid can endure temps below freezing, these fluids are heavier than oil and should be used with either a very short suction lift, a special inlet design , or in situations where the fluid level is above the pump inlet HFD fluids - are a special chemical compound which do not support combustion. They have no water content and can therefore be used at high temperatures with no evaporation problem. These are the heaviest of the hydraulic fluids and require special pump inlet designs or special pump mounting positions. This fluid attacks the most. Common seal materials used in petroleum oil systems |
|
|
How is hydraulic fluid kept clean during storage |
Keep the spout or bung free from moisture or contaminants. If they are stored outside, tilt the barrels at an angle with the bung out of the water line |
|
|
How is pressure created in a hydraulic system |
Is created by a resistance to flow |
|
|
What determines the rate of flow that can pass through a conductor without excessive friction |
The inside diameter of the line determines the rate of flow that can pass without excessive friction, heat and power loss |
|
|
What determines the bursting pressure of a line |
The wall thickness and the inside diameter determine the bursting pressure of a line, the greater the wall thickness the higher the bursting pressure |
|
|
What type of pipe is used for hydraulic systems |
Seamless steel (black) pipe with its interior free from rust, scale and dirt is recommended for hydraulic systems |
|
|
What is the wall thickness of 1” schedule 160 pipe and what size threading dies are required for it |
Wall thickness .250 Inside diameter .815 1.315 is the OD |
|
|
List the thread types used on hydraulic piping |
NPT American national pipe taper NPS American national pipe straight NPTF dryseal pipe taper |
|
|
How is sealing compound applied to the pipe |
Should be treated with a sealing compound on the male end before being threaded into a fitting and tightened, this prevents the threads from galling to each other, also prevents fluid from leaking through the spiral clearance at the roots of the thread. |
|
|
Why is tubing a popular hydraulic conductor |
Because it can be easily bent and flared |
|
|
How is tubing size measured |
Are taken from the outside diameter, the nominal dimensions are given in fractions of an inch or dash number. The dash number represents the outside diameter in sixteenths of an inch. For ex a tube with dash 8 equals OD 8/16” or 1/2” |
|
|
Briefly describe flared and flareless joints |
Flared joints- rely on the compression of the tubing material between the inner and outer walls of the flare. Tube flares are made with a flaring tool kit. The tube can be made single or double flared. Flareless joints - use an intermediate product to grip the tubing. As the nut is tightened onto the end fitting, the intermediate product is compressed. This causes it to bite into and press onto the tubing to create a seal and hold it in place |
|
|
What factors should be considered when bending tubes |
-to ensure a smooth bend, use proper bending tools and an appropriate bend radius for the tubing size. Generally, the correct bend radius is 3-4 times the tube OD Use the bend radius to calculate the required cutoff length of tubing Bend the tube carefully to avoid distortion. Any flattening, kinks or wrinkles in the bends cause turbulence in the flow Leave a straight length of at least twice the nut length between a bend and a fitting. This allows the connecting nut to slide away from the fitting when necessary Form bends properly so that the fittings are in alignment. Misaligned tubes cause stress to the parts to be connected Allow for expansion or contraction due to temperature when short lengths of tubing are used |
|
|
Compare pumps mounted above the reservoir to pumps mounted below the reservoir |
Above the reservoir- the pump must be able to create enough vacuum or pressure drop to overcome the weight and friction of the liquid, the height from the liquid level to the pumps center line (suction lift) Below the reservoir-atmospheric pressure helps to push liquid into the suction side of the pump. This gives the pump the added advantage of being charge (or pressure fed) by the suction head of liquid |
|
|
Describe the basic procedure for tube installation |
Start from a fixed point Use proper hydraulic fittings Use as few fittings as possible by making bends in the tubing. (Every fitting is a source of turbulence as well as a potential leak) Locate the simplest route with the least number of bends Make sure all joints or fittings can be easily reached for maintenance Put the tube line where it will not be a hazard to workers Put the line where it does not interfere with any other equipment |
|
|
What are the purposes of a hose |
Hose is used to connect parts which move in relation to each other, or which are subject to vibration |
|
|
Describe the construction of various hydraulic hoses |
Various types of hose are used the reinforcement determines the pressure rating Low pressure hose has two or more fabric braid requirements, it can withstand pressure from 250-500 psi depending on its ID Hose with one metal wire braid for reinforcement is called single wire braid and is good for working pressures up to about 1500 psi. Two wire braid hose has a pressure rating of up to about 3000 psi. Substituting spiral wrap wire for the wire braid brings the pressure rating still higher up to 6000 psi for hoses with smaller ID |
|
|
Describe the construction of hose end fittings |
Permanent fittings are crimped or swagged on the hose end and are discarded with the hose. A hose crimping machine is required to assemble the fittings Reusable fittings are screwed or clamped to the hose ends and salvaged when the hose is discarded. Hose for reusable fittings can be purchased in bulk and each section assembled as needed |
|
|
Describe the basic procedure for hose installation |
Allow enough slack to avoid kinking the hose at a ridge connection Do not use a taut hose: pressure tends to bulge the hose and shorten it Do not twist the hose: this can be checked by markings on the cover (use fittings to avoid long loops) Foo
Follow specifications for minimum bend radius Install hose |
|
|
What is the major factor in a hydraulic component failure |
Contamination |
|
|
What is the purpose of a magnetic plug in a reservoir |
Plugs are used to attract steel or iron particles and are normally mounted in the reservoir |
|
|
Define a strainer and state its purpose and usual position in a hydraulic system |
