Engineering Expert Witness Blog

     I got my first 10-speed bike when I was in high school.  It was nice, except for one nasty hangup, the brakes were always going out of adjustment.  Once it did this at the worst of times, when I was going down a steep hill.  I squeezed hard on the brake handles, and nothing happened.  The bike started to go out of control in its ascent down the hill, and in desperation I took my feet off the pedals and pressed the soles of my shoes as hard as I could into the road surface.  To my relief my emergency measure was effective.  The harder I pressed into the pavement, the less my shoes slipped, and the more the bike slowed down.  I had good rubber treads on the sneakers I was wearing that day, and the friction between the soles of my shoes and the surface of the pavement was strong enough to stop my runaway descent.  Something very similar occurs during the operation of a centrifugal clutch.

     If you recall from previous articles in this series, when the clutch mechanism spins faster than engine idle speed, the centrifugal force acting upon the clutch shoes overcomes the tension in the springs.  This causes the clutch shoes to make contact with the clutch housing.  But although there is contact, the clutch shoes will initially slip somewhat.  That is, the clutch housing and cutter head won’t spin at exactly the same speed when a faster spin is first employed, although the slip between the clutch shoes and housing decreases as engine speed increases. 

     Faster speed means there’s more centrifugal force at play, forcing the shoes harder against the drum of the clutch housing.  The increase in centrifugal force makes the shoes move tighter and tighter against the housing, and this causes an increase in friction.   Eventually the engine speed will increase to full throttle, the point where the shoes are pressed into the housing so hard there is no more slip.  The cutter head will then turn at the same rate as the engine, and the engine’s power will be fully transmitted to the cutter head, allowing you to trim grass effectively.

     Friction is a double edged sword.  On the one hand it reduces slip between the clutch shoes and clutch housing.  On the other, the friction between the slipping shoes and clutch housing generates a lot of heat, particularly if the grass trimmer is cutting thick grass.  We’ll see how that heat impacts the clutch mechanism components next week. 

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