SETTLE A FRICTION ARGUMENT? 2

[FDS2008]

I have a physics question that I can't seem to answer using textbooks.

I have a vertical pipe that will be clamped to some structural steel. We are trying to calculate the forces required to restraint the pipe based on friction. The clamp will pull the pipe towards the structural base and there will be two different contact surfaces.
1 - The clamp
2 - THe structural steel frame

The two possibilities that we are trying to settle in the design camp are as follows. The first is that the friction force will be doubled (i.e., friction on item 1 + reaction on item 2 = 2 x friction force) since there are two friction surfaces. The second one considers the friction force on the structural steel frame as a reaction of the clamping force and the pipe would opnly need to overcome the friction force of either surface for it to slip axially. This would mean that the resistance is equivalent to the clamping force only.

What do you all think?

 

[rb1957]

my thought was if the common element (the pipe) is sliding then both the other two elements would have a friction force opposing the motion. this assumes that the load is applied to the common element (and reacted by the other two).

if it isn't, ie the load is applied and reacted by the other two elements (and the pipe is only providing a loadpath from one to the other) then the lower friction interface would determine when the structure starts to slip.

 

[ione]

If we take the structure as a rigid body the mutual distance of its points cannot change. So even if the weakest friction force is equalled, than motion won’t happen until the strongest friction force is equalled or overcome. The strongest friction force acts as a constraint.

 

[imcjoek]

Throw me in for the "sum of both friction forces must be overcome to move the pipe" camp.

If the clamp is rigid, that is.

 

[gruntguru]

ione (Mechanical) 10 Feb 11 9:06
If we take the structure as a rigid body the mutual distance of its points cannot change. So even if the weakest friction force is equalled, than motion won't happen until the strongest friction force is equalled or overcome. The strongest friction force acts as a constraint.
No No No. The sum of the two friction forces must be overcome before movement will occur.

Put the pipe in a vice and pull on an end until it starts to slip. Now make one of the jaws frictionles (rollers, super PTFE, whatever). The force required to move the pipe is halved.

Engineering is the art of creating things you need, from things you can get.

 

[rb1957]

since the OP isn't looking at this thread, does anyone care ?

it's a "thought exercise" and the answer depends on the set-up ... it could A, B, or A+B.

 

[corus]

For a pipe in a vice you have two equal and opposite forces on the pipe. If one force has zero coefficient of friction then the force required to move the pipe is halved; ergo I agree with the guru, it is the sum.

Tara

 

[rb1957]

yes, but it depends on the set-up ...

if you're loading the pipe (ie the middle element) and this load is being reacted by the other two elements then it'll slip when the higher friction surface permits at a load equal to the sum ('cause the applied load is being reacted at two surfaces).

if you're loading the other elements (and the middle one is just a loapath between load and reaction) then it'll slip at the lower friction at a load equal to the lower friction capacity.

IMHO, FWIW ...

 

[desertfox]

Hi FDS2008


Take a look at this example fig 5.5 on page 115 it shows the force required to move the block is the sum of the two friction forces:-

http://books.google.co.uk/books?id=4wkLl4NvmWAC&pg=PA112&lpg=PA112&dq=how+to+solve+limiting+

static+friction+problems&source=bl&ots=o9S1EIJvFu&sig=
zckFmhETDw9jvJ2IngH5FvnpYM8&hl=en&ei=eiRWTf3aF4e3hQfDk-meDA&sa=X&oi=book_result&ct=result&resnum=2&ved=0CBwQ6A
EwAThG#v=onepage&q&f=false

 

[ivymike]

I still think that the clamp is flexible, and that slippage w/r/t the wall (to a small extent) can probably happen without slippage w/r/t the clamp, if pipe-wall friction is inadequate. It's the small slippage w/r/t the wall & resulting deflection of the clamp which I suspect might lead to a more important failure, depending on what the system is and what the life expectation is.

 

[gruntguru]

Of course a real clamp will be flexible and slip will occur on the other surface first while the clamp deflects and friction force on the clamp side builds up until it exceeds the static friction limit and starts to slip. With both sides slipping the total friction force is the sum of the dynamic friction forces on the two sides.

