Calculating set screw load capacity on a beveled mounting disk

[ajhanan]

Hi,

I work for a company that designs lighting fixtures. The mounting mechanism for all of our chandeliers, wall sconces, etc. is the same across the board. We have a generic 'wall mounting disk' which has a beveled/tapered edge which is screwed directly into the wall or ceiling. Next we have a generic 'wall plate' which has a slightly larger diameter which fits completely over the 'wall mounting disk'. Lastly we have either 2 or 3 set screws which screw through the side of 'wall plate' and against the angle of the 'wall mounting disk' to create the clamping force.

We normally use 6-32 or 8-32 set screws and our 'wall mounting disk' has a tapered edge of approx. 7 degrees. We have lighting fixtures as heavy as 150lbs which support the strength of these set screws and this angle!

My question: How can I calculate whether 7 degrees is the best angle to use as the tapered edge on the 'wall mounting disk'? How can I calculate the correct number of sets screws needed to support the weight of these lighting fixtures? I need to learn more about shear failure..

THANK YOU SO MUCH FOR ANY HELP IN ADVANCE!!

 

[drawoh]

ajhanan,

I hate set screws. They dig holes in whatever it is you are clamping. Their inherent functionality is to exceed the yield stress of your base. Any more stress results in plastic deformation, and failure. I try very hard to not use set screws.

I have a Stock Drive Products handbook which lists the "axial holding power" of a #6 screw at 250lb, and of a #8 screw at 385lb. That assumes a solid drive shaft, not a sheet metal lighting fixture. Regardless, I would not put my career on the line over it.

Your problem is rather complex. The shear strength of your set can easily be figured out, but this info probably is useless. How strong is your tapped hole? How is the set screw digging into your wall mounting disc?

--
JHG

 

[ajhanan]

JHG - thanks for the quick reply. I am new working at this company, but it has been the standard for years here and our company has never had a problem. However I thought it deemed some analysis as to how secure these fixtures really are. All of the mounting disks are made of steel, therefore it never quite marks up the mounting disk. Even if it did, it's not visible because the wall plate fits right over it. I hope the images came through! In the last image you'll see that the mounting disk would be held by the 3 set screws if it were positioned in place..

http://files.engineering.com/getfil...b5-b6cf-6ba05fd4eef0&file=Mounting_Disk_3.jpg

 

[ajhanan]

http://files.engineering.com/getfil...b1e0720&file=Mounting_Disk_and_Wall_Mount.jpg

 

[tbuelna]

The loading case would be different for a vertical wall mount versus a horizontal ceiling mount. The ceiling mount would likely result in a more uniform load distribution between the set screw contacts. The wall mount might result in an applied moment at the attachment, so the set screw contacts would not have a uniform load distribution.

With this type of attachment using set screws, the minimum number would probably be three equally spaced about the circumference. This would provide a kinematically constrained condition in the radial direction. As for the optimum taper angle of the mounting disc, I think 7deg per side is much too shallow. Assuming the tip of the set screws has a complementary cone angle, I would suggest an angle of around 30deg (or more) per side for your mounting disc. I would also encourage you to use as large a diameter as practical for your set screws. The steeper cone angle and larger set screw diameter will greatly reduce the local contact stress at the screw tip from the supported load. The steeper 30deg+ cone angle at the screw tip/mounting disc contact will also result in the fixture plate being pushed snug against the wall/ceiling surface when the set screws are tightened.

If you want to determine the correct number of set screws to use for a given load, you need to look at contact stresses rather than shear stresses. You can calculate the contact stress level present at the interface between the screw tip and mounting disc by approximating it as a hertzian line contact between two cylindrical bodies.

Good luck to you.
Terry