Suspension Brakes Most liked posts in thread: Torque Setting...

If anyone is interested, when I get back to work on Monday and can look over my reference books I'll publish the K factors for a lot of different lubes, greases and Loctite. (yea that changes the K too).

 

I don't know for sure Chuck, but 10% sounds like a good guess to me. Anti-seize is a very good idea.

 

Torque-tension equation for fasteners

T=KDF
Where:
T=torque (inch-lbs)
D=diameter of screw (inches)
F=force (lbs) "tension in screw"
K=Friction factor (not the coefficient of friction)

The friction factor for dry screws is ~ .15 to .17. The factor for oiled screws is about .12 to .13. This results in a torque reduction of between 13% to 29% for oiled screws relative to a dry torque spec. If you only reduce the torque by 10% you risk stripping the threads or yielding the screw.

Never Seize can reduce the K factor even further.

 

Well, now you've gone and done it. You don't know what the torque should be and I'll bet you're torqued about it. :lol:

Jim

 

If you are using graphite based thread lube then you should reduce the 122 ft-lb torque to no more than about 90 ft-lbs.

 

I don't think the reason this particular fastener is blamed for stripped threads on the trailing arm, is due to corrosion. It is a tapered self tapping bolt, and if it is not carefully threaded in, it can cut new grooves over the originals. I would suspect corroded bolt threads could gall on their way out and back in again, but once again I think attention to feel could minimize the affect.

I've yet to find definitive guidance on lubrication with self tapping bolts and their torque values, but self tapping bolts are in a class of their own, just like applying lubrication alters torque specs. I primarily rely on Carroll Smith's book for answers, but he hates self tapping bolts & screws, and says little about them. What Mr. Smith says in general, it is better to over-torque than under-torque (keep in mind he builds race cars). So, I clean the threads, light lube, thread carefully, and torque to 122 ft-lb.

 

I use lube, so the self tapping threads will "find the path of least Resistance" on their way into the aluminum trailing arm, I torque to original spec, so the bolt will fasten securely for sure. The lower strut bolt is subjected to great loads, I want the insert to handle the shear forces, to do that it can't be allowed to move. Carroll Smith's Nuts, Bolts, Fasteners, and Plumbing, handbook has numerous reports of failures, avoiding failure is my core reasoning.

 

No.

 

Interesting thread.
I like the difference of opinion between the 2 schools: engineers and mechanics. The math of the k-values appeals to my sense of logic, but I'm also afraid to lower the torque by as much as it implies (20%). On the other hand, it's hard to argue against real world experience: I expect Keith has had those bolts in and out many times and would know if 122 ftlbs caused stripping.

btw I managed to strip one of mine last year (there's a thread about it here somewhere). It was the third time reinstalling the shock. I was using anti-seize and and the torque wrench was set for 122. I can't say for sure I didn't cross thread it though - I forgot it was aluminum and I wasn't being super-careful. But anyway ymmv as they say.

 

I consider Carroll Smith's Nuts and Bolts along with all his books a must read.....even more so for us DIY's.... His logic has not failed me yet. 2cents

 

For those interested...

[ame="http://www.amazon.com/Fasteners-Plumbing-Handbook-Motorbooks-Workshop/dp/0879384069"]Amazon.com: Carroll Smith's Nuts, Bolts, Fasteners and Plumbing Handbook (Motorbooks Workshop) (9780879384067): Carroll Smith: Books@@AMEPARAM@@http://ecx.images-amazon.com/images/I/51wiaKeZAXL.@@AMEPARAM@@51wiaKeZAXL[/ame]

 

The dry torque is specified as 122 foot pounds ... If you apply graphite to the threads and then apply the dry torque value you are highly likely to: 1) snap the screw.. or 2) strip the threads in the aluminum. (Which is most likely how the threads striped out of the aluminum with the 122 ft-lbs torque while using anti seize compound in the post above.) If you want to prevent the corrosion, then there is another method. Apply Loctite 222 and reduce the torque by ~15%. It is a low strength thread locker. The Loctite effects the K factor so you'll need to adjust the torque appropriately. It will seal the threads to prevent corrosion but is low enough in strength that it will not be difficult to remove the screw.

I supplied the info and equations above to help show that this is not "opinion", it's well established science.

In deference to Mr. Smith, over torquing can be disastrous, often leading to sudden failure. While I agree that self tapping screws are problematic, there are ways to properly use them. A good reference is The Design and Behavior of the Bolted Joint, by John Bickford.

No offense intense to anyone here.

 

Here is a "cheat sheet" for K factors .....

Cheers... :smile5: