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[Davinhci]

Originally Posted by rldzhao

also be sure to get a low-profile jack with a flat lifting platform. those usually cost around $100. don't get one of those cheap jacks. (nono to second picture)

google a video for wheel change DIY before you do it. it's not hard but one could easily make mistakes as well.

damn my jack looks like the 2nd one, chit

[lucid]

Originally Posted by foosh

Are you only using it for one car, or one application? I've never owned one that "clicks" when loosening.

If you had changed the torque setting (lower) for a different application, you should need to adjust it for loosening if it "clicks" in both directions. If it were 100% accurate, and you hadn't touched it since you had torqued something, you should need to increase the setting to loosen a bolt.

Yep, just on my M3 for swapping wheels. I set it to 88 ft-lbs a year ago and never touched it again. And I'm pretty sure it clicks when loosening the bolts. I'll double check to make sure. My take is that I am probably tightening just a bit over 88 ft-lbs (the click is just a warning--probably actuated by an adjustable spring; you can apply as much torque as you want past it), and that's why it clicks when loosening.

[dawgdog]

I thought I needed to use a breaker bar to loosen the lug nuts, but my mechanic at Group II Motorsports said to just use the torque wrench. I assume he would know what he is talking about.

[foosh]

Here's good reference, which doesn't say explicitly you should not use it to break bolts free, but it does say very clearly you should only use it when you need to make sure you're applying correct torque in tightening. It appears that every time you use a torque wrench, you're diminishing it's accuracy little by little, so it suggests using a regular wrench when you're not trying to assure a certain value, e.g. breaking loose a bolt.

It also says you should set it to zero after every use, because you're stressing the spring, and messing up it's accuracy. Keeping it set at a certain value continually stresses the spring, and it develops a memory which affects accuracy.

http://ecmweb.com/mag/electric_torqu...ches_critical/

Searching Google under "torque wrench calibration" yields a number of good articles. One suggests quality shops, especially engine-build shops, have them recalibrated every 30 days.

[lucid]

Originally Posted by foosh

Here's good reference, which doesn't say explicitly you should not use it to break bolts free, but it does say very clearly you should only use it when you need to make sure you're applying correct torque in tightening. It appears that every time you use a torque wrench, you're diminishing it's accuracy little by little, so it suggests using a regular wrench when you're not trying to assure a certain value, e.g. breaking loose a bolt.

It also says you should set it to zero after every use, because you're stressing the spring, and messing up it's accuracy. Keeping it set at a certain value continually stresses the spring, and it develops a memory which affects accuracy.

http://ecmweb.com/mag/electric_torqu...ches_critical/

Searching Google under "torque wrench calibration" yields a number of good articles. One suggests quality shops, especially engine-build shops, have them recalibrated every 30 days.

That really is a matter of the quality of the spring and matching the right spring to the application. What you are referencing is simply the difference between elastic vs. plastic deformation. If a spring is to spec and constructed with proper materials, it should not deform plastically under load and its deflection vs compression behaviour should be consistent. Meaning, when you unload it should return to its original natural length (elastic deformation), so there should be no residual strain. Think about the springs in your suspension. You car does not go lower and lower with time. However, if you were to load 4000lbs onto your car somehow, they would most likely deform plastically and be screwed up. Same should apply to the torque wrench. If you don't use it to loosen a really overtorqued bolt (outside of the wrench's operational range), and if it is built with the right spring, it should not be off.

[foosh]

No reason to debate it any further. I read about a half-dozen seemingly reputable articles this morning, and one thing they all agree on is the need to release the pressure on the "click type" mechanism. That means cranking it down to near zero after use. There seems to be no debate about that, and even the manufacturers of wrenches instruct one to do so. Virtually all of the articles contradict what you say above.

I will concede that there doesn't seem to be a clear reason not to use it in reverse, except for saving wear and tear, and thus preserving calibration longer. I learned something new today.

P.S. And actually yes, over time car springs do sag and go lower. I've had to replace sagging springs on the 25+ year old muscle cars I've owned.

[lucid]

I will believe it when I hear a credible explanation, and all there seems to be on this topic is opinion, so I stand behind my questioning. A high quality spring should not deform plastically over time as long as it is operated within its specificied operational range. If it is used beyond that, it would need to be calibrated. There can be other reasons for calibration such as temperature changes and installation (positioning), etc. I would be dissappointed if my M3 "sagged" in 10 years, and would demand a spring replacement since that would mean that they used poor quality or wrong spec springs. I doubt that will happen though.

[foosh]

All you have to do is read the available technical articles on the topic. I can't read them for you.

[lucid]

Random webpages are not credible sources of technical information. Modulus of elasticity of steel does not change with time. Nor will it experience residual strain as long as the yield stress of the material has not been exceeded due to improper loading/part design. Check out the notion of a stress-strain curve in any strength of materials book. However, there is the notion of "creep" in materials. Steel can experience creep, especially at high temps over a really long period of time, but I don't see how this can affect a spring in a torque wrench significantly. If the explanation is that creep is indeed a factor for a spring in a torque wrench, I will learn something new today as well.

