Aluminium eccentric

Curious to know whether people consider if aluminium would be a suitable material for items like loco axle pump eccentrics, riding car band brake drums and items 'fixed for life' with pressed on wheels either side?

Lighter, less corrosion (perhaps) and possibly a better bearing material than steel???

Edited By Nick Clarke 3 on 23/08/2018 19:24:24

The probem is that small particles of aluminium wear off and oxidise. Aluminium oxide is used to make grinding wheels.

Brian

Hi Nick I have used high grade aluminium for all sorts of wear parts from motorcycle disk brakes to plastic mould tools but they were all hard anodised and some were PTFE coated, not cheap but very light and wears well.

David

Thanks for answers (and any in the future too).

My reason for asking was what about the long term issues as taking wheels off axels, press fit or Loctite is a non-trivial action in my opinion and thanks for some useful comments on this from you both.

Problem with aluminiums is its picks up at no notice. Rotational will wear quick.

Intrigued by aluminium discs Dave, have had hard anodising and ptfe impregnated stuff done but were slow moving parts.

The brake discs were a trial to reduce none sprung weight on racing motorcycles and was quite acceptable but you had to keep an eye on the coating for wear. The tool coating was done by Acorn Anodising local to where I live. The tool part was an aluminium sleeve with a groove cut in a spiral like a shallow thread in which a heating element was wound and then moulded over to make a pipe fitting. The sleeve was pulled out by a hydraulic puller and then rewound for next cycle, with little wear to sleeve the heating element was about a multimeter diameter and the groove was about .25 mm deep. Quite a pull.

David

"The probem is that small particles of aluminium wear off and oxidise. Aluminium oxide is used to make grinding wheels.

Brian"

If this is the case, how come most IC engines are running with Aluminium pistons.

I see no problem with using Aluminium alloy for eccentrics, provided eccentric straps are made from case hardened steel. However, the environment beneath a Locomotive boiler is hardly conducive to longevity of nearing surfaces.

Old Triumph twin motorcycles had the big end pin running direct on the aluminium con rod. They seemed to work quite well, but for some reason aluminium eccentrics doesn't feel right. As it's a difficult job to replace the eccentric and easy to replace the strap I'd stick to the conventional way. The mass of the eccentric is not going to change your life.

Edited By duncan webster on 25/08/2018 12:25:29

Many small engines use aluminum alloy rods running directly on a steel or cast iron crank, and aluminum pistons in the bores. However, these engines are closed crankcase designs and have a generous splash or pressure feed oil lubrication system, not exposed to dirt ash and grit generally. In addition the aluminum used is a special high silicon die casting alloy not generally available in bar form.

As Bob said the underside of the locomotive is not particularly free of dirt ash and grit. Oil supply to the eccentric is not a sure thing.

I would say any benefit of using aluminum in this case is negligible and there are some risks, I would also recommend the traditional steel/bronze or steel/ cast iron combination for eccentrics on locomotives. They are well proven in small and large locomotive bearing applications.

Is it because the aluminium body of a piston in an IC engine doesn't touch the cylinder? I'd expect most wear to take place between the piston rings and the liner, neither of which are aluminium?

Dave

S.o.D: The rings need to be free in their grooves to work, and this means not constrained by having to act as location for the piston. (Which would mean that the back of the ring was in contact with the bottom of the groove, stopping the ring from moving freely). The aluminium doesn't touch the cylinder very often or very hard because there is a film of nice grit-free oil. Or did I misunderstand your meaning?

Bob R: Most engines (certainly those with aluminium pistons) run with their pistons in a spray of oil which is filtered and renewed. They are not in an atmosphere of ballast dust or ash. And the aluminium is indeed a fancy high-silicon alloy not easy to get even if you do want to make eccentrics out of it.

Cheers, Tim

Edited By Tim Stevens on 25/08/2018 18:42:55

Phil -

I have moved your question to its own thread in the manual machine tools topic where it might get more attention.

It's here: www.model-engineer.co.uk/forums/postings.asp?th=137568#2370761

Neil

If you must use aluminium for an eccentric strap, the crown of an old (reasonably large) piston (old truck, or tractor) would supply enough metal. If possible, use cast iron, much lower friction, and with a locomotive the usual thing is to try and get as much weight as you can for traction, there is nothing gained if you save a few grams in one place, and add blocks of lead for extra traction.

Ian S C