Posts Tagged ‘boss’

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.



Mechanical Power Transmission – The Centrifugal Clutch Mechanism

Sunday, April 15th, 2012
     My journey through engineering school was marked by a cast of colorful characters from around the world.  I remember one Russian professor in particular, fond of extolling the virtues of Russian engineering by the statement, “In Soviet Union steel ingots roll in one door, military tanks roll out other door.”  During that period of history in his homeland, it was not uncommon for all components down to the smallest screw to be manufactured within the same factory.

     That professor taught me all about clutch mechanisms, and whether they’re present in Soviet tanks or grass trimmers they perform the same basic function.  Let’s take a look at one now.

Centrifugal Clutch Mechanism

Figure 1


    Figure l shows my color-enhanced clutch illustration, which makes it easy to identify the different components of a centrifugal clutch.  The main part of the clutch is colored green and it’s respectfully referred to as the “boss.”  I assume it’s earned the title due to its role in keeping all component parts of the clutch assembly together.

     The blue portion shows two clutch shoes.  The boss fits loosely into notches within the shoes.  The curved surfaces on the shoes are composed of a high friction material, and we’ll see why later.  Two springs attached to the shoes cause them to pull towards each other and keep them from falling off the ends of the boss.

     The yellow portion shows the engine shaft coupling.  It’s permanently affixed to the center of the boss.  This coupling has a hole in it that enables the clutch mechanism to be attached onto an engine shaft with a threaded nut or some other type of mechanical fastener.

     Now that we’re familiar with a centrifugal clutch’s parts we can see how they come into play in a real world application, that of an engine shaft.   We’ll explore that next week.