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SETTLE A FRICTION ARGUMENT?
2

SETTLE A FRICTION ARGUMENT?

SETTLE A FRICTION ARGUMENT?

(OP)
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?   
 

RE: SETTLE A FRICTION ARGUMENT?

i think it's either/or not both ... one interface or the other will slip independent of the other, no?

RE: SETTLE A FRICTION ARGUMENT?

I see 3 surfaces, not 2

clamp to pipe
pipe to frame
clamp to frame

The weakest friction interface will let go first.

RE: SETTLE A FRICTION ARGUMENT?

On second thought, the clamp to frame friction would not be involved in this debate.

RE: SETTLE A FRICTION ARGUMENT?

On 3rd thought, I think that so long as the friction (frame to clamp or pipe to clamp) can support the clamp weight, the friction between the pipe and frame is the only criterion for holding the pipe.
I hope this is my last comment on this simple but elusive question

RE: SETTLE A FRICTION ARGUMENT?

no diagram provided, but assuming that we're talking about a round pipe sitting in a saddle with a u-bolt clamping it to the saddle, and that the saddle is rigidly fixed to the wall, then...

two interfaces of concern:
1 - The clamp - to - pipe interface
2 - THe structural steel frame (saddle) - to - pipe interface

friction at each interface must be overcome to have slippage at the interface in question.  Slippage of the pipe w/r/t the steel frame can happen without slippage of the pipe w/r/t the clamp, assuming that the clamp can deflect.  I would anticipate this behavior, in fact, since the clamp definitely can deflect, and would in many instances increase its clamp force as it deflected, thus attaching itself more firmly to the surface of the pipe (still a failure to restrain).  

Unless the clamp is intended to support a large fraction of the supported load via shear and bending of the clamp itself, I would recommend designing the part so that the friction at the supporting structure is more than sufficient to support the full load.  This will avoid axial loading of the clamp (which I assume you want to avoid).

 

RE: SETTLE A FRICTION ARGUMENT?

I think each surface provides its own frictional resistance. How does one surface "know" that the other exists? Multi plate clutches handle more torque at the same spring  pressure.
Despite the basic physics premise that friction is independent of area, i'm guessing the clamp wrapping part way round the pipe will make the greatest contribution, unless there is a saddle at the structural steel as described by ivymike.

My attempt at a first cut would be to consider it half a "riser clamp"
 http://www.pipingtech.com/products/ptpcat/support/clamp/fig90.htm
Note for full clamping force the clamp can not "bottom" on the structural steel after tightening.
http://www.webbikeworld.com/Motorcycle-tires/avon-storm/fjr-1300-front-pinch-bolts.jpg

RE: SETTLE A FRICTION ARGUMENT?

but they wouldn't act in parallel as the op suggests "the friction force will be doubled".

they are separate and one or the other will slip  

RE: SETTLE A FRICTION ARGUMENT?

Is this a schoolwork problem?

rmw

RE: SETTLE A FRICTION ARGUMENT?

Draw us a sketch because I am a little confused with your description.  

RE: SETTLE A FRICTION ARGUMENT?

The pipe does not know that one side is a clamp and one side is a beam, and it does not care.  In a simplified, ideal world, wherever it feels contact pressure there will be friction.  It's up to you to decide how ideal your situation is.  For instance, if the surfaces do not stay parallel then secondary issues arise.   

RE: SETTLE A FRICTION ARGUMENT?


I am not sure I understand the description 100% but..

There is no 'doubling up' of clamping force (if that is what you mean) because there are two surfaces. You have increased contact area and so increased resistance to movement, but it is a function of area.  

RE: SETTLE A FRICTION ARGUMENT?

Hi rb1957.

This is how I pictured his system, even after re-reading it a few times.
http://www.ted-kyte.com/3D/Pictures/U-Bolt-MufflerClamp.jpg
the flat bar represent the surface of the structural steel.  The U-bolt is the clamp.

There will be some friction available at the flat bar/pipe interface as a result of the force applied by the clamp.
There will be some friction available at the pipe/u-bolt interface due to the same force being applied by the clamp.
To slide the pipe axially one force greater than the sum would have to be applied.

If, by chance the clamp friction and the steel frame friction are the same the force required to slide the pipe will be 2X the force required if the clamp were superTeflon (coefficient of friction = zero).

"Adding plates to a clutch unit to form a multi-plate clutch will increase its torque capacity, without increasing spring strength or clutch diameter."
http://www.blu808.com/assets/images/50057-350.jpg

If the question was whether the steel surface friction and the clamp surface friction should each be doubled, then I was mistaken.

