A centrifugal clutch is a clutch

Clutch

A clutch is a mechanical device which provides for the transmission of power from one component to another...

 that uses centrifugal force

Centrifugal force

Centrifugal force can generally be any force directed outward relative to some origin. More particularly, in classical mechanics, the centrifugal force is an outward force which arises when describing the motion of objects in a rotating reference frame...

 to connect two concentric shafts, with the driving shaft nested inside the driven shaft.

The input of the clutch is connected to the engine crankshaft

Crankshaft

The crankshaft, sometimes casually abbreviated to crank, is the part of an engine which translates reciprocating linear piston motion into rotation...

 while the output may drive a shaft, chain, or belt. As engine revolutions per minute

Revolutions per minute

Revolutions per minute is a measure of the frequency of a rotation. It annotates the number of full rotations completed in one minute around a fixed axis...

 increase, weighted arms in the clutch swing outward and force the clutch to engage. The most common types have friction pads or shoes radially mounted that engage the inside of the rim of a housing. On the center shaft there are an assorted number of extension springs, which connect to a clutch shoe. When the center shaft spins fast enough, the springs extend causing the clutch shoes to engage the friction face. It can be compared to a drum brake

Drum brake

A drum brake is a brake in which the friction is caused by a set of shoes or pads that press against a rotating drum-shaped part called a brake drum....

 in reverse. This type can be found on most home built karts, lawn and garden equipment, fuel-powered model cars and low power chainsaws. Another type used in racing karts has friction and clutch disks stacked together like a motorcycle clutch. The weighted arms force these disks together and engage the clutch.

When the engine reaches a certain speed, the clutch activates, working somewhat like a continuously variable transmission

Continuously variable transmission

A continuously variable transmission is a transmission that can change steplessly through an infinite number of effective gear ratios between maximum and minimum values. This contrasts with other mechanical transmissions that offer a fixed number of gear ratios...

. As the load increases, the speed drops, disengaging the clutch, letting the speed rise again and reengaging the clutch. If tuned properly, the clutch will tend to keep the speed at or near the torque peak of the engine. This results in a fair bit of waste heat, but over a broad range of speeds it is much more useful than a direct drive in many applications.

Centrifugal clutches are often used in moped

Moped

Mopeds are a type of low-powered motorcycle designed to provide economical and relatively safe transport with minimal licensing requirements.Mopeds were once all equipped with bicycle-like pedals , but moped has been increasingly applied by governments to vehicles without pedals, based on their...

s, underbone

Underbone

An underbone is a small motorcycle built on a frame consisting mostly of one large diameter tubular component.The stylistic difference between a motorcycle and an underbone is considerable and they are sometimes known as "step-throughs" or even marketed as scooters in the West.The fuel tank...

s, lawnmowers, go-kart

Go-kart

thumb|A [[Kart racing|racing kart]] at the [[Commission Internationale de Karting|CIK-FIA]] European Championship 2008A go-kart is a small four-wheeled vehicle...

s, chainsaw

Chainsaw

A chainsaw is a portable mechanical saw, powered by electricity, compressed air, hydraulic power, or most commonly a two-stroke engine...

s, and mini bikes to

• keep the internal combustion engine
  Internal combustion engine
  The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
  
   from stalling when the output shaft is slowed or stopped abruptly
• disengage loads when starting and idling.

Thomas Fogarty

Thomas Fogarty

Thomas Fogarty , is a professor of surgery at Stanford University. He is also an inventor of medical devices and a vintner.-Background:...

, who is also credited with inventing the balloon catheter

Balloon catheter

A balloon catheter is a type of "soft" catheter with an inflatable "balloon" at its tip which is used during a catheterization procedure to enlarge a narrow opening or passage within the body...

, is credited with inventing a centrifugal clutch in the 1940s although automobiles were already being manufactured with centrifugal clutches as early as 1936.

Advantages

• No kind of control mechanism is necessary
• It is cheaper than other clutches.
• Prevents the internal combustion engine
  Internal combustion engine
  The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
  
   from stalling when the output shaft is slowed or stopped abruptly therefore decreases the engine braking force.

Disadvantages

• Since it involves friction and slipping between driver and driven parts there is loss of power.
• As it involves slipping, therefore it is not desirable in cases where there is heavy load or in high torque requirements.

See http://contentdm.lib.byu.edu/ETD/image/etd223.pdf for a basic description of Dry Fluid Centrifugal Clutch design and operating principle. A device such as this was reportedly used in the Taylor Aerocar roadable airplane of the late 1940s and early 1950s.