Proper Torquing

[viejo]

When torquing down fasteners I generally try to attach the socket/driver directly to the the torque wrench, but occasionally the job requires an extension between the wrench and the socket/driver. My usual methodology is to add 3-5% to the final torque value to compensate for the flex in the extension; I've been doing this for years with no horror stories resulting, but I'm wondering if I'm really operating in a well reasoned matter... or just been lucky. A few days ago I had occasion to be installing the shouldered bolts that fasten the cam cover on a K engine and, as the torque value was rather low (80 in lbs), and accurate torque being critical for both the safety of the shouldered bolts as well as the proper compression of the cam cover gasket, I finished the job wondering if I had applied proper/even torque all the way around as (only) one of the bolts required an extension between the wrench and the driver. I do not have any in-house way to calibrate my torque wrenches against a known value using various length/diameter extensions to evaluate any gain/loss of actual torque applied versus the wrench's "click point".

All guidance/direction appreciated.

Dave

 

[PGlaves]

I would be shocked if the elasticity in a typical extension would be a significant factor at all. That and given that the error or deviation in clamping force for a given torque is often as much as 10% and up to 25% cause me not to overthink what the torsional change in my extension might be.

I suspect the guys among us that used to construct or maintain nuclear containment vessels and some such might shed better light on this than I can, however.

As for those specific bolts, I believe they are shoulder bolts so once they shoulder metal to metal the torque applied is simply to keep the bolts from falling out but no further compression of the gasket(s) occurs.

 

[rangerreece]

Paul, the man with the answers. Thanks for the enlightenment. I've been wondering about this question myself but have been too shy to ask. Didn't want anyone on the forums thinking I'm one of those non-wrenching beemerphiles.

 

[jamesevans]

If you use a long extension and don't align it with the bolt it could result in a significant difference - the wrench would be effectively longer or shorter according to how it was out of line. Keep it all square and nothing to worry about.

 

[PGlaves]

If you use a long extension and don't align it with the bolt it could result in a significant difference - the wrench would be effectively longer or shorter according to how it was out of line. Keep it all square and nothing to worry about.

Certanly angular deflection of a long extension, particularly a wobble extension, could lengthen or shorten the lever arm.

 

[viejo]

Thanks for all the quick responses. I thought there was probably a good chance I was overthinking the issue, but I've always believed that the dumbest questions are the ones you don't ask. The observations about maintaining torsional linearity when using an extension are spot on. (What actually got me thinking about all this was remembering that sometimes using a longer screwdriver apparently helped me back out screws that were being recalcitrant.)

Dave

 

[20774]

The observations about maintaining torsional linearity when using an extension are spot on.

Curious...what do you mean by that?

 

[bc1100s]

(What actually got me thinking about all this was remembering that sometimes using a longer screwdriver apparently helped me back out screws that were being recalcitrant.)

Not being a nuclear containment vessel engineer I may be totally off base but...I can't see how having a longer screw driver helps remove a stubborn screw. Are you saying that the elasticity of the screw driver shaft results in an increase in torque at the tip? Once that (very slight) elasticity is taken up, would the same torque being applied at the handle not be directly applied at the tip? Inquiring minds need to know.

 

[AntonLargiader]

The length of a normal extension has no effect on static torque. Offset wrenches (like the typical driveshaft bolt wrench) usually do based on the amount and orientation of the offset.

U-joint extensions could get tricky if you're trying to be 100.0% accurate but for most purposes they don't have an effect. To calculate an effect from them you'd need to know the exact angle of the yokes and so forth. Wobble extensions are more like CV joints; I can't see there being a predictable and calculable correction that you'd be able to use (or that you'd need).

Using a normal extension and not holding it exactly perpendicular to the fitting or whatever is simulating a wobble extension, not an offset extension, so don't sweat that either.


Edit.

 

[PGlaves]

Not being a nuclear containment vessel engineer I may be totally off base but...I can't see how having a longer screw driver helps remove a stubborn screw. Are you saying that the elasticity of the screw driver shaft results in an increase in torque at the tip? Once that (very slight) elasticity is taken up, would the same torque being applied at the handle not be directly applied at the tip? Inquiring minds need to know.

Yes

 

[viejo]

Curious...what do you mean by that?

Making sure that the extension is always in column with the bolt and at a true 90 degree angle with the torque wrench. Curiously enough I just came across my June issue of the MOA magazine and found Wes Fleming's article on torque.

 

[viejo]

Yes

I've never really questioned the apparently better results of a longer screwdriver before now... just one of those things that popped into my mind while I was in the shop last week, but I'm not the first one to make that observation. It's likely nothing more than a matter of a longer screwdriver probably having a fatter handle allowing the user to get a better grip for applying torque.

 

[bc1100s]

It's likely nothing more than a matter of a longer screwdriver probably having a fatter handle allowing the user to get a better grip for applying torque.

That's what I was thinking. Good discussion, btw.

 

[AntonLargiader]

And a longer screwdriver is easier to keep aligned properly when you're pushing it into the screw. And often easier to apply that pressure to, because you have more room.

The principles regarding extensions would make a good article for ON, or for my website. It's been a while since I've written anything new.

 

[Wkoppa]

I've never really questioned the apparently better results of a longer screwdriver before now... just one of those things that popped into my mind while I was in the shop last week, but I'm not the first one to make that observation. It's likely nothing more than a matter of a longer screwdriver probably having a fatter handle allowing the user to get a better grip for applying torque.

Sears offers a screwdriver that morphs to a t-handle for better grip and downward pressure.

http://www.googleadservices.com/pag...0001&pla&kpid=00947135000&kispla=00947135000P

Hope this link works.

Wayne Koppa
#71,449 Life

 

[20715]

I bet the variance from elasticity of the extension pales in comparison to the range of accuracy of the wrench. But I'm not a nuclear containment vessel engineer either. I did sleep at a Holiday Inn a few years ago.

 

[20774]

I did sleep at a Holiday Inn a few years ago.

I think the effect wears off over time, but I think you're spot on with your observation...motel, or not!

 

[AntonLargiader]

I bet the variance from elasticity of the extension pales in comparison to the range of accuracy of the wrench. But I'm not a nuclear containment vessel engineer either. I did sleep at a Holiday Inn a few years ago.

The elasticity of a typical extension does not change the torque at all. No matter how soft the springs of a car are, its full weight is still on the ground.

 

[pffog]

The elasticity of a typical extension does not change the torque at all. No matter how soft the springs of a car are, its full weight is still on the ground.

What he said, simple physics Newtons 3rd law, Every action has an equal and opposite reaction.

 

[36654]

Not being a nuclear containment vessel engineer I may be totally off base but...I can't see how having a longer screw driver helps remove a stubborn screw. Are you saying that the elasticity of the screw driver shaft results in an increase in torque at the tip? Once that (very slight) elasticity is taken up, would the same torque being applied at the handle not be directly applied at the tip? Inquiring minds need to know.

With NASA-STD-5020 "REQUIREMENTS FOR THREADED FASTENING SYSTEMS IN SPACEFLIGHT HARDWARE" even your typical "S" rider can be a torquing expert......