# Is this a real tortion spring?

Hi guys,

I am trying to figure out if my calculations are correct.

would a torsion spring which produces 173 pounds/rotation and is pulled to 180 degrees be something that is realistic?

I can’t figure out how to convert the ft-lbs over at mcmaster to lbs/rotation…

am I even working in the right units? somehow lbs/rotation seems to be what I want, and I can’t understand what ft-lbs do here… (it’s late… give me a break )

Thanks!
-Leav

p.s.
phew! CD just had a database error! I need to get my heart back up from my pants! :ahh:

Torsion springs produce torque so pounds per rotation is not a unit which can be applied to them. What the unit on McMaster car is referring to is the maximum torque when the spring is fully wound. It should have a linear decay from that point.

So for a 180 degree spring with 40in lbs of torque it would have 20in-lbs at 90 degrees and 0 in-lbs at 0 degrees.

I’m not sure if you know this so I also give a brief explanation of torque.
Essentially 1 ft-lb can support 1 lb on a one ft radius. so a 40 in-lb spring would be able to just barely support a 40lb load at 1 in from the point of rotation.

To figure out how much torque you need to produce a given speed you will have to used rotational physics equations such as torque=rotational acceleration*moment of inertia.

thanks guys…

so my calculations show that I need 173 inch-lbs.

which is about 4.5 more than what mcmaster carries… I hope the local spring shop has what we need… thanks!

-Leav

You could also try and solve that by using several springs together.