Breakaway torque and torque wrenches

[rudy88]

Question about torque wrenches and their proper use:

I have read that "break away torque" will cause a bolt to resist movement after it has stopped, meaning it may take more torque to free it than was used to tighten it. I find this curious because when I see people torquing (usually lug nuts) they often stop while turning the bolt (to turn the ratchet, or just because they are turning slowly and stopped for no reason) and then proceed in increments. Couldn't this give a false high reading when they begin to turn the wrench again?

Let's say for example a bolt needs to be torqued to 85 ft pounds. The operator turns the wrench while waiting for the click and reaches 80 ft pounds. They stop to turn the ratchet and when they go to turn again it instantly clicks because the torque needed to get the bolt started again is higher than the torque achieved while in motion. But the operator hears the click and says the bolt is tight.

Now, I'm not an expert, but wouldn't this bolt be under-torqued? Are you supposed to be "in motion" when you hear the click (on a click type torque wrench)? I made this scenario up and have no idea if those numbers are remotely reasonable.

While we're on the subject, does anyone take thread condition into consideration? If so, how? I've seen lug nuts slathered with anti seize, corroded, stripped, you name it, all tightened using the same spec.

I'd like to hear what you all think

 

[srmofo]

You are correct on all aspects. It just goes to prove that lug but torque isn't that critical. Certainly not as much as some people make it out to be.

 

[zkling]

Yes, static vs kinetic friction. Thus why you are supposed to hit the torque value mid swing of the wrench with smooth motion. Lubrication effects friction factor of the threads. Some compounds will specify a friction factor for the specific compound; and this is why most torque specs are stated as dry/clean threads.

This topic has also been discussed to length many a times on here. I'd suggest you do a search for more in depth reading.

 

[devoncoolman]

All your questions and theory's are correct. Thats why when torquing head bolts typically they want you to lube the threads and under the head of the bolt to reduce this. When torquing heads i try to torque them in one pull if possible. Or when im getting close i make sure i have room to finish in one pull.

Wheel torque is not as critical as engine fasteners.

 

[rudy88]

Alright, thanks. I did do a search on here, and on google as well. I'll try again with some of the terms you all are using. I was able to find general info regarding how lube affects torque, but the one thing I could not find a source for is if the wrench should be in motion when it clicks.

Thanks for the quick responses.

 

[Flat-rate]

Low torque and then angle is close.

Then measuring stretch such as for rod bolts is best.

Or good 'N tite works too.

it's all about how important it is.

4 tight lug nuts is better than 5 loose ones

 

[finn]

For really critical fasteners, use the torque-angle method.

For OEM prototype engines, we would grind the bolt ends parallel to each other and use an ultrasonic device to monitor bolt stretch while tightening.

Probably not something you have in your home garage.

 

[rudy88]

For OEM prototype engines, we would grind the bolt ends parallel to each other and use an ultrasonic device to monitor bolt stretch while tightening.

Now THAT sounds accurate. That's something I'd like to see in action. Why did you have to be so precise for those particular engines?