Bolt Opening Torque Mis-Conception

Measure the temperature of bolt when making up and loosening.

Whats on the bolt when making it up.

Whats the structure it holds?

What movement in the structure?

Is tight tight after wards in terms of "shake it until it fits"?

My guess is that you have to compress material when tightening up, but when loosen the screw the tension releases and helps with the task.

Look at bolt stretch.

The pitch angle of the threads acts (slightly, tan θ) in the direction of loosening.

GA

I agree entirely, surely its always easier to go "downhill" than "uphill" (to put it simply).

Unless of course a good star or spring washer was (correctly) used!!

Here is an article on torque auditing.

It is almost universal that when torquing a fastener, you don't stop and start, willy nilly. You tighten in a continuous motion, and the torque is affected by dynamic friction in the threads and under the bolt heat. Static friction is generally higher, so in general making the bolt rotate after stopping will require more torque than required to reach that point during torquing.

But that is in general. As Tornado mentions, there is a small effect from thread angle and there can be some wrap up of the bolt, if it is long. Galling can occur, making loosening torque quite a bit higher than tightening torque.

So... no simple answer, and the article should give you some pointers.

Sounds like joint relaxation. Research bolted joint relaxation and creep. There's a wealth of info online about what you're experiencing.

Try pushing a car without any wheels up a hill (OK not a very steep one); now try pushing it down.

Would that be an M20 car or an Imperial one?

If the opening torque needs to be greater than the closing torque, then try using a different fastener. The Nord-lock (usual disclaimer) springs (no pun intended) immediately to mind.

I know of nothing, written or logical, that indicates that the opening torque should be as much as the installation torque after a short duration.

Sometimes, when a lubricant is used, it dries out and acts like a glue over a longer term. This has been known loosely as "stiction", but as soon as the bond is broken the unscrewing is easy.

I know of nothing, written or logical, that indicates that the opening torque should be as much as the installation torque after a short duration.

Good mechanics always torque a bolt in a continuous motion. Given that bit of knowledge, then the written and logical explanation can be intuited from this. You can see that static friction is 20 - 50% higher than dynamic friction. Because thread angles are very small, the "sliding downhill effect" is no where strong enough to overcome the effects of static friction.

Specialized fasteners that are torqued to yield or lubricated to a spec before torquing, or have special washers, etc, can change this, but the OP's original expectation is shared by many mechanics.

Odd that your wonderful post, Ken, is marked as off topic.

Here is a quote from an earlier time when this discussion came up at cr4.

My engineer devised an aluminum block with three-each of four separate fasteners sizes that covered 95% of the fasteners we used and set about collecting data. Each set of three identical fasteners was torqued (all dry) to the same value and of course, that value was different for each set. I knew the breakaway torque would be higher than the applied torque but I was unprepared to see how wide the range of break-away torques was. A fastener initially torqued to say 30 in-lb. might require anywhere from 35 to 55 in-lb. to break it free. It also didn't seem to matter whether it was the same fastener in the same hole or if I swapped things around, the variability never improved. We chaulked it up to variability in the fasteners and threaded holes, as well as when and where exactly in the torquing did a specific thread begin to compress against the mating thread in the hole.

In that thread, Nick Name correctly stated:

Under normal conditions it is never less since when bolt is tightened the friction is a movement (sliding) friction and when one tries to get it loose the friction is static one and static friction (after a longer time it even sticks together) is always greater than movement friction.

In my earlier post, I linked to a chart showing static frictions averaging about 35% higher than sliding. In an 8 x 1.25 mm fastener (a course thread) the thread lead angle is about 3.25 degrees, giving a tangent of .056. That value is obviously not anywhere high enough to overcome the difference between sliding friction and static friction. (This is even moreso the case if the thread is fine.) So the engineer in the first quote knew that "breakaway torque would be higher than applied torque" because he understood the theory, and theory and practice in this case mesh.

Nick Name also provided a correct explanation, then.

This article correctly notes that one should not apply the final torque to critical fasteners (such as head bolts) in a small increment for this reason.

