Kips to Foot Pounds

[MiddleTommy]

I have seen many posts on how to convert kips to foot pounds. None of the answers are correct.
I finally found the answer and wanted to document it here.
T = 0.0167 x P x D

T = torque (foot-pounds)
P = bolt tension in pounds( 1 kip = 1000 pounds )
D = bolt diameter in inches

So to get the torque if required to pretension a 5/8" bolt to 2.35 kips

0.0167 x 2350 x 0.625 = 24.53 foot-pounds torque

 

[rb1957]

what an odd way to express "preload from torque" ...

0.0167 seems very low to me ... 0.2 is more typical.

The other thing to consider is the variability in torque created by preload control, typically as much as plus or minus 1/3 (ie the preload created is between 67% and 133% of the nominal (nominal preload = T/D*5). If you need the nominal preload, what'll happen if your torque only gives you a preload of 2/3rds of nominal ? ie design your torque to give 150% of the preload you think you need, then worry in case you got 133% of that (200% of nominal)

another day in paradise, or is paradise one day closer ?

 

[Lomarandil]

Er, as I understand it, the relationship between bolt torque and tension varies based on several factors -- the condition of the bolt and nut threads, thread pitch, etc.

I don't think it's this straight forward (although it may be a handy rule of thumb for standard structural bolts and applications).

I think the 0.0167 is using a coefficient of 0.2 and converting from feet to inches.

 

[drawoh]

...

I think the 0.0167 is using a coefficient of 0.2 and converting from feet to inches.

That's what I am getting too.

T = KPD, where K is the friction factor, 0.2 in this case.

--
JHG

 

[MiddleTommy]

It is a formula I found in the Structural Bolting Handbook by

http://www.steelstructures.com

 

[rb1957]

yeah, you're right about the factor. But depending on the criticality of the fastener I think you should consider scatter in preload from torque control; the handbook won't tell you much about that.

another day in paradise, or is paradise one day closer ?

 

[drawoh]

MiddleTommy,

My machine design textbook (V.M.Faires) proposes a K factor of 0.2 for as-received bolts, and 0.15 for lubricated ones. Faires claims that values of K have "been observed in the literature from 0.1 to 0.34 or more". If you divide these values by twelve, how many decimals accuracy should you use?

--
JHG

 

[MiddleTommy]

Structural talking to Aerospace and Mechanical. We all have different rules.

 

[MiddleTommy]

The notes in my manual say 0.0167 is an industry (Structural Steel) average. Well oiled bolts could be as low as 0.01 and rusty bolts could be as high as 0.025
Proper torque/tension relationship in your scenario should be determined with the use of a Skidmore-Wilhelm device where you tighten to the proper tension and record how much torque it took.

 

[dhengr]

MiddleTommy:
Whichever language we each speak, one thing is for sure; and that is that in most cases torque is not a very reliable way to determine preload on a bunch of random nuts and bolts. Torque is a bit more reliable once you’ve calibrated your torque wrench on a number of nuts and bolts, same size and thread, all from the same batch and in the same condition from being manufactured, handled, stored, cleaned, lubed, etc. Actually, for structural applications, the turn of the nut method is quite reliable once you know the particulars of the nuts, bolts and threads. One full turn of the nut elongates the bolt a specified amount. And, that’s something you can calc.

 

[ajh1]

For structural bolting review the RCSC Specification, available as a free download from

http://www.boltcouncil.org.

Torque calculations without the benefit of a pre-installation test determined with a Skidmore or other similar device are basically worthless for establishing the appropriate torque to achieve a specified tension. This is doubly true if the installation is going to take place in an exterior exposure application as compared to a factory setting.

 

[SAIL3]

as indicated, the turn of the nut method, eliminates all this speculation/guessing....

 

[rb1957]

is 0.0167 any more accurate than 0.2/12 ?

another day in paradise, or is paradise one day closer ?