How does a torque wrench work?
by TeGGer
Note: I recreated this from the Wayback Machine's archive of TeGGer's defunct website because it was significant to another article I host. If TeGGer ever brings his website back to life I will be happy to link to it there. All of TeGGer's other content can, for now at least, still be found on the Wayback Machine.
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. The Craftsman is actually made by a company called Danaher, which also makes the "Kobalt" brand wrenches sold by Lowe's hardware stores.
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. The decision was a no-brainer of course, so I ended up with two torque wrenches, one new and usable, one old and 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, there's no ratchet, plus 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 Arm Pin and Retaining Ring. This enabled the ratchet head to be pulled out of the body tube, with all the other stuff following.
Click on the picture to open the hi-res image, then study the parts and their names. The critical parts for calibration are the Adjustment Screw, both Handle Nuts, and the Grip Assembly.
Here are the Grip Assembly, Adjustment Screw and Handle Nuts, all assembled.
The Grip is clamped to the Adjustment Screw by the Handle Nuts. When you turn the Grip so as to run it up the scale on the Barrel to the torque reading you want, you are screwing the Adjustment Screw into the Barrel, compressing the Torque Spring. The Grip and Screw never rotate relative to each other.
The Torque 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 Handle Nut 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 Endcap off.
When a torque wrench is calibrated, the Grip is moved up and down the Adjustment Screw, so that the reading at the Grip and Barrel markings will match what the Torque Spring is trying to do. At that point, the calibration technician tightens the Handle Nuts together, clamping the Grip and Adjustment Screw in phase until the next time the wrench is sent for calibration.
You would need this Allen key, plus a very thin-wall 11/16" socket, to do the calibration.
Why does the handle have that notchy, clicky feeling as you turn it to the desired torque setting? The "click" is NOT made by the threaded Adjustement Screw itself! It's a function of an interaction between the plastic Grip and a machined feature on the Barrel.
If you look closely at the bottom of the wrench's Barrel (go back and look at the second photo from top), you will see a series of long, shallow slots milled into the Barrel. These are oriented axially, and spaced radially, around the body. (If they were cut all the way through, they would make the bottom of the body resemble a military rifle's flash suppressor.) There is a small nib, or protrusion, on the inside top of the plastic Grip that indexes into the slots. When you turn the Grip, its top flexes sufficient to allow the protrusion to ride out of one slot and fall into the next, giving the handle that "click" feeling as you wind the setting up or down.
There are ten slots of course, one for each pound as you rotate from 1 through 0, up each ten pounds of scale marking.
I'm not sure how the Grip's locking ring functions; it never occurred to me to examine that assembly. I suspect it simply prevents the protrusion from being able to ride out of whatever slot it's in.
We'd better put our wrench back together so we can see how it actually works.
I've already put the Thrust Washer and the Positive Engagement Spring and its Cup back inside, and the Torque Spring is part way in. The Cam Assembly, Pawl and Ratchet Assembly are next.
The Cam Assembly consists of four basic parts: The Cam itself (a steel cylinder that goes all the way from Pawl to Torque Spring), a plastic Cage, a light spring, and a number of Cam Balls. I didn't take the Cam Assembly apart when I took the photos. The Cage is simply there to organize the Balls; it bears no load. The Cam Assembly appears to be there as an anti-windup device, and as a means of preventing drag against the Barrel, so that the Pawl will be affected solely by the force applied by the Torque Spring.
The Torque Spring is now in all the way, the Cam Assembly part-way in, and the Pawl and Ratchet Assembly are waiting.
Once the ratchet head is in all the way, I can install the Arm Pin and its Retaining Ring (hole for that visible at top of the Barrel). The Arm Pin both holds the assembly together and provides the pivot point for the Ratchet Assembly. You'll soon see why the ratchet head needs to pivot.
With the Grip backed down all the way, there is no resistance to assembly. I do not need to push down on the Ratchet Assembly in order to push the Arm Pin back in.
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 Assembly, the Pawl, the Cam Assembly and the Torque 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 Barrel so I could see the Pawl. The Barrel is very hard steel. I wore through a couple of Dremel wheels cutting through it.
Note that the bottom of the Ratchet Assembly is on top, the Cam Assembly below, with the Pawl interposed between them.
I've cranked the Grip up the torque scale, screwing the Adjustment Screw into the Barrel, squeezing the Torque Spring, putting more and more clamping force on the Pawl. At this point, if I tried to remove the Arm Pin at the top of the Barrel, the whole works would shoot out the top with some force, like a jack-in-the-box, and go all over the shop, maybe injuring somebody in the process.
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 Torque Spring's preload, the Pawl is able to rock to one side. The bottom of the Ratchet Assembly now smacks the side of the Barrel, and you hear that special click.
The instant you hear the special click, STOP TURNING THE WRENCH! The wrench doesn't suddenly "freewheel" after the click and automatically prevent additional torque being placed on the fastener, it will keep adding torque until you stop pushing! Correct torque is achieved the instant the Pawl heels over and smacks the side of the Barrel. In other words, the click is an audible and tactile signal ONLY. You are expected to respond to the signal by ceasing to push on the wrench.
The genius of this design is that the Pawl'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 Torque 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 Grip as you reach bottom, you may loosen the lower Handle Nut, 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, use itONLY for its intended purpose (final tightening of fasteners), neve rdrop it or subject it to any sort of impact, keep it in its original container, and it will serve you well for many moons.
Some links...
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 is here.
A nice article from Motor.com, on torque wrench care and calibration:
http://www.motor.com/magazine/pdfs/022002_08.pdf
A rather crude exploded diagram of another design of wrench, from Harbor Freight:
http://www.harborfreight.com/manuals/0-999/239.pdf
