Theoretical Faliure torque Calculation !! 2

[ghpatil1234]

Hello Friends,

I need your expert opinion on below case.

I have failure in my part at 30 Nm torque, so I need to calculate theoretical failure torque so that I can know the safety factor of this sealing thread joint.

Torque Spec : 15-20 Nm
Thread size - M10x1.0
Thread on nut part - 7.5±0.5

Bolt:
Material - En1A
Pic -

http://files.engineering.com/getfile.aspx?folder=73684512-dceb-4aa9-afca-e42e7b28dbef&file=Bolt.bmp

Nut:
Material - Aluminium
Pic -

http://files.engineering.com/getfile.aspx?folder=0be64516-b796-4994-a162-cfd1ddcb07e7&file=nut.bmp

 

[CoryPad]

What failure mode? Internal thread stripping, external thread stripping, or external thread fracture?

To calculate on a torque basis, you need to provide the inner and outer contact radii for the bolt and any surface treatments to determine friction coefficient.

 

[Screwman1]

Take a statistically valid sample size to failure and you will have your failure torque figure. With the friction from the flare, this is not a joint that would lend itself well to a calculated value.

 

[ghpatil1234]

sealing cone found damaged & few thread slippage in nut.

 

[tbuelna]

Based on your OP, we know one thing for certain. Since your parts experienced failure at 150% of the max installation torque, your analysis should probably assume a FoS of less than 1.5 the max installation torque.

 

[ghpatil1234]

Yes, I guess so...
But I need to prove this on paper (means what is pratically happening should be match with the theoretical calculation).

Can anybody help me with some formulas...Actually this application is automotive braking, where we are connecting bundy tube with wheel cylinder.

 

[CoryPad]

Automotive brake line joints like these are validated by physical testing as mentioned by Screwman1 above. The axial collapse of the flare does not correspond to the usual mechanics of static fastened joints.

Some equations to develop a basic understanding of torque and forces are shown here: faq725-536 and here: thread725-35222

 

[tbuelna]

ghpatil1234-

There are numerous technical references that will give you an analytical approach for calculating axial force vs applied torque from a screw thread. But as Screwman1 pointed out, due to the wide variations in the calculated results from factors like friction, to obtain an accurate result you would need to test a statistically relevant number of samples. Unfortunately, a statistically relevant number of test samples in your case may likely number in the hundreds.

 

[Tmoose]

FWIW Table 7.2 here may apply -

https://www.faa.gov/regulations_pol...craft/amt_handbook/media/FAA-8083-30_Ch07.pdf

 

[israelkk]

tbuelnaMANU

It is important to add the variation bolt tensile strength allowed by the bolt spec. Testing one batch will not indicate a safe design unless the break tensile strength of the tested bolt is known and documented in the manufacturer C.O.T. Then the result should be normalized to the minimum allowed bolt tensile strength. The tested bolt may be in the maximum bolt strength allowed by the spec and will pass the torque test but, a minimum properties bolt may not. The variations in the bolt tensile strength can vary up to 20% according to military specs.