Torque capacity of bolted flange 2

[Tagger]

I am trying to confirm how to calculate the approximate torque capacity of a bolted flange. First you take the preload (P) that is generated by a bolt that is torqued to a certain amount. You then multiple P times the number of bolts in the bolt pattern to find the total preload. With the total preload you find the force (F) it takes to slide the two steel faces by F = ?*P with ? being the friction coefficient for steel on steel. With F calculated you take the radius of the bolt circle to find the Torque capacity.
Example:
12 3/4-10UNC Grade 8 bolts torqued give a preload of ~28,200 lbs. per bolt. 12 x 28,200 = 338,400 lbs.
Bolt circle is 9.88" diameter, ? for steel on steel is about 0.8, thus the torque capacity is about 1.3 million lb-in.
Is this correct?
Thanks

 

[CoryPad]

Your method is correct, but your friction coefficient is unrealistic. Regular surfaces don't reach much above 0.3 unless you use a surface treatment or the test is conducted in a vacuum (the vacuum data is promulgated by various sources without mentioning this important detail). Are the mating surfaces machined? Any chance for residual oil? If so, you should use a lower number like 0.1.



Regards,

Cory

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

 

[Xianglu]

Check this tip for info.

http://www.eng-tips.com/viewthread.cfm?qid=112403

Your concept is correct, but ? should be according to lubricaction condition between 0.15 to 0.25

 

[Tagger]

I based it on a clean interface between the flanges. I read in Machinery's Handbook 0.8 in Mark's 10th edition page 3-23 it states "hard steel on hard steel dry 0.78", "mild steel on mild steel dry 0.74". If they were greasy OK .11, .15...I should design for worst case that there is some grease on the surfaces (which are machined).

 

[CoryPad]

As I stated, your numbers are for vacuum conditions:

thread404-68353

Regards,

Cory

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