It's a complex topic. Depending on the design, lubrication may be required or forbidden. And if it's required, it should normally be on the threads only, under the head is usually forbidden.

And it's very important to understand that in most mechanical design with fasteners, the fasteners provide tension, and friction between 2 faces actually carries the load. Too little tension then fasteners can be sheared off. Too much, then the fasteners may not have remaining strength available for loads that act to add tension to the fasteners.

Using a torque wrench to reach target tension is normally only about +/-30% accurate. Usually design margin allows for this but in very critical applications where margin is not feasible, calibration is done on a sample of the same materials, etc, or a more direct measurement done. More direct measurement can mean hollow fasteners that allow you to measure the amount of stretch, use of ultrasonic measurement to measure stretch, washers with integrated strain gauges, or cleverly designed "tension indicating" washers or fasteners. There are many types so I just those keywords for anyone interested.

I am not saying that this design required such complex methods and sizing these fasteners should not be difficult. There is probably a mistake or lack of control in the assembly process.

Lots of good stuff in this comment but I'm confused about the +/- 30% accuracy quote, in my experience it is relatively easy to reach sub 10% accuracy based on comparing different (good quality) torque wrenches against each other and against a calibrated one.

Is there something I am missing here?

It's easy to calibrate torque wrenches to +/-5% of each other for _torque_. Calibrating a torque value to _tension_ of a fastener depends not only on the accuracy of the tool, but on materials, lubrication, temperature, process, etc. +/- 30% actually depends on having a good process. This only accounts for variations in tolerance, surface finish, etc. If you add lubricant under the fastener head when you are not supposed to, you can easily reach +50 or +100% tension.

Ah, I see your point now, I thought you meant it to be a direct cause of the torque wrenches themselves to be off. That makes good sense and aligns with my experience.

After manufacturing tension tends to drop over time so starting off with a '+' may not be entirely bad assuming it isn't extreme and that it doesn't cause the materials to deform more than permitted. The way I understand it: you apply a certain torque to a fastener in order to get to minimum levels of tension and friction (which still have an engineering reserve) on the fastener itself to guarantee a seal and to stop the fastener from coming loose, so under-tension is far worse than over-tension as long as the over-tension does not result in damage to fastener or the materials, and the allowed tolerances for over-tension are quite large (up to +150% or so normally before any permanent deformation would occur).

Unless you are using 'stretch' bolts which tend to elongate to accommodate any over-tension to end up with something quite close to the intended value. This stretching tends to be non-elastic so you'd have to replace a stretch bolt every time you unfasten it or there is a pretty good chance that it will break and/or that the threads under the ending position of the nut will have deformed so that they end up being stripped if you refasten them because the nut will travel a bit further on every refastening.

I presume his point is that you are measuring torque (potentially very accurately) but what you actually care about is tension. It's the variable factors affecting the relationship between torque and tension that results in most of the +/- 30% accuracy in tension, not the measurement of the torque.

The desire of fastener tension is clamping force or fastener stretch in some cases, not fastener torque. I think what the parent is saying is that a perfectly torqued fastener is only about 30% accurate in terms of clamping force.

Ive been working on cars and motorcycles and everything else other than airplanes for almost 25 years now, from motor swaps to transmission rebuild and custom suspension. The only time I ever use is a torque wrench is on engine internals, or smaller fancy areas into aluminum. Everything else, over the years, you learn by feel.

You pay attention to the size of the bolt, the material it's going into, and it's overall job. Yes you'll mess it up a few times a long the way. Hopefully you're a lot more careful when you see that the cost of making a mistake is a difficult extraction or alteration of some sort. Personally, I think you'll run into bolt/thread failure 10x more from improper insertion, dirty threads, corrosion, and overall entropy than you will from over torquing once you learn a few early lessons.

I've done countless brake jobs, tire rotations, oil changes, spark plugs, valve cover gaskets, shock and bushing changes, without a torque wrench. I've yet to ever had a problem from it after going a little too hard when I was 16 yrs old and learning what bolts and materials can hold what.

At the Costco where I get my car's tires swapped out (summer/winter tire swap) or rotated, the technician always tightens all the wheel nuts with a torque wrench.

Then another, different technician comes by and checks all the nuts again with his own torque wrench.

When all is done and I get the sign off paperwork, they strongly encourage me to stop by next time I'm there so they can quickly check the wheel nuts again with a torque wrench.

