Robotics/Design Basics/Mechanical Components

Robotics: Design Basics: Mechanical Components

Gears

Gears are mechanical parts with cut teeth designed to mesh with teeth on another part so as to transmit or receive force and motion. The cut teeth are also sometimes called cogs. Others like in chainsaws and motors can be tweaked. In Robotics Gears are used to transfer rotational forces between axles. They can change speed and direction. The axles can stand in any orientation, however not all orientation can be done with 2 gears. Commonly gears are used to reduce the speed of a motor. When they reduce the speed, the torque of the output axle increases.

Common types of gears as used in Robots are explored below. Each type of gear is used for different purposes and it has both advantages and disadvantages.

See this Wikipedia article on gears. This page will cover how each gear is used specifically in robotics.

Spur gear

Spur gears found on a piece of farm equipment

Spur gears are the best-known gears. These are the simplest form of gears and are commonly used in light machines as bikes, mixers, etc,...
They aren't used in cars as they are very noisy and their design puts a lot of stress on the teeth.

You might use them to transfer rotation from a motor output shaft (coming directly from the motor or from the gear box) to the axle on which the wheels are attached. This poses a limitation: the motor output axle and the wheel axle have to be parallel.

Bevelled gear

Beveled gears are used when you wish to transfer work between two perpendicular shafts that are on the same plane (if the axles were to be extended they would hit). They can have straight, spiral or hypoid teeth.

Worm Gear

This is a gear that resembles a screw, with parallel helical teeth, and mates with a normal spur gear. The worm is in most cases the driving gear, there are however a few exceptions where the spur gear drives the worm. The worm gear can achieve a higher gear ratio than spur gears of a comparable size. Designed properly, a built-in safety feature can be obtained: This gear style will self-lock if power is lost to the drive (worm) however this feature doesn't work if the pinion is powered.

An Example of a Worm Gear and Pinion

Rack and Pinion

Torque can be converted to linear force by a rack and pinion. The pinion is a spur gear and meshes with a toothed bar or rod that can be thought of as a sector gear with an infinitely large radius of curvature. Such a mechanism is used in automobiles to convert the rotation of the steering wheel into the left-to-right motion of the tie rod(s).

Chains & Belts

Chains and belts can be used to transfer rotational motion over a longer distance. The main difference between the two, lays in the fact that a belt can slip. This is not necessarily a bad thing as slipping occurs when the output shaft carries a load that is too heavy. In this case the belt slips instead of the motor being stalled. This protects the motor as it could otherwise be damaged by the stall current.
The disadvantage of belts is that since it can slip, the amount of rotation doesn't remain the same, e.g., the input axle may turn 4 times, while the output axle may turn only 3.8 times. This should be taken into account if the position of the output axle is important.