Bolt Equivalent Torque Test

[jmarkus]

I want to develop a test to determine if a weld stud (already welded) meets its spec as a Property Class 9.8 fastener. From the ISO 898-1 spec, the proof load for an M10 at that class is 37.7kN. If I use T=F*k*d, where k=0.2 for steel on steel (nut is steel, stud is steel), then I should be able to torque the fastener up to 75.4Nm to acheived the proof load. Does this make sense, or am I missing something? If I put an aluminum plate between the nut and the fastener, does that change k?

Thanks,
Jeff

 

[desertfox]

Hi jmarkus

Using a torque figure to provide a proof load in the bolt is not very accurate, although theoretically your formula
above would predict the tension in the bolt it is only an approximation and you could be at least +/- 20% out on the tension figure.
For instance if there is any lubricant on the thread or nut the "k" figure will fall dramatically and for a given torque, your pre load could be several times bigger than that which you calculated.
If you need a figure to prove the failure load of the fastener then you need to measure the elongation of the stud during tightening, this assumes of course that the weld at the bottom of the stud doesn't fail first.

regards desertfox

 

[CoryPad]

The 'fox is correct - friction will vary a lot, making a force determination difficult. You should perform actual tension testing to determine if the stud meets its property class.

Regards,

Cory

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

 

[Screwman]

I agree also. The unknown here is what friction (K) you have on that already welded stud. I would run a study sandwiching a load cell on the stud to develop the "K" factor for that fastener.
Changing anything in the joint stack-up carries a good chance of affecting the K factor. That catches folks all the time when they change things like insert geometry used on spot faces and wonder why they are suddenly breaking bolts.
Remember that to conduct a proof load test you need to be able to measure the length of the fastener before and after the test with a high degree of accuracy. It is going to be hard to do that on studs that are already welded to an assembly. The usual practice is to spot drill both ends of the fastenr and then use a ball anvil mic to measure before and after length. trying to measure without the spot drill has too much gage error to work out usually