guy48065
Well-known member
Lately there have been a lot of questions about torque wrenches...how they work...brand differences, etc. so I thought I'd go find a good image of the guts to post. If you search "torque wrench diagram" you'll find plenty of useful images of what's inside the tube but most are exploded parts diagrams. Here's a good photo of the actual parts (I hope hotlinking is OK and the owner doesn't remove it):
Whether it's a $300 Snap-On or a $15 Horror Fraught they LOOK mostly the same. And typically have similar accuracy specs. The little "pawl" shown is a very critical part. It's sandwiched between 2 hardened discs and is held in place by a shallow recess in each disc. The pawl is hardened and has slightly eased edges, and is small enough that it can rock when the tapered head forging moves. When applied torque exceeds the spring force on the pawl it suddenly tilts and the end of the head taper bangs into the tube--causing the click you hear & feel. That movement is what causes the let-off of torque.
For this action to be consistent over the life of the tool the moving parts need to be accurately made and precisely hardened. Any wear or gouging will make the "break point" change in time, or be inconsistent from one fastener to the next. A Harbor Freight wrench might perform exactly the same as a SO out of the box but how much do you trust the metallurgy and hardening on a $15 collection of simple-but-critical parts?
The tool show has a couple refinements you won't find in the cheapest tools. That slug labeled "cam assembly" is actually a linear bearing that's ringed with tiny ball bearings that allow the spring force to be smoothly transferred to the pawl as the spring is compressed by the adjustment screw. This helps with repeatability--every fastener gets the same torque. The centering washer ("positive engagement spring and cup") is also a nice touch that keeps the spring from twisting and keeps it centered as you twist the handle. The double nuts on the adjustment screw make it easier for a hack like me to set the handle so it's right on the markings stamped into the tube. Vague markings are another source of error.
Whether it's a $300 Snap-On or a $15 Horror Fraught they LOOK mostly the same. And typically have similar accuracy specs. The little "pawl" shown is a very critical part. It's sandwiched between 2 hardened discs and is held in place by a shallow recess in each disc. The pawl is hardened and has slightly eased edges, and is small enough that it can rock when the tapered head forging moves. When applied torque exceeds the spring force on the pawl it suddenly tilts and the end of the head taper bangs into the tube--causing the click you hear & feel. That movement is what causes the let-off of torque.
For this action to be consistent over the life of the tool the moving parts need to be accurately made and precisely hardened. Any wear or gouging will make the "break point" change in time, or be inconsistent from one fastener to the next. A Harbor Freight wrench might perform exactly the same as a SO out of the box but how much do you trust the metallurgy and hardening on a $15 collection of simple-but-critical parts?
The tool show has a couple refinements you won't find in the cheapest tools. That slug labeled "cam assembly" is actually a linear bearing that's ringed with tiny ball bearings that allow the spring force to be smoothly transferred to the pawl as the spring is compressed by the adjustment screw. This helps with repeatability--every fastener gets the same torque. The centering washer ("positive engagement spring and cup") is also a nice touch that keeps the spring from twisting and keeps it centered as you twist the handle. The double nuts on the adjustment screw make it easier for a hack like me to set the handle so it's right on the markings stamped into the tube. Vague markings are another source of error.