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4 Cards in this Set
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BELT DRIVES
A belt drive moves a pulley (follower or driven pulley) from another pulley (driver) by means of a friction force between the surface of the belt and the pulleys.' If there is no slippage between the belt and the pulleys, the linear speed of a point on each pulley will be the same and equal to the linear speed of the belt. If the pulleys are the same diameter, the rotational speeds (r/min) of the pulleys will be equal. If the driver pulley is smaller in diameter than the follower, the follower will rotate at a lower r/min than the driver, and vice versa. ie belt driven pulley shown in Figure 1, the belt drive should be arranged so that the tension(Force F1) in the bottom belt is always greater than the tension (Force F 2)'of The top belts.This increases the angle of contact the belts with the pulleys and reduces slippage, with a resulting increase in power transmission from driver to follower. Belt drives are used to transmit power or to change rotational speeds, or both. |
Rotational Speed
of Pulleys If the belts do not slip (efficiency = 100%), then any point on the rim of each pulley will travel at the same linear speed (r/min) as the belt itself, although the rotational speed (m/s) of the pulleys will vary if they are different diameters. This is illustrated in Figure 2. If there is no belt slippage, a point on the rim of the driver would travel a distance equal to the circumference of the driver for each revolution. |
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Pulley Trains
A pulley train consists of a series of pulleys connected by belts. Pulley trains are used to change speeds and/ )wer at varying speeds to different shops or machines. When considering pulley trains, the principles for simple pulley arrangements can be applied
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GEAR DRIVES
In gear drives, teeth cut into each gear wheel mesh together to transmit the power. For single gears meshing together, the direction of rotation becomes reversed. The ratio of rotational speeds of the gears depends upon the number of teeth on each gear as shown in EX Rotational speed A(r/min) / Rotational speed B(r/min) = Number of teeth A / Number of teeth B |
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Intermediate Gears
Intermediate gears or idler gears, are used to transmit power between gears that are a distance apart. Intermediate gears do not affect the speed of the driven gear, since the same number of teeth mesh with both driven gear and driver. If two intermediate gears were used, then the rotation of the driver and driven gears would be in opposite directions.The intermediate gears do not affect the speed ratio. The idler gear does reverse the direction of rotation of C, which rotates in the same direction as A. An advantage of gear drives over belt drives is that there is no slippage. However, badly meshing or pooriy lubricated gears result in excessive friction and low efficiency. Gears are able to transmit more power than belt systems of comparable size. |
CHAIN DRIVES
Chain drives use special gears called sprockets, which are driven by chains. An advantage of chain drives is that the gears do not have to mesh together and a positive drive can be obtained over a longer distance. The speeds of chain drives are calculated in the same way as for gear drives. Gear Trains A number of gears may mesh together in a train, to produce variations in shaft speeds and directions of rotation. Gear trains are considered in a similar manner to pulley trains. Backlash Because clearance is necessary between teeth that have to mesh, gears can be moved slightly when not driving. This movement is called backlash and, if excessive, can cause extreme forces on the teeth during starting or reversing. |
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Linear Velocity(m/s)= r/min *2(pi)R m/r / 60 s/m r/min*(pi)d / 60 m/s N1/N2=D2/D1 |
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