Maintain a constant velocity for a rotating wheel

[AamsterC2]

TL;DR

Maintaining a constant speed for a wheel being turned by a rope wrapped around its circumference

I want to make a small mechanical calculator with no electric parts, nothing too advanced but a few binary gates capable of some basic functions.

All the examples I were able to find were amazing but were still dependent on electricity to drive the mechanism. One possibility is a spring but I was also interested in trying to make it powered by a weight lifted up and connected to a string that is wrapped around a wheel. As the weight drops it spins the wheel and powers the mechanism. The weight would keep the torque constant but the speed would be variable.

The only thing I could think of would be to have a flexible or expandable surface that would be forced out by centrifugal force and could reduce speed by rubbing against the inside of a drum. This would enforce a maximum speed on the system but I don't think it's the best solution. What techniques/devices exist for controlling this sort of behavior? Or is there some other way that I could drive the mechanism that avoids this problem all together?

 

[AZFIREBALL]

For the hanging weight method; find an old Grandfather clock; it has what you need to control the speed of the rotating shaft. Its the part that makes the tick-tock sound. Also a Grandfather clock type power drive mechanism is better than warping a string around a rod. Plus the rewind process is faster.

 

[jrmichler]

You are correct that speed control by centrifugal force and friction is not the best, but it can work. The old style mechanical record players were driven by a large clock spring. Wind it up, and it would turn the turntable. The speed was regulated by a centrifugal brake. The centrifugal brake had a pair of weights that would swing out from centrifugal force and brake against the inside of a drum, resulting in roughly constant speed.

The clock spring was about one inch wide and packed in black grease. On disassembly, it went SPROING and threw black grease everywhere, including over me and my clean white shirt. I learned this back in elementary school when I took one apart just before leaving for school.

Maybe just drive it with a hand crank?

 

[Rive]

If the mechanical load is constant and you can drop the idea of the rope then maybe you can consider turning it into a water mill. As I recall those can has a fairly stable RPM.

 

[some bloke]

I think a large part of this depends on the velocity you wish to maintain! As some people have said, the mechanism in a grandfather clock is one option. If you need a continuous motion (I believe a grandfather clock will tick in jumps rather than a smooth motion) then the defining factor is the speed of the weight dropping. The terminal velocity of a weight dropping can be dictated by the medium through which it falls - filling a vertical tube with water and having the weight fall through it will give a lower speed than falling through air.

An alternative, if you want a more tuneable system, is to have a long hydraulic piston, and connect both the inputs to one another with an adjustable valve in the pipe. Put a weight on the end of the piston and point it upwards - the speed at which it drops will be dictated by the restriction of the valve in allowing the hydraulic fluid to flow from the bottom to the top. This will result in a pretty uniform speed for the duration of the "fall".

Another alternative, working with the terminal velocity idea, is a set of paddles in an enclosed tank of viscous fluid, which are turned with the mechanism. They will have resistance proportional to velocity squared, so their own terminal velocity should slow the fall of the weight without needing a large tube of the stuff.

I also learned of something called Smart Grease when I was in school, which was specifically designed to only allow movement at a specific speed. A wheel inside a hole filled with the stuff should achieve a constant speed.