Opinions on Lock Washer Varieties

How about a Jet-Nut? (Keps Nut)

If it's good enough for airplanes, it may be good enough for your application.

What sort of dynamic loading would the washer be subject to? How much travel would be expected? It could be helpful if you provide a machine type, or at least an industry.

I might suggest looking at disc springs. They're made of just about every material available in any size. You can predetermine the deflection, preload and fatigue rate better than most lock washers, though they work under the same principle. Here is a Youtube video covering spring stacks. And, would you be surprised my employer has a guide on them?

I don't care for split lock washers myself. I sold fasteners in a hardware store for a few years, and many customers found they still loosened under heavy vibration (like mowers) and also damaged sensitive parts. Something like a belleville washer doesn't have an abrasive edge.

Finally, check out GS's preload springs, spacers, and washers.You may also want to consider compression springs or wave springs.

Thanx Doorman. I've never used the jet nuts, primarily because I haven't had many situations where the one-time-use aspect of jet nuts is appropriate or practical.

As far as the industry, well it's the textile industry. There's not so much "typical" vibration... it's more of a jerking, pulsing type of movement, that can be even worse for certain fasteners. The loads vary considerably. But it is just a machine assembly, and not a building, so that narrows it somewhat. But also, as I said, we've experienced some issues with some very rough handling for voyages that are often across the world, and a few hundred miles by truck on dirt or unmaintained roads, by handlers that couldn't care less.

You might want to consider packaging for transport methods as well....

http://www.brentpackaging.com/faq.php

Thanks. That's a good link. I was just investigating that very thing, searching for crating/palleting options. That might help with at least part of the problem.

And by the way, I'm not trying to solve a problem with one particular washer or one particular application... I'm just asking for general facts or opinions of peoples experiences with different lock washers, through their careers. I've read studies and have used most every type out there. But there is nothing as valuable as experiences from a cross section of engineers and fabricators, to get a good idea of what might work best.

I use the bevel style lock washer. Find them as effective as most.

Have you ever looked at or tried Nord-lock washer?

If so your thoughts on those.

Nord-loks are great. They just have to be taken to a specified torque depending on the application.

from your moniker, I'm assuming you know a lot about bolts. However, I don't quite follow your logic. Lock washers are not intended to provide bearing surface, in fact a properly sized lock washer of the split or serrated variety is in fact slightly smaller than the bearing nut. If sized and compressed correctly, it increases resistance to loosening, often from vibration. An increased bearing surface is obtained by using a traditional washer (often called plain). We use a lot of them, and always use both, since they have different functions. Is this correct, or should I rethink.

For that many fasteners you might want to consider SEMS bolts or nuts with whatever washer you decide on.

Have you considered wave washers?

If you still want a washer locking system from my experience the integral washer head hex bolt and integral washer nut are very hard to beat. I understand a lot of this type is used on railways with great success. I have switched from lock washers on many applications to the integral system and never regretted my decision. At the time I used the Bowman system and I think they were the ones that developed it or perfected it. This system also distributes the clamping load over a larger area for enhanced clamping forces and lower damage to the parts under clamping stress. In my opinion lock washers are a major cause of loosening of bolt & nut fasteners.

GA

Many years ago when I still ran the maintenance department of the company I work for, since we only did a couple projects per year requiring a controls engineer, I was purchasing nuts/bolts/washers from a Rotanium salesman. When I asked for lock washers, which his company sold, he flat out told me they do not work and are a waste of money. The small amount of tension they impart are not enough to lock the bolt/nut in place. If truly locking the bolt/nut in place is required, he told me to go buy a thread locker adhesive, such as Locktite. Other wise, just torque to specs without a locking washer.

These comments are not necessarily my opinion, but are thrown out here for others opinions. (however - I haven't used one since) They also are in reference to the traditional split ring style, not the serrated edge washer. This style also functions under a similar method of imparting tension, so one would assume.....

GA - Here is a site that explains why lock washers should not be used in most applications. http://www.boltscience.com/pages/vibloose.htm

As boltintegrity has stated above, "The important issue is proper initial design. . .".

Adding a lock washer just because it has always been done, it is better to be safe than sorry, or it doesn't cost that much are poor reasons to add a lock washer.