Can be considered as coarse filters, they remove larger solids from fluids travelling in a straight path. Mounted in the reservoir on the inlet of the pump |
|
|
Describe |
Proportional filters - has only a portion of the oil passing through the filtering element. The rest flows directly to the reservoir. With continuous recirc, all of the oil eventually flows through the filter Full flow filters - all the oil passes through the filtering element, this design gives more filtering action but builds up resistance to flow as the filter becomes dirty. It often has a bypass valve |
|
|
Describe the filtering elements |
Mechanical (metal) filters- are considered coarse filters or strainers, and consist of layers of wire screens or discs of perforated metal. They remove the larger solid particles but do not remove water or very fine solids Adsorbent filters - filters contain materials such as paper, wood pulp, fabric waste and wool. They remove fine particles as well as water and water soluble impurities Absorbent filters - remove impurities by both mechanical and chemical means. Bone black, charcoal, fullers earth and other active clays are examples of these filtering materials. These filters remove all solid particles and insoluble sludge, plus nearly all water and soluble, oxidized material ** adsorbent filters may also remove most additives used in inhibited hydraulic fluids |
|
|
Define a micron |
One micron equals one millionth of a meter, 39 millionth of an inch, the smallest particle that can be seen with 20/20 vision is about 40 microns |
|
|
Briefly describe the following |
Gear pumps- is a fixed capacity pump which has two or more rotors External gear pumps-a conventional external gear pump has its gears meshing on their periphery (outer edges), one gear is driven by the other. The gears carry the liquid from the suction port to the discharge port, around the inner walls of the casing Lobe pumps- a lobe pump has larger spaces between its teeth than conventional gear pumps do. Their rotors must be driven by suitable drive gears mounted outside the casing. These pumps deliver a more pulsating flow than conventional gear pumps. Screw pumps - axial flow pump. It may have one, two or three screws which carry the liquid from the suction port to the discharge port. This pump also needs to be driven by external gears. These pumps move the liquid linearly through the pump which eliminates pulsation. There is no metal to metal contact within the pump, which makes its operation very quiet Internal gear pumps - has one external gear rotating within an internal gear. The rotation of the external gear is off-center to that of the internal gear. This arrangement is compact. The crescent seal and the gerotor pump are two commonly used internal gear pumps Vane pumps- may be unbalanced or balanced and have fixed or variable capacity. The variable vane pumps may be able to reverse flow through the system Hand pumps-most basic of piston pumps. Found in equipment such as hydraulic jacks and floor jacks. Design is similar to hydraulic cylinders and are either single or double acting. Check valves are used to maintain correct flow direction. A release valve is used to return the liquid to the reservoir Axial and radial piston pumps - are used in powered hydraulic systems. Both may have fixed or variable capacity, some reverse flow through the system |
|
|
What are the nominal and absolute filter ratings |
Nominal filter rating - indicates that the filter will remove most particles of that size or larger Absolute filter rating - means that a filter will stop all particles of that size or larger |
|
|
How is the filter condition indicated |
Indicators that show condition Gauges with green, yellow and red divisions of the dial Tell tales with green, yellow and red bands |
|
|
What are the advantages and disadvantages of the following |
Inlet side filters - filters all the hydraulic fluid going to the system, risk of cavitation Pressure side filters - protects the valves but does not protect the pump or main main pressure valves Return side filters often considered the best for a fine filters as the fluid is at its highest temp and therefore lowest viscosity. Most common |
|
|
List the purposes of a reservoir |
To provide an adequate supply of fluid to the system Also allows air in the hydraulic fluid to escape, dirt and water to settle out and heat to dissipate |
|
|
How much capacity should a reservoir have |
2-3 times rated pump delivery for 1 min |
|
|
Describe the construction of a reservoir |
Normally made from steel plate with all joints welded. Must meet general industrial specifications. Some have a sloping bottom and a drain plug at the lowest point |
|
|
What is the importance of a vent, a temperature gauge and a sight glass |
Vent- opening must be large enough to let the air move as fast as the fluid is either removed or returned to the reservoir Temperature gauge-is often installed in the reservoir to help monitor any excessive heat buildup in the system Sight glass - important to be able to see the oil level |
|
|
What are the purposes of heat exchangersbrei |
To ensure correct operating temperature of the hydraulic fluid. Heat exchangers are frequently used to cool the oil in a hydraulic system. They can also heat the oil for cold weather startups |
|
|
Briefly describe |
Water coolers - usually consist of a nest of tubes in a shell (cylindrical container) oil flow in one direction and cooling water flow is in the opposite direction Air coolers - the tubes are vertically mounted and have fins for heat removal. Oil flows through the tubes and a fan drives cooling air over the tubes and fins to remove heat Heaters- adding heat to the fluid by use of either electric immersion heaters with thermostat control, by passing steam or hot water through a coil or length of pipe submerged in the tank, by starting up the hydraulic system and pumping oil over the relief valve at maximum pressure to create heat from fluid friction |
|
|
What are the purposes of accumulators |
Store fluid under pressure for future use as a source of potential energy. They also absorb shock waves or dampen pulsations and maintain a constant pressure in the system |
|
|
Describe how pumps are rated |
By their capacity and pressure |
|
|
When using an accumulator, what should be known about it? |
The amount of oil to be added before the gas charge The recommended pressure of the gas charge (precharge is a percentage of the operating systems max pressure |
|
|
What must be done before any work is started on an accumulator loaded system |
