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I am building a kinetic sculpture. In part of the sculpture I have an axle in a sealed bearing. At the end of the axle are three fan blades that catch the wind and rotate the axle.

I can configure the blades so that when there is a slight breeze, I get a nice, slow, pleasing rotation. However, in this configuration, when it gets quite windy, the rotation is too fast, and not pleasing.

I can configure the blades differently so that in heavy winds, the rotation is not too fast. However, in this configuration, a slight breeze will not cause the axle to rotate at all.

Is there a (simple) mechanical way to have my cake and eat it, too - allow for the blades to catch a slight breeze and rotate slowly, while not going crazy in heavy winds?

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  • $\begingroup$ two options - 1. increase load (braking torque) with speed, as in answer below; 2. de-power the blades (ie airfoils) with speed, I.e. rotate them to decrease their angle-of-attack in the air flow. E.g. Sails are made with elasticity in twist to help take gusts more gracefully. So some twisting flex and/or ability to rotate each blade around its longitudinal axis (with elasticity to return it to position at low power) could be an idea. It won't get you a constant rotation speed, but should at least speed up less when overpowered. The two solutions together would help reduce forces in high wind $\endgroup$
    – Pete W
    Apr 1 at 13:15
  • $\begingroup$ Have a look for the wind turbines that are offset on their pivot so they rotate out of the wind as the speed increases. $\endgroup$
    – Solar Mike
    Apr 1 at 18:47

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I performed a search for "mechanical speed governor" and the results were exactly as expected, but the mechanisms presented are unnecessarily complex, in my opinion.

One such answer from Mechanical Question and Answer presents this image:

mechanical governor

Even though this is described as the regulator for a diesel engine and apparently manages fuel flow, the overall geometry is consistent across many such mechanisms.

The rotation imparts horizontal movement of the weights, which then act via linkage to change some aspect of the mechanism in order to restrict speed.

A simpler design with equal effectiveness may be a centrifugal clutch. The website Cars and Motors Online presents an explanation, but the image from the web site is easy enough to understand:

centrifugal clutch

The outer ring or shell is usually attached to the power train of a motor driven system such as a mini-bike and has a sprocket on the underside relative to this photo. The weights in the center of the mechanism are driven by the engine and obviously spins at the engine speed. As the speed increases, the weights inside expand against the springs and impart motion to the shell, which is lined with friction material akin to brake lining or clutch lining. In some cases, the weights are lined, but that's insignificant.

In your case, you would have the spinning mechanism rotate the weight assembly, while the shell (power out) would be affixed to something solid enough to handle the rotation forces. As the spinner reached speed, the friction would cause it to slow to retract the weights.

One could have to select an appropriate spring strength by trial and error to determine the correct combination to limit the speed to the desired level.

On a related note, one could create the mechanism in the first image, which is often considered artistic (and more frequently 'steampunk') in such a manner that the rotating weights contact a friction ring. This would be a more open version of the centrifugal clutch.

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