Lock washers only provide some benefit to keep fasteners from turning in applications where the threaded fastener is in low tension as in a shear application. The way a split or star lock washer works is by digging into the metal to resist rotation in one direction (kind of a mini-ratchet). For this action to work, the surfaces of the bolt and the material being clamped must be soft enough to allow the washer edges to dig in.

In applications where the split lock washer flattens before the design tension is achieved, it does nothing to prevent the fastener from backing out as it only starts to resist rotation after the fastener has started to work loose; which means that the bolted connection has failed because it has dropped below the design tension.

Wow. Split washers are completely ineffective. That's quite a statement.

www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CJ8BEBYwAA&url=http%3A%2F%2Fwww.wclco.com%2Fpdf%2Flockwash%2Flw05-08.pdf&ei=Xlm-T76nE6e16AHgjo1G&usg=AFQjCNHQQrxijJEa8r4MqkntGYZUfngd4Q

They work really well. Try building a keeping a seat on a dirtbike without them,

or a hydraulic pump on a tractor.

so why do they result in superior rates of quality assurance when used, ie fewer loosened fasteners over time. (or lost, sometimes resulting in failure of component)

Not trying to be flippant, but have you actually ever built things with fasteners?

9?

I included a link. It was one of many. It includes a detailed report on the physical attributes that make many split ring washer applications desirable. I've never seen an opinion pole on this subject, but it would be interesting. When someone is not interested in user surveys, they prefer to live in a theoretical world. I like it there, too. But sometimes, it is not very useful.

I'm suggesting that you provide some information that will show me that my preconceptions, based on theory and experience, are wrong. BTW, I do understand that many fastener applications are under/over/not engineered. It must be true that many people use split washers incorrectly. I'm sure I have. Help a brother out.

Contact the major hardware manufacturers, they will in all probability tell you the truth that the spring washers or lock washers are the most common cause of fastener loosening. They make them and sell them because the customer asks for them. I have worked with many skilled persons that cannot place trust in a thread locking compound but insist in the lock washer. Sometimes I can get them to use both. That is human nature. The integral washer head and washer nut work very well in most all cases.

I provided two, one from NASA which does not allow their use. the other from bolt science showing the effects of vibration on a nut plus split lock vs nut alone showing that the nut was in fact superior. Serrated washers do serve some small amount of locking effect in soft materials, however they tear up the surfaces and can be a initiation site for both corrosion as well as cracking. Your link was from a manufacturer of lockwashers, not exactly an unbiased source of information. Nor does it supply any testing basis for it's assertions.

More links:

http://www.kimballmidwest.com/catalog/MarketingText/The%20Lock%20Washer%20By%20Guy%20Avellon.pdf

http://www.eng-tips.com/viewthread.cfm?qid=230741

Thanks for the links. They are pretty light on data as well.

If the joint was properly designed, with appropriate fasteners specified and used, you don't need lock washers, is that correct?

So when I find that the item I'm working on (it's real metal) is frequently loosened., I should re engineer and re build so that I can use a bigger faster. I would usually just use a lock washer, ny-loc nut or lock tite on the existing specified fastener, depending on location, time, service intervals, all designed (in my pea brained mind) to give me increased performance. I guess I could just weld it.

No argument about increased clamping force (or lack thereof). Once the split is compressed, it has the same clamping force as the washerless joint. It retains that clamping force for a longer interval though.

In this instance, it's not about clamp force, it's about longevity and survival intervals. That is why they are used, because they give you increased survival time of vibration induced loosening. If you want to make this about clamping force, and how if I used a bigger or better (ie more expensive) fastener, I would not need a split washer, because the increased clamping force would resist the induced loosening, I hear you. I agree. My Weedeater has a handle on with a 8mm bolt/nut. Guess I should toss the POC, cause it keeps coming off.

No, they don't! did you even look at the first link with the graph showing loss of clamping force with time under vibration? In fact they often contribute to problems because they may loosen but not be noticeable during PM and will continue to shake loose without being noticed until it fails catastrophically. Once the spring has begun to expand the joint has already failed! even the SAE does not recommend them for highly stressed bolts and hasn't since the late 60's when Junkers did his research to PROOVE that they don't work.

http://www.boltscience.com/pages/vibloose.htm

here is the key paragraph: "Work completed during the 1960's in Germany indicated that transversely applied alternating forces generate the most severe conditions for self loosening. The result of these studies led to the design of a testing machine which allowed quantitative information to be obtained on the locking performance of self locking fasteners. Such machines, often called Junkers machines (a video of such a machine can be seen - see the bottom of this article) in the literature - after it's inventor, have been used over the last twenty years by the major automotive and aerospace manufacturers to assess the performance of proprietary self locking fasteners. As a result, a rationalisation of the variety of locking devices used by such major companies has occurred. For example, conventional spring lock washers are no longer specified, because it has been shown that they actually aid self loosening rather than prevent it."

