Does bolt preload reduce the bearing stress on positioning pins?

[Nathanwest58]

Hi all,

I have some detailed design work coming up designing bolted/pinned interfaces, so I’m doing some examples trying to remember what I learned at uni. I would like to calculate the stresses in bolts and pins of a hypothetical mechanical assembly, but I have a question before I get started.

Let’s say I have a steel disc that I would like to bolt onto an upright steel plate, so basically clamped against a wall. I have six bolts all on the same pitch circle diameter. I also have a pair of dowel pins with a location/slight interference fit, on the same pitch circle as the bolts but situated 180 degrees from each other, to position the disc properly on the plate.

Now I was going to start by calculating the bearing stress in the pins induced by the force of the disc “hanging” off of them: the load is the mass of the disc multiplied by gravity. But, wouldn’t the load on the pins actually be much lower than that (or even zero) because of the preload of the bolts?

 

[Baluncore]

When you tighten the bolts, you are clamping the disc to the wall. The dowel pins must survive assembly.

Will you rely on friction, or on the shear strength of the bolts to hold the disc in place? If bolt shear strength is important, then ground bolt shanks and reamed holes will be needed, and the dowel pins must also be rated for shear, but would not be needed with those bolts.

 

[Nathanwest58]

When you tighten the bolts, you are clamping the disc to the wall. The dowel pins must survive assembly.

Understood, thanks.

Will you rely on friction, or on the shear strength of the bolts to hold the disc in place? If bolt shear strength is important, then special bolt shanks and reamed holes will be needed, and the dowel pins must also be rated for shear, but would not be needed with those bolts.

The pins will be used to make sure the disc is in the correct position when it is installed onto the plate. After that, it would be the bolt friction securing the disc.

 

[Baluncore]

After that, it would be the bolt friction securing the disc.

Dowel pin shear will only be important during the assembly procedure.
Friction between surfaces, due to bolt tension, will be important during operation.
Check the coefficients of friction.
Think about what type of gasket material might provide higher friction.
I would consider a paper gasket between bolted steel faces.
A gasket may not be needed if iron is bolted against aluminium.

 

[Nathanwest58]

I would consider a paper gasket between bolted steel faces.

That is a very good idea, thank you for the suggestion.

Dowel pin shear will only be important during the assembly procedure.

Let's say the pins have an interference fit with the disc. But the holes on the plate are slightly larger so there is a very close location clearance fit. I take the disc with the installed pins and slide it onto the plate: it is "hanging" off of the plate by the pins. At this point there will be a bearing stress acting on the pins. Now I install the bolts up to their recommended preload and the disc is pulled against the plate. Would it be correct to say now that the bearing stress on the pins would be relieved because the friction of the bolts is now completely carrying the load?

 

[Baluncore]

Would it be correct to say now that the bearing stress on the pins would be relieved because the friction of the bolts is now completely carrying the load?

Probably not, since tightening the bolts will not relieve the existing static forces on the pins. But the stress on the pins would not be expected to increase beyond the assembly stress.

 

[Tom.G]

friction of the bolts is now completely carrying the load?

Uhmm... not quite...

Friction between surfaces, due to bolt tension,

 

[Nathanwest58]

Probably not, since tightening the bolts will not relieve the existing static forces on the pins. But the stress on the pins would not be expected to increase beyond the assembly stress.

Yes this makes sense now that I have thought about it some. Thanks for the help.

 

[Ranger Mike]

Carroll Smith book - Nuts, Bolts, Fasteners and Plumbing Handbook - MBI Publishing- Mechanical design of the bolted joint - As used by racers, the bolted joint is liable to have several strikes against it going in. The engineers rule is that God intended for bolts in tension to clamp surfaces together into rigid joints (not to be confused with rigid structures). He did not intend for bolts to be used in flexible or even partially flexible joints. The reason is that parts joined in a flexible manner, when loaded, will move in relationship to each other- either in a plane of the bolt axis or perpendicular to it. Either way, the relative motion of the clamped surfaces will produce stress in the bolt in addition to that foreseen by the designer of the joint. When bolting parts together, make sure that you have enough flange material thickness to achieve rigidity, even if you have to add material that serves no other purpose. And never depend on bolts to locate the parts. Always bear in mind that clamping is the function of bolts and that location is the function of dowels.