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?

  • Tightening brass screws like they are wheel nuts

    I first bought a torque wrench 40-odd years ago for tightening cylinder head nuts, the point being that they had to be done up evenly and in the correct order. Everything else was done by feel. Nowadays a service manual will have torque figures for every single nut and bolt. Like wheel nuts? Mine (in fact bolts) require 150 N m, which you won't achieve with the sort of spanner that comes with the car.

    I suspect that less experienced people probably would not tighten terminals sufficiently rather than too much and once you get above the domestic scale, a screwdriver simply isn't big enough.

    Any road, can you imagine doing up every ceiling rose and socket with a torque driver?

  • I have actually tightened down engine cylinder heads, with multiple bolts or screws to tighten it’s not just about the torque, it’s also about using the correct sequence and procedure.

    I am not a particularly good motor mechanic, but there are times I just wince watching people put wheels on cars and the like.

  • I once saw a "mechanic" in one of those quickly fit car parts places, car on the ramps , well above his head, long reach drive shaft on ratchet (not torque ratchet just bog standard forward/reverse ratchet) and he walked around in a circle whilst hold of it to tighten the socket. Can you imagine any mechanic or engineer tightening in such a manner?

  • Now that you have bought them, you have the opportunity to give your students a feel of what tightening screw terminals is like, with quantitative results for the torque, and qualitative results for the conductors, how squashed they are. You could also put more conductors in the terminal and retest. Using different CSAs. Tabulate the results for clarity, but it is the feel you are really teaching.

    Cross-threading is something I see, and try to avoid, virtually every day. Easy to do with faceplate screws, squeak, squeak. Definitely worth teaching.

  • Can you imagine any mechanic or engineer tightening in such a manner?

    Only if he (or she) has a problem with his shoulder. Trust me - my left shoulder needs new bearings.

  • Cross-threading is something I see, and try to avoid

    More likely with the old-fashioned round terminal boxes with the slits across the terminals - very easy to cross-thread. A torque driver isn't going to save you from that.

  • Morning Sparkingchip,

    You are doing well even finding a consumer unit with all the cover fixings in place, usually I fine the cover just held on by one screw and even then not "original" as supplied by the consumer unit manufacturer!

    GTB

  • so they can quickly get a feel for how tight things should be

    Maybe an exercise of making a connections at different tightnesses and then dismantling them and examining the distortion of the conductor might be useful?

       - Andy.

  • And realising that only the reversible (elastic) distortion that actually holds connections tight - the strand spreading  and  any distortion/damage that does not return to its original shape on loosening may increase the contact area but does not contribute to the contact pressure. It may however use up a lot of the torque - so tightening 'identical' wires into 'identical' fixings  has a lot in common with measuring a selection of walnuts with a micrometer, a different very accurate answer each time.


    However as a lesson for students, and important one, as loose terminals are very much a hazard - there is a lot to be said for designs with definite spring pre-load, by using star washers under things that matter.and constant force springs on pipes etc. The stcrew looks simple but it hides a lot of complexity that is often overlooked.

    I;d suggest also that they try torque tests with lids on adaptable boxes, and with and without some sort of spring / star washer under the head.

    Then flash up a slide of these jackbolts.

    https://www.nord-lock.com/insights/knowledge/2012/comparing-superbolt-nut-style-multi-jackbolt-tensioners-vs-hex-nuts/

    The idea is that for really large bolts the rotating of the nut and the ultimate  tensioning up of the bolt can be made independent actions. Seen on things like wind turbines and large ships.

    For making people think about environments with a lot of shake rattle and roll for everything else. this video of various fasteners on a shaker table is worth watching, if a bit of an advert. Note they do not test star washers, if they had then it would show they can be  pretty good.

    The fun starts about 1 minute in,  after the introductory flannel
    .

    Mike

  • I should have been in bed last night, but I had another cup of tea and watched a bit of Shed and Buried on the telly, they were doing up an old three wheeler with a motorcycle engine in it.

    There was a comment when the carburettor was being reattached that if you tighten one of the two bolts fully rather than taking them down together gradually as a pair the carburettor and it’s mounting flange will end up twisted and will leak air.

    Technique is important.