PFR - Be careful from where you get your references. Your link http://www.wclco.com/pdf/lockwash/lw05-08.pdf is from a company that sells lock washers and of course they want to present the positive side of using a lock washer. Even with that said, a careful read of the study says that the biggest advantage of adding the lock washer is the increase of bolt stretch.

As far as experience goes, I would give precedence to NASA's.

I believe the conflict between Rorschach and PFR may be partly due to a bit of miscommunication; what sounded like absoluteness in Rorschach's statement. I agree with him, mostly, about split ring lock washers. They are pieces of crap, quite often. They usually are not very effective and they often don't work at all.

But given the fact of the uncountable multitudes of situations and circumstances in which lock washers might be used, I would be hard pressed to say they NEVER work, and that they ALWAYS make things worse than if no lock washer was used at all.

There are just too many variables of material hardness, type of vibrations, design, environment, lube, torque, etc., to be absolute about any statement regarding split ring lock washers.

But as a rule, I don't like them. They don't work very well in many instances that I've experienced. There are in fact many studies that end with the recommendation that... they are not highly recommended, to one degree or other.

Hence... this thread.

There is some good stuff coming up here. Thanx to everyone for their opinions, and for the various links.

I guess the big question is the different effectiveness of those types that rely on friction or bite (ie, the ratcheting types) and the types that rely on tension (spring types).

It IS absolute. The testing proves it. Once the spring begins to expand the clamping force is lost and the joint has failed. That is the criteria, if the bolt begins to move, the joint has failed. full stop. The spring lock REQUIRES the bolt to begin to move before it does anything, so it is by definition NOT WORKING. A star washer may work somewhat in soft materials in which there are no issues of surface damage or cracking, but a helical spring lockwasher will NEVER prevent bolt loosening. it cannot. it REQUIRES it to loosen before it does anything.

Once the spring begins to expand, the clamping force is lost.

If that spring has a split washer, it takes longer to lose all of its clamping force.

If it does not, it takes less time to lose all of it's clamping force.

That is what you can get from a properly installed split washer.

It is not absolute.

Except in virtually all cases it ACCELERATES the loss of clamping force. Just look at the graph! the nut alone took a third longer to lose all of it's clamping force than the lockwasher did. The lockwasher IS the spring. it is a single turn compression spring.

Thank you, and Rorschack, too. I had forgotten how elemental a bolt in function is, and how many properties it shares with a spring. I don't engineer titanium for NASA. I do, like most on here, I suspect, frequently experience under built devices that need to be repaired, or are in the process of designing new ones, and have to weigh benefits versus cost. I believe reading these posts has potentially improved our products, and certainly my knowledge. Many of the articles had alternate or opposing views. It seems clear that the state of the art in bolt fastening does not include split washers in the vast majority of cases. But they work well in some applications. Correlating maximum load to fastener selection obviously is the key. If you do that correctly, you do not need a split washer.

Now if I could just get that through to the engineer on my Husqvarna.

Out of Box Experience

I have been following this thread for a while now and am surprised that such a problem has not found a unilateral solution. I mean, there was a man on the moon and we still haven't solved this?

The reason I am saying this is that I have, for a very specific application, resolved this problem once and for all. Now I find that my solution could be introduced in other fields as well, like arresting a nut, forever but still make it removable.

Is it not true that the friction of the thread should do most of the work and compensating for movement. The area of friction of the thread is a much larger area than just the washer, be it split or not.

I will do some more research and see if what I am suggesting has not been done before. My solution would require one more machining process to the bolt but if that can solve the problem of vibration and loosening nuts why not do it.

Interesting, Ky.

For your machinery, don't rely on lock washers of any kind. Use bolt stretch. If the bolt length is short, make it longer by adding a spacer. Here is a good link on designing bolted connections: http://www.fastenal.com/content/feds/pdf/Article%20-%20Bolted%20Joint%20Design.pdf

When designing a bolted connection, think of the bolt as a spring. A proper connection tensions the spring to a value that holds the connection together. Too high a tension results in a failed bolt and too low a tension results in a failed connection.