Isolate the accumulator with a shut off valve Discharge or drain the accumulator back to tank |
|
|
Describe |
Weight loaded accumulator - a vertical cylinder fitted with a piston. A packing gland or similar device keeps the fluid in the cylinder as the piston moves. A platform on the piston is loaded with scrap iron, concrete blocks or other heavy material. The force of gravity provides the energy to keep the fluid under constant pressure Spring loaded accumulator-use compression springs instead of gravity to supply resistance. Springs must be evenly loaded to allow even travel of the piston through the cylinder. This type of accumulator does not produce constant pressure through the entire stroke. The springs exert minimum pressure when the accumulator is at a low volume Gas charged accumulator-depend on the compressibility of a gas (such as nitrogen or air) to produce the necessary pressure and delivery. Compressed air is used for low pressures Dry Nitrogen is used for med to high pressures **Pure oxygen is never used with petroleum oil because an explosion or fire may result Non separated accumulator -have no physical barrier between the gas and liquid, they are mainly used on low pressure systems. The low pressure limits the amount of gas that dissolves in the liquid Piston accumulator - are much like cylinders without piston rod. A simple piston accumulator has a free floating piston between the liquid and the gas. Has two sets of required packing. These seal the two chambers and centralize the piston to prevent any metal to metal contact. A bleed hole is used to eliminate any buildup of pressure between the seals Bladder accumulator- have a natural or synthetic rubber bag mounted inside a chamber. This bag separates the liquid from the gas. The bag is molded around the gas valve and mounted through the top of the chamber. The liquid connection is mounted through the bottom of the chamber Diaphragm accumulator- are similar in operation to bladder accumulators. They use a natural or synthetic rubber diaphragm mounted in the center of the chamber to separate the liquid from the gas |
|
|
Describe the operating principles of the following |
A pressure intensifier- when extremely high pressure is needed. It is not possible for the system to create such a pressure, pressure intensifiers are used. Intensifiers for one type of fluid use hydraulic fluid in both sides. Those with two types use compressed air or other gas on one side and hydraulic fluid on the other. The cylinder has two different size pistons connected by a piston rod, or one piston and a plunger. It operates as a force multiplier. The lower pressure act on the large piston are and forces the small piston forward, creating much higher pressure A bourdon gauge - consists of a calibrated dial face and pointer attached through linkage to a flexible metal tube, called a bourdon tube. The bourdon tube is connected to the system pressure. As the system pressure increases the Bourdon tube tends to straighten slightly. This is due to the difference in area between its inside and outside surfaces. This action causes the pointer to move around its dial face and indicate the pressure A Schrader gauge- has a calibrated dial face with a pointer attached through linkage to a plunger and bias spring. The system pressure is connected to the gauge and acts on the plunger. As the pressure increases the plunger is forced against the bias spring. This moves and rotates the pointer around the dial face, indicating the pressure A flow meter - portable testing device. They are rarely permanently attached to the equipment. Used to monitor the flow in a line and to determine the efficiency of pumps and motors A pressure activated electric switch- the most common electrical interface device found in a hydraulic system. They protect operators, equipment and work in progress by sounding alarms and shutting off equipment when the pressure gets too high |
|
|
What units are used to measure pressure |
Bar, kpa, psi |
|
|
Why is the measurement of temperature important |
Indicates whether the heaters or coolers are set correctly as to forewarn its malfunction |
|
|
List the hydraulic fluids available today |
Petroleum oil - most common Fluids with high water content Invert emulsion fluids Glycol fluids Synthesized hydrocarbon hydraulic fluids Vegetable or grain oils |
|
|
List the qualities that should be considered when selecting a hydraulic fluid |
Viscosity Viscosity index Pour point Thermal stability Resistance to oxidation Resistance to rusting Resistance to air foaming Fire resistance Lubricating qualities Long life Cost Disposability |
|
|
Briefly describe the following |
HFA fluids- high water content, are a soluble oil or synthetic chemical emulsion, they generally contain 5% to 10% oil. The soluble oil has a milky appearance and the synthetic solutions have a clear appearance. These fluids have excellent cooling ability HFB fluids- water in oil fluids, are invert emulsions which contains more oil then water. They generally contain only 40% water. HFB fluids should be checked regularily to maintain the viscosity and the water- oil ratio. Water or oil can be added to maintain the required ratio HFC fluids - water glycol fluids, consists of 30%-40% water dissolved into glycol. HFC fluids should be checked regularily to maintain the correct water glycol ratio. Glycol is the same family as permanent antifreeze ethylene , therefore the fluid can endure temps below freezing, these fluids are heavier than oil and should be used with either a very short suction lift, a special inlet design , or in situations where the fluid level is above the pump inlet HFD fluids - are a special chemical compound which do not support combustion. They have no water content and can therefore be used at high temperatures with no evaporation problem. These are the heaviest of the hydraulic fluids and require special pump inlet designs or special pump mounting positions. This fluid attacks the most. Common seal materials used in petroleum oil systems |
|
|
How is hydraulic fluid kept clean during storage |
Keep the spout or bung free from moisture or contaminants. If they are stored outside, tilt the barrels at an angle with the bung out of the water line |
|
|
How is pressure created in a hydraulic system |
Is created by a resistance to flow |
|
|
What determines the rate of flow that can pass through a conductor without excessive friction |
The inside diameter of the line determines the rate of flow that can pass without excessive friction, heat and power loss |
