torque spec vs thread pitch

[MillWeld]

Most parts assembled with bolts have torque specs to assure proper tightening. These specs must assume that the bolts have a certain pitch. I have wheel lugs that are to be torqued to 90 ft-lbs and some of the lugs are 1/2-13 and some are 1/2-20. Torqueing these the same wiil give very different holding forces. What gives here?

 

[Baby Grand]

What kind of vehicle - tractor, truck, hay wagon?
Are the studs all the same on individual hubs, or are they mixed?
Did you get this vehicle new or used.
Ever had the lugs on a hub replaced?


I think you are correct to suspect something's aromatic in Denmark. The holding forces/clamp load/tensile preload would be roughly
(13/20) on the coarse pitch threads compared to the fine pitch threads, assuming similar friction and elastic modulus. Sounds like some would be either over or under torqued.

 

[Toyboy]

The torque spec has more to do with bolt strength and application than coarse or fine threads.

 

[MillWeld]

What kind of vehicle - tractor, truck, hay wagon? trailer
Are the studs all the same on individual hubs, or are they mixed? Mixed
Did you get this vehicle new or used. Used
Ever had the lugs on a hub replaced? No

I put together a tandem tailer axle mixing and matching mobile home hub/drums selecting what I thought was the best. Ended up with 1/2-13 on one side of an axle and 1/2-20 on the other end. I read that lugs should be tightened to 90 ft-lbs but there was no mention of the thread pitch. If 90 ft-lbs is for a 1/2-13 then the torque for 1/2-20 should be about 50 - 60 ftlbs, I think but such info is not readily offered. Am I off base here?

 

[Scotty370]

Millweld- I've had the same thoughts over the years! Wouldn't the same Torque Wrench setting on a new, clean, oiled, fine-threaded bolt, create more tensile stress, than a dirty or used bolt, of a courser thread? Just trying to learn..........~S

 

[whistlepig]

This chart has 1/2-20 specs on it:http://www.imperialinc.com/pdf/A_FastenerTorqueCharts.pdf

 

[ModMech]

All are correct as far as they commented.

The difference between stretch of a bolt NC to NF is minimal at the torque values used. When you are dealing with greater Tq values, such as head bolts, valves etc, correct stretch is critical and we move to "TnT" - Tq and Turn for greater accuracy and repeatability.

The Tq of 90 odd lb-ft on a 1/2" fastener is nowhere near it's peak, or "yelid" strength, so corse or fine is minor. It is significant if the threads are oiled, dirty or galled however, which can result in grossly under torqued lugs on wheels - very dangerous.

 

[RickB]

The torque spec has more to do with bolt strength and application than coarse or fine threads.

A properly engineered torque spec considers bolt strength and thread pitch, not one or the other.

 

[whistlepig]

And thread strength.

 

[MillWeld]

Millweld- I've had the same thoughts over the years! Wouldn't the same Torque Wrench setting on a new, clean, oiled, fine-threaded bolt, create more tensile stress, than a dirty or used bolt, of a courser thread? Just trying to learn..........~S

I think you're right. A bolt/screw is basically an inclined plane - it's just a circular one. I haven't done it yet but I bet if you apply the physics of an inclined palne to a bolt you'll find that a NC and NF threaded bolt torqued to the same value the NF will produce more tensile force. Someone more experienced than me might draw a more accurate conclusion.