Gear FAQ

Q: How do gears work?

A: Consider a motor that applies rotational force (torque) to a shaft.  It is not possible to drive the robot’s wheels directly from the motor output, because in their factory configuration, the motor’s output speed tends to be too high.

Gears allow us to adjust the motor’s rotational force using mechanical advantage, so the robot’s wheels turn with the speed and force that meet our requirements.

The relevant formula is T = Fr

where r is the radius of the gear, T is torque, and F is force.

We use a 2-stage gear.  You can think of F (force) as a constant here.  The output gear (called the driven gear) is larger than the input gear (called the driving gear), and so the torque is increased.    If the output gear is smaller than the input gear, the torque decreases.  Besides the change in torque that takes place when gears are combined, there is also a corresponding change in speed.

In summary, when a small gear drives a large one, torque is increased and speed is decreased.

Gears are combined using their teeth. The number of teeth is not arbitrary, since it is the key means of proper reduction.  Gear teeth require special design so that they mesh properly. If there is any looseness between meshing gears, this is called backlash, the ability for a mechanism to move back and forth within the teeth, without turning the whole gear.

Reducing backlash requires tight meshing between the gear teeth, but that, in turn, increases friction.  For this reason, gears require lubrication.   Another potential problem with gear mechanisms is extra friction caused by gears rubbing against the gear box.  Another potential problem, experienced by the team in a previous year, is a manufacturing error which caused gears to be mismatched in the right and left wheel assembly.  To verify the correct gear ratio, it is worthwhile to count the gear teeth.