What torque settings for accessory screws.

Question

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?

mapj1 · Accepted Answer

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.

gkenyon · Answer

nicemark said: 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.

AJJewsbury · Answer

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.

David Stone · Answer

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.

nicemark · Answer

Yes trashing some brass screws to start with may save some accessories. Good idea if torque settings are not forthcoming.

What torque settings for accessory screws.

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