Torque Question

[StreetRider]

If an extension piece is added to a Torque wrench:
a) does the torque value change with the added distance (length of extension) between the head of the wrench and the socket?
b) does the torque value change if the handle is extended?

 

[Wawazat]

Think. What is torque and where is it measured?

That answer will tell you that no, the torque value does not change.

 

[Wawazat]

Now that I think about it myself, there will be less torque applied using an extension due to the uncalculated torsional deflection of that extension. I do not think that it will be substantial, however I do think that it would be different. Am I wrong?

 

[emry]

No to the extension (unless you deflect the heck out of it or it twists), yes to the handle and also if you use an offset socket.

 

[ckw373]

StreetRider;230330 wrote: If an extension piece is added to a Torque wrench:
a) does the torque value change with the added distance (length of extension) between the head of the wrench and the socket?
b) does the torque value change if the handle is extended?

So, T = F * D where

T = torque
F = force
D = distance

So, if the distance is increased and the force is held constant, it is clear that the torque will go up. Now, the answer to the first question is a little more complicated. The above equation assumes that (in this case) the axis of the torque wrench head is lined up with the axis of the bolt. When you put an extension on the torque wrench and apply force, the extension will not be exactly parallel with the axis of the bolt head when the force is applied and thus, the torque applied to the wrench will not transfer 100% to the bolt. The longer the extension used, the greater the error will be. There is also some bending on the extension (as others have mentioned) that will come into play if the extension is long enough.

This can be easily visualized by thinking about trying to use a 4 foot extension on your torque wrench. The axis of the head of the wrench will not even be close to parallel with the bolt axis once you apply a force to the wrench and the extension itself will bend severely.

If you want to get super nerdo with it, the reason behind this is that the equation is actually:

T = F X D where

the X stand for "cross product" and F is a vector, with a magnitude and direction. You can read all about it here:

http://en.wikipedia.org/wiki/Torque

Hope that helps.

 

[some guy #2]

Unless you are torquing internal engine bolts don't worry about it being exact. If you are worried about it safety wire it.

 

[2trill357]

Torque values are there for a reason.....use them!
Some use them, some don't.......to each his/her own, but they are not some random numbers just thrown in for the hell of it.

It doesn't take much to calculate the correct torque setting when using an extension. Just look online for the manual/instructions for your particular torque wrench (or just use the link Chris gave you) and it should tell you how to compensate/calculate the correct value.

 

[Germany]

Lube the bolt and add 50% of the torque value to it. Then buy a set of easy outs and put a new bolt in at the right specs....


Germany CCS#19

 

[Meat]

Just one quick note on the importance of accurate torque readings: torque (called fastener preload) values are generally set around 70-75% of the yield strength (whether it is the threads bending and shearing, or the tensile yield of the fastener itself). So your fastener needs to be torqued to 200 inch-pounds but you avcidentally torque it to 250 inch-pounds then you have yielded the joint which makes it looser which can cause a laundry list of problems. Oh, and the little 50 inch-pounds that you were off is only ~4 foot-pounds.

This is why there are some joints on my bike that I apply less than the installation torque values and then safety wire. Brake pad pins come to mind.

 

[StreetRider]

Thanks Fellas. Once again I am reminded why I LOVE this organization.

 

[ckw373]

Wawazat;230335 wrote: Now that I think about it myself, there will be less torque applied using an extension due to the uncalculated torsional deflection of that extension. I do not think that it will be substantial, however I do think that it would be different. Am I wrong?

I thought about the torsional deflection a bit, and I don't think the pure torsional deflection, regardless of magnitude, will have any effect. Here's why:

Let's take, for a simpler example, two regular old springs: one soft and one stiff. You put each spring on your bathroom scale and apply 100 pounds of force. The stiff spring deflects very little, while still transferring 100 pounds of force to the scale. It has to transfer all 100 pounds due to equal and opposite reactions. However, the soft spring, while it deflects much more, still transfers all 100 pounds.

Now, let's apply the same thinking to pure torsional deflection. Lets say we have a steel rod and a wooden rod of the same length and diameter. We then apply 100 N*m to each. The steel rod rotates very little and transfers all 100 N*m to the other end, again due to equal and opposite reactions. The wooden rod rotates much more, but again still transfers all of the 100 N*m to the other end. The input to each rod equals the output. The only difference is that you need to twist one rod farther than the other due to the differences in torsional stiffness.

Moving along in our example, let's consider two steel extension rods made out of the same material and diameter, except one rod is shorter than the other. We again apply 100 N*m of torque and the short one twists less than the long one, because the torsional stiffness depends on the length of the rod. The shorter the rod, the stiffer it is.

So, from a pure torsional standpoint, it doesn't matter. All of the torque is transferred.

Please note that in the examples above, I am assuming that when twisting the rods, we are remaining in the elastic region of the material. As soon as the material starts to yield, some of the energy input will then be stored in the material itself, permanently deforming the material. The energy stored into the material doesn't make it into the bolt, and thus input no longer equals output.

Yes, I am a nerdo. Have a happy February 2nd!

 

[Meat]

If you are talking about an extension of the shaft that goes to the socket, then you now have bending of the extension, not just torsion. Your axis of rotation will change and your torque values will be off. Sometimes you must use an extension to reach the fastener but keeping the extension parallel to the shaft of the fastener will help improve the accuracy.

I thought he was talking about extending the handle of the torque wrench, not putting an extension between the socket and the wrench, but I could be wrong.

 

[ckw373]

Meat;230477 wrote: If you are talking about an extension of the shaft that goes to the socket, then you now have bending of the extension, not just torsion. Your axis of rotation will change and your torque values will be off. Sometimes you must use an extension to reach the fastener but keeping the extension parallel to the shaft of the fastener will help improve the accuracy.

I thought he was talking about extending the handle of the torque wrench, not putting an extension between the socket and the wrench, but I could be wrong.

I get what you are saying. I had to read the original question a few times to understand it, but I thought there were two scenarios: 1) lengthening the lever arm of the torque wrench (basically, using a torque wrench that was 3 feet long instead of 2 feet long) and 2) adding an extension between the wrench and the socket. My post above was just saying that, holding everything else constant, the pure torsionals didn't matter.

 

[lemondrop]

Ice Cube says yes