|
|
What determines the bursting pressure of a line |
The wall thickness and the inside diameter determine the bursting pressure of a line, the greater the wall thickness the higher the bursting pressure |
|
|
What type of pipe is used for hydraulic systems |
Seamless steel (black) pipe with its interior free from rust, scale and dirt is recommended for hydraulic systems |
|
|
What is the wall thickness of 1” schedule 160 pipe and what size threading dies are required for it |
Wall thickness .250 Inside diameter .815 1.315 is the OD |
|
|
List the thread types used on hydraulic piping |
NPT American national pipe taper NPS American national pipe straight NPTF dryseal pipe taper |
|
|
How is sealing compound applied to the pipe |
Should be treated with a sealing compound on the male end before being threaded into a fitting and tightened, this prevents the threads from galling to each other, also prevents fluid from leaking through the spiral clearance at the roots of the thread. |
|
|
Why is tubing a popular hydraulic conductor |
Because it can be easily bent and flared |
|
|
How is tubing size measured |
Are taken from the outside diameter, the nominal dimensions are given in fractions of an inch or dash number. The dash number represents the outside diameter in sixteenths of an inch. For ex a tube with dash 8 equals OD 8/16” or 1/2” |
|
|
Briefly describe flared and flareless joints |
Flared joints- rely on the compression of the tubing material between the inner and outer walls of the flare. Tube flares are made with a flaring tool kit. The tube can be made single or double flared. Flareless joints - use an intermediate product to grip the tubing. As the nut is tightened onto the end fitting, the intermediate product is compressed. This causes it to bite into and press onto the tubing to create a seal and hold it in place |
|
|
What factors should be considered when bending tubes |
-to ensure a smooth bend, use proper bending tools and an appropriate bend radius for the tubing size. Generally, the correct bend radius is 3-4 times the tube OD Use the bend radius to calculate the required cutoff length of tubing Bend the tube carefully to avoid distortion. Any flattening, kinks or wrinkles in the bends cause turbulence in the flow Leave a straight length of at least twice the nut length between a bend and a fitting. This allows the connecting nut to slide away from the fitting when necessary Form bends properly so that the fittings are in alignment. Misaligned tubes cause stress to the parts to be connected Allow for expansion or contraction due to temperature when short lengths of tubing are used |
|
|
Compare pumps mounted above the reservoir to pumps mounted below the reservoir |
Above the reservoir- the pump must be able to create enough vacuum or pressure drop to overcome the weight and friction of the liquid, the height from the liquid level to the pumps center line (suction lift) Below the reservoir-atmospheric pressure helps to push liquid into the suction side of the pump. This gives the pump the added advantage of being charge (or pressure fed) by the suction head of liquid |
|
|
Describe the basic procedure for tube installation |
Start from a fixed point Use proper hydraulic fittings Use as few fittings as possible by making bends in the tubing. (Every fitting is a source of turbulence as well as a potential leak) Locate the simplest route with the least number of bends Make sure all joints or fittings can be easily reached for maintenance Put the tube line where it will not be a hazard to workers Put the line where it does not interfere with any other equipment |
|
|
What are the purposes of a hose |
Hose is used to connect parts which move in relation to each other, or which are subject to vibration |
|
|
Describe the construction of various hydraulic hoses |
Various types of hose are used the reinforcement determines the pressure rating Low pressure hose has two or more fabric braid requirements, it can withstand pressure from 250-500 psi depending on its ID Hose with one metal wire braid for reinforcement is called single wire braid and is good for working pressures up to about 1500 psi. Two wire braid hose has a pressure rating of up to about 3000 psi. Substituting spiral wrap wire for the wire braid brings the pressure rating still higher up to 6000 psi for hoses with smaller ID |
|
|
Describe the construction of hose end fittings |
Permanent fittings are crimped or swagged on the hose end and are discarded with the hose. A hose crimping machine is required to assemble the fittings Reusable fittings are screwed or clamped to the hose ends and salvaged when the hose is discarded. Hose for reusable fittings can be purchased in bulk and each section assembled as needed |
|
|
Describe the basic procedure for hose installation |
Allow enough slack to avoid kinking the hose at a ridge connection Do not use a taut hose: pressure tends to bulge the hose and shorten it Do not twist the hose: this can be checked by markings on the cover (use fittings to avoid long loops) Foo
Follow specifications for minimum bend radius Install hose |
|
|
What |
Jkfjka |
|
|
What is the major factor in a hydraulic component failure |
Contamination |
|
|
What is the purpose of a magnetic plug in a reservoir |
Plugs are used to attract steel or iron particles and are normally mounted in the reservoir |
|
|
Define a strainer and state its purpose and usual position in a hydraulic system |
Can be considered as coarse filters, they remove larger solids from fluids travelling in a straight path. Mounted in the reservoir on the inlet of the pump |
|
|
Describe |
Proportional filters - has only a portion of the oil passing through the filtering element. The rest flows directly to the reservoir. With continuous recirc, all of the oil eventually flows through the filter Full flow filters - all the oil passes through the filtering element, this design gives more filtering action but builds up resistance to flow as the filter becomes dirty. It often has a bypass valve |
|
|
Describe the filtering elements |
Mechanical (metal) filters- are considered coarse filters or strainers, and consist of layers of wire screens or discs of perforated metal. They remove the larger solid particles but do not remove water or very fine solids Adsorbent filters - filters contain materials such as paper, wood pulp, fabric waste and wool. They remove fine particles as well as water and water soluble impurities Absorbent filters - remove impurities by both mechanical and chemical means. Bone black, charcoal, fullers earth and other active clays are examples of these filtering materials. These filters remove all solid particles and insoluble sludge, plus nearly all water and soluble, oxidized material ** adsorbent filters may also remove most additives used in inhibited hydraulic fluids |
