Torque factor (or nut factor) for similar alloys

[just_some_shmuck]

I have read several sources that say to know the torque factor (or nut factor - or bolt factor) you need to measure it for your particular situation. I am analyzing joints that have been being manufactured for quite some time. All joints have the same bolt, same threadlocker and the same material under the bolt head. But there are two different materials for the tapped holes.

How similar do the materials need to be so that I can say they are "the same" with respect to the torque factor?

For this specific situation, the materials for the tapped holes are CD 1018 and HR 1020 - but I am more concerned about the general principle.

 

[MotorCity]

What is a "torque factor"?

 

[BrianE22]

If you mean torque/clamp load (=factor?) then a change from one steel to another won't make any difference. It's mostly a friction issue.

 

[MintJulep]

I guess that you mean 'K' in this equation.
T = K F d

where

T = wrench torque (Nm, in lb)

K = constant that depends on the bolt material and size

d = nominal bolt diameter (m, in)

F = axial bolt force (N, lb)

As a guide, I'd say "steel" would all be about the same.

In reality K depends on on plating, surface finish, the presence of other materials (such as Loctite) in addition to material.

Which is why you should measure for a particular combination.

 

[just_some_shmuck]

MintJulep, yes K in your equation is exactly what I was asking about. Another way to ask my question is: Do the differences between CD1018 and HR1020 affect the thread friction?

And it sounds like the answer is: "all steels" (or maybe just "all mild steels"?) will have the same thread friction.

Thanks!

 

[chez311]

It should be noted that the K factor in that equation is a combination of the different geometric (function of diameter vs. pitch) and thread/bearing(under-head) friction factors of the bolt/nut not just thread friction. There are equations to calculate each one of these factors but it is probably a substantial increase in complexity for not much increase in accuracy, plus it requires knowing with reasonable accuracy the individual thread and bearing friction coefficients.

I would agree that theoretically in most cases as long as there aren't any significant differences in coating, lubrication, or hardness/tensile strength your K factor will be very similar between steels of a similar grade. That being said, in reality your K factor as well as resulting clamp load is affected by a myriad of things like differences in coating/plating, surface finish, joint geometry/stiffness, bolt angular misalignment, and introduction of gasket/sealing material between the joint which is why, to echo MintJulep, you should always test each application.

 

[drawoh]

just_some_shmuck,

If you cannot find it anywhere else, your Machinery's Handbook shows the analysis for the stripping of tapped holes. You know the threads. You know the length of engagement. Work out the force required to strip the threads. If the force for both materials is below the force required to break the screw, you have no problem. If there is a danger of the threads stripping, I recommend that you extend the length of engagement or install helical thread inserts, and continue to specify the tensile strength of the screw.

--
JHG