If you then reduce the force until the pipe just stops slipping and reapply (with the clamp still deflected) the force required will be the sum of the static friction forces on the two sides (they will start to slip almost simultaneously).

Engineering is the art of creating things you need, from things you can get.

 

[Jmoore1]

Each side of the pipe has coefficients for frictions, and based on the applied force (equal to the clamping force on both ends), will see a final frictional force. These forces both act axially along the pipe and the sum of these will need to be overcome to move the pipe. If you want to go from there and make assumptions on rigidity and etc you can...but the Sum of all forces must be net greater (negatively or positively) to allow movement, this is basic engineering princples.

 

[FDS2008]

I have to apologize for not getting back to this post as I was not set up for e-mail notification and I got busy believing that no one answered the post. Excuse my unfamiliarity with the process. It definitely generated lots of curiosity. I thank you all for your input.

I have acttached a sketch of the item in question (please ignore the welded tabs as these may be applied at the bottom of the riser to ensure vertical stop).

When I started the post I was in the camp that that as long as you have an axial force that is larger than the highest friction force (assuming the clamp does not bend as mentioned by ivymike and gruntguru), there will be movement (static friction becomes dynamic friction). I had a hard time not believeing this, although there are many of you who made some good points to include both friciotn forces (and there are two friction surfaces).

I looked at the example in the book offered by desertfox. I believe this example is very similar to our problem, even though it is presented in a different way. It shows that we need to include the friction at both surfaces as many of you suggested. I am reluctant but I believe I have to agee.

With respect to the piping slippage, the thermal growth/contraction will generate axial forces that will surely overcome the friction forces of the clamped support, however, our intent is to support the weight of the piping and fluid even after the pipe has grown/contracted.

Thanks again for all your responses and I invite you to continue your posts if you are not satisfied with my reasoning.

 

[rb1957]

so with this set-up both friction faces have to be overcome, so it's the sum of the friction forces.

i'd suggest adding a saddle brkt between the pipe and the chnnl, supporting the inner face of the pipe, to increase the friction area.

maybe clamp collars on the pipe above and below the clamp. now the friction between the pipe and the collars is critical; this load would be transferred into the clamp by bearing from the collars to the clamp.

 

[TrippL]

I would say that first you have to look at is as a static system since static friction would be different than the kinetic friction force.

In the Static system the normal force between the clamp and the bracket would be the same. The Maximum Static friction force, either between the clamp or the pipe, would be the force required to set the pipe into motion.

Not sure where I'm getting in on the discussion so excuse me if I'm late in the game.

 

[rb1957]

i think the normal force is the same on both interfaces, but load along the pipe can be reacted at both interfaces (the clamp and the chnnl) and i don't think the pipe will move untill you've applied enough force to overcome both friction reactions.

 

[TrippL]

Static friction acts as a limiting force. You must overcome both of the friction forces but they are independent (not additive).

The dynamic (kinetic) friction forces would be additive, but this will only happen if the pipe is in motion. Would think that is not the scenario being designed for.

 

[rb1957]

ok, you're pulling on the pipe, there are two friction reactions that ract the applied pull ... the pipe will slip (IMHO) when your pull on it equals the two friction reactions.

 

[ivymike]

...but it may slip w/r/t either interface when the force reacted at a given interface reaches the static friction limit at that interface... which may also result in a sudden change of force at the other interface.

It's not necessarily the case that the load reacted at each interface before slippage is half of the total force.

 

[rb1957]

i didn't mean to imply the force is reacted equally, and i don't see the pipe moving untill it has overcome both friction forces.

assume the two friction forces are F1 and F2, and F2 > F1. asumme load is reacted equally at both interfaces (yeah, i know what i wrote above, just read on before you shoot ...). for Papplied < 2*F1, nothing is slipping. At P = 2*F1, one interface is on the verge of slipping, but the other interface is keeping things from moving. with one surface on the verge of slipping, the other surface is going to react all the further load. At P = F1+F2 both surfaces are slipping.

IMHO

 

[ivymike]

only if both supports are completely rigid. you can have slippage w/r/t one and not the other if a small amount of deflection is allowed.