calibrating torque wrench?
 

I have an old Craftsman 1/2" scale and pointer type torque wrench - how is this calibrated? Can it be checked by putting the square drive in a vise and hanging weights from the handle to check foot pounds?

The pointer ones don't go out of calibration AFIK.
-Chris

I believe Chris is correct but your plan should work to convince yourself of the same.

FWIW, a gallon of water weights 8.34 pounds.

defcon65,
As I recall, from sending out three torque gauges yearly (two dial ounce units and a "beam" pointer pounder) for FAA work, the accuracy is not linear and you can have gauge that is in spec CW but not CCW, and as I recall accuracy is percentage of scale. The problem of hanging something is whether you are hanging it directly off the "post" perse or hanging it on an extension off the post (such as when you use a socket vs. a crow foot you will get two different readings).

You could simply try comparing your beam unit against other units from friends for a quick check, however sending it off to NIST-traceable certificate house is the best method to check (if you really need to; prices vary).

Here are some interesting sites:

http://www.belknaptools.com/
http://home.jtan.com/~joe/KIAT/kiat_3.htm
http://www.teamtorque.com/main.html

The instructions that came with my $100 beam style wrench said that it would still be accuracte if you ran it over with a truck as long as you bent it back so the pointer pointed at "0".
What a ripoff that wrench was, accurate to something like 2% but you can only read the dial to something like 5%. Sigh, live and learn.
-Chris

JEEEZ!!!

Let's try this again in metric!

1l of water weighs 1kg. 1kg on earth will experience 10.2 N of force, straight down. :D

Sometimes I am surprised how productive this country can be, with these non SI units, conversions and lumber that's a 2x4 but not really. :D

SmileWavy

I forgot to mention. You can also connect your wrench to another one, for example using an allen head socket and a regular socket of the same size. That way you can check wrenches against one another. It always gives me a warm fuzzy feeling if my small and large torque wrenches read the same in the overlapping ranges.

George

One last thought on torque, however before I go further, did I ever tell you about the OEM who designed a product for a commercial plane (you have probably flown on this plane before as there are about 2,000 of them buzzing around every minute here in the US, however we wont' mention their name). Anyway the one of the OEM's checks on this product was to use 0-100 foot pound torque wrench to check a specific torque very close to the high end of its scale. They also to use the same wrench to check another function that is between 0-100 inch pounds.

When selecting a torque tool, and unfortunately many wrencher's don't have a complete selection of these tools in their box to cover the various ranges, you want to use a wrench who's mid range falls as close to the value you need to use. Say you are torque'n to 20 ft lbs. Utlimately you would want a tool with a scale from 0-40 ft lbs. or a more commonly found 0-50 ft. lbs. rather than using the 0-100 ft. lbs. Common sense but sometimes we forget. $0.02


Fun Questions:

What does a gallon of "fresh" water weigh at what given temperature, altititude and atmosphere ?

What is the true reading of your torque wrench after leaving it to cook along side your tire swap for one hour on a sunny 100F Texas day?

I think I heard it's also better to "warm up" a wrench by using it several times and then actually going back to torque up the critical fasteners. Is this correct advice?

"I think I heard it's also better to "warm up" a wrench by using it several times and then actually going back to torque up the critical fasteners. Is this correct advice?"

Maybe on a click-type. I hadn't heard that one. On a beam-type torque wrench, the only moving part is the pointer, and that's a bending motion. Oh, and there's the pivoting handle.

Sherwood

Sorry Sherwood, but the "pointer" is staiying put, while the wrench handle moves under the force applied.

I never heard about warming up your torque wrench. There isn't many critical fasteners that are torqued in one step. So, there goes your "warm up". It's built into the procedure. ;)

George

"Sorry Sherwood, but the "pointer" is staiying put, while the wrench handle moves under the force applied. "

George,
Thanks for the correction. I owe that to my dyslexic motor skills. :)

Sherwood

Well, that's a lot of food for thought concerning my $5 garage sale torque wrench! Incidentally, '2x4' refers to the size of rough-sawn lumber. It is then planed 3/16" on all four sides to approximately 1-5/8" x 3-5/8" dimension.

Before it gets too deep remember that torque wrenches are only for "semi-critical" fasteners. A really critical fastener will need some other way to set the bolt tension. The correlation btween friction (the torque) and bolt tension is just not that tight.... think about a scatterplot.

You can measure bolt stretch directly with a stretch-gauge. This is how you tighten rod bolts correctly if the crank shaft is out of the engine case. Unfortunately, is is more or less not practical to measure fastener stretch of a stud since you can't get to both ends of the fastener.

yeh i cheated and got caught. I used 'bolt' to mean bolts, cap screws, studs and other long tensioned fasteners...