torque to failure test

[evmundo2003]

Hi,

I have an issue, while testing the break load of a fastener in Plated form, black self colour and greased forms, I experienced an unusal result in that the greased bolt actually produced a higher break load than the other two bolts. Can somebody explain the reasons why this may occur, for information the jig did not contain a blind hole so the effets of hydraulics can be excluded.

Evmundo2003

 

[Metalguy]

How did you perform this testing?

 

[diamondjim]

Agree with Metalguy,
You need to tell us how you measured the
breaking force. If torque was the only
method, it is a poor method to determine
the breaking force. Turn of the nut may
be more accurate for what you are trying
to do. Actually a tension device would
give the proper results. Maybe this is
what you used?

 

[evmundo2003]

Sorry, the mode is torque to failure using a fully electronic torque wrench and monitoring software tightening the bolts into a pre-constructed jig. this test is conducted in order to achieve the optimum tightening torque for a given fastener/jig. After pondering, I have forgot to test the actual bolts for material integrity. i.e. decarburisation, recarb etc.

 

[CoryPad]

evmundo2003,

Threaded parts that are loaded by torsion have simultaneous tension and torsion stresses. The lubricant decreases thread and/or bearing surface friction, which reduces torsional stress during loading. Thus, the combined stresses (think von Mises) are lower, which allows a higher tension load at failure.


Cory

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

 

[desertfox]

Hi evmundo

As corypad as stated using a lubricant reduces the friction between the mating threads and hence increases the amouunt
pre-load in the bolt for a given torque.
Looking at some formula for calculating combined stress and bolt torque provided by a company called MHH eng co.ltd ("torqueleader&quot.

St=Se/[1+3*{(4*d2/d2+d3)*([P/(3.142*d2)])+[1.155*Ut]}^2]^0.5

where Se= combined tensile and torsional stress

St= tensile stress in fastener

d2=pitch dia of thread

d3=root dia of thread

P=pitch of thread

Ut=thread friction coeff

Normally they recommend that Se is about 90% of the yield stress or proof stress so depending on the grade of bolt Se is a known quantity.The friction factor Ut is dependant on the bolt material and finish for a dry bolt with a self finish Ut can be between 0.1 and 0.16 (steel threads)and can be as low as 0.08 for an oiled thread.
If you look at the above formula and reduce the Ut from 0.16
to 0.08 then the tensile stress (St) in the bolt almost doubles.
In conclusion then a lubricated bolt is more efficient than a dry bolt and its maximum pre-load can be attained at a much lower torque for the same combined stress as a dry bolt.
Also you can see why just using torque as a criteria for bolt failure is not very accurate as the friction coefficients cannot be accuratly determined.

 

[yates]

Cory has hit it on the head, whereas you were just turning the head.
The best thing to determine torque/tension relationship for differently lubricated bolts is by using a Nielsen test-rig.
After all, in a joint you're really interested in the clamping force and the applied torque should only appear in the assembly instructions so as to obtain this clamping force.