Posts Tagged ‘weed trimmer’

Mechanical Power Transmission – Putting the Centrifugal Clutch Together

Sunday, April 29th, 2012

     I’ve never been one to enjoy table top puzzles, yet I love to examine the way mechanical things fit together.  Manipulating parts to see how they interrelate to form an operational system is a pastime I very much enjoy.  In fact, I spend many evenings at my work bench doing just this.  I often become so engrossed in the activity I forget what time it is.  The result is yet another night without TV.  So sad…

     Last week we looked at how a centrifugal clutch mechanism operates when it’s coupled to a gasoline engine shaft spinning at idle speed, and then we depressed the engine throttle trigger to speed things up.  Let’s now introduce a new component called the clutch housing to see how it interfaces with the clutch mechanism to drive the cutter head in a grass trimmer.

centrifugal clutch housing

Figure 1


     The clutch housing shown in Figure 1 resembles a rather short cup.  One end is open, the other closed.

     Figure 2 shows the closed end of the clutch housing connected to the cutter shaft’s coupling.  On the cutter shaft coupling resides a ball bearing which enables the clutch housing to both spin freely and act as a support for the clutch housing.  The open end of the clutch housing allows the clutch mechanism to fit neatly inside.

centrifugal clutch assembly

Figure 2


     Next time we’ll put the assembly shown in Figure 2 into operation.  First we’ll examine how the centrifugal clutch mechanism and clutch housing operate with the engine at idle speed, then compare that to the engine operating at actual cutting speed.



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.



Mechanical Power Transmission – Centrifugal Clutches

Monday, April 2nd, 2012
     I remember the days when trimming grass around trees, fences, and flower beds involved the use of hand operated clippers.  You know, those scissor-like things that require you to squeeze the handles together to move the blades.  Cutting seemed to take forever and there was a lot of bending, stooping, and kneeling which would kill your back and turn your knees green from grass stains.  Worst of all, the repetitive motion of squeezing the handles dozens of times would cramp your hands.  It was a great day when gasoline powered grass trimmers came along.  Just pull the recoil starter cord and you’re ready to go.  It’s fast, easy, and the final result looks better too.

     If you’ve ever operated a gasoline powered tool like a grass trimmer, you probably noticed that the cutter action isn’t immediate once the engine is started.   Instead, the engine enters into a much slower initial speed mode, the idle speed.  The cutter moves only after the throttle trigger is depressed.  This introduces more gas to the engine, causing it to speed up, and this action is due to a device called the centrifugal clutch.

     A centrifugal clutch, or any type of clutch for that matter, serves one basic function, to physically disconnect, then reconnect a gasoline engine from whatever it is powering.  For example, if the engine in a weed trimmer stayed permanently connected to the cutter when the engine was started, it would pose a definite safety hazard.  Even at idle speed, the cutter would immediately kick into high speed operational mode, and if someone wasn’t prepared for this instant response there would be a good probability of injury.

     When a centrifugal clutch is placed between the engine and the cutter, it automatically disconnects the engine from the cutter during starting and at idle speed.  We’ll see how it does that in a later blog.  For now, let’s consider the fact that the idle function serves as a “get ready.”  The user is able to both psychologically and physically prepare themselves to use their tool.  Pressing the trigger revs the engine up and causes the centrifugal clutch to connect the engine to the cutting action.  When the operator takes their finger off the throttle trigger the engine returns to idle speed, and the clutch automatically disconnects the engine from the cutter.  The cutter becomes idle.centrifugal clutch

Figure 1


     Figure 1 shows a gas trimmer and its centrifugal clutch.  The engine is on one end of the trimmer and the cutter at the other.  A hollow metal tube runs between them.  This tube  contains the cutter drive shaft.  The centrifugal clutch and its clutch housing are located in a cone shaped compartment between the engine and the metal tube.  The clutch is connected to the engine drive shaft and the clutch housing is connected at the other end of the cutter drive shaft.  When they’re assembled into the grass trimmer, the clutch fits within the clutch housing.

     Next time we’ll see how the centrifugal clutch on a grass trimmer uses centrifugal force and friction to automatically transmit mechanical power from the gas engine to the cutter.