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?
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?
RE: SETTLE A FRICTION ARGUMENT?
Peter Stockhausen ://www.lin kedin.com/ profile/vi ew?id=3006 4526&t rk=tab_pro
Senior Design Analyst (Checker)
Infotech Aerospace Services
www.infotechpr.net
http
RE: SETTLE A FRICTION ARGUMENT?
clamp to pipe
pipe to frame
clamp to frame
The weakest friction interface will let go first.
RE: SETTLE A FRICTION ARGUMENT?
RE: SETTLE A FRICTION ARGUMENT?
I hope this is my last comment on this simple but elusive question
RE: SETTLE A FRICTION ARGUMENT?
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?
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"
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Note for full clamping force the clamp can not "bottom" on the structural steel after tightening.
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RE: SETTLE A FRICTION ARGUMENT?
they are separate and one or the other will slip
RE: SETTLE A FRICTION ARGUMENT?
rmw
RE: SETTLE A FRICTION ARGUMENT?
RE: SETTLE A FRICTION ARGUMENT?
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?
This is how I pictured his system, even after re-reading it a few times.
http://ww
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?
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?
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?
RE: SETTLE A FRICTION ARGUMENT?
RE: SETTLE A FRICTION ARGUMENT?
Tara
RE: SETTLE A FRICTION ARGUMENT?
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?
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?
RE: SETTLE A FRICTION ARGUMENT?
If the clamp is rigid, that is.
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.
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?
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?
Tara
RE: SETTLE A FRICTION ARGUMENT?
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?
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:-
htt
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?
RE: SETTLE A FRICTION ARGUMENT?
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?
RE: SETTLE A FRICTION ARGUMENT?
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?
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?
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?
RE: SETTLE A FRICTION ARGUMENT?
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?
RE: SETTLE A FRICTION ARGUMENT?
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?
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?
RE: SETTLE A FRICTION ARGUMENT?
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?
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?
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?
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?
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?
RE: SETTLE A FRICTION ARGUMENT?
Take a look at the file I have uploaded.
Without having 2000lbs acting on each side of the pipe you cannot satisfy equilibrium.
desertfox
RE: SETTLE A FRICTION ARGUMENT?
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?
http://ww
RE: SETTLE A FRICTION ARGUMENT?
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?
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?
Its not a problem no need to apologise I didn't think you were arguing with me.
Friction is not independant of area for the reason you have stated see link below:-
http:
RE: SETTLE A FRICTION ARGUMENT?
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?
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?
RE: SETTLE A FRICTION ARGUMENT?
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.