|
|
Define a micron |
One micron equals one millionth of a meter, 39 millionth of an inch, the smallest particle that can be seen with 20/20 vision is about 40 microns |
|
|
Briefly describe the following |
Gear pumps- is a fixed capacity pump which has two or more rotors External gear pumps-a conventional external gear pump has its gears meshing on their periphery (outer edges), one gear is driven by the other. The gears carry the liquid from the suction port to the discharge port, around the inner walls of the casing Lobe pumps- a lobe pump has larger spaces between its teeth than conventional gear pumps do. Their rotors must be driven by suitable drive gears mounted outside the casing. These pumps deliver a more pulsating flow than conventional gear pumps. Screw pumps - axial flow pump. It may have one, two or three screws which carry the liquid from the suction port to the discharge port. This pump also needs to be driven by external gears. These pumps move the liquid linearly through the pump which eliminates pulsation. There is no metal to metal contact within the pump, which makes its operation very quiet Internal gear pumps - has one external gear rotating within an internal gear. The rotation of the external gear is off-center to that of the internal gear. This arrangement is compact. The crescent seal and the gerotor pump are two commonly used internal gear pumps Vane pumps- may be unbalanced or balanced and have fixed or variable capacity. The variable vane pumps may be able to reverse flow through the system Hand pumps-most basic of piston pumps. Found in equipment such as hydraulic jacks and floor jacks. Design is similar to hydraulic cylinders and are either single or double acting. Check valves are used to maintain correct flow direction. A release valve is used to return the liquid to the reservoir Axial and radial piston pumps - are used in powered hydraulic systems. Both may have fixed or variable capacity, some reverse flow through the system |
|
|
What are the nominal and absolute filter ratings |
Nominal filter rating - indicates that the filter will remove most particles of that size or larger Absolute filter rating - means that a filter will stop all particles of that size or larger |
|
|
How is the filter condition indicated |
Indicators that show condition Gauges with green, yellow and red divisions of the dial Tell tales with green, yellow and red bands |
|
|
What are the advantages and disadvantages of the following |
Inlet side filters - filters all the hydraulic fluid going to the system, risk of cavitation Pressure side filters - protects the valves but does not protect the pump or main main pressure valves Return side filters often considered the best for a fine filters as the fluid is at its highest temp and therefore lowest viscosity. Most common |
|
|
List the purposes of a reservoir |
To provide an adequate supply of fluid to the system Also allows air in the hydraulic fluid to escape, dirt and water to settle out and heat to dissipate |
|
|
How much capacity should a reservoir have |
2-3 times rated pump delivery for 1 min |
|
|
Describe the construction of a reservoir |
Normally made from steel plate with all joints welded. Must meet general industrial specifications. Some have a sloping bottom and a drain plug at the lowest point |
|
|
What is the importance of a vent, a temperature gauge and a sight glass |
Vent- opening must be large enough to let the air move as fast as the fluid is either removed or returned to the reservoir Temperature gauge-is often installed in the reservoir to help monitor any excessive heat buildup in the system Sight glass - important to be able to see the oil level |
|
|
What are the purposes of heat exchangersbrei |
To ensure correct operating temperature of the hydraulic fluid. Heat exchangers are frequently used to cool the oil in a hydraulic system. They can also heat the oil for cold weather startups |
|
|
Briefly describe |
Water coolers - usually consist of a nest of tubes in a shell (cylindrical container) oil flow in one direction and cooling water flow is in the opposite direction Air coolers - the tubes are vertically mounted and have fins for heat removal. Oil flows through the tubes and a fan drives cooling air over the tubes and fins to remove heat Heaters- adding heat to the fluid by use of either electric immersion heaters with thermostat control, by passing steam or hot water through a coil or length of pipe submerged in the tank, by starting up the hydraulic system and pumping oil over the relief valve at maximum pressure to create heat from fluid friction |
|
|
What are the purposes of accumulators |
Store fluid under pressure for future use as a source of potential energy. They also absorb shock waves or dampen pulsations and maintain a constant pressure in the system |
|
|
Describe how pumps are rated |
By their capacity and pressure |
|
|
When using an accumulator, what should be known about it? |
The amount of oil to be added before the gas charge The recommended pressure of the gas charge (precharge is a percentage of the operating systems max pressure |
|
|
What must be done before any work is started on an accumulator loaded system |
Isolate the accumulator with a shut off valve Discharge or drain the accumulator back to tank |
|
|
Describe |
