Friction (again)

I put friction on the +y of a support,
But do we include it for the x or z ?
In the case of guide???????

Opinions please

(for the record i don't)

Usually, but it depends on how your restraint is constructed.

In CAESAR II, friction acts in the plane perpendicular to the restraint. So for your "+Y", friction will act somewhere in the "X-Z" plane, depending on the direction of movement.

For a guide, you can have friction parallel to the pipe (due to axial expansion), and maybe up-down also. So if you specify friction on a (Z) guide (with the pipe running in X), then you can have friction in the "X-Y" plane.

Correct is to use Mu=0.1-0.3 for +y restraint. If you use GUI acting are : 3mm-5 mm.


Best regards,

Alin10,

What do you use for horizontal friction carbon steel to carbon steel (ASTM A36)? Thanks for the tip.

Regards.

I use 0.4

it may be 0.4 at time zero and then once it starts to slip it may be 0.04 so there is no simple, magic, coefficient!

Your analysis and consequently the final design should work with and without mu... in other words mu should never be a factor crtical to the designs outcome.

The subject of friction has been discussed a number of times in this forum. Use the [Search] option above to find these posts. This one in particular contains links to a couple of sites listing various values for "mu".

As John stated above, "mu" is not a fixed number.

Firstlly note that when two material sufaces contacts to each other their is creating a friction. Suppose that your pipe axis is in X or in Z direction & your pipe Shoe direction is in "-Y" (means your pipe displacement restricted in "-Y" direction)& your pipe shoe rested on secondary structure then you will put support as a "+Y" & friction factor.

Note that following are generlly friction factor,

A) rubbing suface brtween Carbon Steel to Carbon Steel , friction factor = 0.3
B) rubbing suface brtween Carbon Steel to Stainless Steel(S.S. foil),friction factor = 0.15

Regards,

CPS.

Some Corrections to the previous reponse.

When two surfaces contact each other there will be frictional effects only when there is "tendency to slide" which at the peak ( on the verge of sliding) equals a force co-efficient of static friction times the normal force ) or "sliding" ( this time force equals co-efficient of kinetic friction times normal force).

Friction co-efficient depends on number of factors . Experiment shows that it is by and large independent of the area of contact.

Pl. refer to good mechanical engineering handbooks for reliable data.

Regards

we should always analysis both cases. With and without friction and take the worse case.

My point was that the addition of Mu. to every restraint variable leads to unstable iterations,
when it can be argued that the pipework movement in any other direct apart from opposing gravity is non consistant due to vibrations etc.
Still, some good points.

Now in response to the above comments, we ensure
especially during designs involving nozzle loads that a non friction run is done. But how do we asses that phenomenom of stiction? In most cases running to a nozzle, the nozzle loads are dependent on the friction factors staying low.

Do(should) we assume that pipe shoes are able to lock up???

Well that is the meat of the problem! Adjacent to nozzles if mu is designed out of the solution by use of slide with low mu values or rod or strut type supports/retraints with limited theta angles then friction will be limited and might safely be left out of the model.

A long time ago before programs became sophisticated friction was not able to be included in the model and yet piping systems were succesfully designed by detailing the restraints/ supports so that friction affect was taken into account.