stuck in the same situation (bolts attaching two plates, separated by spacers...) 1

[glulambeam]

thread404-400310

Hi smart people...I also need your help. Please refer to Irene's diagram.

The spacers are 1" in diameter, with a 0.2" concentric thru hole to allow a #10-32 bolt. They are about 2" long and separate two plates as in the diagram in the referenced thread. The spacers can be either steel or aluminum.

1 plate is an aluminum plate that is 0.25" thick. The other plate is a plate that remains unknown other than it is not infinitely stiff (I'd assume it as just another 0.25" aluminum plate) It is assumed the tributary load from this "other" plate onto each bolt is ~115 lbs (the "other" plate is essentially a body intended to be hung from a number of #10 bolts).

does the spacer provide adequate support/confinement to each #10 bolt such that little curvature occurs, and the load transfer into the bolt is shear only (or at least mostly), or can bending on the bolt not be ignored due to the plates not being rigid?

If I compare the relative stiffness of the existing spacer to the bolt, the spacer would take the majority of the load...but the problem here is that the spacer is not
mechanically attached to anything in this system. it's just a floating sleeve through which the bolt passes through. There is a torque range of 3 to 10 in*lbs that will be applied to the bolts, so it's not a whole lot of preload to hold everything together.

Thanks in advance!

 

[rb1957]

without much preload, won't the load wedge the spacer and pry the plates apart ?

without much preload I don't see the spacers working as hoped.

another day in paradise, or is paradise one day closer ?

 

[SwinnyGG]

115 lbs is not a great deal of load- but in order to know if the bolts are in bending, you need to calculate the deflection in the spacer due to the bending moment applied.

 

[glulambeam]

@rb

that's my feeling. It doesn't make sense without sufficient preload being present to prevent wedging/gapping from happening.

@jg

yes, 115 lbs isn't alot, but is it known if any of it can actually get transmitted to the sleeve?

 

[Tmoose]

If the faces of the spacers are not VERY flat, or slightly concave, there may be a tendency to rock on the plate and bend the bolt, pull on it like using a hammer to pull a rusty nail out of a board .

What happens if this system fails ?

edit, I meant to say convex was bad. honest.

 

[SwinnyGG]

yes, 115 lbs isn't alot, but is it known if any of it can actually get transmitted to the sleeve

It must be transmitted to the sleeve. There is no other path for load transmission unless you are allowing the screws to bear on their clearance holes.

 

[Compositepro]

The bolts (screws really, #10 are not usually called bolts) must be pretensioned. As long as the tension on the screws does not exceed the pretension then no gaps will open at the spacer faces and nothing moves except by elastic deformation, which I think in this case can be ignored. The shear is transferred through friction or actual shear of the screw at the face of the spacer. All the moments have to balance out.
A concave surface on the spacer is okay. It convex you have to avoid.

 

[rb1957]

you need the preload stress to exceed the local bending (tension) stress ...

another day in paradise, or is paradise one day closer ?

 

[glulambeam]

Thanks all.

I'm probably going to have the designer thread into the spacer, or better yet, have the spacers removed from the design. It turns out the no10's are limited to a max torque of 10 in-lbs.

If the system fails Tmoose, an expensive piece of hardware falls to the ground, and I'll probably be out of a job. Not exactly a good thing to gamble on.