Posts Tagged ‘mechanical power transmission’

Mechanical Power Transmission – The Centrifugal Clutch in Operation

Sunday, April 22nd, 2012
     Just the other day I unexpectedly experienced the effects of centrifugal force while  driving home from the grocery store.  The checker had packed my entire order into one bag, making it top heavy.  Then en route someone cut me off at an intersection, and I had to make a sharp turn to avoid a crash.  During this maneuver centrifugal force came into play, forcing my grocery bag out of its centered position on the front seat next to me.  It lurched into the passenger’s door, fell over, and spilled its contents onto the floor.  Fortunately the eggs didn’t get smashed.

     In previous articles we identified the component parts of a centrifugal clutch mechanism and learned how centrifugal force makes objects spinning in a circular path about a fixed point move outward.  We can now explore what happens when we couple a centrifugal clutch mechanism to the engine of a grass trimmer.

     Figure 1 depicts the spinning clutch mechanism of a gas engine when it’s just been started and is operating at a slow idle speed.

centrifugal clutch mechanism

Figure 1


     Like the red ball in my previous article on centrifugal force, the blue centrifugal clutch shoes each have a mass m.  They spin around a fixed point P, situated at the center of the yellow engine shaft coupling.  Point P is located a distance r from the center of each shoe.  The shoes in motion have a tangential velocity V, and in accordance with Sir Isaac Newton’s Law of Centrifugal Force, the force Fc acts upon each shoe, causing them to want to pull out from the center of the mechanism, away from the fixed point.  Since idle speed is rather slow, however, the centrifugal force exerted upon the shoes isn’t strong enough to overcome the tension of the two springs and the coils connecting them remain coiled, holding the shoes tightly in position on the green boss.

     So what happens when we press the throttle trigger on the gas engine and cause the engine to speed up?  See Figure 2.

clutch shoes

Figure 2


     Figure 2 shows the clutch mechanism spinning at an increased velocity.  The tangential velocity V increases, and according to Newton’s law, the centrifugal force Fc acting on the clutch shoes increases as well.  The force is so strong that it overcomes the tension in the springs and they extend.  The clutch shoes are caused to move out and away from fixed point P, as well as from each other, traveling along the ends of the boss.

     When we remove our finger from the throttle trigger, the engine will slow down and return to idle speed.  The centrifugal force will decrease and the springs will pull the shoes back towards fixed point P.  The mechanism will return to its previous state, as shown in Figure 1.

     Next time we’ll insert the centrifugal clutch mechanism into the clutch housing to see how mechanical power is transmitted from the engine to the cutter head in our grass trimmer.



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.