How again will you get within 5%? Analytically? You intend to measure the force a bolt produces, then analytically determine the torque, is that right?
that was purely a guess, i'm sure you could have deduced that from the way it was worded without anything to back up that claim.
When I specifiy a bolt torque as an aerospace engineer, my intention is to produce a certain clamping force. If I used that theorhetically derived torque, there would be no clamping force. Why? Because
the overwhelming majority of torque we apply to a fastener is reacted by friction. Different studies have produced different numbers, but generally, 40-50% of torque is reacted in friction under the driven element (e.g. bolt head). Another 40% could be lost in the threads.
understood, the clamping force lost through friction without the same torque applied is something i am well aware of, should i put this in capitals so you don't miss it for a third time?
So how this works in the real world is, we strain gage bolts, get a calibrated torque wrench (actually something a bit more complicated than that) and test how much torque needs to be applied to produce the desired clamp up (as measured by the strain gage). That data is then published in table form for every MIL SPEC bolt and nut combination we use, with lube requirements, washer requirements, and assumptions about hole quality and parent materials.
also understood, not something that needs explaining that there are plenty of tables out there that tell you to install a bolt clean and dry, to X torque and you should achieve X clamping force, that is why i also previously stated that predetermined values have already been tested thoroughly eliminating the need to make my own tables for this purpose. should i also put this in caps so you don't miss it?
You won't get within 50% with the plan you have suggested thus far. This is a very complicated topic as several of us have tried to tell you. I'll leave it here so this thread will be hopefully of interest to the greater population:
pure speculation
1) Testing the torque output of an impact wrench is junk science. I'm quite certain manufacturers set up test rigs of some sort to justify the numbers they advertise. The best rigs would measure torque directly, not by determining torque based on clamp-up pressure as has been suggested here.
your opinion for sure, everybody knows the torque put out by an impact wrench is not the same as say a hy-torque, but you can definitely get a number that has a correlation between the two types of tools. if your impact gun can get a bolt to the same clamping force as a torque wrench, you can say that the torque wrench produces the same "torque" as whatever your torque wrench was set to.
But the real world problem remains that the torque we need to loosen a fastener on a car has to overcome friction, possibly caused by corrosion, which is essentially unknowable. Fortunately, fasteners on passenger cars only get so big and rust can only do so much. My sense is, any gun putting out over 500ftlbs of reverse torque is probably good enough. Beyond that, choosing one manufacturers' product because they say 650ftlbs vs another firms 900ftlbs could make no difference in use. \
i never mentioned using this or anything i was talking about to try to determines what kind of torque it takes to loosen a bolt which could have a million variables to what is making it tighter or looser. this is a good filler though for your paper. most common sense tells somebody that an impacting action work much better at loosening rusted or corroded fasteners because it breaks up deposits on the threads lowering the amount of actual torque it takes to break it loose from a stand still
My guess is what seperates the cordless guns and various air guns is the durability of their batteries or seals. This further negates the manufacturers' test numbers and in my mind validates users' anecdottal experiences. The first time you use your battery gun you may get 500ftlbs. The 10th time you use that same battery, you could be getting appreciably less.
how is this coming into play at any point with any of the previous conversation? the manufacturer can make any claim they want as long as its backed up by proven testing, just like previous peoples claims of just "tighten a bolt and see if you can break it loose" is IR's "nut busting torque" rating, which isn't a real rating of torque, its simply just telling you what you can loosen with enough rattling after it's been tightened to that specified torque. and if you search enough, or ask, i'm sure you can find out how they came to their conclusions of their ratings.
2) Said before but worth saying again: Torque is there to produce preload in the fastener, aka clamp-up on the assembled components. When engineers like me specify torque, what we want is clamp up AND consistent clamp-up. As mechanics,
having the best torque wrench isn't as important as knowing how to torque a fastener correctly. Here are some tips:
this has been mentioned before as well when talking about faux numbers with guns and the difference between a torque rating, a clamp rating, and the friction that causes the difference between the two.
a) Regardless of what the document says- lube everything. No bolt should be installed dry (see * below). Apply a light film of mineral oil (at least) on the threads, shaft, and especially under the head, and on the washer. Make sure all mating surfaces are free from dirt and grit. You can torque either the nut or head side. Choose based on what side is cleaner, smoother, flatter, or harder. You want to torque the lower friction side.
* Dry fasteners do what is called "stick-slip". Torquing bone dry hardware could produce more or less clamp up. When you turn a dry fastener, the force required to over come static friction and get the fastener moving, creates momentum which can just as easily result in over-torquing as under torquing. When I talk about a lubed fastener, I mean something with grease or sealant on it. Our numbers (which everyone else uses) for "dry parts" are typically numbers for fasteners with dry film lube applied by the manufacturer. There are no torque numbers for bone dry, slightly corroded, threads are a little bit gritty, bolts. The reason is (what I've been trying to say all along) friction is a significant contributor to clamp-up and friction of such a fastener is unknowable.
Oil everything! (I use 3-in-1).
you are not a bolt manufacturer and don't know what they do to determine their numbers. take for instance these l9 bolts, we would consistently torque them to the required rating and do it dry because the manufacturer has specific instructions on lubrication and installation.
https://www.kimballmidwest.com/Catalog/MarketingText/L9 Torque Chart.pdf
2. Do not alter the coating as this can result in erratic and unpredictable clamp loads. Do not use supplementary lubricants.
4. When tightening L9 bolts, use the recommended torque values. The self lubricating properties of the system reduce
required torque so that in some cases they may be below those needed to tighten Grade 8 bolts.
even in the case of the fastener NOT having a dry coating lube. when you apply your "3 in 1" you have no idea how that changes the properties of the friction. you should use whatever the manufacturer recommends for install, dry or whatever lube, they will have done the testing already for that lube, different lubes all have different values
b) Final torque must be achieved while the wrench is moving (due to the difference between static and kinetic friction). If, after ratcheting, you begin the next pull and the wrench instantly releases or clicks, you must loosen the fastener and start again.
this is common knowledge, i'm sure anybody who has done any professional installations knows plenty about the mechanics of static friction on a bolt and how to use a torque wrench.
c) Snap On's tests indicate we pull too hard, and for too long
after the wrench clicks. This results in over-torquing, according to Snap On. That's why they made the early warning beep system. But you can learn to just anticipate the click by wrenching slowly. As soon as the wrench starts to release, stop there.
more common knowledge, you can still achieve the same results with a regular wrench as well, you don't need a special electronic one, having a good feel for the bolts, how far the wrench may travel and your speed will keep you from going past the click point.
Someone who started their thread with the statement "torque is a pretty undefined description" telling someone else that they don't understand.