constant torque spring - damping question 1

[DaanW]

Hi all,

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?

Thanks in advance!

 

[gruntguru]

You need a flywheel not a damper.
We need more information. How many revolutions before rewinding the spring? Speed accuracy required? (It won't be very constant using this method)

je suis charlie

 

[DaanW]

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. Thanks!

 

[MintJulep]

Make your drive wheel non-round.

For the 270 degrees where you need a retarding load rub against a braking surface.

 

[gruntguru]

A vane spinning in air would be better than a viscous damper since viscous drag is proportional to speed whereas aerodunamic 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.

je suis charlie

 

[gruntguru]

Combining mintjulep's idea, perhaps drive the spinning vane shaft from a non-round friction wheel. (Friction force is not speed related.)

je suis charlie

 

[PNachtwey]

I agree with grunt guru's first post but blades in the air will not provide much damping at 1 rpm. I didn't see mention of a high speed shaft in the OP's post. Blades in water or oil would provide damping. I have a waterr ower exercise machine that works on this principle.

Why not skip all the Geneva gear and mechanical damping and use a servo motor to directly drive the load or use a gear box? Then you can get all the precision you need need. You can program any motion profile and change gains for different torque requirements during the motion. Modern motion controllers have the ability to execute cam profiles. Electronic damping is much more efficient than viscous or friction damping.



Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

 

[tbuelna]

What you need is a friction brake on the drive wheel that releases during the 90deg portion of travel where the pin engages the geneva mechanism slot. You will need to provide some method of fine-tuning the friction of the brake device to achieve the speed characteristics you require.

 

[DaanW]

Thanks for the responses guys. Some great suggestions here. PNachtwey, indeed, a servomotor would solve many of my problems, unfortunately though, I can't use any electronics for this application. Tbuelna, indeed, that is exactly the route I'm exploring right now. 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.

About the non-round friction wheel, interesting, but I'm not familiar with it. Do you have some more info on that (web page, youtube movie,..) Thanks!

 

[gruntguru]

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.

A mechanism like this Link includes everything you need - including (probably) an aerodynamic speed limiter.

EDIT In fact it looks like the vanes on the aero speed limiter might act like centrifugal bobweights and increase in profile as speed increases.

je suis charlie