Thread stripping calculation/look-up table?

Question

I need to do a hub-pulling type operation on a small piece of equipment that is not made to be taken apart, so no purpose-made anchor points. There are 8 off , 1mm tapped holes with 5 threads per hole , in the aluminium available (used for undemanding normal fixing to the chassis) , if I drill a plate to take 8 screws . The only screws of that thread I have are brass, probably preferable to steel anyway. For a similar hub-pulling where there was purchase , after heating (limited as to amount that can be used ), required about 4Kg of force to extract. Would

4Kg , assuming even distributed loading over the 8 screws be above or below the stripping force ? I found these sites but not applicable for my purpose it would seem

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anorton · Accepted Answer

I would be leery of using these screws for even this kind of light pulling. Even if a single 1 mm screw in a 1.25mm deep hole in aluminum can hold 0.5 kg (I do not know; I have not done the calculation), I know from experience it is very easy to strip an M1X0.25 screw in aluminum even when torquing for preloading. Don't forget the preload and load together have to be less than the strength of the thread. If you do not have any preload (i.e. the screws are loose) then there is a good chance the force will not be evenly distributed among the screws when you pull.

I think I would try to use a method that used some sort of temporary tape or adhesive that is compatible with the moderately high temperature you need. If there is a large enough flat area to mount to, perhaps just viscous high temp grease between the hub and the pulling plate would provide enough suction force when pulled straight up.

dpb · Answer

The _rough_ back-of-envelope estimate here comes out at only about 20  lbf loading each assuming a soft Al.  T6061 w/ a hard fastener might be  closer to 100 lbf.

Ned Simmons · Answer

I get 30 pounds, but maybe your aluminum is softer. And I didn't have the benefit of a napkin.

In any case, if the screws are not silicon bronze or some other high strength alloy, it's probably a toss-up between stripping the threads and breaking the screws.

anorton · Answer

In practice I think both your estimates are overly optimistic. In this tiny, fine thread, the clearance is usually a large fraction of the total thread depth, so the threads just engage near the tips. Usually the failure mode is that the tips round over instead of shearing off. Optical equipment I work with sometimes has these tiny screws.

Bob Engelhardt · Answer

Ah ... good point.  Another nugget of RCM knowledge.  Bob

N_Cook · Answer

Yes I would not say they were a tight fit. Decided to suck it and see and potentially sacrifice half a tapped hole. So 3 turns only engaged in one tapped hole and a spring balance, I took up to 2Kg with no problem. So I'm now confident that 8x five thread screws and a compressible washer/pad of silicone rubber under each screw head , will take the 5Kg via an added hole-matching load-sharing plate

Joe Gwinn · Answer

I'd use belleville spring washers under each screw head, not rubber.  Alternately, a big washer to spread the load onto the rubber.   Joe Gwinn

dpb · Answer

... That seems incredible to me from the initial description...

N_Cook · Answer

Came apart without problem, Al plate with matching holes, silcone pads  and small thin wahers under the screw heads. Frame fitted to the plate  and a pair of circlip pliers in the gap to push abainst the spindle

Thread stripping calculation/look-up table?

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