Break Away Torque, HOw to calculate Options

[Brett W]

We all know that the torque needed to loosen a fastener is usually higher than the torque at which the fastner is tightened. Is there a way to calculate this number.

I just got a new 3/8 Impact wrench and was doing some test with it. We torqued a 1in course thread bolt to 250 ft-lbs. Then used this impact to break it loose. It did it with no problems at 175 psi. Couldn't touch it at 90psi.

ANy body know how to calculate that torque?

[alpha434]

Jeeze. I don't think that there is a way. There are other variables too. You have to take into account the amount of surface area making contact, the amount of pressure, whether or not the fastener has tightened or loosened itself since you put it on. And probly a lot more. The type of surfaces that are making contact and the surface finish of each.

[Brett W]

That's what we figured. We oiled the bolt and welded the head to a piece of steel tubing to keep it from moving. Then we put a 2in 1.25x.125 tube between the head of the bolt and the nut. When we finished testing a 3/8 gun, a 1/2in gun and a 3/4 gun, we had mushroomed the piece of tubing.

The manufacture says the little gun will pull 300ft-lbs in reverse (at 90psi, BS). But is does pull a bolt loose that has been torqued to 250lbs. It is the biggest torque wrench I had tonight. I am going to get ahold of a 3/4 torque wrench that should do at least 400ftlbs.

Anyway I am trying to figure out if how much higher the break away torque is than the measured torque.

[alpha434]

Well, don't do it with the air gun. Use a torque wrench. I would appreciate your posting numbers here. You've piqued my curiosity.

Also, you have to look at the air wrenches mechanism. There is an eccentric hole with particle board fins that flop out and make a seal. It's genarlly impossible to actually get those torque numbers because of all the possible manufacturing imperfections. That's a best case scenerio number.

[Brett W]

I am torquing the nut with the torque wrench and then breaking it loose with the impact.

Check out this impact
(IMG:http://buy1.snapon.com/catalog/OBJECTS/50600/50554.JPG)


http://buy1.snapon.com/catalog/item.asp?P6...ore&dir=catalog

[alpha434]

No no. Use the torque wrench on both operations. The air wrench isn't an accurate tool for measurement. With a torque wrench, you can adjust it to the pound to get your reading. Impact wrenches just beat the hell out of everything.

[McMark]

It sounds like Brett is just trying to verify the manufacturers claims ((IMG:http://www.914world.com/bbs2/html/emoticons/thumb3d.gif)) by torquing the bolt to some value (250 ft/lbs) with a torque wrench. The inital thinking was that tightening to value X would take a roughly equal torque to break it free. Since this obviously wasn't the case, I'm guessing that he want's to know how to accurately dictate the torque needed to break a bolt free.

My advice would be to torque to a value (100 ft/lbs) and then reverse the wrench and gradually increase the setting on your torque wrench until it breaks free. Start at 100 ft/lbs. Does it break free? How bout 105? 110? etc. That'll get you a pretty accurate idea of what you're dealing with.

[alpha434]

QUOTE (McMark @ Mar 26 2006, 11:15 PM)

It sounds like Brett is just trying to verify the manufacturers claims ((IMG:http://www.914world.com/bbs2/html/emoticons/thumb3d.gif)) by torquing the bolt to some value (250 ft/lbs) with a torque wrench. The inital thinking was that tightening to value X would take a roughly equal torque to break it free. Since this obviously wasn't the case, I'm guessing that he want's to know how to accurately dictate the torque needed to break a bolt free. My advice would be to torque to a value (100 ft/lbs) and then reverse the wrench and gradually increase the setting on your torque wrench until it breaks free. Start at 100 ft/lbs. Does it break free? How bout 105? 110? etc. That'll get you a pretty accurate idea of what you're dealing with.

EXCEPT that he plans on breaking the nut free with the impact wrench. Which would not give him accurate results.

I was trying to say the same thing in the earlier post about using the wrench.

[McMark]

You're right I guess I didn't finish my thought. Once he determines the tightening torque that corresponds to a 300 ft/lbs break-free torque then he has determined the setup he needs in order to conduct his test. Everything I typed before is just for setting up the test, it's not actually doing the test.

