Torsion Spring Loading Direction 2

[natepiercy]

Hello all,

Can anyone tell me why torsion springs are always installed such that the applied load closes the coil, rather than opens the coil? Phrased differently: what would be the danger in designing a spring that opposes a force in the "unwinding" direction?

Thanks in advance.
Nate

 

[swall]

You would have tensile forces on the wire I.D. which kills your fatigue life.

 

[israelkk]

You can use it either directions. However, it is weaker in the "open" coil direction due to the stress concentration factor (Wahl Factor) which is larger in the "open" direction. If you design for one direction it is best to use it for the "close" direction. If you design for both directions you should use the "open" stress concentration for calculations.

 

[israelkk]

One more issue is that when the spring is coiled to shape it has favorable residual stress condition (compression stress) at the outer diameter and unfavorable residual stress (tensile stress) in the inner diameter unless it is stress relieved. This will make the spring more weak when is loaded in the open direction.

 

[natepiercy]

Thank you both for your replies. That's very helpful information.

 

[hydtools]

israelkk, I believe you incorrectly stated residual stresses. The outer diameter stretches as the spring wire is wound to form the spring, therefore tensile stress in outer diameter.

Ted

 

[Compositepro]

Hydtools, after the spring is wound and then allowed to relax, the stresses reverse. We are talking about residual stresses in the spring with no external forces on acting on it.

 

[tbuelna]

Wrap spring clutches work by applying a tangential force to one end of a helical close-wound spring that has a tight fit on its ID to an output shaft, which causes the spring coils to expand in diameter and release their grip on the output shaft.

http://machinedesign.com/mechanical-drives/getting-speed-wrap-spring-clutchbrakes

 

[israelkk]

hydtools

When the spring wire is coiled from straight wire to the round shape the wire at the outer side of the spring is stretched permanently. Due to the springback property of the wire, the wire "wants" to return to its previous un stretched condition. This causes compressive stress inside the wire at the outer diameter of the spring. On the other hand, in the other side of the wire, in the inside diameter or the spring the residual stress is tensile stress.

 

[hydtools]

Got it. Thanks.

Ted