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How does a torque wrench work?...

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Torque (tightening force) is critically important when working with any fasteners that will be subject to some kind of load. The bolt (or nut) needs to be "preloaded" with the correct amount of force, so that it will neither break nor come loose and fall off. Automakers specify torque settings for just about all their fasteners, and a torque wrench is used to achieve the requirements.

There are a few different kinds of torque wrench out there, from the simple "beam" type that's available just about everywhere very cheaply, to very sophisticated and specialized ones that measure more than just the bolt's resistance to movement. It is not the purpose of this article to cover all this; any search engine will lead you to that information. The purpose of this article is to show you what's inside one of the most common type of torque wrenches used by the automotive repair industry: the click-type torque wrench. And in this specific case, the Sears Craftsman design.

So, you tighten a bolt with your trusty click-type torque wrench, and eventually you get that satisfying "click" noise when the proper torque has been reached. The "click" is a handy indicator for sure, but what actually makes that click? What's really going on in there?

Well, I found out. How? By accidentally wrecking my original $99 Sears Craftsman click-type torque wrench. You see, every time you put away your click-type torque wrench, you have to back off the setting all the way down, to prevent the internal spring from taking a "set" in the compressed position and throwing the calibration off. One day I was doing just this, when, at the bottom of the handle's travel, it suddenly came loose. It didn't actually come off, but it spun freely and exhibited a half-inch of axial movement that was not there before. Needless to say, the calibration was shot all to heck.

I did some investigation. It turned out a calibration service would have charged me $75 to fix up my wrench, plus it was my responsibility to pay to ship them the wrench, and they were 30 miles away. A brand-new wrench was $99 at the local Sears. A no-brainer of course, so I ended up with two torque wrenches, one usable, one not. Well, you know me. I love to find out how things work. I decided I had nothing to lose by pulling the old wrench apart and cutting holes in it for visibility. So I did just that. The results of the carnage follows...

This is it: the bad wrench. Its scale goes from 20 to 150 ft lbs. It's a half-inch drive Sears Craftsman Microtork 44595, with your standard reversible ratchet.

The beauty of the click-type torque wrench is that it's very handy in tight spots and you don't need to be able to see a dial or pointer of any kind. With the beam or dial types, you need to be able to position yourself and the wrench so you can see the readout, not always easy for the home grease monkey with no hoist.

I took it apart by removing the snap ring and cross-pin. This enabled the ratchet head to be pulled out of the body tube, with all the other stuff following.

I don't know if any of these are the correct terms, by the way. I've named them by function, by what seemed logical. For the sake of consistency, these terms are used throughout this document. If anybody can supply me with the industry-correct terms, I'd be glad to replace mine with those.

Click on the picture to open the hi-res image, then study the parts and their names. The critical parts for calibration are the calibration shaft, both locknuts and the handle.

Here are the handle, calibration shaft and locknuts, all assembled.

The handle is clamped to the calibration shaft by the locknuts. When you turn the handle so as to run it up the scale to the torque reading you want, you are screwing the calibration shaft into the body, compressing the main spring.

The spring's desire to expand itself against this compression is what provides the necessary force for the actual mechanism that makes the "click" sound.

The bottom locknut is the one that came loose on me, resulting in the axial looseness and loss of calibration. I did not discover this until I popped the end plug off.

When a torque wrench is calibrated, the handle is moved up and down the calibration shaft, so that the reading at the handle markings will match what the main spring is trying to do.

You would need this Allen key, plus a very thin-wall 11/16" socket, to do the calibration.

We'd better put our wrench back together so we can see how it actually works.

I've already put the washer and the washer/spring back inside, and the main spring is part way in. The anti-windup assembly, tilt block and ratchet head are next.

The main spring is now in all the way, the anti-windup assembly part-way in, and the tilt block and ratchet head are waiting.

Once the ratchet head is in all the way, I can install the cross-pin and its snap ring (hole for that visible at top of body tube). The cross-pin both holds the assembly together and provides the pivot point for the ratchet head. You'll soon see why the ratchet head needs to pivot.

With the handle backed off all the way, there is no resistance to assembly. I do not need to push down on the ratchet head in order to push the cross-pin back in.

Tilt block relative to its surrounding parts

Let's backtrack a bit.

I'm showing you these two images to help you understand better the next ones.

Shown here, from top to bottom, are the ratchet head, the tilt block, the anti-windup assembly and the main spring.

See how the anti-windup assembly and the ratchet head both have little squares sunk into them? The tilt block is trapped between those squares under pressure of the main spring.

The wrench is completely assembled now.

I had previously cut a hole in the body tube so I could see the tilt block assembly. The body is very hard steel. I wore through a couple of Dremel wheels cutting through it.

Note that the bottom of the ratchet head is on top, the anti-windup assembly below, with the tilt block interposed between them.

I've cranked the handle up the torque scale, screwing the calibration shaft into the body, squeezing the main spring, putting more and more clamping force on the tilt block. At this point, if I tried to remove the cross-pin at the top of the body, the whole works would shoot out the top with some force, like a jack-in-the-box, and go all over the shop.

Now what happens if we actually turn the wrench and try to tighten a bolt? Keep reading...

Click!

Once the turning force you apply overcomes the clamping force applied by the main spring's preload, the tilt block is able to rock to one side. The bottom of the ratchet head now smacks the side of the body tube, and you hear that special click.

The genius of this design is that the tilt block's rocking motion means there is just about zero frictional wear. Thus the only thing that could possibly have a significant effect on torque is fatigue of the main spring itself. The inside of the wrench is slathered in grease, so moisture and rust shouldn't be a problem unless you drop the wrench in the ocean or something.

This type of torque wrench is good for about 5,000 cycles, BUT...two things are critical: 1) you MUST back the pressure off to the very bottom after every session, and 2) you MUST be VERY careful when dialing the pressure back down. If you're too vigorous in cranking the handle as you reach bottom, you may loosen the lower locknut, thus throwing the wrench's calibration off completely, just like I did.

The moral of the story? Treat your torque wrench with the care a precision instrument requires! Treat it gently and properly, never drop it, keep it in its original container, and it will serve you well for many moons.

Finally, you may have wondered from time to time just how good/consistent torque wrenches are. The leaflet that came with my wrench warned that tolerance up or down was about 4%, so a scale reading of 75 ft lbs may actually result in anywhere between 72 and 78 ft lbs actually applied to the bolt from one example to another of the same wrench.
Reader Joe Dille has created a page that tests a bunch of wrenches against a common loading method, with interesting results.
His page page is here:
http://home.jtan.com/~joe/KIAT/kiat_3.htm