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Post by wvengineer on Dec 18, 2012 20:51:21 GMT
Today, I had a couple of the mechanics where I work changing out a motor coupling for me. The coupling that was supplied with the machine was undersized and kept slipping on us. While they were torquing the coupling bolts, one of the mechanics had a question:
Will a torque wrench be accurate if you are using it with an extension?
I'm not completely sure on the answer. You will get some twisting on the shaft, but will that take take enough torque to throw your reading off?
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Post by freegan on Dec 18, 2012 21:35:09 GMT
Although, with most torque wrenches, the torque limit is set on the handle the limiter mechanism is at the hub, therefore an extension bar would have no effect other than increase the chance of you going beyond the 'click'.
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Post by srmarti on Dec 18, 2012 22:15:02 GMT
If I recall correctly, the extension will likely have no noticeable effect. Think the topic has come up before on the former MB forum.
I am assuming OP is discussing a bar between the socket and the torque wrench to reach a nut or bolt.
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Post by the light works on Dec 19, 2012 2:19:24 GMT
I agree that since the torque is measured at the hub, (either with a clicker or a needle scale) then unless you are bending the handle with your cheater bar, or accidentally twisting the adjuster on a clicker, the final length of your handle doesn't make a difference.
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Post by the light works on Dec 19, 2012 2:21:42 GMT
ah, socket extension, not cheater bar - unless you are actually twisting the extension - which would take a LOT of torque, then it shouldn't absorb any torque.
a crowsfoot type fitting would skew the result, though. (to clarify in case we have different terminology, I refer to a fitting that is an open wrench jaw with a socket for the ratchet drive.)
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Post by wvengineer on Dec 19, 2012 12:55:39 GMT
Yes, I was referring to using a socket extension, not a cheater bar.
The situation yesterday, we were using a 3" long Allen wrench socket, 6mm in size. That is a lot smaller diameter than your typical 1/2" extension.
We were only torquing that motor coupling to 30 ft. lb. So I wasn't worried about it being off. How much would you have to be torquing something to in order for it to have a noticeable effect?
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Post by c64 on Dec 19, 2012 13:55:24 GMT
This makes no difference as long as the extension isn't braked by anything.
"The sum of all forces must equal Zero", so the torque seen by the wrench is the exactly the other way round at the bolt. When the wrench "sees" +100Nm, the bolt must have -100Nm or something is seriously wrong with the universe.
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Post by the light works on Dec 19, 2012 14:47:56 GMT
when I am fighting seized lugs with small diameter allen keys, I can get an alarming amount of twist in a 6 inch allen key. however, that "sum of all forces" does make a lot of sense once it is spelled out. since a torque wrench is a measure of force, rather than motion, it doesn't matter whether you are putting two full twists in the extension - the torque at one end will still be the same as the torque at the other.
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Post by c64 on Dec 19, 2012 15:18:24 GMT
when I am fighting seized lugs with small diameter allen keys, I can get an alarming amount of twist in a 6 inch allen key. however, that "sum of all forces" does make a lot of sense once it is spelled out. since a torque wrench is a measure of force, rather than motion, it doesn't matter whether you are putting two full twists in the extension - the torque at one end will still be the same as the torque at the other. Correct, but only true for static systems. As long as the bolt is rotating or the extension keeps deforming, forces are converted to motion. Once the system had settled, the sum of torques is zero. By the way, classic torque wrenches had a thick lever and a thin pointer attached to their hub. A scale on the lever with the handle measures the deformation of the lever which is proportional to the torque you apply to the hub.  Those classic tools are now advertised as "Torque wrenches with direct reading". My dad has one of those he uses for his car, it came with his furniture delivery truck he had bought in 1968. |
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Post by srmarti on Dec 20, 2012 16:22:37 GMT
The socket extension can just be considered a torsion spring. Under static conditions it doesn't change the torque. An analogy that might help would be driving a car or truck onto a scale to weigh it. The springs in the suspension don't change the weight indicated on the scale.
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Post by wvengineer on Dec 21, 2012 0:38:01 GMT
If you are torquing a bolt, can you consider it a static condition? The bolt is being rotated (moving, therefore dynamic) until your torque wrench clicks. It is only after you reach your set point that you stop and you can consider it static.
