Engineering question : Max torque of bolt into 10mm 5083 aluminium plate?

[wipe_out]

Hi,

Hoping someone has some idea on this..

I have to put M8 bolts into 10mm thick 5083 aluminium.. The aluminium plate has M8 holes drilled and tapped into it..

Anyone have any idea what torque the bolts would take before I risk striping the threads on the plate?

 

[CreakyDecks]

Unless you have really good torque wrench I'd say do it "by feel". It's going to be a really low, like nine or ten foot pounds. One finger and thumb on spanner.

 

[Avocet]

Unless you have really good torque wrench I'd say do it "by feel". It's going to be a really low, like nine or ten foot pounds. One finger and thumb on spanner.

Wot he said! The problem with tightening by torque is that it's not an exact science because you never know how much of the torque being applied by the wrench is just overcoming friction between the boat head and the washer and between the threads and the aluminium and how much of it is actually clamping the joint (and therefore trying to rip the threads out of the aluminium). I agree it won't be much though. The first post on the following thread has a handy "rule of thumb" table (in imperial units).

http://www.enfieldmotorcycles.com/forum/index.php/topic,10638.0.html

Your 5000-series stuff is likely to be a bit stronger than the cast stuff on an old Enfield bike, but probably not as much as you might think! Can you get hold of some scrap aluminium and practice? I'd measure the torque to strip half a dozen, average it, and then go for 75% of that.

 

[cliff]

Can you get hold of some scrap aluminium and practice? I'd measure the torque to strip half a dozen, average it, and then go for 75% of that.

Ah, a practical "inginear" at last. Don't forget the thread pitch will also effect torque possible before pullout

 

[wipe_out]

Can you get hold of some scrap aluminium and practice? I'd measure the torque to strip half a dozen, average it, and then go for 75% of that.

Unfortunately not.. I only thought about it afterwards and should have ordered another section with tapped holes to test on..

 

[wipe_out]

The probelm with going by "feel" is I tend to have a rather heavy "feel".. That's why I like using a torque wrench but without a value to torque to I guess its back to "feeling"..

 

[jerrytug]

What job is it, last time I did that it was making an inspection hole in an aluminium tank.
Perhaps you can use a thread sealant which will stop it coming undone (like loctite) and prevent corrosion ( like duralac).

If you are relying on the strength of the aluminium threads for something apart from squeezing a rubber gasket, maybe consider the various inserts you could press in to the aluminium?

Can't you get behind and use nuts?

 

[CreakyDecks]

The probelm with going by "feel" is I tend to have a rather heavy "feel".. That's why I like using a torque wrench but without a value to torque to I guess its back to "feeling"..

That's when you order the helicoil kit

 

[rob2]

It may be of interest to look at engine manuals, unless you intend to go the empirical route. My little Mitsubishi (Sole) engine has M8 CH bolts with a torque setting of 23-29 lb/ft. However the general torque table gives figures for materials listed as 4T/7T/10T of 7.2-9.4/10.8-15.9/18.1-25.3. I guess if I knew what the material specifications meant I could offer this as advice..? I guess I'd actually use the lowest figure and Loctite the threads, which should help prevent any corrosion caused between the dissimilar metals.

Rob.

 

[wipe_out]

What job is it, last time I did that it was making an inspection hole in an aluminium tank.
Perhaps you can use a thread sealant which will stop it coming undone (like loctite) and prevent corrosion ( like duralac).

If you are relying on the strength of the aluminium threads for something apart from squeezing a rubber gasket, maybe consider the various inserts you could press in to the aluminium?

Can't you get behind and use nuts?

It is an inspection cover on a tank..

Unfortunately I can't get behind it to put nuts on.. The top plate will bolt into the bottom plate..

I will need some corrosion prevention and was thinking Hylotyte or Duralac, just not sure which is best in a fuel tank.. Seems hylotyte needs raised temperature to cure but is probably better in a fuel tank application..

Hoping I can get enough pressure to give a good seal (still haven't come up with a good way to test the seal afterwards other than filling the tank and looking for a leak)..

 

[elton]

When designing engines and such like, engineers specify torque using calculators such as this http://www.tribology-abc.com/calculators/e3_6a.htm, and document their recommendations in the engine service schedule.

Anything other than a rigorous calculation is going to be an experienced guess, at best. The usual ideal is to tighten as close as possible to the elastic limit of the bolt.

 

[Rigger Mortice]

(still haven't come up with a good way to test the seal afterwards other than filling the tank and looking for a leak)..

Pressure test?

 

[jerrytug]

It is an inspection cover on a tank..

Unfortunately I can't get behind it to put nuts on.. The top plate will bolt into the bottom plate..

I will need some corrosion prevention and was thinking Hylotyte or Duralac, just not sure which is best in a fuel tank.. Seems hylotyte needs raised temperature to cure but is probably better in a fuel tank application..

Hoping I can get enough pressure to give a good seal (still haven't come up with a good way to test the seal afterwards other than filling the tank and looking for a leak)..

