Centrifugal clutch

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A chainsaw clutch. The chain wraps around a sprocket behind the clutch that turns with the outer drum.

A centrifugal clutch is a clutch that uses centrifugal force 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 while the output may drive a shaft, chain, or belt. As engine RPM increases, 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 amount 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 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 RPM, the clutch activates, working almost like a continuously variable transmission. As the load increases the rpm drops, disengaging the clutch, letting the rpm rise again and reengaging the clutch. If tuned properly, the clutch will tend to keep the engine 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 mopeds, underbones, lawnmowers, battlebots, go-karts, chainsaws, and mini bikes to:

keep the internal combustion engine from stalling when the blade or weapon is stopped abruptly
provide low load during starting
allow engine to idle

Thomas Fogarty, who also invented the balloon catheter, is most often credited with first inventing the centrifugal clutch in the 1940's, although automobiles were being manufactured with centrifugal clutches as early as 1936 [1] (Dr Fogarty was born in 1934).