Calibrating Torque Wrenches

[ahramin]

I've seen Mike Busch and several other mechanics state that torque wrenches should be calibrated every year. I'm not an AME, just an owner, but mine haven't been calibrated since they were new 10 years ago. Pulled one out the other day and found that somehow it had been left in the case at its highest setting rather than backed off all the way so I decided to get them both calibrated.

Called Canuck Tire head office, who assured me the local store could send them out for calibration. Go to the local store, and they've never heard of such a thing. They ask the mechanics, who tell them they have never calibrated a single torque wrench in the shop ever. When I explained that head office said they could send it out, they gave me 2 new torque wrenches to get rid of me.

So I guess I'm good for torque wrenches for a while, but where do you people get these things calibrated every year and how much does it cost?

[iflyforpie]

Send it to Vancouver Calibration. Mine was $45 with a certificate.

WRT to settings... I've been told that it is just as bad to gronch it down to the lowest setting for storage. I always store mine with about a quarter turn off the lowest stop, so there is no tension but no binding either.

[ScottS]

Send it to Vancouver Calibration. Mine was $45 with a certificate.

WRT to settings... I've been told that it is just as bad to gronch it down to the lowest setting for storage. I always store mine with about a quarter turn off the lowest stop, so there is no tension but no binding either.

That is how I store mine too. Back it off, quarter to half turn on. I know they need to be calibrated, but I haven't had mine done. If I started putting together engine blocks, I certainly would.

[Skidoo62]

I have mine recalibrated once a year. Intercity industrial supply in Thunder Bay sends them out for me

[DanWEC]

After the words "Canadian tire" nothing should be expected to go right!
Do you guys have a Snap-on truck that visits? Usually they can grab it. Alternatively any industrial supply should be able to do it.

[ahramin]

The new ones came with instructions, including an address in Surrey for repair and calibration. The cases on the new ones were much flimsier, so I kept the old dirty cases.

Thanks everyone.

[Chris M]

FWIW, I've seen a couple Costco locations that have a gizmo in their auto service waiting room that displays the torque applied. Bring a wrench, set whatever torque, snap it in and see if it clicks at a number that matches what you set it for. I don't know if I'd trust it for my little 3/8" drive wrench, but for my 1/2" it does well. You won't get a certificate for it, but for peace of mind you could bring the wrench along while getting groceries and see if the two agree.

[dayot538]

I'm surprised that AMEs aren't required to certify their wrenches on a regular basis - it's been my experience in the oil patch that much less critical torques (out-of-the-way steam lines, that sort of thing) require a QC person - often an engineer - to witness the wrench being tested on a something-or-other-ometer before anyone is allowed to use it, and then afterwards that signature goes on a form with the serial number of the wrench, date, a pile of other signatures and a blueprint showing what was tightened. All that for a pipe in the middle of a forest!

[iflyforpie]

In an AMO, all measuring tools have to be calibrated and tracked. If you have personal measuring tools, they will have to be added to the calibration system if they are to be used on aircraft.

Depending on the QA system set up and individual customer requirements (like US Airlines with RII, Required Inspection Items.... a dual inspection for all critical tasks), they do require an inspector to witness torques and of course a billion signatures and stamps.

[azimuthaviation]

Calibrations have to be completed in accordance with manufacturers instructions, that includes toerances. Fluke doesnt specify a schedule, they say the closer the intervals are together, the more accurate the reading will be. Pretty basic stuff.


Snap-On makes a digital torque meter with a built in calibration feature. I know one AMO who has one and thats what he goes by, its in his AMO that was TC approved. Doesnt have to spend a penny on calibrations.

EDIT: Torque wrench, not torque meter

[azimuthaviation]

it's been my experience in the oil patch that much less critical torques (out-of-the-way steam lines, that sort of thing) require a QC person - often an engineer - to witness the wrench being tested on a something-or-other-ometer before anyone is allowed to use it, and then afterwards that signature goes on a form with the serial number of the wrench, date, a pile of other signatures and a blueprint showing what was tightened. All that for a pipe in the middle of a forest!

Calibration and metering are huge in the oil patch. You know what a 0.1% difference between the measured readings and actual output on a high pressure gas line amounts to? Probably millions of dollars. Plus the pressures these vessels are under, several hundred PSI. Can cause a very loud bang if something goes bad.

Also monitoring equipment must be constantly calibrated and tested, H2S, SO2, O2 CO2, LEL's. Lots of stuff out there can kill you in many ways, and happens every year in Canada.

