Centrifugal Clutches for Small Engines

Richard L. Parish  |  4/26/2006 5:14:25 PM

Figure 1. Centrifugal clutch on string trimmer.

Figure 2. Typical clutch driving components with spring-loaded fly weights. Weights move out under centrifugal force and engage inside of drum.

Figure 3. Typical centrifugal clutch drum.

Centrifugal clutches are common on many grounds maintenance machines, including string trimmers, chain saws and some lawn mowers. They offer an inexpensive way for manufacturers to provide automatic declutching of a tool when the engine is throttled down to idle and automatic engagement when the engine is revved up.

How Centrifugal Clutches Work
A centrifugal clutch consists of a metal drum fastened to an output shaft, sprocket or sheave and containing a pair of internal fly-weights driven by the engine. The fly-weights may be restrained by springs or may be formed into a simple, single stamped piece. As the rotational speed of the internal portion increases (when the engine is revved up), centrifugal force causes the fly-weights to move outward, either stretching the springs or deflecting the stamped piece. When the fly-weights move out, they contact the inside of the clutch drum and cause it to rotate also, thus gradually engaging the clutch. When engine speed decreases to idle, the fly-weights retract and disengage from the drum, allowing the output to stop.

When and Where Centrifugal Clutches Are Used
Centrifugal clutches offer some advantages: they are less expensive than other clutches, and they are automatic, thus no engaging linkage or control mechanism is required. On many types of garden and grounds maintenance equipment it is desirable to be able to start the engine before the mechanism is engaged and allow the mechanism to stop when idled down.

Limitations of Centrifugal Clutches
Centrifugal clutches have two main drawbacks. First, their engagement is not very positive, thus they are not appropriate for transferring significant torque or power; they have a tendency to slip if loaded very heavily. Second, they do not engage or disengage reliably. This creates a safety hazard since a clutch may not engage and stop the mechanism reliably when the engine is idled down. If an operator relies on the clutch to stop the head of a string trimmer or the chain of a chainsaw when the engine is idled, he could be seriously injured when the string head or chain continues to rotate.

Safety Issues
One safety issue has just been addressed: The lack of positive shutdown. You should never rely on a centrifugal clutch to stop the motion of a machine. You should always turn off the engine before working on a string trimmer head, saw chain or anything else controlled by a centrifugal clutch. Another potential for injury occurs if an operator should try to run a partially assembled centrifugal clutch. If a centrifugal clutch is operated with the drum removed, the fly weights are very likely to fly off forcefully and seriously injure the operator or a bystander. The engine should never be started unless the drum is in place over the clutch, and the drum must be restrained correctly so that it cannot slide off the clutch in operation.

In summary, centrifugal clutches are a low-cost way to control the motion of a lawn or garden tool. They are generally adequate for the jobs to which they are put but have some functional and safety limitations. They must be used with care.

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