What torque is needed to compress a spring?
What torque is needed to compress a spring?
(OP)
Hi all,
I need to know how to calculate the torque needed to be applied to a bolt in order for him to compress a spring. As the force of the spring rises with its deflection I'm expecting some trouble. In my scheme (check attachment), the bolt compresses a spring that pushes a steel ball in a brake. The bolt will be actuated by a servo motor. As I want to control the brake dynamically, I need to know at least the torque range that the servo has to perform.
Can anyone help me?
BR
I need to know how to calculate the torque needed to be applied to a bolt in order for him to compress a spring. As the force of the spring rises with its deflection I'm expecting some trouble. In my scheme (check attachment), the bolt compresses a spring that pushes a steel ball in a brake. The bolt will be actuated by a servo motor. As I want to control the brake dynamically, I need to know at least the torque range that the servo has to perform.
Can anyone help me?
BR





RE: What torque is needed to compress a spring?
RE: What torque is needed to compress a spring?
The torque vs. bolt force relationship obviously exists, but it may not be very reliable for what you want to do. The torque vs. bolt force is based on (affected by) so many variables, that except under ideal conditions it can vary quite a bit. Try this on for size..... you know the spring rate to a fairly good degree of accuracy, that is, lbs./ 16th inch of compression ; you know the pitch on the bolt thread, that is, 16th inches of movement per bolt revolution; that gets you your force on the bolt and ball fairly accurately. Now, back out your torque range for the servo-motor operation/size, but still control things by the number of degrees of bolt revolutions. Any number of good Mech. Engineering refs. should give you some ranges of torque vs. force on the bolt.
RE: What torque is needed to compress a spring?
Thanks for the answer. I knew it would be almost empirical to get a solution but I think it might be ok the way you present. The only doubt I have is if the relation torque/force on the bolt is related to the shear stress instead of the normal stress, as it's mostly used in steel structures calculations.
BR
RE: What torque is needed to compress a spring?
For a very approximate torque to axial force relationship you could use:-
F = T/(0.2*d)
where T = torque
F = axial load i.e. spring load
0.2 is a friction factor for a non lubricated bolt
As stated be dhengr there is a lot of variables in torqueing bolts and I have no idea how accurate you wish to be.
You could use a torque wrench to tighten the bolt up and then measure the spring compression, from that you get the spring load and the torque.
RE: What torque is needed to compress a spring?
RE: What torque is needed to compress a spring?
Torque friction factors are wildly inaccurate. This provides control over force generally adequate to ensure a bolt is torqued down. I would be surprised it was repeatable enough to make your brake work.
The nice thing about springs is that you can get fairly gross movement for a given force. Design your brake actuator to move some fairly long distance like 10, 20 or 30mm. Work out the compression of your spring.
You know the spring rate, right?
--
JHG