Pulley drives, chain drives, and
similar systems are used for three main reasons: (1) to structurally
isolate but mechanically couple a motor and a load, (2) to decrease
speed, and (3) to increase
torque. Gears address the second and third functions but the
level of accuracy required to achieve proper tooth mesh requires the
driving and driven gears to be mounted on the same structure.
Belt and chain drives are far more forgiving of misalignment than gear
trains. Motors usually like to go fast. It's their nature. Only
rarely are gears, belts, or chains used to increase speed. They are
almost always used to decrease speed while increasing torque.
The demo shown above illustrates the principle of drive ratios using timing belts and timing pulleys. If you
turn the knob attached to the little pulley with your right hand
while lightly restraining the motion of the knob attached to the big
pulley with your left hand, you will feel how the torque is amplified
by speed reduction. If you turn the knob attached to the big pulley
while lightly restraining the motion of the knob attached to the little
pulley, you will see how speed is gained but at the expense of torque.
The relationships between speed and torque are ones of simple
arithmetic. Pulleys and gears are wheels. When a wheel of one size
turns a wheel of another size, the speed of the two wheels is simply
the ratio of their diameters. The torque at the driving and the driven
wheels is the same but inverted relationship. A gain of 30% speed
corresponds to a loss of 30% torque. Some power is lost in turning the
wheels and belts, but it
should be much less than half of the input power. Otherwise something
has gone wrong in the design.
For wheels with teeth (sprockets, gears) the diameter used to determine
speed and torque ratios is the pitch
diameter , which is somewhat smaller than the outside diameter
of the wheel. In discussions about sizes of toothed wheels the size of
the wheel is usually designated by the number of teeth on the wheel,
not its diameter. One reason to do it this way is because the pitch
diameter is hard to measure directly.
In the demo, the big timing pulley has 40 teeth, the middle one has 24
teeth, and the small one has 12 teeth. Therefore, the ratio of the big
to middle wheels is 1 : 1.66. The ratio of the middle and small
wheels is 1 : 2. The ratios multiply down the train of wheels, so the
ratio of the big wheel to the small wheel is 1 : 3.33. The little
wheel turns three and one-third turns for every turn of the big wheel.
In longer gear trains, the whole ratio from front to back can be had by
multiplying all the driving gear teeth together, then multiplying all
the driven teeth together, then dividing those two products to get the
total ratio.
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The timing belts and timing pulleys shown
above are from SDP-SI and Small Parts, Inc. See the sources
page .
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