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I guess i'm a torque wrench fanatic I torque everything .....I know I'm a sick puppy .
I’m glad someone here is as nutty as me about tools, but I’ve met a few people far nuttier than me. I have lots of torque wrenches ranging from Snap-On, Mac, Proto, Sturtevant Richmont, CDI, Sears, etc, including many torque screwdrivers, etc. I also had one of my calibration engineers test various torque wrenches and we graphed all the data for comparison and the results were very interesting, but this was ~15+ or so years ago. When it comes to torque wrenches, my favorites and most commonly used torque wrenches & torque screwdrivers are my Proto dial gauges & screwdrivers. I don't own any super high end digital mega-dollar torque tools that are good to 0.01 % and I'd like too, but the accuracy of Proto’s dial torque wrenches are plenty good for our applications and in fact they’re certified for use by NASA and other high end applications.
A good torque wrench will usually come with a graded torque sheet that shows exactly how accurate it is at various increments throughout its torque range. You’ll find these sheets come with wrenches like Proto, Snap-On, Armstrong, CDI, Sturtevant, Mac, etc, but not with the wrenches from places like Home Depot, Autozone, Harbor Freight, Sears, etc. Just because a torque wrench says its accuracy is within 3% doesn’t mean it will be that accurate through its entire torque spectrum. Most torque wrenches are only accurate to their specification in the meat of their torque range with the outer limits being less accurate, but their accuracy generally suffers much more at the lower end of the scale than anywhere else. This is one reason you need more than a single torque wrench if you plan to torque various sized fasteners. I can guarantee you that if you use a Sears, Harbor Freight, Huskey, etc, half inch clicker type torque wrench that has a range of 0 to 100 lb/ft to torque a smaller fastener to lets say 8 lb/ft that it will not be accurate and you’ll risk stretching the bolt (tightening it past the yield zone) or breaking the bolt as opposed to keeping it in the elastic zone.
Bolts have a certain amount of elasticity to them and if you’re a science nut, then you’ll know that steel is more elastic than rubber. When a nut is tightened, it compresses against a surface and stretches the bolt. When the nut is loosened, the bolt will rebound like a spring to its original size if it’s kept within its elastic limit. Fastener can be cycled this way thousands of times or more or even millions of times with high quality bolts such as from ARP when properly used. It’s this spring tension that provides the clamping force which keeps things together, but if you exceed the elastic limit of the bolt, then you’re into the yield zone and the bolt will no longer rebound correctly and will stay permanently stretched, thus coming loose rather easily, especially if heat and vibration are present. You can generally tell you’re in the yield zone when the nut eventually becomes easier to turn while tightening it, which is just before something breaks. Generally, you want to tighten fasteners within 50% to 60% of their elastic limit, which is where nuts and bolts tend to stay together, but this number can be a moving target with the use of heat, lubrication, vibration, etc. This may all sound pretty basic to some folks and perhaps it’s too much info for others, but it would take a lot of time to get into the dirty details of fasteners because this topic can get deep very quickly. Truly understanding fasteners is a very complex business when you get right down to the nitty gritty, but fortunately we don’t or shouldn’t need to get to that level as a user. Here’s a quote I saved from ARP “Automotive Racing Products” who specializes in making high quality fasteners for various markets such as aerospace, automotive, racing, government, industrial, etc. “To thoroughly understand it all would require at least 4 specific engineering degrees and 20 years of hands on experience in each.” OK, I’ll be the first to admit I don’t understand it all and probably never will, but learning is a life experience for me and I enjoy it.
There are several different ways to properly tension a fastener. Measuring the amount that a bolt has stretched is the most accurate way, but it’s not very practical for most folks here. Another way is to turn the fastener a predetermined amount of degrees after it has been properly snugged down to a given point, but this is only valid if the fastener has been previously calibrated by measuring the amount of stretch which isn’t very practical for most people. Another way is to use fasteners with direct tension indicators (DTI fastener components) and we may see more of these in the future once costs come down. The most common way to tension fasteners besides using your calibrated wrist is to use a torque wrench based on the specs given in service manuals, but those values are only good with OEM components under specific conditions. If you replace an OEM bolt with another bolt, the torque specs can differ if the new bolt is made from a different material, or has a different surface finish (plating, polished, black oxide, etc), or is lubricated, heated, etc, so keep that in mind if you swap out a bolt here and there. The most common change you’ll see is lubrication being added to bolts with the use of anti-seize or more commonly engine oil getting on the threads. In those cases you’ll need to subtract ~20% of the specified torque value to be safe, but this number can vary considerably based on the type and amount of lubrication used.
Probably the most significant thing you can do on ‘new’ fastener applications to make sure they stay properly tensioned is to first torque it to spec, then loosen it a half turn, then torque it to spec again, then loosen it a half turn again and then torque it once more. The reason for cycling a new fastener a couple times is because its friction will be higher the first time it’s tightened, but it will lessen with each cycle until it finally evens out. There’s a pretty significant change in friction from the first to second cycle, but after that the difference is usually quite small and cycling it three times is good enough for what we’re doing. Keep this in mind if you replace your spokes and torque them with a torque wrench (i.e. www.fastco.com).