So please, don't "do it by feel". That's how you Boeing. I know I would never patronize your shop again if I saw you being so callous.

A true craftsman double and triple checks his work objectively with other craftsmen, not be proud he can do it blindfolded with one arm behind his back.

I don't work at a mechanics shop.

Costco prior to probably 15 years ago used to just throw wheels on and hit them with air guns without a torque wrench in sight.

Mechanics shops snapping wheel studs was a common problem for years. A decent mechanic would know to never use a high torque impact gun on something with decently low torque threshold, as I said in my OP about experience. But that shows you how many years and millions of cars were/are on the road without caring about exact torque.

The reason Costco does this, and does it twice, is to make sure the lig nuts at tight on the first place... So you don't leave with one loose. The fact that they use a torque wrench is just because they got tired of replacing wheels studs.

The size of a bolt head is directly related to it's target/max torque due to the increased leverage from the larger rotation. This wasn't obvious to me at first but it's helped for rough guesses when I didn't know exact values.

Torque specs only apply to clean and clear threads that are free of corrosion and grease -- unless otherwise-specified.

Threads that are this way do not get stuck half-way.

If the threads are already boogered up for whatever reason, then the torque specification is already wrong: The threads aren't clean, clear, and free of corrosion and grease. This can result in under-tensioned fasteners when using a torque wrench as a guide, since boogered threads (rather obviously) can present an impingement that allows a torque wrench to click off before proper tension is reached.

However, grease is not a magical antidote to this condition.

If the threads (boogered or not) are greased, then the torque spec is also wrong: Greased threads are also not clean and clear, and free of corrosion and grease. This can result in over-tensioned fasteners when using a torque wrench as a guide, since grease is (rather obviously) a lubricant -- allowing things to slide more-freely in a way that doesn't allow a torque wrench to click off until somewhere beyond ideal tension.

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So what do, then, as a home-gamer with a bicycle or maybe a car in the rust belt (but never an aircraft) full of dissimilar metals that are constantly rotting?

You could perhaps kit up to do the Junker test such as in DIN 65151, and make a study of how different greases affects things. You can even make a career out of publishing your studies.

Or: Just make sure the threads are clean, clear, and dry, and then assemble with an anti-seize paste which does not have lubricating qualities that affect final tension yield. (Permatex makes some, as do others.)

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(And in aircraft, always do what the engineers say. If the engineers are wrong, then: Stop doing whatever it is that you're doing, and consult them.)

Only grease fasteners if specified by the manufacturer, otherwise you're going to apply much higher than specified tension at the same torque value.

I need manufacturers to give "frame spent 3 decades in the salt belt, questionable metal compatibility and won't be taken apart until it breaks again" torque specs.

It's like they think I'm going to take this thing apart and put it back together every year.

j/k, finding torque specs for a decades old steel bike is a lost cause

a good machinists handbook will give you torque specs that take into account fastener size, material, and thread-locking style.[0]

mine is much older than linked version, but i doubt they'd revise that whole section out of it.

[0]: https://www.amazon.com/Machinerys-Handbook-Toolbox-Erik-Ober...

In the car world that torque is measured in ugga duggas from the impact wrench

I keep a toothbrush covered in grease for just this purpose.

It’s also a magnetic one so I can stick it to stuff and it stays put.

> I keep a toothbrush covered in grease for just this purpose.

Speaking from personal experience, this is also useful for brushing a robot's teeth.

Where does one find a magnetic toothbrush?

It’s an old Sonicare brush head (a couple of fairly strong magnets are mounted to the bottom), there are also some kinda neat suction cup toothbrush holders you could use

There are different torque-to-tension equations for lubricated and unlubricated fasteners, of various materials and platings.

A properly designed service manual will specific if the fastener should be lubed. If it doesn't say lube/thread locker should be used, then the fastener should be torqued dry.

In bicycle applications it is common to elide explicit instruction to use grease. Nevertheless it should be used.

Yes, that's the way aircraft fasteners are put on.

There's another reason to grease them - the grease keeps the water out which prevents corrosion.

In working on my cars, I always use a bit of grease when assembling fasteners. I've never had one come loose, nor have any rusted themselves on.

In some cases (some spark plugs are what I can think of) explicitly tell you NOT to grease them, as that will let you over-torque them and potentially damage threads (aluminum heads, steel plugs). So using grease is not always the recommended procedure.