Tension can change due to thermal expansion if the bolt is made of a different material that than material being clamped.

Tension can also change due to erosion of the bolt face or materials being clamped.

Establishing the proper tension in the bolt requires taking in these and other parameters.

Making the bolt longer by adding a spacer facilitates a longer spring which is easier to set and maintain at the proper tension.

GA to Rorschach, BoltIntegrity, Phys

Great thread, very informative.

thank you all

In my experience I've found lockwashers to be somewhat worthless, especially for applications where a secure clamping force is required. My company manufactures gearmotors and we have found that an integral serrated hex washer head screw works best, as far as retaining a screw is concerned. I believe they're made in many sizes of stainless steel, so galvanic corrosion would also be alleviated.

Excellent observation and in my opinion dead on, Integral head washers and nuts have been used on many applications from automotive to rail vehicles with great success for at least 40 years that I am aware of. More engineers should be aware of the quality of this design in their vibration applications. Heck use them on everything put the mechanic out of work.

For the facts (not opinions) on fasteners and locking methods, as determined by NASA,reference my link (comment #13). Pages 10/14.It covers the good, bad, and ugly for various environments and conditions for most all commons locking fastener methods:washers,chemical,thread design, etc.

Thank you HiTek. Yes I've seen that NASA report before. I've got that info. But what I'm asking for is actual opinions from the several centuries of real world experience that we have here on CR4. And THAT is a horse of a different color!

Okay, so it seems I'm getting quite a number of recommendations for screws with integral washers. Very interesting. I must confess, I don't have a lot of experience with utilizing them. I guess it's something I should look into, particularly total cost comparisons and ready availability; as well as any additional opinions about them. I'm wondering if there are different types or styles.

Has anyone else had opportunities to use these fasteners?

Thanx folks. I appreciate it.

On my Peugeots I find a variety of locking techniques used:

Internal serations, external serrations, both internal and external serations (on the same washer).

Nyloc nuts

Where bolts absolutely Must not be able to self loosen they use a thin metal plate under the bolt head, or nut with fold up tabs.

The head bolts, however use only a flat washer - note that these are screwing into a cast Iron block and clamping down an alloy head ....... Note that this is a general rule, spring, star, serrated washers must NOT be used against aluminium alloys.... flat washers only because with vibration the star, serated and spring washers will wear into the surface and release the tension.

I haven't seen any conventional spring washers on them, now I know why!

Thank you

In my experience, I have not found any automobile or truck manufacturer using lock washers on any component that must be kept tight. I have seen split lock washers used on brackets holding accessories such as water pumps, or air conditioning brackets in place, but these are also disappearing in the last 20 years.

One of the applications that has constant shock loading, and is repeatedly fastened, and then un fastened over the course of many years are the wheel fastening system on vehicles. No one uses lock washers. What we do see is an increased surface area between nut, and wheel.

I do not believe in coincidence, Something drove ALL of the major manufacturers to come to the same decision.

Hey... that's an excellent point. No locking washers of any kind on wheel lugs! I've never consciously noticed that before. And I'd say it's pretty dang important that those nuts don't come off!

The reason there are no lock washers (or serrations, safety wire, locktite, etc) on wheel fasteners is because they have been properly engineered with very wide safety margins so that they don't need them. There is sufficient clamping force from the large studs, large nuts, large bearing surfaces, large torque, correct bolt stretch (thanks CR4 members) to prevent loosening.

Isn't the question the OP has relative to dealing with normal assembly practices for typical applications where he did not design the fastener length, size, material, torque spec, etc? He is taking the role that should have been designated earlier in the design process (leaving this selection to the fabricator might not be wise) The fact is, almost any fastening requirement, with minimal failure rates, can be met with a big enough bolt, can't it? Is that the most cost effective way to do it? not usually, in my opinion. It's simply overkill.

Well actually, I AM designing the machines. Or rather... re-designing someone else's design to alleviate the loosening fastener problem we seem to be having during prolonged and rough shipping, and to a lesser degree, during operation.

