Last week, Aldi was selling a torque wrench with 1/2 inch drive for $29.99. I was a bit shocked when I saw the price because that’s not a lot of money for a torque wrench.
Honestly, I was a bit skeptical of its performance and a bit hesitant to part with $29.99 for what could be a dud. However, I did go and check it out in-store before completely dismissing it. The wrench came in a durable molded plastic case and came with an extension bar, 17mm, 19mm and 21mm sockets. I was sold, so I shelled out $30 and brought the thing home.
First impressions are that its not too bad. I’m no torque wrench expert however the quality seems pretty reasonable for its price. The ratchet works, the ratchet is reversible and the whole device feels pretty substantial. Its stated specifications are; adjustable range of 28-210Nm with an accuracy of ±4%. The measuring scale is a bit difficult to read because of the bright chrome finish, but I think this can be fixed by rubbing a paint marker over the indentations and wiping of the excess paint to emphasise the scale markings.
However, is it accurate? There’s not much point tightening bolts to a required specification if the wrench itself is way off the mark.
Testing this thing is pretty straightforward. All you have to do is load up the wrench with a known mass, measure the distance of the lever and do some simple maths. All you are doing is following the definition for torque;
That is, torque (τ [tau]) equals the product of the pivot length(r) and the force (F). The pivot length is easily measured and I can work out the force by simply adding a known mass and multiplying by the acceleration of gravity.
Conveniently, the distance from the centre of the ratchet to the end of the wrenches handle is a whole round number, 400mm. This is the length of the pivot and is the value for “r” in the torque formula.
To create the force that is exerted on the wrench, I used some free weights I had lying around. I had 4x 2.5kg plates and 4x 5kg plates, so the maximum mass I can test is 30kg. Now I have to admit that I have no way of accurately measuring the mass of the free weights so I am making the assumption that all the weight plates are as per their stated value.
Using some bits and pieces, I cobbled together a hook to hold the weight plates and some nylon rope to hang the hook from the wrench. I’ll assume that the hook and rope are of negligible mass.
OK, so I have a test setup, how did I test the wrench?
Generating the test force was done using the weight plates. The plates have a known mass, for example the larger plates are 5kg each and since my garage is situated on planet Earth, everything inside my garage experiences an acceleration of 9.8ms^-2. When the 5kg plate is sitting on the garage floor, it is exerting a force of 5 x 9.8 = 49 Newtons on my garage floor (F = ma). Therefore if I hang the 5kg mass from the handle of the torque wrench, it will apply the same force, 49N, on the torque wrench.
Now, I’m a metric person, so I like to measure torque in Newton metres (Nm) and this definition literally means the torque force applied by the specified number of Newtons on a lever 1 meter in length. However, my torque wrench is not 1m in length but is 400mm (0.4m) in length. So all I have to do is multiply my test force value by 0.4 to calculate the test torque value. It is also important to remember that the wrench must be horizontal, so the test force is acting perpendicular.
Testing the wrench involved me working out the theoretical torque my test weight would generate, setting the wrench to the same torque setting and slowly loading up the wrench with the test plates. When the wrench is at the specified torque, an audible click is heard and that tells you the wrench is at the set torque. Releasing the load resets the wrench. As I slowly loaded the wrench I listened for the “click”. If the click occurred before the all the weight plates were hanging, then the wrench’s setting needed to be increased. If the click didn’t occur, then the wrench’s setting needed to be decreased.
|Mass (kg)||Ideal torque (Nm)||Measured torque (Nm)||Error (%)|
|7.5||7.5 x 9.8 x 0.4 = 29.4||30.5||3.7|
|10||10 x 9.8 x 0.4 = 39.2||41||4.6|
|12.5||12.5 x 9.8 x 0.4 = 49||50||2|
|15||15 x 9.8 x 0.4 = 58.8||60||2|
|20||20 x 9.8 x 0.4 = 78.4||79||0.8|
|25||25 x 9.8 x 0.4 = 98||98||0|
|30||30 x 9.8 x 0.4 = 117.6||118||0.3|
As you can see, at low values of torque, the wrench is not great, in fact it exceeds its specification of ±4%. However once the wrench starts to get loaded up its performance appears to be quite good. Unfortunately, I don’t have enough weight plates to test the wrench’s full range, but for what I expect to use the wrench for, the range I tested is enough (to achieve 210Nm with this wrench, I’d need to hang 53kg from the wrench). Don’t forget that I have made the big assumption that the weight plates are their stated value (I don’t think there would be very rigorous quality control on the precision of plate’s actual mass in the factory that make weight plates).
All I can truly declare from this quick little experiment is:
- the $29.99 Aldi torque wrench actually works
- it response is somewhat linear,
- at low values of torque it may exceed its specification. But, if low values or torque are important to you, then you should use a wrench with a range that covers lower torque values
- at higher values of torque it is well within its specification of 4%
So there you have it, the Aldi torque wrench seems reasonable value when you consider its price of $29.99. Also, if you aren’t doing any critical work where you need a torque wrench with a traceable test certificate this may suit your needs. If you do have a torque wrench, testing its accuracy is a straightforward job that you can perform at home.