Best audit procedure for bolted joints?

[adi1]

What is the best audit procedure for the bolted joints?

For example If Assembly torque – 20-30 Nm
Audit with in 15 min – 17-33 Nm (+/- 15%) for hard joints
And 14-36 Nm (+/-30%) for gasketed/soft joints

Is this a good assumption to start with? Valuable suggestions needed.

 

[Dinosaur]

We use turn-of-the-nut method and calibrate the procedure using a skidmore device. Another method uses Direct-tension-Indicating washers (DTIs). These are tested using "feeler" gages after the bolts are tensioned. A third method is very similar to DTIs except they are loaded with a paint that squirts out when the DTIs reach load. Checking torque has been done in a large variety of bolted applications but is very unreliable. The condition of the bolts varies wildly and this variation in friction results in a wide variation in tension.

If you're not making a bolted connection using High strength bolts, ask someone else. All my experience is with high strength bolted connections. Good Luck.

 

[MintJulep]

What is the purpose of the audit?

To see if your assembly workers are bothering to tighten the fasteners in the first place?

To see how much of the design preload you are actually getting and retaining?

Given the tenuous relationship between torque and actual achieved preload, any audit using torque provides about zero engineering value, although it may make the MBAs happy.

If you really want to know if you are retaining preload, ultrasonics are probably the best bet.

If you just want to know if your workers are doing their jobs, 85% of the assembly torque without rotation.

 

[CoryPad]

adi1,

I agree with Dinosaur and MintJulep that post-tightening torque checking provides little engineering information.

However...

The automobile industry regularly wastes its time with this procedure. Your values are typical starting points for joints, but are adjusted as needed based on acquired data.

One (decent) way of defining audit torque is: advancing a previously tightened, at-rest fastener in the tightening direction less than 5 degrees. This should correlate within 15% of tightening torque for hard joints, assuming your company specifies tight tolerances on part dimensions, friction coefficient, surface roughness, etc.

Regards,

Cory

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

 

[adi1]

Cory and Minjulep,

Yes the audit purpose is to check if our operators are doing thier job. we just wanna check the retaining torque.
we got some datamytes which when rotate the fastener less than 5 degrees give you the torque and gives a sound and light signal.

red - out of spec (high)
green - in spec
yellow - low spec

I am just looking for the general rule of thum. you guys both came up with 15%. Thats what I came up with too.

But our company is really not tight in tolerances on part dimensions, friction coefficient, surface roughness, etc.

so dou you thinks its best to go for +/- 20%? or still go with 15%?

and also please let me know the general rule of thum for soft joints? 30%?

Thanks

 

[CoryPad]

If I had to, I would start with +/- 15 % for hard joints and +/- 30 % for soft joints and then adjust with actual data as needed.

Regards,

Cory

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

 

[adi1]

Thanks Cory