Engineering Expert Witness Blog

     In previous articles we identified the component parts of a centrifugal clutch mechanism and learned how centrifugal force makes objects spinning in a circular path about a fixed point move outward.  We can now explore what happens when we couple a centrifugal clutch mechanism to the engine of a grass trimmer.

     Figure 1 depicts the spinning clutch mechanism of a gas engine when it’s just been started and is operating at a slow idle speed.

Figure 1

     Like the red ball in my previous article on centrifugal force, the blue centrifugal clutch shoes each have a mass m.  They spin around a fixed point P, situated at the center of the yellow engine shaft coupling.  Point P is located a distance r from the center of each shoe.  The shoes in motion have a tangential velocity V, and in accordance with Sir Isaac Newton’s Law of Centrifugal Force, the force F_c acts upon each shoe, causing them to want to pull out from the center of the mechanism, away from the fixed point.  Since idle speed is rather slow, however, the centrifugal force exerted upon the shoes isn’t strong enough to overcome the tension of the two springs and the coils connecting them remain coiled, holding the shoes tightly in position on the green boss.

     So what happens when we press the throttle trigger on the gas engine and cause the engine to speed up?  See Figure 2.

Figure 2

     Figure 2 shows the clutch mechanism spinning at an increased velocity.  The tangential velocity V increases, and according to Newton’s law, the centrifugal force F_c acting on the clutch shoes increases as well.  The force is so strong that it overcomes the tension in the springs and they extend.  The clutch shoes are caused to move out and away from fixed point P, as well as from each other, traveling along the ends of the boss.

     When we remove our finger from the throttle trigger, the engine will slow down and return to idle speed.  The centrifugal force will decrease and the springs will pull the shoes back towards fixed point P.  The mechanism will return to its previous state, as shown in Figure 1.

     Next time we’ll insert the centrifugal clutch mechanism into the clutch housing to see how mechanical power is transmitted from the engine to the cutter head in our grass trimmer.

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