Constant force spring by using a bunji cord.

Can you explain further what you mean by constant force? Maybe give the actual application you have in mind.

Perhaps this would work: Try using a spool (constant radius), stretching the cord full extension and winding the (stretched) cord onto the spool. Then attach it (at a distance equal to the unstretched length of the cord from the spool) to a spring scale (like for weighing fish). Have someone allow the spool to unwind, calling out the angular increments, while someone else records the force at the scale. Use the force/angle information to build a varying-radius creel (helical cord-bed) so that the torque at the shaft is constant (T=F*r, so r = T/F(angle)). Use a gear, pulley, sprocket or other fixed-diameter element on the shaft to extract the constant force.

I keep reading "Perhaps this would work". I think it is a little over my head but the more I read it the more it sinks in and I think I am understanding it. I am going to keep studing this note becouse I think it might be the solution I need.

I am building a prototype trailer axle assembly that collects, stores and reuses momentum from slowing or stopping the trailer. I want to keep it as simple and light as possible so I thought I will use a bunji, like a bunji jumper might use, to collect, store then reuse the power to spin the trailer wheel forward. A constant force spring is like a large watch spring that somehow keeps a steady force applied from beginning to end. I think that would be good for my trailer axle but the large spring is to heavy for the small prototype I am building. Thanks for your input.

It's a great idea. It seems like the challenge would not be whether or not you could use a bungi, but instead, how would you provide enough slip between the cord and the spool to utilize the entire length of the cord. If you have a spool that slips, well then you would have trouble reapplying the torque that you have stored in in the cord. I think it could be done and done so quite simply. It almost seems like you need some kind of spool that has a differential rotation. I will continue to think on this. Something else you might consider is a flywheel, but this might not meet your weight requirement.

Wow, I appreciate you guys input. I think some day I will be moving in the direction of hydraulic, but for now I am going to try simple. The slip that is needed between the spool and the cord has been the problem. I tried stacking roller bearings end to end on a shaft and letting the bunji wind arround them and that seems to take care of a lot of the friction but the power it puts out is great at first and as it winds down it looses strength so I am trying to find a way to even the power output from start to finish. Thanks for your input.

I read rv guest's comment. It seems like he is really on to something. I still think friction between the cord and the spool would be a problem. Perhaps he has used or seen a setup similar to what you are trying to construct. As far as the constant force goes, the force will still be proportional to the stretch of the band. This is just the way elastic materials behave. However, in such a situation as you would apply it it shouldn't matter as a greater force when begining acceleration would be beneficial.

Also, I'd like to emphasize that you'll need a spool with some radius variation, as stated above. Even with bunji cords, a spring in its definition is always a device that gives a force proportional to a deformation. So, no constant force springs exist alone.

This is a really good idea. Are you thinling of actuate the "spring" together with the brakes? So, it's also needed to preview some device that knows the difference between the driver releasing the brake to apply throttle, and the driver releasing the brakes to get out of the car... easily done with parking brake actuation or so...

And, at the moment of the power release, I think maybe an acceleration limiter may be needed, like a dampener, or even a variable area restriction orifice in a line that somehow absorbs wheel energy like a pump... I know, you'd like to keep it simple, but my engineer sense is bipping. Toy cars that run under springs tend to be uncontrollable in the begining of the power release due to wheel slippage.

Personally, I still believe more in an electric motor that works as a generator when brakes are applied. It's less prone to failure and the power is controllable as desired.

But still an interesting idea. I'd like to read more about it.