Need to buy a 1/4 drive low range torque wrench is there anything to avoid or good makes to look out for?
quote:
Originally posted by v8kid
Need to buy a 1/4 drive low range torque wrench is there anything to avoid or good makes to look out for?
Unless properly calibrated they are as much of a guess as doing it by feel.
We have britool, snap on, stahlwille etc, in work and they don't calibrate the torque wrenches any more as they aren't accurate enough so
there is a bench mounted torque setter. even on the best ones there can be 10 or 15Nm difference between the wrenches dial and its read out on the
calibrated machine.
What about these electronic in line read outs are they accurate?
These days nearly all the torque wrenches sold are from just a couple manufacturers and rebranded.
The Bergen brand is generally very good value for money I have a lot of Bergen speciality tools that are identical to big brand names but were a third
of the price.
You will find Bergen tools from a number of eBay sellers.
US Pro and Neilson are similar brands that offer decent quality at a good price.
You can check the calibration of a torque wrench fairly easily with a cheap digital lugage spring balance scales.
When storing a torque wrench between use always back the setting right back to zero to reduce the tension in the spring to prenvent it drifting out of
calibration.
If you really want a quality one norbar are a good choice, but the price will make you cry....
BT advice is probably about the best practical solution for on a budget; find a half decent one then check it yourself with a simple bit of maths and
a digital scale (Force * distance = torque).
Dan
Regarding calibration you can do this yourself at home. Here's the method - sorry if I'm teaching to suck eggs.
I inherited 2 torque wrenches a couple of weeks back. One was the old bending lever with dial indicator type. The other a twist and preload spring
type (that clicks).
Clamped the drive in the vice, then hung a 16kg kettlebell (verified by bathroom scales) mass on the lever arm. On the twisty I adjusted until it just
clicked, on the bending arm just hung the mass.
My presumption was the bending arm materials were the same as the day it was made, the dial was fixed solid, it zero'd well, so I expected the
calibration to be good. Wrong! It was reading 20% less than the calculated torque 16kg x g (9.81m/s²)x centre of drive to centre of gravity of mass
(0.27m).
The dial indicator however, was given to me with the spring wound up (and likely been like that for 10 years). I had very low expectations for this
one, but with the same test method, at 42Nm it had an accuracy of 0.9% !!!
Now I was doing up some fasteners at 40Nm so this was the point at which I was interested in accuracy, but to check over the wrench fully, I would do
the same test but at different torque settings, varying either the mass applied or the length from pivot to CoG.
One way I tried (before the 16kg kettlebell) was an ikea bag progressively filled with cans of oil, bottles of water, tools etc, checked on the
bathroom scales for mass. The centre of mass is easily found - it's just where the strap hangs when you lift it.
If this wasn't the case there are videos on youtube of a threaded preload adjuster internal to the wrench that you can modify to remove or add an
offset across the whole range. However if you have a 'drift' error then you can't do much about it without changing the spring I think.
N.B. This doesn't take into account mass of the lever, repeat variability, speed of application and some other assumptions. However, it is pretty
empirical mechanics, so I think it's a reliable test!
Thanks Chaps You've come up trumps - I'll go for Bergen and calibrate it before use with motivforz's deadweight method
Cheers
quote:
Originally posted by ste
Unless properly calibrated they are as much of a guess as doing it by feel.
We have britool, snap on, stahlwille etc, in work and they don't calibrate the torque wrenches any more as they aren't accurate enough so there is a bench mounted torque setter. even on the best ones there can be 10 or 15Nm difference between the wrenches dial and its read out on the calibrated machine.
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