Also, fasteners should not be lubed arbitrarily. Torques are often specified for the dry condition and not the lubed condition.

Dry torque specifications.

wet torque specifications

When to use wet and when to use dry torques.

There is a good section on "torque to yield" bolts in the above article.

Bottom line....in twenty years of fixing airplanes, we were told to use wet torque because dry might gall and stick when tightening up. Wet torquing was more consistent. Some owners manuals specify dry torque. Fine. The owner's manual trumps "standard" practices and I won't argue with the owner's manual. My "standard practice" was always to use wet torque, a much lower but specified value. None of my airplanes ever fell out of the sky. The motorcycles on the Gimli racing circuit often fell apart though...they always used dry torque. Investigation of breaking torque values indicated that equipment which was installed with dry torque had much more variable torques than equipment which was installed using the "wet" chart above. Many blown engines resulted from wonky torquing caused by dry torque. The mechanics were all good men who didn't forget to torque the bolts, they were just victims of circumstances.

Getting them to accept "wet" torquing was impossible. So I left them to their smoking wrecks.

I wish I could find the research notes I took at the time....

Yes, I agree with the point made in your Popular Mechanics link, which reinforces my point that fasteners should not be arbitrarily lubed, but that instead the engineer's recommendation must be followed. For moderns engines, especially, getting correct bolt tension is more critical than it was in our day, with Honda for example, using ultrasonics to gauge bolt tension.

Per Popular Mechanics:

Most times, the specified torque value assumes clean and dry parts. Clean means no dirt, rust, dried-up ­gasket sealer or anything except shiny metal. Wire-brushing the threads will help remove rust or sealant. Engine fasteners, like head bolts or main cap bolts, are often specified to be torqued with 30-weight engine oil wetting the threads and washer. If you're installing a fastener that has a dry torque spec, and the threads and bolt face are oiled, you'll need to reduce the torque by 15 to 25 percent, because the slipperier surfaces will decrease friction. Teflon-bearing lubes or moly-sulfide engine assembly lubes can reduce friction enough to require a 50 percent reduction in tightening torque. Even casually substituting a zinc- or cad-plated bolt or washer for an unplated one calls for, respectively, a 15 or 25 percent reduction in applied torque, because the plating acts as a lubricant. Fail to heed this advice and the fastener will be seriously overtightened. You'll either snap it or crush a gasket to the point where it leaks.

Read more: How to Use a Torque Wrench - How to Tighten Lug Nuts - Popular Mechanics
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Airplanes and racing motorcycles are at opposite ends of the spectrum in expected reliability. I used to work on racing motorcycle engines that were shot after an hour of racing: that had nothing to do with bolt torque. General aviation aircraft engines are typically good for 1600 - 2000 hours between overhauls. And of course, as in airplanes, all the critical fasteners were safety wired.

In thousands of miles of riding racing motorcycles, many years of twisting wrenches for teams and personally, I've never seen a motorcycle fall apart from a fastener torqued to manufacturer's spec (usually dry).

Gaskets blowing, loss of oil, once in a while something else. The bike was often being gone over after being laid down to find out if anything could be salvaged. When there was signs of blown gaskets, we checked the torques. When analyzed, the breaking torque was often as much as fifty percent different between adjacent screws. Which does not prove that much come to think of it, there are no doubt many reasons for breaking torque to be so wonky.

The race engines were torn down a LOT regardless if they had been laid down. Some after every race. Hands were dirty, screws were not ultrasonically scrubbed clean, bolts were almost always re-used, and time, as always, was never enough. And people put a lot of money into the sport. You would think that something as simple as torque would be easy...

I figured I was on to something. But then I got posted out of the area, and never got into it again. Maybe the OP can pick up where I left off.

BTW, I love the hydraulic "tourquing" systems they got now. They pull the bolt away from the workpiece, you hand tighten the nut, and take off the pressure. Presto, perfect torque. Coming to a dealer near you .... someday.