Dan T

 

RE: SETTLE A FRICTION ARGUMENT?

2
Draw a FBD and set the pipe in motion. The two contact surfaces will both contribute friction and the contributions must be added. For a rigid clamp the static friction contributions must likewise be added. A flexible clamp adds complication as mentioned previously.

It is not a function of area either - just normal force and friction coefficient.

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

RE: SETTLE A FRICTION ARGUMENT?

I took another look at this and come to the following conclusions

1 So long as the pipe does not slip, it is impossible to allocate the distribution of friction but the sum of the 2 is surely the longitudinal force on the pipe.

2 As the longitudinal force increases, slippage will occur at both interfaces simultaneously at a force equal to the sum of the static friction forces at the 2 interfaces. My reasoning is that you can't have one interface slipping and the other static, since the very act of slippage must be inclusive.

3 Looking at 2, it may be possible that the distribution of friction at forces less than the limit may be proportional to the static coefficients, but that is a gut feel, since it would neatly fit the simultaneous slippage, I postulated.

 

RE: SETTLE A FRICTION ARGUMENT?

You have two contact areas. Let's assume each one is characterized by its own fiction factor (µi). Each contact area represents a boundary. Compute the normal forces acting on each surface (Ni). Friction is independent of area. Compute the product of each friction factor by the relevant normal force. The higher µi*Ni will rule the motion.

RE: SETTLE A FRICTION ARGUMENT?

some of the posts have made me think (!?) about static friction vs dynamic friction ... for static friction i think the lower frcition interface will rule, but maybe under dynamic friction, ie if both interfaces are sliding, the total friction is the sum of the two interfaces ...  

RE: SETTLE A FRICTION ARGUMENT?

Friction has nothing to do with area, but with the reaction force being applied. Calculate that force and you have the friction force resisting any applied loads.  

Tara

RE: SETTLE A FRICTION ARGUMENT?

rb1957,
The only difference between static and dynamic (kinetic) fiction is that the dynamic friction factor is lower than the static one. When you equal or overcome the higher µi*Ni (being µi the static friction) motion will happen.

RE: SETTLE A FRICTION ARGUMENT?

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.  

RE: SETTLE A FRICTION ARGUMENT?

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.

RE: SETTLE A FRICTION ARGUMENT?

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.

RE: SETTLE A FRICTION ARGUMENT?

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.

RE: SETTLE A FRICTION ARGUMENT?

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.

RE: SETTLE A FRICTION ARGUMENT?

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

RE: SETTLE A FRICTION ARGUMENT?

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 ...

RE: SETTLE A FRICTION ARGUMENT?

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

RE: SETTLE A FRICTION ARGUMENT?

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.   

RE: SETTLE A FRICTION ARGUMENT?

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.

RE: SETTLE A FRICTION ARGUMENT?

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.

RE: SETTLE A FRICTION ARGUMENT?

(OP)
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.   
      

RE: SETTLE A FRICTION ARGUMENT?

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.

RE: SETTLE A FRICTION ARGUMENT?

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.

RE: SETTLE A FRICTION ARGUMENT?

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.

RE: SETTLE A FRICTION ARGUMENT?

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.

RE: SETTLE A FRICTION ARGUMENT?

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.

RE: SETTLE A FRICTION ARGUMENT?

...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.

 

RE: SETTLE A FRICTION ARGUMENT?

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

RE: SETTLE A FRICTION ARGUMENT?

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.
 

RE: SETTLE A FRICTION ARGUMENT?

RB1957,

Ok, I have come to terms that my first try was wrong (haha) and that the frictional forces from both surfaces would have to be overcome to move the pipe (the frictional forces are additive).

Would the added forces be the sum of the static max friction forces (Fsmax) or the kinectic friction force of the smaller force plus the Fsmax of the larger friction force?

Even though there is no motion yet, the smaller static frictional force would have been reached and reduced to the kinetic frictional force correct?

Let me know your thoughts, done twisting my head around this one for the day.


 

RE: SETTLE A FRICTION ARGUMENT?

i don't think dynamic friction would play a part unitll the surfaces were actually slipping ... i see it as something like yielding, below the static friction both interfaces are attracting load, as one approaches slipping it'd offload to the other (that's still stable).

if we allow one side to slip relative to the other, then we'll have to include effects like the clamp rotating (slightly), so that one edge digs into the pipe (clearly increasing the allowable load), and how much deformation does the clamp (or the chnnl) do to the pipe ?

RE: SETTLE A FRICTION ARGUMENT?

