What is the proper formula used to determine torque when the threaded fastener intentionally bottoms out in a blind hole?
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I don't think the math/design is different... Other opinions?
See:
http://www.engineersedge.com/calcula...orque_calc.htm
http://www.engineersedge.com/calcula...alculation.htm
http://www.engineersedge.com/fastener_thread_menu.shtml
I guess my question is....why are you intentionally bottoming out the screw? You'll keep torquing down until it starts self tapping in the tap drill portion of the hole....then SNAP!!!
Last edited by Marky; 07-19-2011 at 11:08 AM.
I am a little confused. Are you clamping something flexible, compressible, solid or just bottoming the bolt out?
What torque are you looking for? Anything from zero to a little-less than the failure point might be an acceptable torque depending on what you are trying to achieve.
Why would you want to do that? All the theoretical analysis and formulas for screw fits and torques are based on the assumption that the screw is in tension. In this case it is in compression, and actually that compression would only be seen on the bottom thread or two. I think any screw used like that would come loose very easily.
Set Screw?
Very good point!
I guess that's why they tend to come loose so easily.
Took a quick look at a handy-dandy slide chart I have from a bolt manufacturer and they give recommended seating torque for set screws as well as socket heads. There is somewhat of a curve to the relationship between seating torque in tension vs. compression but it has some bumps in the curve and I'm sure it has something to do with their recommended minimum screw length. As a percentage of the seating torque of a cap screw... some smaller set screws are as low as 50% and then as the diameters go up (as do the recommended minum set screw lengths) then the torque value for set screw as a percentage of cap screw goes up to a point where it is actually higher... then it drops back off again to the point where a 1-1/2" set screw should only be torqued to about 25% of what a cap screw would be.
Myself... I don't calculate the torque requirement for any bolting applications I encounter. I look at my particular application and materials and length of engagement etc.... then I look at my handy dandy slide chart... or whatever source is nearby like maybe a rebuild manual or whatever and I decide from there where the torque needs to be. I figure no sense re-inventing the wheel. I also trust that the mechanical values for various steels etc. that are given to me are accurate, and apply my personal fudge factor to it depending on the situation.
I can't help but continue to guess as to why you are bottoming this bolt out. My first thought was set screw application but the way you worded it... it seems almost more like you might just want to leave some bolt hanging out there and still be tight in a hole? If by any chance that is the case then a jam nut might be your answer? Rather than compressing the bolt stretch it with a nut. Or maybe you could tolerate a thin wall sleeve or tube so that the bolt could be stretched but still have bolt hanging out?
Oh... another thing... better put a dog or half dog point on that bolt of yours if you think you will ever want to remove it someday.
Good luck!
Bob
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