Posts Tagged ‘metallurgy’

Mechanical Power Transmission – The Centrifugal Clutch Feels The Heat

Sunday, May 27th, 2012

     Ever get out of bed on a cold winter morning and feel as stiff as a ladder?  Summer’s heat doesn’t have the same effect on aging joints as winter’s chill, and many retirees have been motivated to move into warmer climates because of it.

     Heat can change the properties of metals like steel, too.  By properties, I mean qualities such as hardness and stiffness–where hardness relates to steel’s ability to resist wear and denting, while stiffness relates to its ability to resist a force that is trying to bend it.

     Obviously, if things get hot enough, say in the thousands of degrees Fahrenheit, steel will soften and eventually melt into a blob of glowing liquid.  At lower temperatures the change will be less dramatic, but its atomic structure will be undergoing change nonetheless.  Varying temperatures cause atoms to become energized, causing them to move around within their atomic structure.  Depending on how quickly things cool back down, the iron and carbon atoms that make up the steel can end up in different locations, causing a permanent change.  The steel could end up softer or harder.  For example, slow cooling hot steel in air makes it softer, while rapid cooling, such as when you submerge hot steel quickly into cold oil, makes it harder.

     How does heat play a part in the ongoing discussion of the centrifugal clutch in a grass trimmer?  Well, friction between the shoes and housing generates heat as a result of centrifugal force.  Clutch springs are made of steel, which is hard and resistant to bending.  But during operation they may heat up to hundreds of degrees, then slowly cool down again when the grass trimmer is shut off.  Without getting into a complex explanation of metallurgy, this slow cooling makes the steel in the springs softer, and with time they will lose their stiffness and weaken.

     Over time the springs become so weak they are unable to overcome the centrifugal force acting on the clutch shoes, causing the clutch to fail at its task of disengaging the cutter head from the engine at idle speed.  In other words, as soon as the engine is started, the cutter head will rapidly begin to spin.   With these conditions in place, the cutter head poses a threat to anything or anyone making contact with it. 

     Next time we’ll look at another cause of centrifugal clutch failure, that is, component wear due to friction between the clutch shoes and clutch housing. 


centrifugal clutch springs damaged by overheating