Shaft retaining adhesives

I would knurl them slightly so there is somewhere for the Loctite to sit. If there is no gap then there will be very little adhesive in the joint.

You don't need any clearance with loctite, even interference fits are OK.

Neil

Thanks for further replies. Both the Truloc 231 and Granville Bearing Fit have now set - so maybe 48 hours. It must be because of the oxide coating on the ali.

This experience made me realise that I don't actually understand how these adhesives work. I've done some Googling - Henkel say that they're anaerobic and are 'activated' by 'metal ions' - they seem to suggest that iron and copper are most effective. I get the anaerobic bit, at least in a vague way. Molecular oxygen is a diradical which must somehow inhibit cross-linking. But I don't understand where the metal ions come in, and haven't found any clear explanation out there. I'd be grateful for any pointers to a detailed explanation of the chemistry. Out of curiousity mainly.

Rob

See here for a simple explanation.

**LINK**

Edited By mark smith 20 on 15/10/2017 00:37:53

Thanks for the link Mark, the explanation given there is a bit hand-waving but nonetheless useful. I'll figure it out eventually when I can get access to academic journals.

As an experiment I tried making an ali/ali joint with a gap of about 0.05mm (ie 0.1mm on diameter) and bunged a few mg of fine copper powder into the hole. Applied adhesive to male part, put it together with some twists to distribute things, and it siezed within minutes. 24 hours later and I can't break the joint by hand. Copper is evidently a powerful catalyst for the polymerisation process.

Rob.

Loctie 620 is a high temp high strength glue. Use it with their primer when gluing Al. Prime both parts, allow primer to dry, then apply the 620 to both surfaces, and assemble. It can have a gap to about 0.1mm or so on diameter. The larger the gap, the weaker the joint.

Neil

That's something I wished BMC had done when they made the gear lever for my 1974 Marina. The ball was secured to the shaft with some sort of adhesive and after a few years it let go, allowing the shaft to slide downwards through the ball and throwing the linkage out of whack. I spent some challenging journeys without reverse gear until I tracked down the problem and sorted it.

George

The detailed chemistry of anaerobic adhesives can be quite complex and subtle. The adhesives are mixtures of acrylic monomers which polymerise easily by free-radical mechanisms. The free radicals are generated by decomposition of an organic peroxide. However, if there is oxygen around it reacts with the radicals and stops the polymerisation from happening. That’s why the adhesives are stored in polythene containers which allow oxygen to permeate.

The other important factor is that metals like iron and copper are very efficient catalysts for decomposition of peroxides to make free radicals, so that if the liquid adhesive is trapped between two surfaces containing iron or copper then radicals are produced rapidly, oxygen is used up through reaction with those radicals and can’t be replaced, and fast polymerisation (adhesive curing) takes place. Aluminium does not catalyse peroxide decomposition, so if the surfaces are aluminium then removal of oxygen still allows polymerisation to take place but the absence of catalysis means it’s a lot slower. The aluminium oxide is a bit of a red herring - even soluble aluminium compounds are not catalytic.

In reality it’s a bit more complicated because the requirements for long-term storage and efficient curing are challenging and need all sorts of additives to get the required control, but those are the basic principles.

Many thanks Norman, that makes sense and explains things at the sort of level I was looking for. I'd come to the conclusion myself that the mechanism must be more complicated than the curing of eg epoxies and cyanoacrylates. I'd looked at the MSDS for Loctite 638 but hadn't identified the role of cumene hydroperoxide.

Apart from learning about gluing aluminium togetether, my internet researches have revealed that there's quite a lot of confusion and misinformation about adhesives out there (OK, as with everything else!) - a well-informed series in MEW would be good!

Rob.