×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

U-Bolt Stress Analysis

U-Bolt Stress Analysis

U-Bolt Stress Analysis

(OP)
Hi All,
I need some advice/feedback on how to properly analyze a U-bolt. The particular scenario under which this bolt is used is simple enough: It hangs over a pin and then the threads protrude through a restrained plate. The way I'm looking at this, tightening the nuts will develop a tensile preload in the threaded sections. Given the support condition at the inner radius of the U portion where it contacts the pin, the tensile loads on the threaded sections should develop a shear load and a moment at the U center plane. However, we've calculate the resulting bending stresses at that location using initially curved beam equations and the stresses seem unreasonably high. I attached a sketch showing the setup and FBD. This seems so simple, but we're getting equivalent stresses that are higher than the yield strength of the material, even when applying the rated load in the analysis.

RE: U-Bolt Stress Analysis

Lodgera:
This design question has been asked and discussed before, do a search and see what you can find in the Mech., Structural or Welding and Fastening forums. That is most likely where the pertinent threads would show up. If you have reasonably good fit btwn. the U-bolt and the pin, that is, pin o.d. and U-bolt i.d., you should really not have much bending as you show in your FBD. “F” is just a tensile stress/force along the axis of the U-bolt, and this is reacted by radial forces, and friction, around the contact half of the pin and U-bolt. There is probably some frictional/gripping advantage to having some ground flat surface on the bearing area of the U-bolt. The plate under the nuts on the U-bolt will have a bending moment which is FL/2 and you just make this thick enough, stiff enough to tolerate this loading from the nuts.

RE: U-Bolt Stress Analysis

I would get a mechanical engineering handbook and check out the section where you have contact stress between two curved surfaces of similar and dissimilar diameters.

RE: U-Bolt Stress Analysis

(OP)
I've looked through these forums at length and still cant really find a detailed answer to this question, only other statements that there shouldn't be much bending. I understand in reality the reaction at the U bolt ID will be distributed and radial . . . are you suggesting that these reactions will, if the bolt is sectioned at the apex of the U as shown in my sketch, result in a zero sum on the moment equation such that there is no moment acting on the cross section? How would you draw the FBD? Can you show this result analytically?

RE: U-Bolt Stress Analysis

The source of your error is using the wrong tool. Winkler-Back (assuming that's the solver you are using for curved beams) assumes just that- a curved beam.

The situation you are describing is not a fixed, curved beam. Because you have cylindrical surfaces in contact, this becomes a hertzian stress problem. Start there.

RE: U-Bolt Stress Analysis

I believe that you are using the wrong approach ....Lab test results trumps analysis

Maximum loads on U-bolts has been extensively investigated empirically by the nuclear power industry evaluating pipe supports in the 1980s....

Suggest that you contact some of the major pipe support vendors (ANCHOR, Power piping etc) and discuss this topic.

As I recall, the variety of end conditions (nuts attaching the u-bolt to the structural member) can have a significant affect.

Some fun reading when you cannot sleep some night .......

http://www.nrc.gov/docs/ML0732/ML073230518.pdf

MJCronin
Sr. Process Engineer

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources