clutch

I invented this clutch mechanism in response to a problem with one of my early Technic models. The model was a motor driven merry-go-round. Inherant within any merry-go-round is a large rotating structure that supports the seats. The problem was that when the motor was turned on, the merry-go-round started with a sudden jerk - most unlike the smooth and gentle start of the real thing. A similar problem occured with the braking. It stopped like it had hit a brick wall. Associated with this motion was the second problem of overstressing the gears in the drive-train. Overstressing gears is not part of my building philosophy so a solution had to be found.

The key component of this clutch is the differential assembly. Its usefulness is derived from the fact that it has one input and two dependent outputs. In the photos the output axle is marked with a white axle joiner and the actual clutch is the small yellow wheel with the string wrapped around it. The mechanism as presented only works well when the output is connected to a load with a large amount of friction and inertia. If the load is small, you can modify the mechanism by adding a similar structure to act as a brake and link it to the clutch control lever so that the brake releases as the clutch engages. The operation of the clutch is as follows:

The clutch is disengaged when the string around the yellow wheel is loose. As the motor drives the differential housing, the frictional load on the output shaft stops it from spinning. This results in all of the output being directed to the yellow wheel and it spins merrily. When the yellow control lever in the left of the top photo is slowly moved downwards, the string around the yellow wheel tightens. This results in the yellow wheel slowing and the drive energy is smoothly transferred to the output shaft that starts to rotate. When the clutch is fully engaged, the yellow wheel is stationary and all of the input drive motion is transferred to the output shaft. Notice that in the configuration of the top photo, the output shaft spins at twice the rotational velocity of the differential housing and in the same rotational sense.

	Top view of the clutch.
	This view from below shows the control lever in the clutch "released" position. The shock absorber spring is required to hold the control lever in either of its two extreme positions so that you do not have to hold it continuously. If you don't have a shock absorber a similar mechanism can be built using a rubber band. My first clutch used the rubber band method. The tail end of the string is tied to a friction pin. (I forgot to trim the string.)
	The control is now in the clutch "engaged" position. You will note that very little movement of the string is required (about 1/4 a revolution of the axle). This makes it somewhat difficult to get the string at exactly the correct length. The two half bushes allow for easy fine adjustment of the string length by sliding the left half bush back and forth along the axle.
	Two other configurations are possible. In this configuration, the output axle spins at the same speed as the input axle but in the opposite sense. (The control lever and string have been omitted from these photos but are still required.)
	In this third configuration the output axle spins at half the speed as the input axle and again in the opposite sense.