[Joe Ricard]

Hmm, SO who really gives a rat's ass how much torque it takes to get a bolt off. As long as it comes off. However you guys were tippy toeing around the real important part. Impact wrenches do not do a good job of obtaining exact torque.

Torque wrench for my lugs ONLY. engine assembly is the same. My preference is the deflecting beam. but If I had enough money the click Snap on would be ok.

SOmething nobody talks much about is "running Torque" Which is the torque it takes to trun a Nylock or other self locking nut before it reaches the contact surface.

[groot]

Brett,

Considering we test all joints for clamping force and break-away torque (and I do mean all joints), I don't think there's a very good way to calculate.

Too many variables.

[Brett W]

I got these two new impact wrenches that are supposed to be the best ones on the market. We were a little slow in the shop last night and were kickin' around the idea of testing or impacts to see how strong they really are.

SO we grab this 1in bolt (1 7/16 head) and run it down with a 3/4 impact at 90 PSI. Then we grab the 3/8 gun and try and break it loose. My new 3/8 gun could knock loose everything the 3/4 gun could put on. We were concerned that the 3/4 gun wasn't getting enough flow out of the little 20gal home compressor at John's house. So we grab a torque wrench and tighten the bolt to 250FT-LBS, then tried to use the little gun to break it loose. The little gun could not do it at 90 PSI.

So we head over to my shop which has an 80gal compressor capabable of 175 psi. We turn the pressure up and let the little gun go to it. Well it takes the nut right off. So we were trying to figure out how much torque the gun really put out.

Anyway, as far as testing breakaway torque, I figured you could build a test rig that would estimate the amount. Take a long pull handle and start carefully hanging wieghts on the end of it until the fastener would just start turning. That should give a pretty decent estimate of the breakway torque.

This is what happens when you have downtime and new tools. (IMG:http://www.914world.com/bbs2/html/emoticons/smash.gif)

[groot]

You definitely have too much time on your hands. How about building some of my rear suspension?

[Brett W]

It only took a couple of hours last night. I had spent the last 10 days at the shop, 12-14 hours a day and was just winding it down for the weekend.

Hell in the last week I built a complete manifold to tailpipe 2.5in exhaust for a Supercharged Scirocco, a solid motor mount system for the VW, and a custom front crossmember and half width radiator for a Civic.


Anyway my new tranny showed up today so I am now back to my race car.

Here is the bolt and the damage to the tubing:

[jd74914]

Hey Brett, is there anything special about the honda in your blog?

[Cap'n Krusty]

No way would I subject my air tools to 175 PSI. We run oures at 125 max, and lower than that for everyday work. The Cap'n

[Brett W]

I normally leave our reg set at 100psi. We were just testing my new toy.

What would make the Cervix special? It has full custom suspension, relocated battery, custom front crossmember, half width radiator, custom engine built for boost. Just a regular 90si. Gets 30mpg in town and 35-40 on the highway. Will get a turbo when I solve the intercooler issue.

[jd74914]

QUOTE (Brett W @ Mar 27 2006, 11:05 PM)

I normally leave our reg set at 100psi.  We were just testing my new toy. What would make the Cervix special?  It has full custom suspension, relocated battery, custom front crossmember, half width radiator, custom engine built for boost.  Just a regular 90si.  Gets 30mpg in town and 35-40 on the highway.  Will get a turbo when I solve the intercooler issue.

Cool. I was wondering since I remember you talking about intercoolers and didn't see one on this car (IMG:http://www.914world.com/bbs2/html/emoticons/cool.gif)

Hyjack off.

[LvSteveH]

Actually, I think a fairly predictable formula could be developed, or at very least a good rule of thumb. Your test rig has some issues though. You need to be torquing against a flat and unyielding surface, like a thick plate. The tubing is deforming and has the potential to act as a lock washer. If you used heavy wall tubing and a 1/2" plate as the surface, that would have very repeatable results.

[Brett W]

Sorry, the tubing is just a spacer because the bolt is shouldered and too long. It only deformed after we hammered on it with a 3/4 impact. I figured for my test fixture I would use a piece of square tubing to take up the gap evenly. It would be something very thick to keep it from deforming.

I built an intercooler for the Civic but it is too big and the outlets are kinda in the wrong place. So it will be going on the 914 race car as it is kind of a generic shaped intercooler.