Torque value for M3 screw into Ali?

[littlebean]

as it says really, we've built a wind tunnel model that has adjusters held in place via steel screws into ali.
having watched some people working on it, it looks as though we need to supply a torque screwdriver pretty sharpish.........

 

[gtae07]

I don't think Ali would appreciate you putting screws in him...

 

[RoninB4]

Length of screw and thickness of Al?

 

[littlebean]

Ali doesn't mind...

Screw about 10 mm long into 5mm tapped hole

 

[dutchgray]

If you can use helicoils or better still solid thread inserts in the aluminium, threads in aluminium in regular use will get chewed up pretty quickly.

Torque value, enough to hold the adjusters still in use plus a bit for safety.

 

[Rabid Badger]

Those threads aren't going to hold up regardless of the tool used.

Is it possible to move to larger screws (M4 or M5) with threaded metal inserts?

Are these screw heads in the air path? If not, could they be replaced with thumbscrews? That would make it harder to over-tighten and add some convenience.

 

[littlebean]

Thanks for your thoughts
Can't put inserts in and can't have thumbscrews - they're sliding adjusters to set flaps etc so need to be as small as possible. Hence I can't go larger either, I guess we'll 'nip' them up and then try to easier the torque required to release and go from there

 

[cvairwerks]

M3 socket set screws, recommended torque is .9Nm....... Question about the model and flow rates...This a large model or small, and is it going to be in a steady state flow or a dynamic flow? I did transonic model design eons ago, and the models we built for a PhD project, still get pulled out and run in the Ludweig Tube tunnel every couple of semesters. We ran them in flows between about 800 fps and 1300 fps.

 

[littlebean]

It's around 1.5 metre wingspan, adjusters go into the wings & tailplane and slide to set the control surfaces. Tunnel is low speed, roughly 35mps and constant air flow

 

[timgunn1962]

It's difficult to recommend anything without a lot more detail.

I did a short "electrical upskilling" course a few months ago, aimed at mechanical maintenance guys to give an introduction to the rudiments of electrical maintenance. It was stuff I've been doing for the last 4 decades or so and I was put on the course to see whether it would be worth putting a bunch of our guys on it. The thing that really stood out to me was just how few folk had any "feel" for screw tightness.

The wiring code here in the UK calls out use of torque screwdrivers and I'd thought it was ridiculous: anyone with a modicum of competence should have no problem tightening terminals. We had a panel wiring exercise on the course using DIN-rail terminals and I was able to pull out every single wire that the other guys in my group had tightened. I now see the reason for specifying torque drivers.

It shouldn't have been a surprise: I'm utterly useless with a keyboard or touchscreen and I think it's probably a generational thing.

If you, or someone else, has been doing this stuff for years and has a feel for it, the best approach is probably to use a torque indicating screwdriver to tighten the screws, note the peak torque reading and supply a preset torque driver to that value.

 

[littlebean]

I think that's the route we'll go, between the two of us around 80 years experience, but the model is being used by people with no experience hence trying to protect it

 

[Steve_P]

Torque value should be the recommended for that size and type of screw. You should do an experiment and if you can break the screw, or cam out the head, before the threads strip, then you have enough thread engagement. Whether or not the screw size is adequate is another issue.

 

[WildBill]

It will be a failure contest between the threads stripping and the M3 breaking, thats a tiny thread for aluminium. I would at least put some type of insert in the aluminum. You could also get a torque screwdriver, we use them at work for small stuff. Like this -

Micro-Adjustable Torque Screwdriver

{ "1": "Micro-Adjustable Torque Screwdriver" }

www.lindstromtools.com

 

[Rabid Badger]

Would a nylon screw work? They won't strip the threads in the aluminum. You could leave some extras so they can swap to a new one when they get worn. They also tend to resist backing out once tightened.

 

[PCustoms]

Keep in mind the torque value is only required to reach the max recommended clamp force of the fastener.

Your application might need a much smaller clamp force.

 

[AEAdam]

Jeez just answer the question. 12-15inlbs should do it.

See my chart here

I like/have the adjustable CDI torque driver 5-40inlbs.

(also good for competitive shooters - found mine on eBay for around $50 iirc)

 

[Rabid Badger]

Jeez just answer the question. 12-15inlbs should do it.

1) There are practical issues that will cause significant problems later if they aren't addressed now.

2) The highest torque rating I found for an M3 BHCS was 1.35nm. The only figure I found for steel M3 screws into aluminum was 1.0nm, and that was with helicoils. That puts your "12-15 inlbs" figure anywhere from the absolute max to 70% above it. Seems like a recipe for disaster.

 

[cvairwerks]

I think that's the route we'll go, between the two of us around 80 years experience, but the model is being used by people with no experience hence trying to protect it

LOL...tell them to follow the procedures or you'll feed them to the fan....We didn't let any of the undergrads or master's students do any of the model installation or removal from the test section. It was all done by myself, one other assistant or the lab machinist. Even the PhD we were working for wouldn't touch the model once it went into the test section.

 

[littlebean]

LOL...tell them to follow the procedures or you'll feed them to the fan....We didn't let any of the undergrads or master's students do any of the model installation or removal from the test section. It was all done by myself, one other assistant or the lab machinist. Even the PhD we were working for wouldn't touch the model once it went into the test section.

feed them to the fan............
Unfortunately too many students, too many tunnels running at once and not enough of us for us to do everything - we run tunnels from 1m/s up to Mach 12

 

[littlebean]

we've now made a tool that (severely) limits how much torque you can put on the adjuster screw as a short term measure but will be looking to get approval to purchase a torque screwdriver or two, possibly one preset so it can't be fiddled with, to cover as many bases as possible

 

[littlebean]

thanks for all your thoughts and suggestions, much appreciated

 

[cvairwerks]

feed them to the fan............
Unfortunately too many students, too many tunnels running at once and not enough of us for us to do everything - we run tunnels from 1m/s up to Mach 12

LOL...we could go to about Mach 1.4 on our Ludweig tube. We had a tiny hypersonic tube we could go to about Mach 6 on, and I was working on designs for a hypersonic that was capable of Mach 20 with an 8"x10" test section. We also had an arc tunnel in work that was capable of about Mach 8. Only place I ever worked where we had to call the POCO dispatch a day before, if we wanted to run our big compressor at full load or the arc tunnel, so that there was enough generating capacity online. We had about 2.5 megawatts coming in to our little test building on campus. I rarely messed with our low speed stuff, other than helping load a model once in a while.

 

[littlebean]

The one time one of our faster tunnels failed we switched off the local grid in a 5 mile radius - leccy board weren't happy for some reason

 

[cvairwerks]

LOL... first time we hit the load button on this compressor, we caused a bit of a power sag. We had the 500 hp cooling water motor on line, along with the 10 hp lube oil pump, then dropped another 1750 hp at full load onto the line.....Lots of flickering and things going on and off on campus.

 

[littlebean]

Lots of flickering and things going on and off on campus.

for some reason students, academics and facilities don't seem to like that.............