Proof stress on commercial steel machine screws 1

[chassisdeeziner]

ARGGH!

For those of us buying &quot;commercial steel&quot; machine screws in the <0.25&quot; major diameter...does anyone have a recommendation on the allowable proof stress for these &quot;commercial steel&quot; parts?

Background:
The SAE grade 2 spec doesn't apply to small machine screws as far as I can tell.

My fastener supplier says they warrant a UTS of 60 ksi on commercial steel fasteners(which correlates with SAE grade proof stress of 33ksi).

I've tried using 75% of that proof stress in the T=kFd torque to preload formula and it just &quot;feels&quot; too loose.

Another fastener supplier says I can count on 50ksi proof strength.

Anyone have any thoughts?

 

[mechwitch]

dear chassisdeeziner,

I've been having much the same problem recently. I also find even if you're given information from the supplier, in practice torques can be way off the mark.

I'm in a similar industry to yourself and appreciate that this information is critical. The way I now get around it is for each new screw size in each new material I do torque tests.

It may not be exactly what you're after, but it's very effective, and accurate. If your customer needs calcs as proof - you could always work it backwards!

 

[chassisdeeziner]

Mechwitch....thanks for the reply.

Yeah...this is a frustrating topic and I've resorted several times to doing torque testing as you suggest. Unfortunately, that always leaves me with the sinking feeling that perhaps the fasteners I used for the testing weren't of the lowest proof stress allowed by &quot;commercial steel&quot; spec. Which in turn leads to worrying about what happens when we do get a lot of screws with the minimum proof stress and ...BAM....fastener failure.

How do these numbers seem to you for zinc plated steel fasteners going into a zinc plated PEM:
#4-40 - 3 in-lbs +/-0.5
#6-32 - 6 in-lbs +/- 0.5


Chassisdeeziner
LeCroy
Innovators in Instrumentation

 

[mechwitch]

I'd say you were in the right ball park with those figures but i'd watch out in using exactly the same materials for screw and nut - I've been having a lot of trouble with them galling and stripping threads.

Have you had any of these problems too??

 

[chassisdeeziner]

I try to always use zinc plated steel fasteners with zinc plated steel PEMs or nuts. There are times when I wind up using stainless screws (they are readily available with a nice black chrome finish) in which case I tend to throttle the torque back for the lower proof stress of the stainless compared to carbon steel.

I also tend to always use some sort of Nylok patch on the threads to keep the fasteners from rattling loose....that really messes up my torque calculations as the assembler has to overcome the additional friction of the locking patch. I haven't found a nice way to predict that additional torque and therefore wind up testing to make sure the speced torque can overcome the patch,



Chassisdeeziner
LeCroy
Innovators in Instrumentation

 

[alpharam]

Though Nylok prevents macro shifting, it is rather tolerable to micro shifting. Reasonable assumption is that the torque would drastically increase with relatively microscopic shift in the final phase of tightening the screw. This shift can be easily covered by nylon elasticity. I don’t have an exact formula for you, but in most cases Nylok factor can be rather negligible. Correct me if I’m wrong.