As far as "a big enough bolt"... well, no. I'm not having a bolt failure. I'm having loosening issues, and would like to stop the practice of using split ring lock washers within my company; among other modifications, like proper assembly practices by our assembly crew. But the point of my original question is that I'd like to replace split rings with something else. I'm trying to find out what is the best option. I'd like to have my ducks in a row when I present the solution to the rest of the company. It won't be easy to completely eliminate split rings. Most people are not willing to accept their inherently faulty design, since they have been in use "forever".

Are the fasteners being tightened sufficiently? Is the hardware made of SS? If the threaded half is not SS, I would think about using flanged lock nuts. I have been switching to Grade 8 flanged bolts and nuts where I can, on my limited use. Again, JMHO. Good luck.

Everything is SS of one sort or other. There are very few threaded connections... by that I mean, they are nearly all nut/bolt combinations... not threaded holes. As far as are they being tightened properly... we have a typical bunch of regular blue collar guys that work in the assembly area of the company. Welders, fabricators, assemblers, etc. I can assume they are doing the best they can, but their skill levels are all over the place. There are several hundred nuts and bolts that get assembled during the course of a month or two long build. All the guys are lending a helping hand in one place or other. Sometimes some of the engineers and designers get their hands dirty as well. The human/training issue is one matter. I'm trying to deal with the mechanical aspect best as I can, so that we have the best chance of getting quality connections on the finished product, despite the human factor. So step one is to find a substitute that I can present, to replace the split rings.

Can you do a side by side test of the current fastener system and the flanged nut-bolt design? Then bolt together something and attach to say, a forklift that has no suspension, and let it bang around for a while and watch the results. Better in your shop, than on the other side of an ocean.

Write a work instruction and process control card for each operation, specify in detail, with illustrations, the parts to be used, tightening torque, tools to use, etc, etc.

Leave nothing to chance, or to the choice or discretion of the operator, spell it out in one syllable language so there can be no misunderstanding or interpretation, have each operation signed off as completed.

The work instructions should be laminated, the process control cards should travel with the assembly until it is serial numbered and packaged for shipping, then be retained in your records - traceability.

Ensure all the tools are in good condition and where necessary (eg torgue wrenches) calibrated regularly.

There will be initial pain in the manufacturing process but long term improvements with fewer customer complaints, returns, field failures, etc.

Sounds a bit like overkill to me. I would more favor education and training. If the workers understand what is happening, they should adopt the new procedures. Of course they need to be part of developing the procedures, not just dictated to. There is plenty of intelligence on the shop floor--maybe not the education we have, but the intelligence and experience.

After assembly, assign individuals to perform QA. Their job would be to properly re-torque each fastener then apply a torque strip across the end of the bolt and the nut.

Proper re-torque would require: (1) loosen the bolt, (2) re-tighten to proper torque, (3) apply torque stripe. Then move on to the next bolt.

A second person is required to do the re-torquing. It cannot be the same person that did the assembly. The problem with relying on the same person is that they believe that they did it right the first time and they do not see the reason to put the wrench on the fastener the second time. They would just apply the torque stripe or check the box on the paperwork. A lazy person would skip the step just because he is lazy. A highly motivated individual might skip the step sincerely believing that they are doing a good thing for the company by saving labor.

Doing assembly work it is very easy to miss a step because you are bored, thus the the work instruction and process control card. It is not a perfect system, but it does help, and yes the workers do need to be trained, not only how to do a job, but also the reasons for the procedures.

I also work in a small company and we have implemented these procedures because we have male and female technicians, engineers and managers with varying skill levels and experience doing assembly work. Also when someone is interrupted part way through an assembly it is often difficult to remember exactly where they were up to, the process control card when properly used helps them to remember because it records the last completed step.

You forgot the most important aspect : the geometry! The majority have a conical surface so that the friction is increased by the 1/cos effect!

To give an example a bolt with a thread M12x1.25 with a conical geometry requires a torque of 206 Nm and about 3/4 of it is located at the head(conical) contact, the same bolt with a flat head brings same preload with only 128 Nm but only 40% is located at the head level. The flat solution was used for aluminium wheels and after a time abandonned for the conical but with stronger alloys.

But in some cases it for different reasons impossible to design the fastener as it should be so that some device is needed to maintain its initial position.

If you look at many solutions you will notice one common aspect : if the device uses the clamping force it fails (split washers are the best example) it works properly only if the position is maintained by a torque independent of the clamping force. Those are the deformed threads and other solutions making usage of a local elasto-plastic deformation independent of the clamping force.