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Post by c64 on Dec 21, 2012 0:49:25 GMT
If you are torquing a bolt, can you consider it a static condition? The bolt is being rotated (moving, therefore dynamic) until your torque wrench clicks. It is only after you reach your set point that you stop and you can consider it static. Actually this is a problem, indeed! If you use an impact wrench which has a specific torque and you use it "all the way", the bolts will break since there are two effects which will keep the bolts turning: 1) The momentum of the wrench and bolt 2) Gliding friction is a lot lower than static friction. That's why a mechanics is taught to stop the wrench numerous times while tightening bolts. You need to stop the wrench and then start it again until it won't start turning the bolt any more. Then you have dialled in the minimum torque but not much more which is OK for most applications. If you need a specific torque, you can't use an impact wrench at all and need to use a manual torque wrench turning the bolt a fraction of a revolution at a time. |
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Post by the light works on Dec 21, 2012 1:36:19 GMT
If you are torquing a bolt, can you consider it a static condition? The bolt is being rotated (moving, therefore dynamic) until your torque wrench clicks. It is only after you reach your set point that you stop and you can consider it static. Actually this is a problem, indeed! If you use an impact wrench which has a specific torque and you use it "all the way", the bolts will break since there are two effects which will keep the bolts turning: 1) The momentum of the wrench and bolt 2) Gliding friction is a lot lower than static friction. That's why a mechanics is taught to stop the wrench numerous times while tightening bolts. You need to stop the wrench and then start it again until it won't start turning the bolt any more. Then you have dialled in the minimum torque but not much more which is OK for most applications. If you need a specific torque, you can't use an impact wrench at all and need to use a manual torque wrench turning the bolt a fraction of a revolution at a time. not true - you can use an impact wrench set to leave it short of the desired torque, and finish it with the torque wrench. |
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Post by srmarti on Dec 21, 2012 21:21:56 GMT
If you are torquing a bolt, can you consider it a static condition? The bolt is being rotated (moving, therefore dynamic) until your torque wrench clicks. It is only after you reach your set point that you stop and you can consider it static. Sure. I stop when the torque reading is holding steady and the bolt isn't turning. Then again, I use a beam or dial type torque wrench. I don't have one that goes click. |
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Post by c64 on Dec 21, 2012 22:53:32 GMT
not true - you can use an impact wrench set to leave it short of the desired torque, and finish it with the torque wrench. This needs to be very far off, so far that an impact wrench is a waste of money and you can use one of the inexpensive "non-impact" ones.  And since impact wrenches are for coarse work only, anything which requires a precision torque wrench shouldn't come even close to an impact wrench! So either use an impact wrench wisely (pumping the trigger and letting it stop when you feel the torque is about to become right) or don't use it at all! |
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Post by c64 on Dec 21, 2012 22:57:56 GMT
Sure. I stop when the torque reading is holding steady and the bolt isn't turning. Then again, I use a beam or dial type torque wrench. I don't have one that goes click. You'll love them and they are not that expensive any more. They are not quite as precise as the dial kind but faster to use and you get very close to your desired torque every time and real quickly since you don't need to look. Just push until it clicks and stop, your reaction time is compensated by the ⅛ turn the wrench lets go after the click. The downside is that you always need to relief the spring inside whenever you don't use them or they become inaccurate fast! |
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Post by srmarti on Dec 22, 2012 1:13:48 GMT
I don't need to use them often enough to justify replacing what I already have though. If I used them to earn a living, it might be worthwhile.
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Post by silverdragon on Dec 23, 2012 7:27:20 GMT
I had to read this through, the whole way, and it was only half way through I realised an "Extension" was between the wrench and the socket, not a bit of pipe on the handle...........
Dont know why I was thinking that way, but lets move on....
Ok, so the extension is the one you use to get at a nut buried deep in the machine where you cant swing a mouse, let alone a spanner....
The extension bar would transfer all the torque to the wrench, it cant be any other way, because if it is, where is the torque going anyway?... you cant have say 40 torques at one end of a shaft and only 30 at the other end unless both ends are moving at different speeds.... (Say that moment of delay between hard acceleration and slowing down on a prop shaft....)
That is my understanding, if I am wrong, please explain, and keep it simples please?....
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Post by the light works on Dec 23, 2012 14:52:22 GMT
you pretty much got it - with the point that was brought up that if you were to put a meter on the spanner end, and a meter on the socket end, the meters would show slightly different rates of torque WHILE the nut was turning - because some of the resistance is being released. (like the prop shaft delay)
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Post by c64 on Dec 23, 2012 15:33:46 GMT
I had to read this through, the whole way, and it was only half way through I realised an "Extension" was between the wrench and the socket, not a bit of pipe on the handle........... Dont know why I was thinking that way, but lets move on.... Ok, so the extension is the one you use to get at a nut buried deep in the machine where you cant swing a mouse, let alone a spanner.... The extension bar would transfer all the torque to the wrench, it cant be any other way, because if it is, where is the torque going anyway?... you cant have say 40 torques at one end of a shaft and only 30 at the other end unless both ends are moving at different speeds.... (Say that moment of delay between hard acceleration and slowing down on a prop shaft....) That is my understanding, if I am wrong, please explain, and keep it simples please?.... In a static system, all forces sum up to zero. No matter how fancy your system is, this rule is always true. So as long as you don't add additional levers to the system, no forces are translated in the system. As long as the extension is straight (and not banana shaped or something), the torque of the bolt is exactly the same as the torque of the hub of the wrench. The only thing which makes a difference is dynamics. As long as the extension is changing by deformation, torque is then translated into movement. But once everything has settled, the bolt stops turning and the deformation of the extension and bolt is done with its reshaping, then you have a static system again. |
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