Derating torque values for oil/grease

Discussed this with an engineer friend the other day. Theory is to reduce torque values when the threads are no longer 'dry', for which most torque values are supplied except for a few rare ones. It probably means I've been doing a few bolts up too tight for a while now. Does anybody do it with any #'s by lubricant or just by feel? Or can of worms?

 

I would assume the values are set accordingly. Take my Gsx-R shop manual for example. They give the value for the head bolts and also instruct you to use an oil/molybdenum mix on the threads. The value given would have taken the lube into account. Same for a case cover bolt. They give a torque value and dont instruct you to use any lube so the value given would account for no lube.

 

Your Engineer friend needs to go back to school. For dry threads you would INCREASE the torque values to achieve the same bolt stretch as dry threads would eat up some of the torque (actually power - torque over the time you tightened) in overcoming the friction of the dry threads.

Torque bolts is all about achieving a certain bolt "stretch"...literally the amount of strain you induce in the bolt (indicative of the amount of stress imparted to the bolt). It is determined by the material of the bolt and the base material you are clamping.


EDIT: Ok. The 3x Sapporos I drank plus your double negative threw me. We are saying the same thing. But honestly, I wouldn't worry about it. That sounds like the "hair splitting" of a Physicist, not an Engineer.

 

Isnt that what he said? Torque needed decreases when the thread are no longer dry.

 

Yes, just edited. It was a result of the feared double-negative-Sapporo error.

 

I saw the edit, just too lazy to redo mine after I wrote it.


As for the original post I wouldnt be too concerned unless they were really important torque to yield bolts.



Most of the torque to yield bolts i've done they want some sort of really thin lube on the threads.

 

Yes +1. That is standard shop practice in industry.

 

30%-50% reduction in torque values for lubricated threads, depending on your source.
http://www.zerofast.com/torque.htm (approx 50% reduction for lubrication)

http://raskcycle.com/techtip/webdoc14.html (20-30% reduction)

It is true that lubrication reduces run-on torque (torque required to install the fastener), so reducing your final torque value is a good idea. It's up to you who you want to follow.