[ruddersup?]

Is there adjustment on a torque wrench to calibrate it or do you throw it away?

[NeverBlue]

In an AMO, all measuring tools have to be calibrated and tracked. If you have personal measuring tools, they will have to be added to the calibration system if they are to be used on aircraft.

...and in general this is why most of the Avionics AMO's supply all crimpers, meters, and test equipment...so that there is control of calibration and it is not left up to the AMEs to supply and calibrate their own stuff.

...this is also why my tool costs as an AME-E over 3 decades have been pretty low...I would say less than $1000 over 30 years.

[AirFrame]

Those of you who have calibrated your torque wrenches... Do you get any indication when you get it back how far out of calibration it was, or how much adjustment was needed to bring it back into calibration?

I'm curious whether a lot of these calibrations consist of someone checking five or six torques and then putting it back in the box with a calibration certificate, with no adjustments needed...

[ahramin]

That would be my impression Airframe. The new ones I got in exchange had torque certificates showing low, mid, and max values tested and all were within 1%. Dunno what the limit is.

[DonutHole]

I believe the tolerance is set by the manufacturer. I bought a jet 1/4 and was hitched at... Though their tolerances are tighter than snap on (2% over 4%) the jet document requires calibration every six months... pain in the ass

[Skidoo62]

When my wrenches come back each one has a calibration form listing the different settings they were tested at and the actual torque at each of the settings. If an adjustment is made they add another column listing settings and actual torque after the adjustment.

[CID]

The primary point of calibration is to check accuracy. Adjustment is secondary. It doesn't really matter if it needed adjustment or not, what matters is that you have a measurement that is traceable to a standard within reasonable tolerances.

Any mechanic worth his/her salt will calibrate torque wrenches at the prescribed interval. And THAT is why I never take my car, much less my lawn mower to Canadian Tire.

[Alloy]

Hello all!

With click type torque wrenches, calibration is based on cycles; CDI requires calibration after 5,000 cycles. As far as I have seen, because I've never met an AME that counts clicks, an annual calibration (paid for and tracked by the AMO) is done to ensure accuracy is maintained.

I've got a list of Canadian CDI authorised repair and calibration centres, and would be happy to forward contact information for the closest cal center to you. Please send me an e-mail and I would be happy to helpl: ian@vmaerospace.com

[sportingrifle]

My snap on guy can check them in house but has to send them out for certification or adjustment. He said that so long as the haven't seen 10 000 clicks, been dropped, or stored cranked up, they should be still in calibration. So you can see, that for a small 1 man shop, annual calibration might be overkill. For a busy large shop with many people using the wrench, it wouldn't be.

Having said that i bought a used one on Ebay that was waaaay out.. By the time I bought it, repaired it, and certified it, I could have bought a new one. And here is a thought....If you have one of the old style beam wrenches, (the kind where you bend the beam against a fixed scale) how can it go out of calibration if nothing is bent? Wondering if we haven't outsmarted ourselves with digital click electronic wrenches. Also, all the calibration in the world if you don't know how to use it. I have seen several apprentices from fancy schools and one licensed AME not using them correctly.

Cheers Sportingrifle

[ahramin]

I'm pretty sure I know how to use a torque wrench - and Mike Bush's article on it certainly seems to cover everything - but why don't you post a little course in this thread just in case sportingrifle? What are the errors you see?

[sportingrifle]

First off, for anyone really interested in hardware assembly, this book is amazing: http://books.google.ca/books/about/Carr ... 1HmmRCN7YC
While written for race car types, it starts off with the theory of bolted joints and all of the issues are 100% applicable to aircraft applications.

To understand torquing fasteners, you first have to understand that the idea is actually to stretch the fastener a certain amount and pre-load it to a greater degree than that which it will experience in use. Torque is an approximation of this, but only about 1/3 rd of the applied torque actually loads the fastener. The rest is used to overcome the friction of the nut against the washer, the threads against each other, and to a lesser degree the friction inherent in self locking nuts. Several years ago I did some unscientific testing by torquing 1/2" AN fasteners to a specified bolt stretch, while lubricating the threads and bearing surfaces with moly. It turns out that approximately 1/3 of the torque is used to overcome the friction of the bearing surface, 1/3 to overcome the friction in the threads, and the remaining 1/3 to actually stretch the bolt. Almost all of the errors in torquing fasteners results in under-torqued fasteners. BTW, Lock-Tite threadlocker has minimal lubricating properties and seems to change the torque values by less than 2%. I called Lock-Tite and their tech rep confirmed this and stated that the products were intentionally designed to have a minimal effect on torque specs.