"Presto, perfect torque"[i/]

Perfect bolt tension and consequently perfect clamp load. Torque is actually zero!

Could you give more info about the test assembly, the bolt pitch and length, materials,thread lubrication and used tooling for the operation.

Not a full answer but something to think about:

The tire rims on a car (often called wheels) most often require more torque to remove the lug nuts or bolts than to tighten them. This is why the normally used power tool to install/remove them is a pneumatic impact hammer. Most often they are made so that the maximum counter-clockwise torque (lefty loosey, righty tighty) is greater than the maximum tightening clockwise torque. This is done this way to accommodate the increase torque required to remove the nuts/bolts.

This is not an absolute property of all torque wrenches but one of most of the good to best models.

Food for thought!

Good Luck, Old Salt

I have never owned or used a torque wrench for LH threads, but I have owned and used a great many for normal RH threads. LH ones are out there of course, but seldom.

None of the ones I have had used the torque mechanism for undoing a nut or bolt. They all were working as simple wrenches when undoing......this could explain your thoughts....

Some of the ones I have used when tightening, once the mechanism had "clicked" to indicate torque value achieved, then simply locked up and you could tighten the bolt/nut even further!!! I believe this is to save your knuckles!

I had a young man do this each and every time, he could not understand the need to stop at "click"!!! Luckily we caught him at it and no bad units were shipped.

One of the possible reasons for loosening torque being higher at loosen time on car wheels is that corrosion can play a role after months (days?) of usage and join wheel and bolt/nut together, as I believe most mechanics know about...its nothing unusual then. But who measures it??? Who has the tool to measure it? Who uses the tool, even if its available?

As I said, unless you have a torque wrench that measures in both directions (the cheap Beam ones of 40 years ago come to mind), how would you even know? Look here for a few pictures of one if the name is new to you:-

http://en.wikipedia.org/wiki/Torque_wrench

I believe I still have one in the garage somewhere....

I believe often its simply down to the imagination of the user that the undoing is at a higher torque, is in not having a method of measuring or not even bothering to measure even if the tool is to hand.....basically, this value is of no interest to most people.

Loosening nuts put on by some car/tyre dealers, who obviously did not torque correctly, can be a trial, even though by law here, all must use calibrated torque wrenches!!!!

Yes and no. My mistake, in the third sentence my intention was to state "pneumatic impact hammers". Yes, there is quite a difference between these two tools.

Perhaps you may also have made a similar malfunction of the Ulna and Radial attached digits. "I have never owned or used a torque wrench for LH threads, but I have owned and used a great many for normal RH threads. LH ones are out there of course, but seldom." From all the more skilled and knowledgeable mechanics, engineers and professors I have learned so much from, catalogs, texts books, catalogs, seminars, etc. (the type that you probably also learned from) that I learned so much from (not a dig, its a complement to you and myself), I have never seen or handled a torque wrench that could not be used in both directions (RH threads- tightening in the CW direction and loosening in the CCW direction) (LH threads- tightening in the CCW direction, loosening in the CW direction). I have checked the 1/2 dozen or so of my torque wrenches in the various tool boxes, although I think I lent one or two to my brother, and each one can be used in either direction. To verify this I used each one with both RH and LH 1/2"-20 cap screws and appropriate nuts. The more advanced click types ones, with the ratchet mounted on the head (socket) end, required changing the direction with the knob/lever at the head in order to accommodate the change in direction but each worked on the test bolts and nuts in both directions. The old beam types also worked in both directions by looking at beam deflect in the opposite direction. I don't have an electronic type because I haven't been able to justify its purchase to the wife and my beer budget isn't large enough to accommodate purchasing one and also beer. Besides I haven't needed one, the old ones are good enough just like me.

Name is not new to me, as you see I simply used it incorrectly in my previous post. I learned about them when I was about 10 years old fixing my go-cart (this was over a half century to me). Since then I have graduated to bigger and better things but haven't forgotten my wrenches. Neither have I forgotten my handy impact hammers, both pneumatic and battery. Neither have I forgotten about a drip pan for a grid leak and hinges for a swinging choke.

