By worf - Fri Jan 08, 2021 8:27 pm
- Fri Jan 08, 2021 8:27 pm
#48865
Torque - as far as nuts and bolts are concerned - is a direct measure of friction of the thread surfaces sliding over each other. It is only an indirect measurement of clamping force.
Friction, as we know from our first physics book, is the normal (clamping) force times the coefficient of friction. The last can vary significantly based upon condition of threads. New, shiny, electroplated threads will have a low coefficient, whereas old crufty, much-used threads will have a higher-than-new coefficient.
The actual clamping force of the fastener has to do with elongation of the fastener. The number of turns of the bolt or nut is what directly determines clamping force. So, an old crusty bolt turned to X N-m will have a lower clamping force than a new bolt turned to X N-m because the friction is higher. A lubricated bolt will have a higher clamping force because the coefficient is lower thereby allowing more turns before X N-m is reached. In applications where the only purpose of the fastener is to keep to pieces together the specific clamping force is not as important as the friction force of the threads that thereby keep vibration from loosening the fastener.
However, when we get to heads, actual clamping force within a small range is critical. That clamping force can be calculated from the material properties, thread pitch, and the number of turns a fastener is given. So, the directive to Torque to 20 N-m and then do 90° turns serves to first, ensure thread engagement and to then elongate the fastener by a calculated amount so as to provide a specific clamping force.
The lubrication on "torque-to-angle" serves the purpose of minimizing the bad effects of the thread surfaces sliding against one another: reduces chance of galling or otherwise damaging threads, makes the fastener easier to turn smoothly, etc.
Edit, Addition: So, in a torque-to-angle application the final torque is almost irrelevant.
N_Jay wrote: ↑Fri Jan 08, 2021 4:04 pm If it were me, I would do the 20nm in two steps (Maybe 10nm and then 20 nm) to make sure everything seats evenly.This is a fine idea. For super important big things, like heads, I always progressively "creep up" on the torque value.
N_Jay wrote: ↑Fri Jan 08, 2021 4:04 pm The fact you are tightening by angle and not by torque eliminates some of the variability due to thread lubrication.This is correct but the explanation doesn't sit well with me in the context of head bolts/studs.
Wet (oiled) threads put more force (stretch) on a bolt or stud than dry threads at the same torque.
Torque - as far as nuts and bolts are concerned - is a direct measure of friction of the thread surfaces sliding over each other. It is only an indirect measurement of clamping force.
Friction, as we know from our first physics book, is the normal (clamping) force times the coefficient of friction. The last can vary significantly based upon condition of threads. New, shiny, electroplated threads will have a low coefficient, whereas old crufty, much-used threads will have a higher-than-new coefficient.
The actual clamping force of the fastener has to do with elongation of the fastener. The number of turns of the bolt or nut is what directly determines clamping force. So, an old crusty bolt turned to X N-m will have a lower clamping force than a new bolt turned to X N-m because the friction is higher. A lubricated bolt will have a higher clamping force because the coefficient is lower thereby allowing more turns before X N-m is reached. In applications where the only purpose of the fastener is to keep to pieces together the specific clamping force is not as important as the friction force of the threads that thereby keep vibration from loosening the fastener.
However, when we get to heads, actual clamping force within a small range is critical. That clamping force can be calculated from the material properties, thread pitch, and the number of turns a fastener is given. So, the directive to Torque to 20 N-m and then do 90° turns serves to first, ensure thread engagement and to then elongate the fastener by a calculated amount so as to provide a specific clamping force.
The lubrication on "torque-to-angle" serves the purpose of minimizing the bad effects of the thread surfaces sliding against one another: reduces chance of galling or otherwise damaging threads, makes the fastener easier to turn smoothly, etc.
Edit, Addition: So, in a torque-to-angle application the final torque is almost irrelevant.
Last edited by worf on Fri Jan 08, 2021 8:31 pm, edited 1 time in total.
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Heinlein’s Corollary: Never underestimate the power of human stupidity.
The Reddit Conjecture: Sufficiently advanced stupidity is indistinguishable from malice.
Worf’s Razor: Never attribute to stupidity that which is adequately explained by laziness.
Worf’s Identity: Sufficiently advanced laziness is indistinguishable from stupidity
Worf's Law: Once you've mitigated risk from stupidity and laziness in your endeavors, failure is usually the result of insufficient imagination.
My 928 Inspection Guide