clamping force, bolt torque dry vs. lubricated

[kyle4501]

I had posted this on another thread, but thought it merited it's own thread, so here it is again.

I just found this site:
http://www.derose.net/steve/resources/engtables/bolts.html#largebolts

Of course, the charts are just values calculated using industry accepted formula T = KDP where T = Torque (inch-lb.), K = torque coefficient (dimensionless), D = nominal diameter (inches), P = bolt clamp load, lb.
K values vary depending on the surface: waxed (e.g. pressure wax as supplied on high strength nuts) = .10, hot dip galvanized = .25, and plain non-plated bolts (as received) = .20.
Torque has been converted into ft/lbs by dividing the result of the formula by 12

( I didn't verify all the numbers, but some of the ones I checked seemed off (the 7/8 bolt chart seems to be copied from the ¾ chart, so use at your own risk.  ) 

The clamp load = 0.75 ( proof load / stress area )
(the proof load is about 80% of the tensile strength)

The clamp load of a 3/4"-16 gr8 at 419 ft-lbs is 33,566 lbs!  -  Helps explain how easy it is to warp the hub or distort the tapered seat in the rim with that much pressure. . .   

Over torquing a bolt loads it closer to its breaking point  -  Helps explain why some studs pop off. . . 


I hope this helps.

[RichardEntrekin]

Nice post Kyle.

In another life, on a planet far away, I worked as a Maintenance Engineer in a Nuke Power Plant. I worked on the big valves. We were real interested in them not leaking when we put them back together.

In a nutshell, torquing a dry bolt has two issues. One, on an overall basis you produce less clamping force by torquing a dry bolt. Two, and much more important, is that the clamping force variation from bolt to bolt is far higher with dry bolts than it is with lubed bolts. That is intuitive when you think about it. The variation bolt to bolt produces leaks and warpage in the parts you are joining.

If you are concerned about the bolt or nut backing out, then use loctite. It provides lube while wet and locks it down when it sets.

[kyle4501]

Richard,
You are too right, but the main point I am trying to make is the reduced torque needed when threads are lubricated, and the associated risk of over tightening the bolts/ nuts.
(AND that different lubes have different torque reduction factors.)

Manufacturers recommend dry torque because the variations in clamping load aren't as critical as excessive bolt tension when it comes to wheels on a bus.

When I've been involved in the design of critical bolted joints, we did not calculate the torque - we calculated the bolt stretch required to attain the desired clamping pressure. We measured bolt stretch as applied torque was increased until the desired length was reached. This was the torque value specified. (We did this on a bunch of bolts & evaluated different lubricants to ensure accuracy and KNOW the variation.)

When exact clamping loads are needed, we use hollow bolts so we can measure each bolt as it is tightened, then there is no guessing at how much applied torque was to overcome friction.

Proper procedure saves more money in the long run than popular legends.    

[Hard Headed Ken]

This is just some additional information about measuring clamp loads. A company call Tech-Scan manufactures a thin film that measures clamp load. The film has an output connection that is interfaced with their software. It can be calibrated to give an accurate reading. They can produce the film in the shape of a head gasket or whatever you were doing the research on. I used it to measure the varying pressures that were generated between the rubber door seal and the body in automotive applications.

Ken

[kyle4501]

There is some cool stuff out there - if you know where to look. 

My problem is I often don't even know I should be looking.