Why are centrifugal clutches used?

Written by pauline gill
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Why are centrifugal clutches used?
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Besides disengaging the engine from the drive train at idle, centrifugal clutches provide several functions to simplify driveline designs on small multipurpose vehicles, lawn tractors and some mowers, power landscaping equipment, mini-bikes and go-karts. They are available in a variety of output types including drive-chain sprocket, V belt, continuously variable transmission belt drives, and coupled shaft drives. Flexible configurations provide for virtually every engine type on the market.

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Description

Drum type centrifugal clutches consist of a round steel brake drum, two or more segmented brake shoes with a thick layer of friction brake material, and a spring-driven retraction mechanism to keep the shoes centred and clustered to the hub while the engine is idling.

Engagement

As engine speed and power is increased, the centre hub holding the shoes spins faster and centrifugal force acting on the shoes starts to overcome the spring force and pulls them away from the centre hub and towards the inside of the drum. As the shoes engage the drum, the spinning friction turns the drum with the shoes. This starts to rotate the rest of the driveline, and the drum gradually is locked to the clutch hub by the combination of high centrifugal force and friction. When the throttle is released, the engine idles again and the shoes retract, freeing the drum to continue to turn with the driven load.

Advantages

Relatively simple and inexpensive in construction, centrifugal clutches are rugged and reliable. Careful selection of springs to match engine RPM characteristics allows an engine to operate in its peak torque range throughout most of the load acceleration process. Centrifugal clutches are also simple and inexpensive to implement, as there are no separate pedals or levers to purchase, integrate, or adjust. Once operating at normal engaged RPM, centrifugal clutches operate as one solid locked mechanical component, with no accumulated bearing wear while driving the load.

Disadvantages

Since centrifugal clutches usually engage under full or near-full power, the drums and shoes heat quickly which could cause overheating when stopping and starting repeatedly in a short period. This could cause chattering, grabby engagement if the drum cannot dissipate the heat fast enough. An overheat condition could also affect the lubricant in the centre bearing on which the drum freewheels during idling.

Variations

Centrifugal clutches can also be part of small constant-velocity belt transmissions that are popular on some go-karts and snowmobiles. Here, three or more rotating engagement weights actually increase the diameter of a driving pulley while its corresponding driven pulley decreases its own pitch diameter. This system is also used on scooters and mopeds. It can deliver a range of drive ratios based on RPM of over three to one, providing either strong accelerating force or high top speed when needed.

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