Thread forming screw head 1

[Kinsrow]

I have a #4 thread forming screw with T8 drive head and another with T7 head drive. If I'm applying the same torque on both screws with all other parameters to be same (the only the difference is T7 and T8), would I get less clamping force on the joint with the screw that has T8 drive?

Thanks
Kinsrow

 

[CoryPad]

Assuming that neither drive system plastically deforms (absorbing some of the input torque), then both screws will have the same clamping force.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Kinsrow]

I thought that when we have bigger drive head, the more torque can be transmitted to the threads body thus increasing the clamping force.

Please correct me.

Thanks

 

[CoryPad]

If the input torque is the same (which you stated it was), and if the drive system does not absorb energy by plasticity or crack propagation, then both drive sizes will transmit the same torque, and hence, both will develop the same clamping force.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[apdesign]

Kinsrow,
CoryPad is right on the money with his answer which could be translated as:
"Torque is torque (as long as you're along the same axis"

APDESIGN

 

[dimjim]

Torque by definition is a force times the
radius. It does not matter whether it is
a slot, phillips, hex, or whatever drive.
The force times the radius is the same.
The stress on the different T drive heads
does increase as the T drive gets smaller.
This would be analogous of useing a 1/4 or
1/2 drive head. You can get the same torque
for either, but you might break the drive
head. The torque is created by the force
on the handle of the driver irregardless
of the sixe of the drive head.

 

[Kinsrow]

It made a lot of sense now..Thank you Cory. DimJim explains it very well.


Thanks!
Kinsrow

 

[Screwman]

Cory is correct. I have done testing on those size parts with thread forming screws and if the input torque is the same and the thread and head size is the same, then the clamp load will be the same.
I feel that an attempt to consolidate drive sizes is going to be the offshoot of this question and I need to tell you that it is not quite as easy as it seems. If you go to the smaller drive you will be applying a greater proportion of it's endurance limit at the seating torque and as a result your tool life may suffer. Putting the larger drive may cause heading problems and head strength issues. Drive size changes need to be looked at carefully; we spent several manyears creating head standards for our last drive system development project.

 

[Kinsrow]

The seating torque is 4 in-lb. So either T7 or T8 will do just fine I would think

Kinsrow

 

[Philrock]

As stated above, the drive size certainly is not a factor. However, the o.d. of the screw head bearing surface is a factor, because a good portion of the tightening torque goes into overcoming head bearing friction. If the T8 screw has a larger head bearing diameter, it will develop less clamping force for a given input torque, because less torque will available to work on tightening the threads.

 

[Kinsrow]

I guess I probably mentioned it wrong. The T8 head is slightly larger than T7 head ( thread is the same ) My first impression was that I will get less torque using T8 head. Since I'm going to T7, I'm going to have more clamping force with doesn't hurt me.

 

[MintJulep]

Phil hits on a point that everyone else missed.

Friction under the head is a factor in determining how much of the input torque goes toward achieving preload.

However a larger head may not necessarily lead to higher friction forces, no matter what they taught us in school abou F = mu x N.

Contact stresses, deflection and surface finish also all come into play.

So, as is always the case with threaded fasteners, if you really need to know how much preload you are achieving, you have to test your specific application.

 

[MintJulep]

And oh by the way.

With a thread forming screw the head friction to all of the other things that "absorb" torque without producing useful preload is much smaller than for regular threaded fasteners.

So in the particular case under discussion, the difference in head size is probably trivial, and probably less than the scatter from joint to joint.

 

[CoryPad]

MintJulep,

I fail to see how we missed a point when:

with all other parameters to be same

If the bearing surface is allowed to be different, then all things are not the same. We weren't allowed to consider that option, hence the original answers of "it doesn't matter", with the caveats of no drive system plasticity or crack initiation/propagation.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.