How Do You Calculate the Required Torque for a Square Threaded Screw?

[stinlin]

Square threaded screws...

Homework Statement

http://img180.imageshack.us/img180/6789/statics2kq3.gif

Picture ^^

High-strength bolts are used in the construction of many steel structures. For a 1-in.-nominal-diameter bolt the required minimum bolt tension is 47.25 kips. Assuming the coefficient of friction to be 0.35, determine the required couple that should be applied to the bolt and nut. The mean diameter of the thread is 0.94 in., and the lead is 0.125 in. Neglect friction between the nut and washer, and assume the bolt to be square-threaded.

Homework Equations

tan(theta) = L/2*pi*r
Q = M/r

The Attempt at a Solution

All I was able to figure out is that...well, I wasn't. I tried drawing some pictures, but my big source of confusion is that I don't know how to draw a diagram for the thread and where the numbers all go. :( Please give me an umph in the right direction!

 

[turbo]

The threads on most bolts and nuts are triangular in cross-section. Is your instructor trying to simplify thing for you a bit by assuming that the cross-section of the threads are rectangular and not triangular? If so, can this help you?

 

[stinlin]

This is an introductory statics class, so we do square-threaded screws for now. The problem that I'm having is that I don't know how to address the bolt tension and what that does and how it affects this couple I need to find. The answer is given to be 737 lb*ft.

 

[stinlin]

Is this a legit diagram, with T being the bolt tension? Then I could solve for Q and get some large number, multiply it by the diameter of the bolt and divide by 12 to get into feet, right?

If so, why do I use the bolt diameter and not the 1 in. diameter?

 

[FredGarvin]

The threads on most bolts and nuts are triangular in cross-section. Is your instructor trying to simplify thing for you a bit by assuming that the cross-section of the threads are rectangular and not triangular? If so, can this help you?

Check out Acme threads. They are almost square in section.

 

[AlephZero]

Your diagram is the right idea. Imagine you have a piece of paper wrapped round the bolt and draw the thread on it. Then unwrap the paper and you will get something like your diagram, where the thread is an inclined plane.

The numbers you were given mean the diameter of the unthreaded part is 1 in. That's the part where the tension has to be 47.25. The mean diameter of the thread (half way between the outside and the base of the thread) is 0.94. That's the radius where the force from the nut is applied and converted into a torque.