Using a torque wrench with an extension?

[SilveradoCanada]

If you must use an extension to reach a fastener with your torque wrench, does it effect the actual torque being applied?

I mean just a regular extension, not the torque limiting type, and of course taking care to keep the socket/extension/TW aligned with the axis of the fastener.

[CKTA]

Yes, but not by much. I THINK for every inch extension you need to add about 2 ft./pds.

[XLR8N]

What type of bolt are we talkin about?

[SilveradoCanada]

No bolt in particular, rather just a general curiosity.

[adam728]

If you mean an extension such as a crow's foot, then you need to calculate for the extra distance to the fastener's centerline from the torque wrench's drive point.

A regular extension will not change the torque value as long as you can keep things aligned while torquing. The centerline of the square drive still aligns with the centerline of the extension/socket/fastener, so you are not changing the mechanical leverage.

This effects torque
http://penforhire.files.wordpress.com/2007/08/2197-ed-korns-torque-extension.jpg


These do not
http://jbtoolshop.co.uk/images/SS142A.jpg

[SilveradoCanada]

Thanks adam728,

I suppose I could clarify; will using a standard extension like those in your second pic cause any variance in the accuracy of the TW? I suspect not, so long as fastener, tool and wrench head are axially aligned...

[chengny]

Torque = force X lever arm, that's all there is to it - in the lab. In real life:

This can be filed under general torquing information - as opposed to any specific torque values/sequence.

When making up any joint it must be remembered that the primary objective is not the amount of torque (twisting force) on the fasteners. It is the degree of compression at the interface (or on the gasket) of the two pieces being brought together. Since it is nearly impossible to measure that compression as pounds per square inch (between a cylinder head and engine block for example) it has become standard engineering practice to use the amount of torque applied to the bolting. The idea being that the thread pitch, bolt diameter and the applied torque will yield a predictable compression force.

This works just fine - in the lab. In the real world torque can be affected by any number of variables.

For example, consider a bolt with ragged threads. It will have a difficult time being screwed into the bolt hole. Your torque wrench is "stupid". It has no idea why it is encountering resistance to turning. So, if the threads are bad enough the torque wrench will "click out" well below the clamping force that is actually required.

A more common reason for weak flange to flange loading is lack of lubrication. And here it must be remembered that the actual torque value is only reached at the very last stages of tightening. If you thread a clean, lubricated bolt in to a properly tapped hole there will be no increase in torque at all (machine threads only) until the underside of the bolt head makes contact with the flange surface (or washer if used). When the bolt "shoulders out" any further rotation will cause it to stretch and be under tension. This tension is what forces the two components together. If there is no lubrication between the underside of the bolt head and the washer it is turning against, the increased friction will directly increase turning resistance. Your torque wrench will interperet this this as the bolt having reached the desired tension. But it will be a false reading. You will get your click at 65 ft/lbs and thing that the joint is made up to spec, but it isn't. It is going to be loose.

So (for those of you who haven't left the room from boredom) the most important things you can do to get a properly torqued joint are;

Be sure that the threads are properly sized and completely free of foreign material. Run a tap into the bolt holes and use brand new bolts (or be sure they are immaculate).

Lubricate everything. The threads, the flange surface, the washer (under side and top side) and the bottom of the bolt head.

[SilveradoCanada]

Thanks chengny,

I've always believed in and practiced lubricating bolts, washers, etc. They not only thread and tighten easier, but a good coat of grease helps avoid finding them rusted tight the next time.