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Why You Need a Torque Wrench (or Two…)
I’ve got a confession to make.
I'm “that guy.” Call it OCD, call it being anal-retentive, but I'm always making an effort to be as technically correct as possible. One aspect of that effort has been the use of torque indicating or torque limiting tools when tightening fasteners.
It started after I put new valve plates and gaskets on a Carlyle 06E compressor. As I was always taught, all the torque you needed to apply to a fastener was the torque you could apply with a normal-sized combination wrench. So, that is exactly what I did. The compressor gaskets failed and were bypassing the very next day because I didn't get those bolts tight enough. The tech who took that call asked me if I had and used a torque wrench. At first, I didn't understand.. Sure, I tightened the bolts. No, he asked, did you TORQUE them to specifications? 90-100 ft/lbs was the spec and, while you can get that kind of torque out of a standard length ¾” wrench, you're working at it.
That question started a dive down something of a ‘rabbit hole’ for me, and I'm going to share some of what I learned.
I started with one torque wrench, which I used just to tighten the bolts on compressor heads. From there, I've expanded to have three torque wrenches (¼, ⅜, and ½ drive) and a torque limiting screwdriver, mostly for electrical connections.
What is torque? Simply put, torque is a measurement of the amount of force required to turn a fastener. For bolts, torque is normally measured in ft/lbs or in/lbs. To help you understand, those are foot-pounds (ft/lb) or inch-pounds (in/lb). A foot-pound is one pound of force applied on a lever one foot long measured from the center of the fastener.
As we continue to increase the torque applied to a bolt, the male threads on the bolt move deeper into the female threaded hole while the part being fastened, a compressor head, for example, prevents the head of the fastener from following them. This stress results in a stretching of the bolt. That stretch applies what is called ‘clamping force’ to the assembly. This stretching permanently deforms and weakens the bolts, and sometimes proper assembly requires new bolts. Lubricant applied to the fastener makes it easier to turn, which decreases the torque required to achieve the same clamping force. Be careful always to follow manufacturer guidelines.
One place where torque wrenches are making inroads in our industry is with ductless mini-splits. While I haven't broken down and added one of these to my kit yet, I do have an easy way to torque flare fasteners using a regular torque wrench: crowfoot wrenches.
These little guys allow you to turn any ratchet or breaker bar into a handy wrench. To use them with a torque wrench, however, requires a little extra step.
Remember how we measure torque? It's based on the distance between the center of the fastener to the point where force is applied. Well, a torque wrench is calibrated to have force applied on the knurled part of the wrench handle and centers that force on the centerline of the drive spindle. Adding a crowfoot wrench to the end of the wrench changes the center of the applied force. We need to account for the extra length of the crowfoot and the extra leverage that it creates.
For this, use your required torque force as TA to solve the math. That said, I've found very little actual difference when using a crowfoot wrench, and since often torque values are given in a range, it isn't really necessary to calculate the difference,. Just set your wrench to the low end of the torque range and use the crowfoot. That will generally keep you within the specified torque range.
Another trick, where possible, is to install the crowfoot at a 90° angle to the drive. Doing this makes the actual and effective length of the tool the same and allows direct use of the tool without calculations. Keep in mind that a standard socket drive extension won't affect your torque wrench settings because it doesn't affect the length of the tool in the direction that matters.
I hear a lot of guys argue against using a torque wrench because they can tighten things up just fine without one. Probably so. I did a lot of jobs prior to the one I mentioned earlier without using one, and I was “just fine…” or was I? Did I tighten those flanges evenly, or did I warp the flange by over-torquing one side? Did I over-torque that flare and set myself up for a leak later? Did I tighten all the bolts evenly, ensuring even clamping force on all those gaskets? A torque tool simplifies things for us. Tighten to the specified torque, and you're done. You don't have to think about that variable anymore. It's as tight as it's supposed to be and no tighter.
It's one less thing to worry about.