Archive for May, 2012

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

Mechanical Power Transmission – The Centrifugal Clutch and Friction

Sunday, May 20th, 2012

     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. 


centrifugal clutch friction


Mechanical Power Transmission – The Centrifugal Clutch and Metal Fatigue

Sunday, May 13th, 2012

     When I’m under a lot of stress I sometimes have the nervous habit of grabbing a paper clip, straightening out the bends, then repetitively bending it back and forth.  Eventually the wire reaches a point where it just breaks apart.

     My paper clip broke due to metal fatigue.  Metal parts are said to become fatigued when they’re subjected to forces of a repetitive nature such as occur due to twisting and bending.  The metal cracks, then eventually breaks due to the stress.

     So what’s happening when metal becomes fatigued?  Figure 1 shows the simplified atomic structure of a sample metal.

Figure 1


     When the metal is deformed, such as during bending, its rows of atoms are forced to move with respect to each other as shown in Figure 2.


Figure 2


     The movement of rows of atoms leads to an alteration in structure, breaking bonds between atoms.  This results in small cracks forming along the metal’s surface, cracks which eventually migrate deeper inside the metal with each subsequent bend.  With time the metal will become so compromised by the cracks that breakage occurs.

     Metal fatigue can occur in centrifugal clutch mechanisms as well.  Power tools such as grass trimmers typically operate between idle and working speeds many times during a day’s usage.  As we learned in previous articles, when the engine runs at idle speed, the springs in the centrifugal clutch mechanism stay retracted.  As the engine speeds up, the centrifugal force acting on the clutch shoes extends the springs.  Successive extensions and retractions cause the metal in the springs to bend, and over time they, like my paper clip, will become fatigued and metal springs will break.  

     Next time we’ll continue talking about centrifugal clutch failures and learn how the springs of a clutch mechanism can fail without its metal being brought to the breaking point.





Mechanical Power Transmission – The Centrifugal Clutch Powers Up

Sunday, May 6th, 2012

     Energy, or power, requires direct contact to transfer.  In most cases.  One notable exception to this rule of physics that I know of is the martial art of Tai Chi.  But when we’re talking golf, for example, if you don’t’ make contact with that ball, it ain’t gonna fly, no matter how many swings you take.

     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.

centrifugal clutch assembly

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.

centrifugal clutch power transmission

Figure 2


     With the engine spinning faster the centrifugal force, Fc, 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.