 |
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. The names used throughout this document are taken from a Danaher parts diagram found on a Sears Web site. Thanks to johngdole for finding that diagram and for discovering that the Crafstman and Kobalt wrenches are both made by Danaher. 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. |