Torsion Spring for Opening Lid

[amoncur]

I am designing a lid that pivots out of its frame on one end by means of two torsion springs around the pivot axis. The lid weighs 0.4 pounds and the center of mass is 7 inches away from the pivot axis. Thus, the moment about the axis is 7 x 0.4 = 2.8. Since there will be 2 torsion springs, I am looking for springs with a 2.0 in-lb rating to be safe.

My question is this: does the leg length of the torsion spring have an effect on what torque specification I should be looking for? For example, McMaster 9287K145 has a torque rating of 2 in-lb, but its legs are 1.25" long. I can only accommodate a leg length of .75". If I chop off the leg length, will the McMaster spring still be 2 in-lb? Or am I thinking about this incorrectly?

 

[israelkk]

The legs are part of the spring and if are shortened the spring rate will increase and you will get more than 2 lb-in resulting in higher stresses in the spring.

 

[hydtools]

Shortening the leg will increase the force on the leg. The torsion will remain the same.

http://www.engineersedge.com/spring_torsion_calc.htm

Ted

 

[israelkk]

hydtools

The spring legs considered added coils to the number of coils of the spring body and their length is included in the active number of the coils of the spring. Therefore, by shortening the legs you decrease the number of active coils. At the same angular deflection the spring will give more torque and larger stresses in spring wire.

 

[zekeman]

"he spring legs considered added coils to the number of coils of the spring body and their length is included in the active number of the coils of the spring. Therefore, by shortening the legs you decrease the number of active coils. At the same angular deflection the spring will give more torque and larger stresses in spring wire."

Correct, but the spring stress of 2 in-lb would not change.
And the equivalent loss of the number of coils due to a decrement of 1 inch of leg is
.5/(pi*d)
d= pitch diameter of spring

 

[israelkk]

zekeman

If you apply the 2 lbf-in torque on the spring the stress will be the same. But usually the spring designed for same angular deflection of the lid therefore, the torque will be higher, the stress will be higher. May be slightly higher but still higher. If the spring is subjected to cyclic operation that may affect the life cycle.

 

[amoncur]

So how do I reconcile the moment imposed by the lid on the spring with the torque of the spring (assuming the spring legs will need to be shortened)? Since the torque does not change based on leg length, it seems I cannot simply match a torque specification to the moment calculated from the lid. How can I "translate" spring torque into an equivalent moment value?

 

[TheTick]

You need to consider more for your spring rate selection. There is also preload and the resulting lessening of the force on the spring as it unwinds.

I used to do this type of problem a lot (2-3 times a month over 4 years). We always started with a graph of torque vs. lid angular position. First curve was torque due to lid weight & CG position (which lessens as lid raises). Second curve was spring torque, which was linear with a slope of k and offset from zero by preload * k. This graph would show necessary preload and spring rate needed to lift lid and would also reveal if lid would close on its own or have a balance point. Note: Graph does not account for momentum.

 

[hydtools]

Based on your spring data, the spring rate is 2.0/90 = .022 lb-in/deg. The 2.0 lb-in torque is at 90 degree rotation. You can figure spring torque at various angles between 0 and 90 using that spring rate.

Ted