Torque Spec-dry vs oil

[qtrhorse]

I have a 1"-8 thread with a 110 ft.lbs. max. torque spec. This seems quite sufficient if the threads were always dry. However, between the the application is contantly contaminated by oil, in takes about 150 ft.lbs. to get the same results, a formula or lab test backed data sheet would be nice to justify.

 

[NickE]

Wrong way man...

A lubricated thread requires less torque for the same tensile bolt load.

110ft-lbs seems kinda small for a 1"-8 thread.

Nick
I love materials science!

 

[qtrhorse]

My application is on a pull stud on a CNC machine holder, we have found that it takes about 150 ft. lbs. to keep the oily stud from coming loose however, the manufactures spec is 110 ft.lbs. max., I don't know if that is dry or wet. I know what works, just trying to justify it.

 

[Lou Scannon]

I'd say the fact that 150 lb-ft works, and 110 lb-ft doesn't, justifies it. That assumes you know what you're talking about when you say it works.
I don't see that you're flirting with any particular failure mode by torquing a 1" thread to 150 lb-ft, except possibly damaging the threads at the end of the stud due to interference if they didn't use a bottoming tap.

By the way, this seems a little off-topic for this forum.

 

[Fabrico]

Uh, where's the logic here? If it tightens easier wet, shouldn't it loosen easier too?

 

[NickE]

because hes' now developing enough force and stretch in the bolt for it to hold tight. IE: the preload is now sufficient to overcome the service loads.

 

[Fabrico]

The harmony of that with your earlier statement, would depend entirely on what you consider service loads. Especially since the lower (dry) value is sufficient to overcome them. If he must apply more torque to the bolt when oiled then when dry, then he is developing substantially more force and stretch in the bolt than what is needed to reliably do the work. If this is true than it is not a matter of the preload now being sufficient to overcome the service loads, but a matter of the preload keeping the bolt from loosening because of something else such as vibration, sudden speed change/reversals, etc. No? I still fail to see logic or need for justification in the original question. Even cutting oil lubricates.

 

[patprimmer]

I like Hemi's answer

Regards

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[qtrhorse]

Thanks for all the great input!

When tightening the pull stud it distorts the taper on the holder, the tighter you go, the worst it gets. Yet, you must torque to 150 ftlb. to keep from coming loose.

Only option is to tighten to spec and use a Loctite 371 or 377 to keep it from loosening.

If someone knows of a better, more vibration-proof threadlocker that will hold in the oily environment.

 

[Tmoose]

Are you using keys in the spindle nose?
I worked for a spindle company for about 7 years. Cat, SK, HSK interfaces. Brutal conditions, interupted cuts, high speeds. We had plenty of problems, but I don't recall stud loosening ever being a problem. I suspect stud loosening is a symptom or indicative of a more basic problem, like maybe inadequate drawbar force, improper spindle taper form, a torque wrench that is calibrated way wrong, or the use of torque wrench "extensions" being mis-interpreted.