Torque wrench testing

I have a Britool torque wrench I bought new many years ago when rebuilding motorbike engines etc.

Apart from having it professionally checked for accuracy can something be rigged up at home to test it.

Paul

Do make sure that the pointer on the torque wrench can move freely

Fairly straightforward. Grip the square part in the vice (perhaps grip an old socket in the vice so you don't damage the square bit) and suspend a known weight at 0.5 meters from the centre of the square. So for example, hang a 25kg sack of spuds at 0.5m = 25 x 0.5 = 12.5 KgM. If you can extend the handle by putting some tube onto it - say to 2.0 m. then 2.0M x 25 Kg = 50 Kgm. There will be a slight inaccuracy due to the weight of the tube, but even that could be calculated if you knew the weight of the tube.

Another way is to mount the square end in the vice (as before) and stand on a set of scales. Grip the torque wrench handle and measure the distance that your hand is from the square and note it. Now push down on the handle and measure the difference in weight of the scales. Difference in scale weight x noted distance etc etc.

Paul,

A simple drum and line arrangement, with a dead weight hanging, should be all you need.

The drum can be reduced to a short sector [as on a beam engine] if you prefer.

MichaelG.

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Edit: acknowledgement to SB 5, who said something similar

P.S. ... Norbar has some useful calculators, etc.

Edited By Michael Gilligan on 08/06/2015 08:10:44

Edited By Michael Gilligan on 08/06/2015 08:30:49

Thank you for the suggestions this Britool torque wrench clicks when the desired setting has been reached it is set on the handle graduations by a screw.

Paul

Digital weighing scales that are used for fishing or weighing luggage, easy to check they are reasonably calibrated by hang a known weight on the end. There are purpose made calibration devices for torque wrenches but the cost may be prohibitive.

Bob

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Paul,

That's absolutely ideal for the method I suggested.

Any arbitary choice of drum diameter [or length of lever], and any arbitary choice of weight ... then keep re-setting the torque wrench until it clicks just as the weight starts lifting off the ground.

To all practical purposes, you can assume that the scale graduation on the wrench is linear; so you should only need a couple of spot-checks.

MichaelG.

MANY years ago, at one of my former places of employment, the checking device was a 12 inch diameter pulley fixed to a shaft (running in bearings - preferably ball type) with a hexagonal end to the shaft. Clamped to the pulley, and running just over halfway around its circumference was a wire with a weight carrier on the end (Just a circular disc fixed to the wire.)

The weight carrier was loaded withe required weight and the torque wrench, set to the nominal torque applied to the hexagon, via a suitable sized socket. If the wrench did not click/break at the nominal setting, the torque wrench was adjusted until it did when lifting the set load. So any error was obvious, and could be recorded for calibration purposes. Being 12 inch diameter the toque setting was obvious.

If an indicating type of wrench was being checked, again any error was obvious.

By incrementing the weight, the wrench could be calibrated so that any necessary corrections could be applied.

Incidentally, I found that, despite their appearance, the "bendy bar" type of indicating wrenches were quite accurate, and consistently so. Which was more than could be said of a very well known and respected brand of "Guaranteed for Life" of dial torque wrench, offered for use in an industrial environment. The Britool wrenches were usually very good. But any type of "click" or "breakback" wrench should be brought to the setting point, gently. Snatching, could result in a hammer blow which would make repeated readings quite inconsistent. Obviously, the same applies when using the wrench to tighten a fastener.

Lubricated fasteners will produce more consistent loads in the fastener, but probably greater than when the threads are dry.

It is taken as read that the threads are clean and undamaged, if consistent tensile loads are required.

Howard

I've got a Britool torque wrench same style 1970's vintage, lost the ball bearing many years ago, anyone know what size ball bearing?

I thought it may have been around 1/4"

You could use a milk container with water if you haven't a suitable weight. Should be a little more accurate than sack of spuds ! You could use the supermarket veg scales to check.

