Fastener Torque vs Clamping Force 5

[AirRider04]

Is there a simple way to calculate torque if the clamping force of a fastener is known(or vis versa).

Thanks

Air

 

[swall]

Torque(in-lbs)=.2xbolt diameter(inches)xclampload (pounds). The .2 factor is a starting point. In actual practice, this factor can vary substantially depending on type of surface treatment on the bolt and presence/absence of lubricant.

 

[BATman2]

AirRide04,

Handbooks typically state:

T = (K*D*W)/12

T=torque
D= dia.
W = Clamp load
K=0.09 to 0.3 depending on factors affecting friction.
0.3 for non-plated fastener, 0.12 if anti-seize compound is used.

Typcially W = 75% of proof load of fastener

Batman2

 

[flamby]

Batman,

Your formula is basically the same as given by swall with the added confusion of changing inches to feet. swall's formula is correct even without stating the units, but he did. Your formula is incorrect without stating the units and you did not.

Ciao.

 

[BATman2]

Flamby,

The obvious is now stated. Note that Swall is correct but did not offer guidance on the clamp load, no offense to Swall). If the wrong value is used the joint can loosen or bolt will fail. Selection of clamp load is often the biggest issue, not units.


T (in-lbs) = K *Dia (in)*W (lbs)

T=torque
D= dia.
W = Clamp load where 75% of proof is typically used.
K=0.09 to 0.3 depending on factors affecting friction.
0.3 for non-plated fastener, 0.12 if anti-seize compound is used.


Batman2

 

[diamondjim]

I think what has been shown is that the
formula gives such a great spread of values
that it is almost meaningless as a method
to calculate either torque or clamping force
from their inverse for any degree of
reliability.

 

[Rjeffery]

there is no relation, in the real world, between torque and tension or clamping pressure. Check the Commentary in the AISC manual. If your nut and bolt are out of tolerance in any way it will effect apparent clamping pressure. gauled threads, debris will make torque look high but the clamping pressure will be low. over sized nuts or undersized bolts or even lubrication will provide torque values but higher plamoing pressure.

torque should only be used ao assure uniformity of the twisting force applied...

 

[BATman2]

Diamondjim and RJeffery,

I fully agree with your points except the twisting force, that too would be affected.

This is a typical formula used to estimate torque. Obtaining a proper value on friction alone is diffcult, The only thing that counts is bolt stretch. If you must know the clamping load to a high degree of accuracy, you must some how measure actual bolt stretch. Often this is not practical. If you can not measure stretch,(hard to do in the field) and must resort to torque, you'd better ensure you're buying quality hardware that meets IFI, SAE or other standards, and have significant margin (MS of 1 at least)in your joint design.

Batman2

 

[diamondjim]

BATman2,

I would rather see turn of the nut
method used as a reverse clamping method.
At least you can calculate the stretch
by the degree of turn of the nut which could
be related more accurately to force
and elongation.

 

[BATman2]

Diamondjim,

While turn of nut would seem to be a better approach, it too has issues (what doesn't..)

Take a look at the following discussion:

http://www.eng-tips.com/viewthread.cfm?qid=87855&page=1

Thread 87855

If the stiffness of the joint is relatively low, turn of nut will not provide the correct value. The amount of bolt extension needed to obtain the desired preload is so low, its difficult to determine if the nut is stretching the bolt or flexing the base material.

Batman2


Batman2

 

[Tmoose]

Experiments made with my left index finger in the jaws of the bench vice whilst tightening the vice handle with my right hand strongly suggest the statement "there is no relation, in the real world, between torque and tension or clamping pressure " may be a bit harsh, although the relation can be pretty variable. My conclusion after completing multiple vice experiments on myself and others was greater torque = greater clamping force.

http://www.textronfasteningsystems.com/eng_tools_f/boltjointdesign.html

http://www.textronfasteningsystems.com/PDFs/socket_tech/inchshc.pdf

 

[Rjeffery]

Tmoose,

As you say, if you use the same screw, the same torque applied to the screw should provide the same clamping force. If you use 100 or 1000 or 10000 different, screws and applied the same torque to each of these screws the clamping force should be the same in each case PROVIDED, all the screws were exactly the same. In the real world, that is never the case.

AISC has indicated that they have removed any table or formula that proports to relate torque to tension simply because of this.

I stand by my statement that in the real world, there is no (exact) relation between torque to clamping force to tension.

 

[Tmoose]

"As you say, if you use the same screw, the same torque applied to the screw should provide the same clamping force. If you use 100 or 1000 or 10000 different, screws and applied the same torque to each of these screws the clamping force should be the same in each case PROVIDED, all the screws were exactly the same."

I did not mean to say that at all .
All I said was greater torque = greater clamping force.
When rotating my snow tires yesterday I used my old Craftsman torque wrench and try to hit 75 lb-ft. In my heart I know it was really +/- something, like practically any other measurement I make, no matter how many decimal places my calculator displays. But my despair is not so great I will abandon torquing those lug nuts. I think it may be significant that the one wheel I had loosen back in 1968 did not recieve the torque wrench's verification.

 

[EngJW]

I usually use the above equation to estimate the clamping load, given the torque. I might want to know the force holding a joint together, the unit loading on a gasket, or the stress in a clamped joint. The torque can be from a standard torque chart unless otherwise known, and a high or low value of the nut factor can be used depending on the situation.

I know it is not the most accurate equation, but sometimes you need a quick estimate or a place to start.

 

[BATman2]

Jilwoodward,

EXACTLY right. You need to start somewhere considering the function of the joint, range of parameter, marigins etc. The torque-tension relationship is an estimate and must be treated as such when designing a joint.

If clamp load it must be known, the only way is to test the joint somehow or measure bolt elongation. If very high precision is needed, special bolts with tigh tolerances.

Batman2

 

[davrusso]

What about the case of an extrememly low clamping pressure, where no deformation of the bolt or threads takes place? In one instance we would like to use small bolts to achieve a clamping pressure of approx. 50 PSI on a glue joint. It would be simplest to relate the torque to the clamping force, where the friction of at least the bearing force of the nut on the washer can be reduced with special thrust washers/bearings.

 

[KalleH]

Volvo corporate standard 5511,21, which can be found on the following adress has excellent tables to determine clamping force / torque relationships depending on thread and base friction.

Please, see the adress below:

http://www.tech.volvo.se/standard/eng/

 

[desertfox]

Hi AirRider04

Have a look at this link to free demo software you can download to calculate what your asking for.
Also if you go to the home page of this site you can view
theoretical information about bolted joint design.

Regards desertfox

 

[desertfox]

Sorry here is the link

http://www.boltscience.com/pages/download.htm?

regards desertfox