Bolt Torque Question 1

[Q519]

I have a bolt without any friction information.
T=FDK
D=0.008m
Tmax=20Nm
K= ??? unknow friction factor

It was tested and Tavg(old)=2Nm.


The same bolt was coated and the friction has increased.
It was tested and Tavg(new)=9.8Nm

I know the relation 4.65Tavg(old)=Tavg(new)

If both bolt was torqued to Tmax=20Nm on a torque wrench, would the clamping force be the same?

ANY HELP OR GUIDANCE WOULD BE APPRECIATED. THANKS

 

[desertfox]

Hi

I'm not completely following what you are saying, how are they testing the bolt???
However if you torque tighten two bolts with the same thread and both are either lubricated or not, then in theory the clamping force would be the same but in practice the clamping force could vary between each bolt by as much as 40% because torque is not a very accurate way to control clamping force.

 

[Q519]

It was torqued wrenched tested to see if the friction has increased.
So Tavg were measured for both bolt condition.

This is the same bolt, same thread, only thing changed was the coating on the bolt thread.

 

[tbuelna]

You need to establish what the prevailing torque is with the thread joint before and after coating. Prevailing torque is that existing in the thread joint being tightened before any axial preload is produced.

The increase in wrenching torque you describe after coating the bolt threads is likely due to interference between the mating threads caused by excessive coating thickness. The prevailing torque does not contribute to axial preload. However, the prevailing torque does contribute to the self-locking capability of the fastener.

 

[CoryPad]

It is unlikely that the friction conditions are the same for the coated and uncoated bolts, therefore the two types will not have the same clamping force when tightened to 20 N[·]m.

 

[Q519]

OK let me rephase the Question.

How do i find out what current friction factor is experimentally???

Then i can do the same experiment on the new coated bolt to find out friction factor experimentally.

 

[desertfox]

Hi

Try this link:-

http://www.google.co.uk/url?sa=t&rc...34GICA&usg=AFQjCNHaE5pB0hnpMxbFsa-HtSWcn2UtpA

A question for you, are you using some kind of prevailing torque nut or are these just ordinary threads.

 

[Ron]

No. The clamping force is a function of the torque, which is a function of the friction. If the friction increases, the same torque will not produce the same clamping force. The clamping force is inversely proportional to the friction.

 

[CoryPad]

This is the method to use for the experimental work:

http://www.iso.org/iso/catalogue_detail.htm?csnumber=27788

 

[ajh1]

Download the RCSC Specification, free at

http://www.boltcouncil.org.

Take a look at Section 7 on pre-installation testing. This is generally done with a Skidmore tension device. If you use a calibrated wrench in the test you can establish a specific torque to use to get to the tension you desire in the bolt.

 

[Tmoose]

Seems like the angle torque method, in reverse, would work.

If the old tightening method presumably results in the fastener providing satisfactory clamping, measure the range of angles to which they are tightened.
Then, tighten a bunch of the new fasteners to the same angle while monitoring the maximum torquage.