Engineering Expert Witness Blog

     Last time we looked at a gas powered trimmer’s engine, centrifugal clutch mechanism, clutch housing, and cutter head and how they’re assembled together.  With the centrifugal clutch assembled into the grass trimmer, let’s refer to Figure 1 to see what it looks like when we start the engine and run it at low, idle speed.

Figure 1

     Figure 1 represents a view from the back of the clutch housing, revealing the centrifugal clutch housing inside.  At idle speed there are only a few millimeters of space between the blue clutch mechanism shoes and red clutch housing, but the important point is that they’re not touching the clutch housing.  Because they’re not, the engine’s power can’t be transferred from the engine to the clutch housing, and it remains stationary, that is, the clutch housing doesn’t spin.  Since the grass trimmer’s cutter head is coupled to the clutch housing, it also remains stationary.

     Figure 2 shows what happens from the same viewpoint when we press the throttle trigger, making the engine spin at operational speed.

Figure 2

     With the engine spinning faster the centrifugal force, F_c, acting upon the clutch shoes overcomes the tension of the clutch mechanism springs, and the shoes move away from each other along the green boss.  They will eventually make contact with the clutch housing, enabling power from the engine to transfer to the clutch housing via the centrifugal clutch mechanism.  The clutch housing and cutter head spin along with the engine, and we can now cut grass.

     When we let go of the engine’s throttle trigger it again slows to idle speed, the shoes no longer touch the insides of the clutch housing, and the housing and cutter head stop spinning, as we saw in Figure 1.

     Next time we’ll talk about centrifugal clutch failures, things that can go wrong with them and keep them from operating properly.

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