why use nut factor instead of coefficient of friction? 2

[electricpete]

http://home.houston.rr.com/electricpete/BoltTorqueVsPreload.ppt

The above-linked formula from Machinery's handbook appears to provide information to relate bolt preload force Q to appplied torque F*R, depending on bolt radius r, thread pitch p, and friction coefficient mu.

The more common relationship between these variables is expressed in terms of empirical nut factor k, where
k = (F*R) / (Q*r)

Why do we use an empirical value for nut factor k rather than using an empirical/experimental value for mu in conjunction with the Machinery's Handbook formula? Is k more constant accross a range of applications than mu?


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[MintJulep]

Friction where?

Between the threads of the nut and the threads of the bolt?

Between the face of the nut and the lock-washer (if you are foolish enough to still use one), or the flat washer, or the material being fastened?

Paraphrasing from Bickford:

The nut factor, k, is really an experiemental constant. It covers friction at all of the interfaces, torsion and bending in the fastener, deformation of the mating threads, and other things.

What this means of course, is that if you really need to know preload in a fastener, and are going to use torque control to tighten the fastener, then you really MUST test your particular application and determine k for your application.

Do not rely on handbook values (except perhaps as guidence in selecting upper and lower bounds for a sensitivity analysis).

 

[CoryPad]

There is no good reason why nut factor k is used instead of longer equations that use actual friction coefficients.

Your example (a power screw with no head that clamps a part) is a perfect example of why k is not good. Since there is no head, there is no head friction, which in typical bolted joints, produces a reaction torque equal to approximately 50% of the input assembly torque. Thus, the k calculated from that example cannot be used on other configurations. However, if I determined the friction coefficient for those mating parts with no head, then needed to know the preload for the same joint with a headed fastener, I could use the long equation without problems.

You used the word bolt, so I assume you are asking about fasteners to clamp a joint. If so, there is a Fasteners section in Machinery's Handbook that covers torque vs. preload better than your example.

Regards,

Cory

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[electricpete]

Thanks. I am interested in both bolts and power screws but I had forgotten the point that there is a major difference due to friction at the bolt head. I didn't see anything in a quick skim of the fasteners section but I'll look again.

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[CoryPad]

I have the 26th Edition of Machinery's Handbook. Section 9 covers Fasteners, starting on page 1452. Within this section is a subsection titled Torque and Tension in Fasteners, and begins on page 1475. It has a comprehensive treatment on preload vs. torque.

Regards,

Cory

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[electricpete]

Cory
I don't know how I missed that. That should provide me with more than enough reading material on this subject.
Thx.

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[pso311]

Mint Julep,

Curious...why is it foolish to use a lock washer?

 

[MintJulep]

pso,

faq404-1257

 

[EnglishMuffin]

MintJulep - Just out of interest, why did lockwashers ever become prevalent in the first place ? They used to be all over the place on cars and electrical equipment, and in some cases still are. Were no tests ever done ? When was it discovered that they are in fact useless ? Wouldn't the kind with teeth on both sides that bite into the metal maybe do at least some good, if only on the first tightening in mass production operations ?

 

[electricpete]

What if you have a gasket in your joint that shrinks over time after being initially compressed.

If no lockwasher was present, then only a small amount of shrinking of the gasket would remove all preload from the bolt (the distance would be the difference in length of the unstressed bolt and bolt under tension).

But if lockwasher were present, then it would have to shrink a lot more before that lockwasher returned to it's original shape.

So the lockwasher gives an ability to cope with changing conditions that might otherwise completely remove the preload of the bolt (which removes friction to prevent rotation of the bolt), right?

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[Tunalover]

EnglishMuffin-
The lockwashers were never tested in an authoritative way for the simple reason that NO ONE EVER WANTED TO PAY FOR IT! I know it's mindboggling that there is no such literature out there because I did an exhaustive search.

Instead we must simply rely on our peers and fastener experts to voice their (overwhelming) opinions that the helical split lockwasher has a neutral effect at best.




Tunalover

 

[MintJulep]

e-pete,

Go out to your shop floor and compare the torque it takes to compress a split lock washer totally flat to the torque needed to get your desired fastener preload.

If a joint loosens to the point that a lock washer becomes a spring again, the joint has already failed.

 

[Compositepro]

I've seen many joints where the the bolts is mainly a shear pin and will still function well after the nut becomes loose. A split lock washer does increase the time between the nut becomes loose and the time that the nut and bolt fall out of the hole and are lost. Are there better solutions? For many applications, sure. But there are many alternative solutions to any problem.

 

[electricpete]

what about belleville washers... the same problem?

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[dimjim]

yep

 

[MintJulep]

A Belleville "washer" is really a spring. Notice that almost all engineering texts discuss them in the spring chapter, not the nut, bolt and washer chapter.

So, any time you are considering a Belleville in a joint, imagine the same joint with a helical compression spring. Does it seem absurd? If yes, then a Belleville is absurd too.

Bellevilles do have their place; in joints where keeping a constant - but not necessarilly high - preload is required. High-power buss bars are a good example.

The Germans and the French have managed to convince themeselves that Bellville washers can compensate for embeddment. They haven't convinced me yet.