This demo show a hand crank operated worm
gear drive. Worm gear drives have two main components, the worm
and the worm gear, or worm wheel. Worm drives have four
salient features.
- The input shaft (the one with the hand crank
that turns the worm) is at a right angle to the output shaft (the one
with the steel "mallet" object attached).
- The reduction ratio is high compared to a normal spur
gear assembly. That is to say that a large number of turns on the input
shaft results in comparatively few turns of the output shaft.
- The torque gained in a single stage of gearing is
rather high, as would be expected from the large amount of speed
reduction between input and output.
- In the case of worm drives it is very common, as is
the case in this demo unit, for torque to be able to move only from the
worm shaft to the worm wheel shaft. This is different from the other
gear, belt and chain drives in the display case where torque can be
applied at either end. If you try to rotate the "mallet" object
attached to the worm wheel shaft you will find it impossible or nearly
impossible to turn. Please do not be too aggressive testing this last
statement.
In the Drive Ratios page, it is explained that as speed is lost, torque
is gained, and the proportionality between speed and torque in belt,
chain, and spur gear drives, is the ratio of the wheel diameters.
Its is also explained that for toothed wheels like gears, the
proportionality of speed and torque is based on the number of teeth on
the respective wheels. This proportionality holds true for worm drives
as well, with one odd twist. The worm only has one tooth. It just wraps
around the worm four or five times. The worm wheel has about 30 teeth,
giving a gear ratio of 30 to 1. (30:1).
Other things to note on this display are the mounted bearings
supporting the worm gear shaft, the bronze bushings supporting the worm
shaft and the structural material from which this display is
constructed. Lexan® polycarbonate sheet has optical properties
similar to acrylic sheet, but highly superior mechanical properties in
terms of strength and toughness.
Worm gear drives are usually acquired in a pre-manufactured form,
rather than built by hand. W.W. Grainger, Surplus Center, and MSC are
sources for prebuilt worm drive units. Most right angle gear drives are
found in the form of gearcases on gearhead motors. These motors usually
have high torque and a low output speed.
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