What assumptions to use with a threaded stud under load

[MacomMech]

I have a stud with threads at both ends loaded in bending. Should I treat the whole stud as one diameter or reduce the diameter of the threaded parts to the thread minor diameter?

Reducing the diameter at the ends will give localised high stresses.

The stud mufactured using 304 SS bar stock with the threads machined.

Many thanks

 

[unclesyd]

So be it.

Single point threads will also give you large stress risers and depending on how you are loading the stud could be quite detrimental to service life.

Are you taking about true bending or shear?

 

[rb1957]

by "loading in bending" do you mean that there is a transverse load applied to the stud (ie, putting it in bending). in this case it could be considered to be a simply supported beam. if the stud is fully threaded, i'd us the minor diameter as the effective section of the beam. if the margin is -ve, consider plastic beam as a possibility. or could the ends provide some moment reaction (which reduces the peak moment in the beam) ?

like unclesyd noted above, maybe you need to worry abit about the fatigue effects on the stress concentration of the thread.

 

[MacomMech]

The stud is 8mm diameter, the threads are only at the ends, 15mm one end 25mm the other (both M8)
Two studs are screwed into a flange, a manifold slides into the studs and a pump fits on the end, the whole lot is clamped together using 2 M8 nuts and washers.

I have assumed its bending, but I suppose there is a bit of shear going on at the interface between the manifold and the flange.

 

[rb1957]

now it's getting complicated !

let's not "fuss" whether the studs have shear loads applied or maybe couples. it sounds like the studs are cantilevered into the flange ... if you wanted to many the cantilever more effective you could add a nut to the stud, screw the stud into the flange, then tighten the nut up against the flange.

one thing to figure out is are the two studs working together or separately ...

anyways, you can determine the loading on the studs (free body diagram), though i suspect that your peak moment is where the stud is screwed into the flange ... for the stud i'd use the minimum thread daimeter as the effective section.

question ... is the flange good for the job required of it ? the moment is being transferred across the threads ...

it occurs to me that i'm assuming (essentially) horizontal studs supporting and offset pump; though this is equivalent to vertical studs with horizontal loads. a completely different setup is vertical studs with axial loads ??

 

[MacomMech]

I was trying to keep it simple. Yes the studs are horizontal, and all loads as far as I can tell are on the centre line so each stud has an equal share.

The COG of the pump is above the centre line of the studs, until now I have ignored any moment caused by the pump, apart from it's weight

 

[MintJulep]

The limiting cases are:

Vertical load entirely supported by friction at the interface - in which case the studs are in tension only.

Vertical load entirely supported by the studs - in which case the studs will have a combined bending and shear load applied.

If there is a gasket sandwiched in things are somewhere in between.

I'm assuming here that the flange face is vertical.

 

[rb1957]

ok, so you've got two studs, horizontally projecting from a vertical flange, supporting a pump ... so the studs are loaded by the weight of the pump and imbalance forces from the pump, and other pump forces. the studs are in the horizontal plane (beside each other) rather than in the vertical plane (above each other).

if you have the studs threaded into the flange, the moment is taken out over the threads ... sounds a bit dodgy particularly if the stud is dodgy for minimum thread diameter.

 

[unclesyd]

This setup is very similar to an in line engine exhaust manifold with a turbo attached.
We have a number of high pressure piston pumps with the outlet manifold attached the same way but with a caveat that we always use 3 or more bolts. As stated above the idea of more bolts is to keep the bolts in tension. On several of the heavier manifolds there are pins,like a dowel pin, added in the mix to prevent unwanted movement.
as mentioned by the OP there should be enough bolting to prevent the first bolt from the pump to act like a pivot.

As stated above it would be nice to have a few more details like is there a gasket or Oring or what?

Is there any additional support for the pump?

How is the ancillary piping handled?