[foosh]

Oh good heavens, I suppose you don't believe stuff in the BMW owner's manual either.

If I read, in a torque wrench owner's manual, I should dial it back to zero after use, I think I'll continue to do it. If I read in a manual on a manufacturer's web-site that calibration is recommended every so often, I'm gonna believe that. I gotta think there are a bazillion calibration articles and shops for a reason. You can do whatever you want.

[lucid]

Questioning is a good thing. There is no evidence here that using a torque wrench to loosen bolts as long as they are not torqued more than the operational range of the wrench results in calibration issues (unless you have a poor quality wrench, but if you do, it will also go out of whack when you use it to measure the torque after tigthening anyway). There is no credible technical explanation for that that has been provided that I am aware of. You gave advice in that regard, and I questioned your advice, and that's appropiate.

Dialing it back to zero is another issue. I questioned that as well and then offered one possible technical explanation for it. If that is the case, great, at least I'd know.

[foosh]

Yes, read post 28.

I conceded I could find no evidence supporting the commonly-held belief that it shouldn't be used to loosen a fastener, other than a slight extra amount of wear and tear from use. Since there's no logical reason to know the torque to break something loose, that is the only reason for not using it for that purpose.

There's plenty of credible evidence that the more they are used, the more often they need calibration. There's plenty of credible evidence from tool manufacturers on the need for calibration, and the need to relieve pressure on the spring to maintain calibration.

[lucid]

Originally Posted by foosh

There's plenty of credible evidence that the more they are used, the more often they need calibration.

I didn't see evidence/explanation for this being a significant factor. Manufacturers must be conservative in their suggestions because of liability reasons. They must assume that the user will use the tool to torque or untorque things beyond the specified operational range of the tool (and hence throw off the calibration). As I said, temperature changes can be an issue. That might be why they recommend calibration with use.

Foosh, this is all cool--at least from my perspective. I am just wary of myths, and try to use what I know to probe things. Nothing personal.

[foosh]

Lucid,

I'm not taking it personally, and I find it fascinating. I've wasted a lot of time on this today for that very reason. I'm in the aerospace safety biz, and I've been hearing about problems with torque specs, operator, and tool error for years. There is little room for error as result of improper assembly or maintenance of an air- or spacecraft. Improper torque is a frequently-cited culprit.

Here's an excerpt from an article I just pulled describing Boeing's quality assurance program on torque specs. It's rather interesting, and it not just the cheap tools that are culprits. The military also has very strict recalibration stds.


Calibration Conundrum

Boeing's SDC is the world's largest satellite manufacturer, with more than 200 commercial communication satellite launches since 1963, in addition to military and weather satellites and spacecraft. The SDC's million-square-foot manufacturing facility has more than a dozen satellites under construction at any given time, primarily its 601 and 702 series of body-stabilized communications models, as well as two lines of mobile communications satellites. Pre-launch testing includes subjecting them to spin rates of 30 to 100 RPM, as well as vibrations and sound similar to what they would experience during launch (up to 50,000 pounds of force and 165 decibels).

To test their ability to withstand the rigors of space, they are placed in a thermal vacuum chamber and subjected to temperatures of minus-320 degrees to plus-250 degrees Fahrenheit while powered up and with instruments operating.

The facility has received both the Software Engineering Institute's Capability Maturity Model Integration (CMMI) Level 5 (Optimizing) certification and the Aerospace Standards (AS) 9100 Certification. AS9100 consists of the ISO 9001:2000 standard with additional aerospace specific requirements. Part of the SDC's quality control program, for example, is complying with ANSI/NCSL Z540-1-1994 calibration standards.

“To meet those requirements, a torque verification system has to include maintaining the history of each torque and the calibration data,” says senior calibration engineer William K. Jinbo. “A quality control person would actually monitor the whole process of the bolt torquing from the beginning of obtaining the wrench to the very end after the torquing is done. Then they would have to maintain the data and history of the torque.”

It also meant that each torque tool needed to be sent outside for calibration, an expensive proposition considering that SDC has 3,500 torque tools.

“I had a task to improve the quality of the torque tools that were causing the over-torque,” says Pham. “To achieve this, we first had to conduct a study of root causes using the standard Six Sigma tools. This entailed looking at the people, systems, processes and methods as well as the torque tools themselves.”

One thing that Pham and his team looked at was whether the gages were properly calibrated or were the culprit. They conducted a study of repeatability and reproducibility and found that the tools were out of calibration, some by as much as a factor of 1.5 or two. This was despite the tools being sent to a third party for calibration.

But even though they were within tolerance, they were still subject to operator error. The tools were spring-loaded and different operators would pull it different amounts after hearing the click. The solution that was in place to rectify this issue was to have an inspector standing there also listening for the click to ensure the component was not under- or over-torqued. But even this wasn't consistent.

