In this 'ible I will go over how I fixed my torque wrench (a cheapo-clicky style I got from an auto-parts store) which had stopped working, and re-calibrated it. Its mostly a copy/paste but I have edited it a good bit. Enjoy!
I found out the hard way that my click-style torque wrench no longer "clicks". I sheared off a bolt. Upon playing with the wrench some more, I discovered that it no longer operated properly, it didn't click for any torque setting. Since I got it rather cheaply at an auto parts store, it wasn't worth shipping off for "warranty service", so I decided to take it apart to at least see the internals and find out how it works, and attempt a repair. As it turned out, the fix was even more simple than I expected, and my wrench is now fully functional again. As I was unable to find any info out on the web about these things, more than just forum postings of hints (and mostly asking questions that went unanswered) and a couple how-to-calibrate pages, I decided to post this page for the benefit of all. The following is a description of what I did, along with pictures and my analysis of how it works.
Step 1: The Wrench
This as you can see, is a Torque Wrench. This particular one is a simple twist the handle to set the torque one, with the torque settings engraved in the handle, read like a micrometer (the internals of the ones with read-out windows with turning digits are similar). To keep from accidentally adjusting the torque while you are torquing something, there is a knob on the end that you screw into the handle to lock it in place. The ratchet is reversible like a normal ratcheting wrench is, via the switch, but the click is only functional in the tightening direction (unless the head itself is reversed, see later in this 'ible).
The way this wrench is supposed to work is this: you dial in the torque by twisting the handle until the reading engraved on the handle matches your requirements. You turn the knob at the end to lock this adjustment in. You then place the socket on the head and tighten the bolt/nut until the wrench makes an audible "click". As this is a "precision" measuring device, it should only be used to measure and adjust the torque of fasteners, it should not be used as a drive ratchet. What I mean by that is you should pre-tighten the nut/bolt at least finger tight using another ratchet or tool so you are only using the torque wrench to check and adjust that tightness to your required torque.
The way it functions is rather simple: the ratchet head connects to a lever arm held in place by a bearing resting in a notch on that arm under pressure from a spring. The higher this pressure is, the higher the torque it takes to overcome the force of the bearing escaping the notch. Once that torque is reached, the lever arm escapes suddenly, "Clicking" to the other side of the main body of the wrench.
Step 2: Disasembly
Step 3: Disassembly Part 2
Step 4: Disasembly Part 3
To remove the assembly, the handle has to be removed. The piston has a screw in it that keeps the lockout knob from unscrewing all the way out. This also keeps the pins that are in the calibration sleeve from falling out. Once this retainer screw is removed, the lockout knob can be fully unscrewed from the piston. Then the knob, calibration sleeve and piston can be separated from each other. Once the calibration sleeve is removed, the handle can be unscrewed until it comes off the wrench body. Once the handle is gone, the pin holding the piston in place is easily removed and then the rest of the internals easily slide out of the wrench body.
Step 5: The Lockout Knob and Clicker Functionality
Looking at the other parts that came out you see how the adjustment handle simply puts more and more tension on the big spring. This spring pushes the roller bearing piece down onto the sloped and notched end of the lever arm that extends from the ratchet head. This arm is attached to the main body with a single pin down near the head, creating a lever. The force you exert on the handle gets transmitted via the spring and bearing to the lever arm. The more tension put on the spring by adjusting the handle, the higher the force on the bearing assembly, the greater torque required to let the bearing escape the notch and let the arm "Click" to the other side of the body. When the torque is released, the spring pressure forces the bearing back into place, causing another "click". A bearing and grease is used so that friction forces are not as involved as they otherwise would be.
Step 6: Re-Assembly
Following the steps backwards, re-add the spring, piston, calibration sleeve, lockout pins, washers, knob and retaining screw, piston retaining pin and handle. Leave the main lock nut off for now as we will calibrate the wrench.
Step 7: Calibration
I used a vice-grip locking wrench and 40lb dumbbell to calibrate this one. The vice-grip locks to the socket head of the wrench, and has a handle long enough to create a known torque with the dumbbell. The dumbbell rests on the vice-grip's handle at a known distance from the center of the ratchet head (8.25"). This creates a torque of 40*8.25=330in-lb/12=27.5ft-lb. The whole assembly rests on the rounded ratchet head, and you can try to lift the weight using the wrench. When the calibration is correct, it will click right around 27.5 ft-lbs when the weight is just barely lifted. Note that this will not be completely accurate, and if specific and precise torque is required you should use an actual calibration rig. This is just a quick and simple way to get your wrench in the ballpark.
To do this, tighten the calibration sleeve until it is about half-way into the handle. Set the handle to read 27ft-lbs and test the torque. If it clicks before the weight moves, back the handle off the spring and tighten the calibration sleeve again. If the weight can be lifted without it clicking, loosen the calibration sleeve. Repeat this process of lifting and adjusting until you can repeatably barely lift the weight. Once this setting is found, tighten the lockout knob, then reinstall the main lock nut and tighten it to lock the calibration sleeve in place. Your wrench should now be back to normal.
Other calibration rigs use a long bar attached to a socket that fits the wrench. The bar is laid out horizontal and in a way that it can pivot at the ratchet head. Weight is then suspended from the end of the bar with string, and a similar process as above is done to set the calibration. More professional an accurate rigs simply have a socket the wrench is attached to, and then display the torque as it is applied via dial gauge or other readout. The wrench can be adjusted so that it clicks at the proper torques across a wide range of settings.
Step 8: The Ratchet Head
The ratchet head is kept together with two screws holding a plate to the main body. Once the screws are removed, the plate easily comes off exposing the internals of the ratchet head. The ratchet works by two pivoting spring loaded metal bars interacting with splines on the socket head itself. A cam attached to the direction lever pushes one bar out of the way to control which direction the ratchet will turn. Since the torque wrench only clicks to one side, only one direction can actually measure torque. The other direction is for convenience of using the wrench as a normal wrench (not recommended as it could throw it out of calibration). To change direction of the torque arm, simply remove the socket head and flip it over so the socket points out the other side of the wrench. This one allows it as the openings in the plate and the head are identical, others might not depending on design.