×
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

Simple Clamping Force / friction question

Simple Clamping Force / friction question

RE: Simple Clamping Force / friction question

IMO divided. Green arrows are 6000N (assuming vertical down arrow is meant to be on the inside surface of the clamp, Assuming this isn't a trick question (currently your support plate isn't attached to anything so will just move with the bar....) red arrow is dependant on friction factor you use between steel and support / U bolt.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

RE: Simple Clamping Force / friction question

(OP)
Thanks for the input LittleInch.
So the force to slide the bar out of the U-bolt is 12,000*friction coeff?

That's what I was thinking but I'm not yet convinced. Only 6000 N is applied to the assembly after all.


RE: Simple Clamping Force / friction question

6000N on each face of the block. but the CoF is different on both sides (should be much lower on the bar side), so Force = 6000*(CoF1+CoF2)

another day in paradise, or is paradise one day closer ?

RE: Simple Clamping Force / friction question

'cause CoF used above is dependent on the area of contact, no?

another day in paradise, or is paradise one day closer ?

RE: Simple Clamping Force / friction question

(OP)
No I don't think that's right rb1957.
Coefficient of friction is determined by the materials only.
Friction depends only on the normal forces and the friction coefficient. It is not dependant on the contact area.

RE: Simple Clamping Force / friction question

it seems pretty reasonable to me that the bar will have a lower CoF on the block than the plate will.

test it ... bolt two bars together around a block, bolt two plates around a block; which is harder to move ?

another day in paradise, or is paradise one day closer ?

RE: Simple Clamping Force / friction question

The force is 6000 N multiplied by the number of slip planes, in your case 2. You can learn more about slip critical connections by reading section 5.1 of the Guide to Design Criteria for Bolted and Riveted Joints here:

http://boltcouncil.org/files/2ndEditionGuide.pdf

RE: Simple Clamping Force / friction question

(OP)
Thanks Cory!

RE: Simple Clamping Force / friction question

With U bolts it's not a simple as 6000 x F1 + F2. the bolt tends to deform sideways and bend. Given the very small surface area of the bolt / bar contact point, there is also the not inconsiderable issue of deformation of the steel bar under this sort of contact pressure, creating a small arc like deformation of the bar, be it elastic or plastic deformation.

For locations where axial force is the issue, you really need to have a definable flat area to grip the pipe to avoid these issues or simply clamp the bar between two flat bits of metal.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

RE: Simple Clamping Force / friction question

I'm with CoryPad on this.  Sliding force = 2x6000xFrictCoeff.

However there is a possible further complication.  Once slippage begins the bolt assembly might rotate infinitesimally, which would lead to what I believe are known as "binding forces".  It might be this sort of effect that underlies rb1957's feeling that the plate's friction will be greater than the bolt's.

RE: Simple Clamping Force / friction question

What? No free-body diagram?

I you split the system and diagram it, you will see it must be 6000N on each side (assuming "ideal" bolt that does not bend).

RE: Simple Clamping Force / friction question

(OP)
Thanks all for clearing up my doubts.

Sliding force = 2x6000xFrictCoeff.
This is precisely what I presented to a customer who laughed and told me I was wrong.

I showed on a fbd how there must be an equal reaction force on the opposite side of the U-bolt, but he maintained that if the tension in the bolt is 6000 , it must be 3000 each side.

Obviously I realise that in reality that assembly would rotate and bind etc. before coming off. I just wanted to clarify the theory.

RE: Simple Clamping Force / friction question

Yep, the free body diagram as mentioned by TheTick would explain it all. Basic Static problem without getting into all other details such as deformation, slip critical etc., making a mountain out of a mole hill. The pull needed to overcome the static condition is 6000 x static Cof on top + 6000 x static Cof on bottom

RE: Simple Clamping Force / friction question

The friction between the round bar and the plate will not be the same as the friction between the flat plate and the flat bar.

The round U Bolt is made of different material and has a different finish; it will not be the same.

It is [the load * cof] of the flat plates

Plus

the [load * cof] of the flat plate and the round U bolt.

Charlie
www.facsco.com

RE: Simple Clamping Force / friction question

thought so ...

another day in paradise, or is paradise one day closer ?

RE: Simple Clamping Force / friction question

I assume you have a load attached to the block? Then the block would have to slip before any load is taken up by the u-bolt on the other side of the plate. So for zero slippage, you can only count on one side of the block producing friction.

RE: Simple Clamping Force / friction question

(OP)
Thanks Brian E22. That is a good point.

RE: Simple Clamping Force / friction question

i'd've thought there'd be some friction for the bar ... less than the plate side, but more than zero.

if you had the plate trapped between two U-bolts there'd be some friction, wouldn't there ?

another day in paradise, or is paradise one day closer ?

RE: Simple Clamping Force / friction question

Whenever there is more than one path for the load to go you have to look at relative stiffness to determine how much of the load goes along each path. In this case the load path from block to plate is very stiff. The path going through the u-bolt, however, includes a transverse load on a near cantilever section of the u-bolt. The cantilever section is not stiff in bending compared to the stiff solid block so little of the load would pass through it.

One approach to equalizing the 2 load paths would be to apply the external load to the middle of the cantilever section of the u-bolt. That may, of course, not be practical in your application. Another approach would be to make the block load path more compliant or the cantilever load path more stiff (in bending I'd think but maybe also in shear).

RE: Simple Clamping Force / friction question

With the diagram the OP provided on 03Oct14@07:29 I suspect the situation transcends analysis.  Depending upon the bending stiffness of the u-bolt, the bar will probably begin to move before the plate does.  Depending upon the amount of relative movement, the bolt will be put into additional tension.  Friction forces increase.  By an unknowable amount.

Impossible to model mathematically.  Probably also impossible to get reliably reproducible results in a physical experiment.  Murphy's Law will prevail:  if you want to induce "binding", it won't happen;  if you want to avoid binding, it will happen.

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