So the first issue is reusing hardware with galled plating surfaces. Torque specs for fasteners are developed with the idea that the specified friction reducing plating is in new condition. This is a bigger issue with larger or higher strength fasteners. The second issue is that the torque specs are developed for straight threads, not self locking nuts. The torque required to overcome the self locking feature should be added to specified torque. This is a fairly minor issue that is affects the smaller (AN3 and 4) fasteners more, but I bring it up because in the smaller fasteners it is more than 4% of the torque - ie greater than the calibration tolerance of the wrench. Luckily very few AN3 and 4bolts with self locking nuts are used in critical tension applications. The wing attach angles on the Harvard and DC-3 are the only two that come to mind. When doing these up, we test the toque of the self locking nut and add this to the torque value specified.

But the big issue is the "break away" torque required to get a fastener moving against friction.....it is significantly higher than the force required to keep the fastener moving. In the larger fasteners you can feel this as you start to tighten the fastener. So the correct way to torque a fastener is to start it moving and make sure it continues to slowly tighten without stopping, until the specified torque is reached. The problem comes when the application of torque is stopped just shy of the specified torque value - usually because the wrench has bumped up against an engine mount or some other obstacle. After re-positioning the wrench it goes "click" as it reaches the preset torque, but it has clicked against the break away friction torque, not the steady state fastener pre-load torque. The fastener is not sufficiently preloaded and prone to fatigue failure. The solution is to guess by experience how far the wrench will likely have to be swung, and organize it so that the final torque is reached with the wrench still turning.

Luckily, there are very few critically loaded fasteners in an aircraft, but piston engine hold down studs and crankcase through bolts are 2 examples. The catastrophic failure rate of engines that have had cylinders recently replaced is many times higher than engines that haven't, and under-torqued fasteners for all of the reasons mentioned above are thought to be a big part of the problem.

And one last funny story. Several years ago I was installing a propeller on a radial engined airplane - one that required 780 ft. lbs of torque on lubed threads! Now I am sure that there are torque wrenches out there are that big but I don't own one, and besides, a 4% calibration error is nearly +/- 30 ft/lbs.! The Hamilton Standard installation manual calls for a better way - you to hang off an appropriate length of bar (780 divided by your weight ). So after coming from my aviation medical examiner's TC certified scales, I was hanging 3'9" out on a bar when a TC Airworthiness guy walks up and asks what I was doing. I explained that I was torquing the prop nut and he went mental, quoting torque wrench calibration requirements and CARS, and all manner of other things. It was civilized until he said I looked like a monkey hanging from a tree, was obviously a hazard to aviation, and grounded the airplane. The next day I sent a very strongly worded letter to his boss along with the appropriate page from the Ham. Standard installation manual, and suggested that said fellow might be wearing the installation bar if he ever came into my hanger again. He subsequently transferred to Ottawa. The point is, the standards are important but an understanding of the issues and some practicalities are equally important.

Not familiar with the article you mentioned, where is it? Apologies if you already knew this stuff, you sound like you do but I posted for those that may not.

Cheers Sportingrifle.

[photofly]

Very interesting post; thanks for taking the time to write it.

That book isn't cheap!

[Pavese]

Very interesting post; thanks for taking the time to write it.

That book isn't cheap!

Amazon at CDN$ 308.50 and CDN$ 763.59 seem disconnected from reality........

$27.95 on Smith's web site, all 6 of the series for $99 http://www.carrollsmith.com/books/purchase.html

Carrol Smith's books are legendary in the motorsports world, this is one of a series well worth reading for the transferable knowledge.

From "Engineer to win": "Murphy was actually an early aviator. He could have easily been a racer, the two species are very similar in nature and outlook. The rest of Murphy's real laws are variations on the same theme, which leads us to Smith's second law of motor racing: THERE IS NO SUCH THING AS MATERIAL FAILURE-- ALL FAILURES ARE HUMAN IN ORIGIN."

My first passion was motor racing, it runs deeply and the parallels are striking.

D

[BeechjetYKZ]

My airline sends their tools for calibration to a company with the same name as those vertical orange things they put under the wings of aircraft while parked. They have locations in Toronto, Ottawa, and a few others. Google it.