I love my wrenches, figuratively, since they have assisted me in so many ways both avocationally and vocationally. Knowing them, how to use them properly and the knowledge about the things they were being used on has kept the paychecks coming during periods of drought in the chemical engineering profession. Of great importance to me are some words of advice my father gave to me- 1) take good care of your tools and they will take care of you; 2) Know a trade. For the 2nd I learned several trades. This wisdom was given to me by a guy who was a MIT magna cum laude graduate in EE. He never flaunted it but he sure enjoyed his tools and using them! No, I don't have that type of degree but I have his tools and many more of my own including torque wrenches.

If I make a simple mistake in the future please make a posting such as "instead of torque wrench do you mean impact wrench as you did above?" Sarcasm tends to pi_s me off and I don't like to be nor do I like to waste my time, your time and others time because someone chose not to help clarify a obvious mistake.

Also my keyboarding is nothing to brag about but it will be for the rest of my life. I prefer to work with my tools and not a keyboard. The tools help me relax from having to keyboard!

Finally, I have used the wrenches to loosen RH thread bolts in several ways. For example we were interested in what torque they loosened at to verify that none had worked loose (or looser) since last measured. Found several surprises that way.

Good Luck, Old Salt

Please change the words "torque wrenches" to "impact hammers" in the third paragraph. This error was caused by spending too much time keyboarding instead of having wrenches in my hands. Today the preferred tools are pipe wrenches, pipe cutters, reamers, tri-stand, the 36" and 48" pipe wrenches, the compound pipe wrench, pipe dies and of course the oiler and bucket.

Thank You, Old Salt

Because of the variable friction caused by the metal to metal contact of the bolt threads and the under-head friction, like Lyn said, look at bolt stretch as a positive way to tighten bolts. The sliding friction caused during the tightening can give artificially high torque readings.

By using bolt stretch, or in some cases "torque to turn" where you tighten the bolt a set amount of degrees after a certain torque, causes the bolts to stretch (strain) into the elastic yield range, but not into the plastic range (permanent deformation). This gives the joint the ability to hold torque when exposed to thermal expansion or stress induced dimensional conditions.

If it is not a critical joint, then make sure to use a liberal amount of lubrication on the treads and under the bolt head to limit the amount of actual torque is "wasted" during the tightening process.

In some cases, the bolts may get relaxed due to threads rectification after tightening. Any sort of problem with the thread may get relaxed after torqing. Gas turbine lock nuts are tightened and loosened several times to ensure the torque does not decrease after tightening due to relaxation.

Did you properly lubricate the threads before torquing them?

Without lube, torque numbers are always suspect.

what was the bolt pulling through ?

mild steel ?

repeat the test using hardened or high tensile steel plate that the bolt is pulling through.

many times i have found that a large high tensile bolt would squash a very small depression into mild steel

if you want to get really fussy , undo the bolt then examine the steel surface under a microscope or measure its thickness where the bolt was pulling.

cap screws are more prone to this due to the small contact area under the cap

I am sorry ! even very sorry since I wrote an "academic " answer and the system does not accept such answers and declared its impossibility to process the text. And this although I respected the rules as I did in the past many times.

If you send by the private channel your e-mail address I shall send you the text. I wanted to make it available to all but the system does not agree.

May be the "master" will convince the system that even "academic" answers are of general interest.

Best regards

Nick Name

Here are some comments from an associate that has solved many fastening issues in the automotive parts industry. You may approve or you may not but this is how he sees the original question.

About the torque:
Nothing strange about that. The coarse threads usually behave like that thus in aerospace the coarse threads are not used at all. In aerospace only fine threads are used. There are many explanations on the internet why the residual torque for the coarse thread fasteners always less than the initial torque (if any locking feature is not not used other way the residual torque could be higher).

I tried a second time but again the "system" did not want to process my answer although I did not change the form in comparison with previous answers.

I had text and a jpg picture that is all. Some think is wrong after last changes.