Weight loaded accumulator - a vertical cylinder fitted with a piston. A packing gland or similar device keeps the fluid in the cylinder as the piston moves. A platform on the piston is loaded with scrap iron, concrete blocks or other heavy material. The force of gravity provides the energy to keep the fluid under constant pressure Spring loaded accumulator-use compression springs instead of gravity to supply resistance. Springs must be evenly loaded to allow even travel of the piston through the cylinder. This type of accumulator does not produce constant pressure through the entire stroke. The springs exert minimum pressure when the accumulator is at a low volume Gas charged accumulator-depend on the compressibility of a gas (such as nitrogen or air) to produce the necessary pressure and delivery. Compressed air is used for low pressures Dry Nitrogen is used for med to high pressures **Pure oxygen is never used with petroleum oil because an explosion or fire may result Non separated accumulator -have no physical barrier between the gas and liquid, they are mainly used on low pressure systems. The low pressure limits the amount of gas that dissolves in the liquid Piston accumulator - are much like cylinders without piston rod. A simple piston accumulator has a free floating piston between the liquid and the gas. Has two sets of required packing. These seal the two chambers and centralize the piston to prevent any metal to metal contact. A bleed hole is used to eliminate any buildup of pressure between the seals Bladder accumulator- have a natural or synthetic rubber bag mounted inside a chamber. This bag separates the liquid from the gas. The bag is molded around the gas valve and mounted through the top of the chamber. The liquid connection is mounted through the bottom of the chamber Diaphragm accumulator- are similar in operation to bladder accumulators. They use a natural or synthetic rubber diaphragm mounted in the center of the chamber to separate the liquid from the gas |
|
|
Describe the operating principles of the following |
A pressure intensifier- when extremely high pressure is needed. It is not possible for the system to create such a pressure, pressure intensifiers are used. Intensifiers for one type of fluid use hydraulic fluid in both sides. Those with two types use compressed air or other gas on one side and hydraulic fluid on the other. The cylinder has two different size pistons connected by a piston rod, or one piston and a plunger. It operates as a force multiplier. The lower pressure act on the large piston are and forces the small piston forward, creating much higher pressure A bourdon gauge - consists of a calibrated dial face and pointer attached through linkage to a flexible metal tube, called a bourdon tube. The bourdon tube is connected to the system pressure. As the system pressure increases the Bourdon tube tends to straighten slightly. This is due to the difference in area between its inside and outside surfaces. This action causes the pointer to move around its dial face and indicate the pressure A Schrader gauge- has a calibrated dial face with a pointer attached through linkage to a plunger and bias spring. The system pressure is connected to the gauge and acts on the plunger. As the pressure increases the plunger is forced against the bias spring. This moves and rotates the pointer around the dial face, indicating the pressure A flow meter - portable testing device. They are rarely permanently attached to the equipment. Used to monitor the flow in a line and to determine the efficiency of pumps and motors A pressure activated electric switch- the most common electrical interface device found in a hydraulic system. They protect operators, equipment and work in progress by sounding alarms and shutting off equipment when the pressure gets too high |
|
|
What units are used to measure pressure |
Bar, kpa, psi |
|
|
Why is the measurement of temperature important |
Indicates whether the heaters or coolers are set correctly as to forewarn its malfunction |
|
|
List the hydraulic fluids available today |
Petroleum oil - most common Fluids with high water content Invert emulsion fluids Glycol fluids Synthesized hydrocarbon hydraulic fluids Vegetable or grain oils |
|
|
List the qualities that should be considered when selecting a hydraulic fluid |
Viscosity Viscosity index Pour point Thermal stability Resistance to oxidation Resistance to rusting Resistance to air foaming Fire resistance Lubricating qualities Long life Cost Disposability |
|
|
Briefly describe the following |
HFA fluids- high water content, are a soluble oil or synthetic chemical emulsion, they generally contain 5% to 10% oil. The soluble oil has a milky appearance and the synthetic solutions have a clear appearance. These fluids have excellent cooling ability HFB fluids- water in oil fluids, are invert emulsions which contains more oil then water. They generally contain only 40% water. HFB fluids should be checked regularily to maintain the viscosity and the water- oil ratio. Water or oil can be added to maintain the required ratio HFC fluids - water glycol fluids, consists of 30%-40% water dissolved into glycol. HFC fluids should be checked regularily to maintain the correct water glycol ratio. Glycol is the same family as permanent antifreeze ethylene , therefore the fluid can endure temps below freezing, these fluids are heavier than oil and should be used with either a very short suction lift, a special inlet design , or in situations where the fluid level is above the pump inlet HFD fluids - are a special chemical compound which do not support combustion. They have no water content and can therefore be used at high temperatures with no evaporation problem. These are the heaviest of the hydraulic fluids and require special pump inlet designs or special pump mounting positions. This fluid attacks the most. Common seal materials used in petroleum oil systems |
|
|
How is hydraulic fluid kept clean during storage |
Keep the spout or bung free from moisture or contaminants. If they are stored outside, tilt the barrels at an angle with the bung out of the water line |
|
|
How is pressure created in a hydraulic system |
Is created by a resistance to flow |
|
|
What determines the rate of flow that can pass through a conductor without excessive friction |
The inside diameter of the line determines the rate of flow that can pass without excessive friction, heat and power loss |
|
|
What determines the bursting pressure of a line |
The wall thickness and the inside diameter determine the bursting pressure of a line, the greater the wall thickness the higher the bursting pressure |
|
|
What type of pipe is used for hydraulic systems |
Seamless steel (black) pipe with its interior free from rust, scale and dirt is recommended for hydraulic systems |
|
|
What is the wall thickness of 1” schedule 160 pipe and what size threading dies are required for it |
Wall thickness .250 Inside diameter .815 1.315 is the OD |
|
|
List the thread types used on hydraulic piping |
NPT American national pipe taper NPS American national pipe straight NPTF dryseal pipe taper |
|
|
How is sealing compound applied to the pipe |
Should be treated with a sealing compound on the male end before being threaded into a fitting and tightened, this prevents the threads from galling to each other, also prevents fluid from leaking through the spiral clearance at the roots of the thread. |
|
|
Why is tubing a popular hydraulic conductor |
Because it can be easily bent and flared |