Just drill more holes and use more bolts, then things won't be so critical!
When you make the rubber gaskets, assuming you are cutting them yourself, allow plenty of clearance around the bolts, double the bolt diameter, to give the rubber room to spread when it is squashed.
I personally would not use any complicated thread sealant, on reflection, a smear of grease will be fine IMO, after all, if it gets in contact with salt water you will have other issues

edited to add, don't make the holes too near the edges of the tank or cover, it is basically the area of slightly squashed rubber which makes the seal.

 

[wipe_out]

Pressure test?

I don't have the kit to do it.. Compressor is no problem but pressure gauge, valve and a way to seal up the filler, vent and fuel take off for long enough to see if there is a pressure drop are an issue..

Other option, since the tank is completely empty, is to fill it with water.. If it all works out and the seal is good I can pump out the water and then perhaps use a heat gun (or even a hair drier) to heat the air in the tank and evaporate the remaining water..

 

[wipe_out]

Just drill more holes and use more bolts, then things won't be so critical!
When you make the rubber gaskets, assuming you are cutting them yourself, allow plenty of clearance around the bolts, double the bolt diameter, to give the rubber room to spread when it is squashed.
I personally would not use any complicated thread sealant, on reflection, a smear of grease will be fine IMO, after all, if it gets in contact with salt water you will have other issues

edited to add, don't make the holes too near the edges of the tank or cover, it is basically the area of slightly squashed rubber which makes the seal.

I have LOTS of bolts..
Using M8 and about 50mm spacing.. Hole centers are 20mm in from the edges..

Will have 2x3mm nitrile rubber gaskets, one top and one bottom so it's effectively double sealed..

PDF of plates..
http://1drv.ms/1lnGi8Q

 

[rob2]

The usual ideal is to tighten as close as possible to the elastic limit of the bolt.

That would be difficult to achieve in aluminium plate! You're right, of course, those are the calculation parameters for a matched nut and bolt, but I believe the figures in engine design often have more in common with this applicatin as the (usually mild steel) screw is often tapped into weaker materials, such as an alloy cylinder head or indeed a steel strap, where the lack of thickness will limit the area of engagement.

Although this is teaching Gran to suck eggs, the screws may be seen more as a temporary fixing with the sealant doing the real work, so the most important thing is to torque them down slowly and evenly, working round jumping to each screw that is nearly diametrically opposite. This will ensure one achieves an even thickness of sealant and avoid distorting the components, by keeping the stresses low.

Rob.

 

[Avocet]

Yes, with only 10mm of thread engagement into aluminium, that's never going to happen. Personally, now that I know it's an inspection plate, I wouldn't do them very tight at all. A lot depends on how thick the plate is. We know the bottom plate (with the tapped holes) is 10mm thick, but it's VERY important that the top plate is also very stiff. If anything, over-tightening is counter-productive as the plate then bends in between each fixing and you lose clamping force on the gasket. If anything, it's best to have less clamping force and more fixings AND a relatively soft gasket with a relatively small surface area so that it readily deforms to follow both surface profiles.

 

[wipe_out]

Yes, with only 10mm of thread engagement into aluminium, that's never going to happen. Personally, now that I know it's an inspection plate, I wouldn't do them very tight at all. A lot depends on how thick the plate is. We know the bottom plate (with the tapped holes) is 10mm thick, but it's VERY important that the top plate is also very stiff. If anything, over-tightening is counter-productive as the plate then bends in between each fixing and you lose clamping force on the gasket. If anything, it's best to have less clamping force and more fixings AND a relatively soft gasket with a relatively small surface area so that it readily deforms to follow both surface profiles.

The top plate is also 10mm so the whole thing will be extremely rigid..

The plan is to have two 3mm rubber gaskets the size of the plates (no centre cut outs or anything other than the bolt holes) creating an inner seal and an outer seal meaning hopefully no chance of a leak even if the inner gasket fails in years to come.. The contact surface is probably about 50mm wide around the whole in the tank..

 

[Bilgediver]

When designing engines and such like, engineers specify torque using calculators such as this http://www.tribology-abc.com/calculators/e3_6a.htm, and document their recommendations in the engine service schedule.

Anything other than a rigorous calculation is going to be an experienced guess, at best. The usual ideal is to tighten as close as possible to the elastic limit of the bolt.

Unfortunately this results in too high a figure being used when the bolt is being screwed into a lower strength material such as aluminium...

More than a few folk have also damaged their GRP hulls in way of keel bolts by utilising engineering tables .

 

[Avocet]

The top plate is also 10mm so the whole thing will be extremely rigid..

The plan is to have two 3mm rubber gaskets the size of the plates (no centre cut outs or anything other than the bolt holes) creating an inner seal and an outer seal meaning hopefully no chance of a leak even if the inner gasket fails in years to come.. The contact surface is probably about 50mm wide around the whole in the tank..

I have no experience of this, but my gut feeling is that 50mm is WAY too big. Even for very soft rubber you'll need a hell of a clamping force to squeeze those babies down tight enough to make sure they touch all the way round! I'd have thought something like a large section O-ring might be more likely to be able to deform enough all the way round to form a continuous seal.