One for the engineers

On-going debate at work re a customers want to have a torque setting on a Philips headed machine screw. I not sure you can but what’s your thoughts?

You can get a Phillips bit to fit on a torque wrench, so you could set a torque. But i think if you want to specify a torque you have the wrong fixing, a hex or torque headed screw would be more suitable. if its a Phillips screw the user will just attack it with any old screwdriver.

Agree. Possible in theory but unlikely to work in a practical situation. What’s the application?

Its actually for the gear rack in a stair lift (stannah type). The customer want is maximum 4Nm which is very little. Above that the thread strips as it goes into aluminium. My concern is the bit/head starts to lift depending on the pressure applied downward so it becomes subjective?

You certainly can use a torque setting on a Phillips machine screw, we do all the time, however the settings are generally low and the tools and screws are small (m4 and less) oh and a calibrated screwdriver is mucho expensive!!!!

but as a Philips head is tapered doesn’t it depend on the pressure applied i.e. down force? cant get my head around it tbh. if its hex head or bolt etc then I understand the forces applied are direct but on a tapered type Philips head I don’t get it.

Weren’t Phillips heads designed to cam out at a certain torque anyway to prevent over tightening? Never works though.

Surely the torque setting would apply so long as the driver was seated?

Taper is a red herring. Think of the forces in their component parts. Push down as hard as you like to stop the bit “climbing out”, the torque is unaffected as its perpendicular to it.

Yes you can. I use torque setting screwdrivers every day at work, on M1.6 upward, regardless of the head type. Most are Pozidrive but as above, it makes no difference, as long as you have the right bit for your driver.

understand but this has to be dependant on the force applied down?
child pressing down has to be less force than adult pressing down and required force is minimum 4Nm

Pressing down force is independent of rotational force, basically. Your child stair lift fitter may not be able to press down hard enough to do the job, in which case sack him/send him up chimney, but if he does press hard enough then torque setting is valid.

You are right to an extent that the downward pressure increases the drag on the clutch assembly but you shouldn’t need massive amounts of downward force if the tools and screw are of good quality. As a rule we use the weight of the tool plus a small amount of pressure on the manually set tools. If you are pushing so hard on a machine screw to get it to turn then there are other issues!!. Having said all of that my experience is in electronics assembly so everything is pretty low torque and small and may not relate well to your application…

but if you need to set a procedure to follow and you don’t know the ability/build/power of the employee/potential future employee then how can you set a procedure to folo of tighten to 4Nm?

a ‘warning, do not over tighten’ sticker, combined with a policy of not employing eegits? Or am I being hopelessly optimistic?

Wouldn’t it just be easier to specify a hex head?

Make the head type one that doesn’t suffer from cam out; hex/hex socket/Torx? Must it be Phillips?

yeah that’s what I think hex head all he way – but its the customer specification. I think I need to tell them to change the spec to hand tighten or change fastener to hex.

Not being funny but as a rule we have written process instructions and we train people how to use a £300 screwdriver… Sounds daft but it normally stops tool damage and damage to parts.. 😉

no real concern over the tools its the principal of setting a standard that’s not achievable. we use a mix of air tools and Makita drills. but not sure how we set and measure this with a Phillips type fixing.

Is the fitter going to use a torque sensing screwdriver or simply whatever her has to hand? If it’s, as is likely, the latter, you’re wasting your time trying to tell them to do so and its just a legal cover up for bad design. Ie you’re trying to blame the fitter when the thread strips.

4nm is not much, it’s hand tight at best and you’d have to be a ham fisted Buffoon to screw it up. ( see what I did there). My worry would be that giving a torque figure would encourage a feeling of “this ones got to be tight” and actually lead to more stripped threads.

Robertson screw

I’ve got a torque setting screwdriver that will to 4.5Nm, at work. There’s no reason this screw couldn’t be set with such a manual driver. If you’re setting to 4Nm, in aluminium, that’s about an M6 screw? If so you’ll need a PH3 screwdriver bit? Plus an analyser/meter to set the screwdriver with, which should be calibrated regularly.

There shouldn’t be much risk of a bit this big camming out at such a relatively low torque.

I was going to say change it to a Torx screw and get a 4Nm torque key made up and supply it with the kit. But then they might come back to you if the torque key goes out of calibration so best put the onus on them to supply and use their ow torque wrench.

I’ve got a Britool adjustable torque screwdriver in the tool chest, found it in my dads PC repair stuff he was throwing out. I remember using an identical one at Uni when i needed to torque up some bolts in composite test pieces to 1Nm. At such a low torque it should be fine to spec it for a philips head but it does seem a bit wrong and i’m in the change to a different head type camp.

We have some kit at work with critical torque settings. So critical that we can’t run the risk that they just use a standard tool and tighten to what they fancy. Therefore the fastener has a shear off head. They just tighten until it breaks. It leaves behind a very shallow hex in case it needs to be removed down the line.

If it’s that critical, you’ve got to consider that your fitter will ignore a process that has a simplier alternative. If they say they can trust them, then they could also train them to do it “hand tight”.

Or design the problem out, put a helicoil in at point of manufacture and let them go crazy with the tools.

Use this?

http://www.tredz.co.uk/.Ritchey-Torque-Key-Set_76302.htm?sku=242422&utm_source=Google&utm_medium=AdwordsProductAds&utm_campaign=Adwords&gclid=Cj0KEQjwk7msBRCJj67khY2z_NIBEiQAPTFjv-fYBxs__MynKSRY5ENvhDsO2OQff9IhkBWXtxtQx6EaAjD68P8HAQ#