[foosh]

Originally Posted by lucid

I didn't see evidence/explanation for this being a significant factor

Here's more evidence:

http://www.jlwinstruments.com/torque-tips.htm

[lucid]

Foosh, thanks for the info. I read over what you referenced, but still don't really see an explanation as to why exactly a wrench's calibration would drift off with use unless in the absence of misuse. I am not saying it doesn't ever need calibration. I am trying to understand why. My suspicion is that the real reason is actually operator error or poor design (in the case of a low quality wrench) as opposed to some kind of natural governing law about how steel springs behave--barring the potential creep issue I mentioned which can be dealt with by setting it back to 0 as you say (assuming the wrenches we are using use steel springs to provide the warning). What you referenced kind of supports this view actually. Once these things are built and end up in the hands of users, they will do all sorts of things with them that they are not designed to do, such as dropping them and using them to torque things that are way beyond the specs of the wrench. The stress-strain theory explains why that would throw the calibration off. It also suggests that as long as the yield stress of the material is not exceeded the spring behavior should be consistent as the deformation remains in the elastic domain.

So, bringing this back to topic, I suspect that if I have a torque wrench that is rated at 150 ft-lbs, and if all I do with that wrench is to torque and untorque bolts at 88 ft-lbs, never drop it or use it as a hammer (and let's say always dial to 0 after each use), I still don't see why it would lose its calibration over time if it indeed is employing the right spring for the application (one that won't deform plastically at less than 150 ft-lbs of torque).

[foosh]

There is no disagreement among the dozens of write-ups on torque wrench calibration I've read today, that the more it is used, the more the calibration will drift off over time, even if used only within its spec. The National Institute of Standards (NIST) has a certification program that provides guidance on this. I haven't seen a single dissenting article on the principle that more "wrench-cycles" will require more frequent re-calibration and refurbishment, just like a 100K mile engine will be more worn than a 20K mile engine. It is axiomatic. So if you use the same wrench for loosening and tightening, you're putting 2X cycles on it, hence the rationale for using a cheap breaker bar, which can't go out of spec, unless you break it in half.

You're right, that a rarely used tool, that is never dropped (a single drop and all bets on accuracy are off), never exposed to climatological extremes, or never used beyond limits, theoretically will not need calibration very often. However, aerospace applications require yearly NIST-certified calibration.

The last link I posted from a calibration service that is NIST-certified, stated that some torque wrenches are not designed to be used "bi-directional," while some are, and that if you have one that is not, some manufacturers will void the warranty if it has been used to break bolts loose. Granted, this is the area where there is less evidence.

In your specific case, one good reason why your infrequently-used wrench may be out of spec is that you said you set it a 88lbs when you purchased it, and have never changed it. There is also universal agreement that a click-type torque wrench should be stored with the spring unloaded (e.g. near zero).

OK, I think we've now officially beaten this one to death.

[lucid]

Originally Posted by foosh

There is no disagreement among the dozens of write-ups on torque wrench calibration I've read today, that the more it is used, the more the calibration will drift off over time. The National Institute of Standards (NIST) has a certification program that provides guidance on this. I haven't seen a single dissenting article on the principle that more "wrench-cycles" will require more frequent re-calibration and refurbishment, just like a 100K mile engine will be more worn than a 20K mile engine. It is axiomatic.

An internal combustion engine is not really an appropiate anology for a spring that is being loaded and unloaded. One has to think about where exactly the wear and tear is. In the engine there are all sorts of tolerances that are changing due to extreme use/forces/wear. If the explanation is that there is wear in the interface between the spring and whatever moving part that is actuating the spring, then I must say you are either considering a tool that is seeing very high use or are in an environment that requires very high precision (like the aerospace industry). None of those seem to apply to a torque wrench that I would own and use personally. My argument has been that the spring itself cannot be the source of the calibration error unless it has been mis-selected for the application or abused. If you are saying the wear and tear associated with the moving parts that are actuating the spring are the source of the error, then at least we have an explanation, which is what I have been after all along by asking "why?" I will check the calibration on my wrench and report back. It should be straight forward to check, but the problem is I don't know how well calibrated it was when I first got it, so the more meaningful test would be to check its calibration over time with my current usage pattern to see if it goes off.

[foosh]

It's not what I'm saying, it's what experts on this subject say. I'm done.

[lucid]

I think the calibration issues that might potentially be associated with the wear and tear of the actuating parts of a mechanical torque wrench someone like me would own to swap wheels or do other minor stuff in the house or on the car is negligible (as long as it is not abused).

I just checked the calibration on my wrench. I applied approximately 45.4 ft-lbs of torque to it. It did not click when I set it to 45 ft-lbs. It clicked when I set it to 44 ft-lbs. No need for concern. I've been using this wrench to bolt and unbolt wheels for over a year and I've set the adjustment to 88 ft-lbs once when I got it and left it there with the exception of this little test. I swap out wheels and pads several times a month on average due to frequent track days.