(OP)
I'm going to throw another wrench into the original question, as I spoke to a few more engineers regarding the friciotn question and I am still not convinced about the doubling up of the friction force. I realize my last thread indicated that I was, but now I am not so sure.

The new example is that of a regular clamp (two half moons that clamp around a pipe and bolted together).  Assuming one of the clamps is welded to some structural steel to support the vertical pipe (omit any bending influece to maintain an ideal situation), if you bolt up the clamps tight, will there be two friction forces??  I believe you will have one firction force equivalent to the clamping force (2P) where P is the tenson on the bolts.

What does not make sense with the double friction force argument is that by tensioning two bolts with a force of say P=1000 lbs each (total of 2000 lbs), you would get a friction force that is calculated from a normal force that is double this, or 4,000 lbs.  It would seem that energy is being created here.   I spoke to a hanger engineer (designer, not applicaitns engineer)and they were on board with this conclusion.

Let me know your thoughts!

RE: SETTLE A FRICTION ARGUMENT?

Look at a free body of the pipe and the applied clamp forces.
The force normal to the pipe surface is, in your example, 2,000lbs.  In balance there are two equal and opposite normal forces acting on the pipe.  The friction force due to each normal force is mu*2000.  There are 2 friction faces.  Therefore, the total friction force is 2*mu*2000 = mu*4000.

Ted

RE: SETTLE A FRICTION ARGUMENT?

how about this way to look at it ... what is the friction area ? there are two sites providing area, and i don't think the pipe'll slip before all the friction is overcome.

if you have a table (with 4 legs) standing on a floor, and you push on it, you have to overcome the friction at all 4 locations before the table will move, no?  admittedly the normal force at each is 1/4, but i think the analogy works.

in your case (of 2 1/2 clamps bolted together), i think you're getting hung up on the 1,000 lbs ... each 1/2 clamp is applying a 2,000 lbs force to the pipe (think load and reaction).

byw, not sure i'd ask a designer a theoretical strnegth question ... but that clearly depends on the designer ...

RE: SETTLE A FRICTION ARGUMENT?

That explains it. The normal force is distributed throughout the pipe surface. Although there is multiple materials and contact surfaces. That normal force will be distributed evenly over all of the contact area. Therefore, there is 1 normal force and one coefficient of friction which would be the average of the 2 materials' static friction coef's.

RE: SETTLE A FRICTION ARGUMENT?

DESERTFOX,

The clamp will exert a pressure on the pipe that will be distributed evenly around the entire pipe or whatever the contact area is for the pipe. That pressure will be divided by the contact area to get the normal force in order to determine the friction force.  

RE: SETTLE A FRICTION ARGUMENT?

Hi Trippl

The force I have shown is the resultant force at each side and secondly the chances of getting an equally distributed load around the pipe is very unlikely.
In fact your link in your second post is in agreement with my sketch ie equal and opposite forces.

desertfox

RE: SETTLE A FRICTION ARGUMENT?

Desertfox,

I was not arguing with you, you are correct. However, the reason friction is not dependant on area is because the force produces a pressure which (theoretically) distributes evenly over the surface of the friction interface, in this case that will be the contact area of the clamp and structure.  

RE: SETTLE A FRICTION ARGUMENT?

Desertfox,

Doesn't the website you sent state that the friction force is independent of area, which is what I stated before?

RE: SETTLE A FRICTION ARGUMENT?

Hi Trippi

Yes it does but your reasoning was incorrect:-

[/quote Trippi   the reason friction is not dependant on area is because the force produces a pressure which (theoretically) distributes evenly over the surface of the friction interface]

The above statement is not the reason friction is independant of area which is why I left you the link.

desertfox

RE: SETTLE A FRICTION ARGUMENT?

The force is evenly distributed as a load over the "effective" contact surface. How is that not correct reasoning? The link you sent is an explanation of the microscopic interactions between the surfaces in obtaining accurate coef's of friction. Please explain to me the correct reason why friction is independent of area.

 

RE: SETTLE A FRICTION ARGUMENT?

To understand why friction is independent on the surface area we need a microscopic view of the contact.  Practical surfaces are rough, and they only touch through contact "points" or "junctions". The figure shows two rough surfaces sliding past each other.  When the load increases (bottom) the asperities become flattened by elastic deformation.  This increases the effective contact area, and therefore friction.

The effective contact area A' is only a small fraction of the apparent macroscopic area A. One can derive an average shear yield stress at the contacts,

t = F/A'


Perhaps you can point out in your original statement where the word "effective area" is.                                   
 

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