What torque settings for accessory screws.

As a newish teacher I am trying to get students to fix things with the correct amount of torque.

Tightening brass screws like they are wheel nuts does not give materials much of a lifespan.

So I bought a couple of torque screwdrivers, so they can quickly get a feel for how tight things should be.

However now I look at data sheets I can't seem to find Torque info for anything smaller than a a miniature circuit breaker.

The data sheets now often include other everyday information like Altitude.

Does anyone have some guidance for smaller brass screw torques, or know of a manufacturer that publishes such data?

  • I must admit I never had any "numbers" for torque - just enough so you can feel the screw just biting into the conductor is my rule of thumb - checked with a wiggle of the wire afterwards and a check that the screw then doesn't need any further tightening. Similar with compression plumbing fittings - you just turn until you can feel the increase in resistance as the olive biting into the pipe, and then just a fraction more. But then maybe I'm just old fashioned.

       - Andy.

  • For small fixing screws the relationship between torque, and what you are really trying to achieve, a given degree of extension (stretch) or compression of the core of the bolt is very variable as the thread friction dominates over the 'worm drive' action of the thread which after all is just the problem of pushing things up a ramp, but  wrapped around a stick.

    To illustrate this by example  a simple bench top demo can be done to the students to show that a light smear of grease on an M4 mild steel bolt can almost halve the torque required to reach the head shearing tension - or less destructively and more pertinant to terminals, to graunch right though a 1mm wire core..

    For terminals things are complicated by the fact the yield element is the wire being clamped, not the screw body itself.


    Very few makers are keen to publish figures for this reason, and a blind adherence to a fixed torque can lead to problems.
    A starting point for steel in M3 and up here but for brass I'd start by halving everything. You may expect it to scale with UTS of the material but below about M10 threads  it generally does not.

    Mike.

  • Experience trumps a torque screwdriver.

    I had a discussion with the technical guys at a major consumer unit manufacturer a few years ago, because the quality of their screws was so bad I could actually pull some of the wires out with my fingers from the neutral and earth bar when the screw was at the correct torque.

    Another issue was they stated the same torque for a 1.0 mm conductor as a 10.0 mm, the 1.0 mm were crushed and the 10.0 mm barely nipped up.

    I am not convinced obsessive use of a torque screwdriver always improves the quality of an installation.

  • I agree with the above posts. Where items need to be set to critical torques, there are a whole set of instructions for the assembly, everything to thread lubrication with a specific lubricant, or even met5iculous cleaning of both parts, and certainly small screws will very rarely have a torque specification. The "modern" way to set torques is mpre complex, first the bolt is torqued to a fairly low value, then a higher one and then a further rotation of so many degrees is applied. This technique does need a reasonable length of bolt which is not engaged in a thread to stretch (actually to be like a very strong spring) but is much more reproducible. I am not a great fan of torque settings themselves, it depends far too much on everything else being specified as well, particularly the conductor size and makeup wires, and breaking brass screws is a serious hazard. Note that many screws are now made of steel for this reason! It is a matter of practice, as tight as "is reasonable" for the screw size works quite well. learning with a steel tapped bar and a selection of small brass screws would be an interesting exercise.

  • I bought a torque screwdriver mainly for accessories a few years ago to demonstrate that I had one to the assessor, if asked. I also had a table of different manufacturers' settings.

    Differential expansion and contraction over time is caused by heat during load cycles; this leads to increased resistance over time. Although the torque settings may be correct at the intitial installation, periodic retightening is required; as found on cyclically loaded circuits such as socket circuits. Better to use spring type connections when available, since expansion and contraction is actually beneficial to the connection; at this time I think that they're only available for lightly loaded circuits however. Don't use lubrication of any kind to such screws and torque screws to your maximum "feel" by experience.   

    Jaymack   

  • I tighten the screws in distribution boards with a torque driver (i.a.w. manufacturers' instructions), but seldom with a cross point one because the risk of damaging the screw head is too great.

    Other than that, the size of the screwdriver determines the maximum torque which can be applied - if you think of a watchmaker's screwdriver, it can only be operated between finger and thumb.

    Screwdriver handles for 1/4" bits (and perish the thought, tommy bars or electric screwdrivers) need to be used with caution because the size of the handle can be too big for the smallest screws.

    Many years ago, a railway fitter told me that instead of a torque wrench, one two or three grunts was good enough.

  • As a newish teacher I am trying to get students to fix things with the correct amount of torque.

    Tightening brass screws like they are wheel nuts does not give materials much of a lifespan.

    So I bought a couple of torque screwdrivers, so they can quickly get a feel for how tight things should be.

    However now I look at data sheets I can't seem to find Torque info for anything smaller than a a miniature circuit breaker.

    They are starting to appear on some accessory instructions, like socket-outlets and connection units - typically 0.5 to 1.5 Nm depending on the manufacturer. I seem to remember Schneider and Crabtree now quote them (e.g. on the decorative metal finish range they quote 0.5 Nm), but also more recently even some of the cheapest products from a rather popular outlet that also does mail order.

  • one two or three grunts was good enough.

    In my younger more athletic  days I  recall helping the mechanic do a roadside clutch swap on our Leyland FG  series crew truck and , and his comment was much the same. It seemed to work very well afterwards.

    Mike.

  • Hands up everybody who has never had/used a torque screwdriver

  • I have seen consumer units with a label detailing the torque for the screws that hold the front cover on, now that is obsessive and doesn’t improve safety.