An Introduction to Mechanisms

• Gears work a little like pulleys without the need for a belt. The two rotating parts have teeth that mesh together passing the motion from one gear to the next.

• We can use gears to change:
  • The direction of motion
  • The speed
• The advantage of a gear over a pulley is the meshing prevents slippage so greater forces can be applied.

• Velocity ratio = number of teeth on the driven gear / number of the teeth on the driver gear = input speed / output speed

Output speed = input speed / ratio

Here is a worked example:

• An idle gear is used to change the direction of the output/driven gear or used to connect gears where a large centre distance is needed.
• They do not change the speed ratio.

• Gear trains, two or more gear joined together, can be simple or compound.
• The difference Compound gear trains have more than one gear on a shaft.