|
|
How is tubing size measured |
Are taken from the outside diameter, the nominal dimensions are given in fractions of an inch or dash number. The dash number represents the outside diameter in sixteenths of an inch. For ex a tube with dash 8 equals OD 8/16” or 1/2” |
|
|
Briefly describe flared and flareless joints |
Flared joints- rely on the compression of the tubing material between the inner and outer walls of the flare. Tube flares are made with a flaring tool kit. The tube can be made single or double flared. Flareless joints - use an intermediate product to grip the tubing. As the nut is tightened onto the end fitting, the intermediate product is compressed. This causes it to bite into and press onto the tubing to create a seal and hold it in place |
|
|
What factors should be considered when bending tubes |
-to ensure a smooth bend, use proper bending tools and an appropriate bend radius for the tubing size. Generally, the correct bend radius is 3-4 times the tube OD Use the bend radius to calculate the required cutoff length of tubing Bend the tube carefully to avoid distortion. Any flattening, kinks or wrinkles in the bends cause turbulence in the flow Leave a straight length of at least twice the nut length between a bend and a fitting. This allows the connecting nut to slide away from the fitting when necessary Form bends properly so that the fittings are in alignment. Misaligned tubes cause stress to the parts to be connected Allow for expansion or contraction due to temperature when short lengths of tubing are used |
|
|
Compare pumps mounted above the reservoir to pumps mounted below the reservoir |
Above the reservoir- the pump must be able to create enough vacuum or pressure drop to overcome the weight and friction of the liquid, the height from the liquid level to the pumps center line (suction lift) Below the reservoir-atmospheric pressure helps to push liquid into the suction side of the pump. This gives the pump the added advantage of being charge (or pressure fed) by the suction head of liquid |
|
|
Describe the basic procedure for tube installation |
Start from a fixed point Use proper hydraulic fittings Use as few fittings as possible by making bends in the tubing. (Every fitting is a source of turbulence as well as a potential leak) Locate the simplest route with the least number of bends Make sure all joints or fittings can be easily reached for maintenance Put the tube line where it will not be a hazard to workers Put the line where it does not interfere with any other equipment |
|
|
What are the purposes of a hose |
Hose is used to connect parts which move in relation to each other, or which are subject to vibration |
|
|
Describe the construction of various hydraulic hoses |
Various types of hose are used the reinforcement determines the pressure rating Low pressure hose has two or more fabric braid requirements, it can withstand pressure from 250-500 psi depending on its ID Hose with one metal wire braid for reinforcement is called single wire braid and is good for working pressures up to about 1500 psi. Two wire braid hose has a pressure rating of up to about 3000 psi. Substituting spiral wrap wire for the wire braid brings the pressure rating still higher up to 6000 psi for hoses with smaller ID |
|
|
Describe the construction of hose end fittings |
Permanent fittings are crimped or swagged on the hose end and are discarded with the hose. A hose crimping machine is required to assemble the fittings Reusable fittings are screwed or clamped to the hose ends and salvaged when the hose is discarded. Hose for reusable fittings can be purchased in bulk and each section assembled as needed |
|
|
Describe the basic procedure for hose installation |
Allow enough slack to avoid kinking the hose at a ridge connection Do not use a taut hose: pressure tends to bulge the hose and shorten it Do not twist the hose: this can be checked by markings on the cover (use fittings to avoid long loops) Follow specifications for minimum bend radius Install hose lines so parts can be easily reached for maintenance. Keep hoses from rubbing on fixed objects and keep moving objects from rubbing on them Keep the hose away from high heat sources if the hose cannot be moved, insulate it |
|
|
What standards do fittings and couplings have to meet |
ANSI American national standards institute SAE society of automotive engineers ISO international standards organization |
|
|
Briefly describe |
Threaded fittings-threaded ends for port connections can be NPT (tapered), NPTF (tapered dryseal), or NPS (straight) threaded fittings are most practical on tubing sizes up to 7/8” Flanged fittings - for tubing sizes above 7/8” diameter Quick disconnect couplers - quick disconnect are often used where hoses are meant to be removed periodically. Not all couplers mate with couplers from all manufacturers |
|
|
When designing a hydraulic system from where do you start |
The starting point is usually the work to be done The decisions to be made are Which actuator is to be used and the valves to control it The size and type of pump which will supply the required flow and handle the necessary pressure The size of the reservoir Any other component needed for the system |
|
|
When circuit reading from where do you start |
Following the flow of fluid from the pump to the actuator. This allows you to see what is open to fluid flow and what is closed to fluid flow...always make sure you can get back to tank |
|
|
Why is the reservoir symbol shown over and over on most system drawings |
To clarify the flow path back to the main reservoir |
|
|
What is the major factor in a hydraulic component failure |
Contamination |
|
|
Referring to figure 76 is the solenoid activated bleed down valve normally open or normally closed |
Normally open |
|
|
What type of approach should be taken when troubleshooting hydraulic systems |
Requires a logical approach. Successful troubleshooting begins with creative communication with the operator. It continues with identification and isolation of the problem, safe shut-down of the equipment clean and orderly removal and repair of the problem and follow up on the repair |
|
|
Is talking to the operator of the equipment usually productive when troubleshooting hydraulic systems |
Yes |
|
|
What are the reasons for the loss of speed and the loss of force in a hydraulic system |
Pump creates flow. Loss of speed is a reduction in volume Resistance creates pressure. Loss of force is a reduction in pressure |
|
|
What are the steps taken to shut down the equipment safely |
