centrifugal clutch

To understand how it works, you have to understand what a clutch is for. Basically, a clutch is necessary whenever you want to connect two rotating shaft together, but so that one of them is not totally dependent on the other. For example, in a chainsaw, you want the engine to be able to idle, without the chain spinning. The same for a rc nitro car; otherwise, you wouldn't be able to stop the thing.

How does it work? The parts of the centrifugal clutch are:

• Clutch bell: This is an empty cylinder that connects to the output shaft. The inner surface of the cylinder is usually coated with a grippy rough surface. For example, in rc nitro, this connects to the wheels. It looks like this side on:
  

                   +------
                   |~~~~~~
                   |
  output <=========|
                   |
                   |~~~~~~ <--- rough, grippy surface. 
                   +------
  

• Drive shaft: this is the shaft from the engine. It goes inside the clutch bell, but doesn't touch anything directly.
• Clutch shoes: These are connected to the drive shaft at one end. There are usually two or three shoes. They usually have a grooved shape. They look like this:

  
  +-------------+                          +-   -+
  |   +---------+                          | \_/ |
  | O |                                    +-----+
  +---+
    ^==== this part's connected              ^== Note groove
          the drive shaft, but hinged
          through the o. 
  
  PLAN VIEW                              SIDE VIEW
  

  They are usually more circular than what is shown above, so that the three shoes or whatever are basically equally sized segments of the circumference of a circle.
• Circular spring: This spring fits in the groove around the clutch shoes.

When the drive shaft starts to spin because the engine is revved, the clutch shoes spin out due to centrifugal force (but they fight against the spring). When they do so, they make contact with the rough surface of the clutch bell. This makes the clutch bell spin which in turn drives the output shaft. When the engine stops revving, the spring around the shoes pulls them back in so that they're held tight against the drive shaft; not in contact with the clutch bell. In this way, when the engine is idling, the drive shaft spins, but since the clutch shoes aren't touching the clutch bell, the output shaft doesn't spin.

By adjusting the tensile strength of the spring, the weights of the shoes and the diameter of the bell, different idling speeds can be set.

ymelup talks about a centrifugal clutch that engages the drive when the rotation speed increases above a certain amount - such as is used in chainsaws.

Centrifugal clutches can also be built the other way round, so the clutch disengages the drive above a certain speed.

This is often used in road cars to prevent the car over-revving - when you hit the red line (or often a little above the red line) a centrifugal clutch kicks in (kicks out?) to disengage the drive and prevent you damaging the engine.