| I remember the days when trimming grass around trees, fences, and flower beds involved the use of hand operated clippers. You know, those scissor-like things that require you to squeeze the handles together to move the blades. Cutting seemed to take forever and there was a lot of bending, stooping, and kneeling which would kill your back and turn your knees green from grass stains. Worst of all, the repetitive motion of squeezing the handles dozens of times would cramp your hands. It was a great day when gasoline powered grass trimmers came along. Just pull the recoil starter cord and you’re ready to go. It’s fast, easy, and the final result looks better too.
If you’ve ever operated a gasoline powered tool like a grass trimmer, you probably noticed that the cutter action isn’t immediate once the engine is started. Instead, the engine enters into a much slower initial speed mode, the idle speed. The cutter moves only after the throttle trigger is depressed. This introduces more gas to the engine, causing it to speed up, and this action is due to a device called the centrifugal clutch. A centrifugal clutch, or any type of clutch for that matter, serves one basic function, to physically disconnect, then reconnect a gasoline engine from whatever it is powering. For example, if the engine in a weed trimmer stayed permanently connected to the cutter when the engine was started, it would pose a definite safety hazard. Even at idle speed, the cutter would immediately kick into high speed operational mode, and if someone wasn’t prepared for this instant response there would be a good probability of injury. When a centrifugal clutch is placed between the engine and the cutter, it automatically disconnects the engine from the cutter during starting and at idle speed. We’ll see how it does that in a later blog. For now, let’s consider the fact that the idle function serves as a “get ready.” The user is able to both psychologically and physically prepare themselves to use their tool. Pressing the trigger revs the engine up and causes the centrifugal clutch to connect the engine to the cutting action. When the operator takes their finger off the throttle trigger the engine returns to idle speed, and the clutch automatically disconnects the engine from the cutter. The cutter becomes idle. Figure 1
Figure 1 shows a gas trimmer and its centrifugal clutch. The engine is on one end of the trimmer and the cutter at the other. A hollow metal tube runs between them. This tube contains the cutter drive shaft. The centrifugal clutch and its clutch housing are located in a cone shaped compartment between the engine and the metal tube. The clutch is connected to the engine drive shaft and the clutch housing is connected at the other end of the cutter drive shaft. When they’re assembled into the grass trimmer, the clutch fits within the clutch housing. Next time we’ll see how the centrifugal clutch on a grass trimmer uses centrifugal force and friction to automatically transmit mechanical power from the gas engine to the cutter. ____________________________________________
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Posts Tagged ‘engine drive shaft’
Mechanical Power Transmission – Centrifugal Clutches
Monday, April 2nd, 2012
Tags: centrifugal clutch, clutch, clutch housing, cutter, cutter drive shaft, drive shaft, engine drive shaft, engineering expert witness, forensic engineer, friction, gasoline powered grass trimmer, grass trimmer, hand held gasoline powered tool, hollow metal tube, idle speed, injury, mechanical power transmission, recoil starter, safety hazard, throttle trigger, weed trimmer, weed wacker
Posted in Engineering and Science, Expert Witness, Forensic Engineering, Innovation and Intellectual Property, Personal Injury, Product Liability | Comments Off on Mechanical Power Transmission – Centrifugal Clutches
