# Constant Torque Spring - Damping Question

Question:

I have a constant torque spring that rotates the drive wheel of a Geneva mechanism. I am facing the following problem. I would like a very slow rotation of the drive wheel whenever the drive wheel is not driving the Geneva wheel of the Geneva mechanism (which is 270 degrees of a rotation). However, when the pin of the drive wheel rotates the Geneva wheel by 90 degrees, I need a lot of torque because there is a load connected to the Geneva wheel. Therefore, I need to use a high torque constant torque spring (7.50 in-lbs.).

I think that a rotational damper can help me slow down the rotation of the drive wheel (it is a timing mechanism, and the rotational velocity should be ~1 rpm), but when getting to the point of rotating the Geneva wheel, I would need the damping to be gone (because there, I need full torque).

Does anyone have a suggestion for a rotary damper that is capable of doing this? Or are there other ideas/designs that can solve the same problem in your opinion?

The spring will do 4 revolutions before rewinding. The speed accuracy is important. For 270 degrees of each rev, the rotational speed should be 1-2 rpm, while for the other 90 degrees of each rev (when the pin of the drive wheel moves the Geneva wheel), the rotational speed does not matter too much, but I need high torque for that part of the rotation (about 7.5 inch*pounds). I can’t see how a flywheel would solve the problem, can you elaborate a bit more.

I can’t use any electronics for this application. I am exploring different rotary dampers that provide the friction I need to slow down the rotation to achieve the speed characteristics I need.

The challenge here is to have a small (can’t be much larger than 1 inch diameter and 1 inch tall) rotary damper, that costs very little (single digit dollars). I’m exploring some options now, if you have any suggestions, please let me know. I guess using a non-ideal cheap damper is one option and attach a gear box that gives me the speed I need, but that seems to take up quite a lot of space.

You need a flywheel not a damper.

A vane spinning in air would be better than a viscous damper since viscous drag is proportional to speed whereas aerodynamic drag is proportional to speed squared. It would need to be geared up from your 1 rpm. Add a flywheel to the high speed shaft.
The benefit of a flywheel is to store energy from the spring when not needed and release it when needed. The damper only consumes energy meaning you need a bigger spring.
Perhaps a clockwork mechanism would be better all round.

Start with a round friction wheel (assume friction pad acting radially on the periphery). Remove material to reduce the radius at the sectors where you want to eliminate friction.
I still have a problem with using friction to control the speed of a disc which has a constant torque (spring) applied. Friction will apply a constant retarding torque regardless of disc speed ie it provides a torque control not a speed control.