Lower or mechanically secure all suspended loads Release all pressure in the system Discharge all accumulators and intensifiers Isolate the electrical control system and power supply using correct lockout procedures |
|
|
How is contamination of the hydraulic components prevented |
Keep the work area clean Layout the parts in an orderly and systematic manner for successful service Follow the specified repair procedures |
|
|
What is the procedure for starting up equipment |
Ensure the repaired part is mounted securely Ensure that all connections (hoses, piping, linkage, electrical etc) are fastened correctly and according to manufacturers specifications Adjust units for safe startup condition if necessary Ensure that all reservoirs have sufficient fluid levels Bleed or prime components as required Inform all personnel that the equipment is to be started Remove all safety interlocks Start equipment and observe for any problems |
|
|
What follow up should be carried out after the equipment is repaired and back in service |
After the equipment has been in service for the time specified by the company policy, ask the operator for any comments. If necessary reexamine the equipment |
|
|
What causes the following noise problems |
Cavitation in the pump- clogged strainer Dirty filter An obstruction in the inlet line Fluid viscosity too high Operating temperature too low Excessive pump speed
Relief valve noise - setting too close to another valve setting Worn poppet seat |
|
|
What causes the following excessive heat problems |
Excessive load on pump or motor - relief or unloading valve set too high Worn bearing Mechanical binding Pump coupling misaligned Motor coupling misaligned
Rise in fluid temperature - system pressure too high Dirty fluid or low fluid level Incorrect fluid viscosity Faulty cooling system |
|
|
What is the purpose of a magnetic plug in a reservoir |
Plugs are used to attract steel or iron particles and are normally mounted in the reservoir |
|
|
What causes the following flow problems |
Low flow rate — Flow control set too low Relief or unloading valve set too low Flow bypassing thru partly open valve External leak in the system Variable capacity pump inoperative Incorrect pump speed Internal leakage due to worn parts Excessive flow rate: Flow control valve set too high Variable capacity pump inoperative Incorrect pump speed Incorrect pump size |
|
|
What causes the following pressure problems |
Low pressure: Pressure reducing valve set too low Pressure reducing valve damaged Damaged pump or actuator
Erratic pressure: Air in the fluid Worn relief valve Contamination in the fluid Defective accumulator Loss of charge in accumulator |
|
|
What are the causes for the following actuator problems |
Slow movement: Low flow rate or pressure Fluid viscosity too high No lubrication on moving parts Sticking servo valve Worn or damaged actuator
Erratic movement: Erratic pressure Air in the fluid No lubrication on moving parts Erratic command signal Repair interconnecting wires Misadjusted or malfunctioning servo amplifier Malfunctioning feedback transducer Sticking servo valve Worn or damaged actuator |
|
|
What are the causes for the following cylinder problems |
Cylinder leaks but not visibly - between the piston and the cylinder walls Between the piston rod and the piston Bent cylinder rod- an overload in compression An accident with moving equipment The use of too small rod diameter |
|
|
What are the common problems of pumps and motors |
Internal leakage past their rotors Damaged shaft bearings and seals due to misalignment External leakage at the housing or fittings |
|
|
List common valve problems |
Spool sticking due to -foreign material lodged between the spool and the valve body -broken spring -detents have come out of position
Spool not shifting due to burned out solenoid
Excessive leakage due to -defective o rings -scored spool or valve body |
|
|
Describe three types of valve mountings |
An individual valve with foot mounting An individual valve with mounting plate or sub plate mounting Stacked valves |
|
|
List the potential problem areas with pilot valves |
Sticking spool Loose coil Broken armature Broken spring Worn pin between the armature and the spool Speed control adjustment on pilot operated valves |
|
|
What procedure should be followed when overhauling valves |
Try to obtain a manufacturers service manual and/or parts list Valves are usually disassembled by removing the end caps and extracting the spool and other parts from the body If no service manual is available pay special attn to the position of the spool. It works properly only when installed in one direction Replace o rings and seals at every overhaul. Coat them with hydraulic fluid before installing If the valve is put away in storage for future use -lubricate all internal components to prevent rusting -seal off all ports or openings to keep out contaminants |
|
|
Define a strainer and state its purpose and usual position in a hydraulic system |
Can be considered as coarse filters, they remove larger solids from fluids travelling in a straight path. Mounted in the reservoir on the inlet of the pump |
|
|
Describe |
Proportional filters - has only a portion of the oil passing through the filtering element. The rest flows directly to the reservoir. With continuous recirc, all of the oil eventually flows through the filter Full flow filters - all the oil passes through the filtering element, this design gives more filtering action but builds up resistance to flow as the filter becomes dirty. It often has a bypass valve |
|
|
Describe the filtering elements |
Mechanical (metal) filters- are considered coarse filters or strainers, and consist of layers of wire screens or discs of perforated metal. They remove the larger solid particles but do not remove water or very fine solids Adsorbent filters - filters contain materials such as paper, wood pulp, fabric waste and wool. They remove fine particles as well as water and water soluble impurities Absorbent filters - remove impurities by both mechanical and chemical means. Bone black, charcoal, fullers earth and other active clays are examples of these filtering materials. These filters remove all solid particles and insoluble sludge, plus nearly all water and soluble, oxidized material ** adsorbent filters may also remove most additives used in inhibited hydraulic fluids |
|
|
Define a micron |
One micron equals one millionth of a meter, 39 millionth of an inch, the smallest particle that can be seen with 20/20 vision is about 40 microns |
