Page 141 - Build Your Own Combat Robot

Build Your Own Combat Robot
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Mounting Gear Assemblies
Now that we’ve covered gear assemblies and methods of gear reduction, we
should mention the relative difficulty of constructing a gear reduction power
transmission using off-the-shelf gears. The most difficult part of the process is the
extreme precision required in the placement of two adjacent gears. If they are
placed too close together, the gears will bind and not turn freely. If the two gears
are too far apart, “gear slop” will occur and actual gear slippage might occur. To
place the gears at a proper spacing, you must calculate the center distances and
make the exact distance measurements of the two shaft’s centers on the gearbox,
and then carefully drill and bore the holes using a milling machine.
It is best to bolt the two sides of your gearbox together before drilling to ensure
that all holes on one side align with those on the other side. You must use ball bear-
ings or bronze bushings to support the gear shafts, and they need to have accurate
holes bored to allow the bearings to be pressed in firmly. Remember that when you
drill those first holes and later bore them out for your bearing assemblies, any mis-
takes in placement will mean that you will have to start from scratch, which will
mean two new sides for your gearbox. Plan accordingly and measure carefully.

Securing Gears to Shafts
The second difficult part in building your own gearbox is fastening two gears of
different diameters on a single shaft. If the gears are to rotate freely like an idler
gear, you don’t have to fasten them securely to transfer torque between gears.
However, if you intend to construct an assembly like the one shown in Figure 6-2,
gear number 2 and gear number 3 or sprocket number 2 and sprocket number 3
must be securely attached to the shaft to transmit torque between them. This will
more than likely require a hardened steel pin protruding through the gear’s hub,
through the steel shaft, and through the other side of the hub. If the hub extends on
both sides of the gear, a second pin is recommended. You should always use a
hardened steel pin—never use a cotter pin. Another method to secure gears and
sprockets to a shaft is to use a keyway and a square key stock that was illustrated
in Figure 3-13. Many gears and most sprockets already have one keyway ma-
chined on the inside bore. A local machine shop can easily add a matching keyway
in your axelshaft. Whatever you do, do not use set screws to secure a gear or
sprocket to a shaft. They will not hold together.
You must also be careful to align the gears with each other within the gearbox.
Securing the shafts against side-to-side slop can be accomplished using collets fas-
tened on the shaft inside the bearings. If this all sounds a bit complicated, you’re
right. It really is complicated for the first-time machinist. A better way to go to
achieve speed reduction is to use a chain and sprockets. The distance between
sprocket shaft centers can be a lot less precise to accomplish the same ratio of
speed on the sprockets. If your robot needs a gearbox, you should use a gearbox
that has already been designed and manufactured.
The most common term for these gearboxes is speed reducers. A wide variety of
speed reducers are in use, including parallel shaft speed reducers, where the input