Most torque wrenches are only accurate to 4%. I have a widely inaccurate but consistent old 60's snap-on wrench that I have to use with revised torque values ( on paper).

Most industrial checking equipment is by electronic torque analysers these days.

Hi

I though the following might be of interest as it's relevant to torque tightening.

Torque wrench calibration is one thing but out of calibration tools are not the main source of error when tightening a bolt to a pre defined load.

A higher proportion of the torque value is needed to overcome friction between the mating threads and the underside of the nut in contact with the joint/washer known as the Spot face. This friction has to be known/controlled to get close to a predefined bolt load. This friction can be controlled by using a suitable lubricant or anti-seize compound.

Therefore, a specified torque value should always be accompanied with the lubricant to be used.

For example:

M12 grade 10.9 bolt to be torque tightened to a bolt load of 70kN.

Using KS Paul Easyrun 100 (low co-efficient of friction lubricant) Torque value = 98Nm to achieve 70kN bolt load

Using KS Paul PBC (higher co-efficient of friction lubricant) Torque value = 167Nm to achieve 70 kN bolt load

For this example I have chosen to commonly used lubricants used in industry that are at the opposite ends of the co-efficient of friction scale. It can be seen that if you were given a torque value of 167 Nm but used Easyrun 100 to lubricate the threads and nut spot face you would dangerously overload the bolt. Vice-versa will apply resulting in a bolt seriously under loaded bolt and perhaps subject to eventual failure due to fatigue under cyclic loading.

To summarise, a torque value without a specified lubricant is meaningless and therefore, so will be the torque wrench calibration. The correct bolt preparation is an essential part of the process.

Here's a youtube video

Nigel

You need to know if the specified torque is dry or with lubrication as this can effect the amount of clamp load applied by the fixing. It is worth noting that if the torque setting specified is for a dry clean thread then adding a lubricant can over stress the fixing causing a failure. It is one of the reasons angular torque is used on the likes of head bolts as it is far easier to control the amount of stress in the bolt when the job is done in the field.

Bob

Angular tightening implies tightening to yield,

This technique involves tightening to an initial "snug" torque followed by a known angular rotation which produces a fairly consistent axial load to just above the yield point of the material. This is now a standard aftermarket means of tightening to yield.

This means that the fastener takes on a permanent extension, and uses the fastener most efficiently, but is why some manufacturers insist that the fasteners are not re used.

Many years ago whilst involved in commissioning, what was at the time, the worlds largest multi spindle yield tightening machine, we spent several months repeatedly checking each of the 32 spindles. The end result was that W range, 1/2 UNF bolts were put into yield with a tensile load of 9 tons, and took on a permanent stretch of 0.001" at each tightening. It was considered that these fasteners could be reused, before needing replacement. During the trials, we found, like other manufacturers, that the lubrication needed to be controlled; although not as closely as our suppliers first feared).

Tightening to a torque, or by torque plus angle, whilst departing from the specified lubrication conditions, or the fastener material specification, will cause problems, either in too low a clamp load, or by overloading the fastener.

The load in the fastener can be measured using a load cell. This is effectively the same as fitting strain gauges to the fastener, since they are contained within the load cell.

On very large fasteners, where torques would be so large as to be difficult, or impossible, to apply, it is common practice to use hydraulic nuts, such as "Pilgrim" nuts. Here the load is applied by a hydraulic pressure calculated on the basis of the dimensions of the nut, to extend the nut to apply the required axial load, and then maintained by tightening another nut onto it.

Howard

I have a couple of Norbar torque wrenches and as I knew the people in our torque tools calibration dept. I got them checked. It surprised me how hard it was to get a consistant reading from a click type wrench. the calibration guys had the feel for the calibration rig but my first tries were all over the place. I had thought of the click type as being reasonably idiot proof but it appears even they need the right feel to get consistant results. The calibration guys knew I would struggle to get consistant results so had a good laugh at my efforts. It turned out the Norbar wrenches had held their calibration well and gave good readings accross their range.

Mike