One of the most efficient solutions was developed by a company : NORD LOCK.

The comparison of locking efficiency is now standardized based on a machine developed about 30 years ago in Germany (called the Junker test) and which generates a transversal load on the assembly since this is the ONLY disturbance leading to a clamping loss. The results presented as well by Boltscience shaw very well the differences for an M8x25 (clamping length) the preload was fully lost after about 6 seconds. The Nord-Lock presents a slight loss at the test start (about 5 %) and stays at same level.

I have used these in the field with good results but at a steel mill on the run out tables they were only slightly better than no washers but a light duty thread locking compound worked well. Extreme vibration on the rollers as they tried to grip the billets that were at the coolest a cherry red and only 6 inches away from the pillow blocks we were trying to keep fastened. I would say an extreme condition.

As you may notice I insisted that such solutions are good in the case that the bolt itself cannot be designed to work properly.

I did not mention an important detail. If the parts are secure against sliding (even very small) even short bolts can be a possible solution. The main problem in getting loose are the sliding minute movements since those decrease in a tremendous way the thread friction and due to the preload the bold slides along the thread in un - tightening direction, preload decreases and thus amplitude of sliding increases the phenomenon is unstable and the bolt gets loose.

No threads in the machine's holes. Does that mean that all the bolts are passing through clearance holes only and all are nut & bolt configurations? Could it be that the shifting weight of the heavy machine components exceed the clamping forces?

I am wondering if the various personnel who are assembling the machines use calibrated torque wrenches and know how to use them. Do torque specifications exist for each of the 500 fasteners?

I've been on vacation for the past 1.5 weeks and have thus missed all of the recent fun. Seems like it's time to get back into the fray...

With all of the continuing emphasis on "applying the proper torque" it's no wonder that problems of loosening are being experienced. Come on people, wake up; it isn't torque that you should be concerned about but rather bolt stress!

What good is applying the proper torque if that application doesn't result in the correct bolt stress. Answer: * None.

The stress within a bolt as a result of preloading is that which develops clamp load. Clamp load must be in excess of both the static and the dynamic loads which will be seen by the joint in operation. If the clamp load is less than either of the two, the joint may leak, the fastener may break in fatigue and, the nut may shake itself loose.

Throwing a lock washer onto the joint is a lame attempt to address the very likely possibility that the joint won't be tightened properly.

* "Torque" is simply a measurement of the resistance felt when tightening a fastener. Dozens of variables will conspire against the misguided false assumption that a certain torque will always result in a certain bolt stress. A consistent torque input applied to every fastener on a joint may result in some of the fasteners being too loose while others are too tight.

Stop thinking in terms of torque and begin to think in terms of bolt stress and many of your problems will go away.

Torque, stress, stretch all important but if the fastener is not designed properly then the end will result in the same. Torque or stretch a fastener with a spring lock washer and watch the (lock washer) spread out, now where is your torque or stretch in relation to the procedure? A quality bolt with same quality flat washers and thread locking compound is light years ahead of spring washer. The spring lock washer is nothing more than tradition. Useless piece of junk most of the time. Flange head bold and washer properly designed is as bullet proof as you will find add some thread locking compound if needed and apply proper tightening procedure and you will be hailed as ( the Man ) . Sorry if I appear to the trying to (teach a muskrat algebra ) only two possible results--you will waste your time and pee off the muskrat.

Proper fastener design would include torque, stress, stretch. Your first sentence is nonsensical.

When you watch a lock fastener spread out, you are watching a fastener with improper design (inadequate clamping force) extend the interval between the beginning and the end of life serving the fastened load. If you want to totally discard the split washer function, you will still achieve superior fastening power by extending the bolt stretch potential. Yes, we all agree here that a bigger, more robust design would be superior. But, don't be a muskrat.

PFR, the problem is that there are so many different factors that go into the torque required to generate the proper bolt tension that it is a very unreliable indicator of bolt stress. You can have two identical (within manufacturing tolerances) bolts side by side in the assembly assembled with the same torque and they can have vastly different stresses, particularly if some kind of thread lubricant is involved (and that may just be the grease on the assembler's hands). Torque wrenches also tend to have a accuracy of +/- 20% and that is when they are used correctly which they usually are not. there are specially designed fasteners to indicate bolt stress directly that are used in critical applications.