Proof stress on commercial steel machine screws
Proof stress on commercial steel machine screws
(OP)
ARGGH!
For those of us buying "commercial steel" machine screws in the <0.25" major diameter...does anyone have a recommendation on the allowable proof stress for these "commercial steel" 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 "feels" too loose.
Another fastener supplier says I can count on 50ksi proof strength.
Anyone have any thoughts?
For those of us buying "commercial steel" machine screws in the <0.25" major diameter...does anyone have a recommendation on the allowable proof stress for these "commercial steel" 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 "feels" too loose.
Another fastener supplier says I can count on 50ksi proof strength.
Anyone have any thoughts?
RE: Proof stress on commercial steel machine screws
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!
RE: Proof stress on commercial steel machine screws
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 "commercial steel" 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
RE: Proof stress on commercial steel machine screws
Have you had any of these problems too??
RE: Proof stress on commercial steel machine screws
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
RE: Proof stress on commercial steel machine screws