What torque is needed to compress a spring? 1

[BernardoRelvas]

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

 

[BernardoRelvas]

The image is this one

 

[dhengr]

BernardoRelvas:
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.

 

[BernardoRelvas]

Hi dhengr,

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

 

[desertfox]

Hi BernardoRelvas

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.

 

[tbuelna]

I was not able to open the sketch file you linked, so I do not know the details of your mechanism. As others noted, the relationship between wrenching torque and axial force at the bolt tip can vary greatly due to factors such as friction and interference in the bolt threads. And there are also factors like how the contact frictions at the bolt tip and spring face are reacted by the spring body. The friction at this contact can cause torsional wind-up in the compression spring wire which will have an effect on the axial force at the spring. There is also a difference in the axial force at the bolt tip due to the prevailing and running torques. The axial bolt force for a given wrenching torque will be much lower when the prevailing torque is added to the running torque.

 

[drawoh]

BernardoRelvas,

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