aluminium for gears

Chaps,

I plan to make a clock at some stage and hope to incorporate some ideas of my own. One is to use aluminium for the gears. I have not seen this suggested before. Are there good reasons not to do this ?

I can think of positives (light weight, easy machining, cost) but only one negative and that is the potential for wear. However to counteract this, I was thinking of anodising. This will give a coating harder than the steel and will reduce friction as well.

But I have probably forgotten something obvious and would appreciate your thoughts.

Gerry

Anodised onto Anodised may well pickup depending on lubrication used also it would help if the meshing areas were polished prior to anodising otherwise you would have any tool marks turned into nice hard cutting edges

Paul

No doubt someone with more competence on the matter will tell you more, but my experience shows that ali-ali pair does not work. There will be welding and galling pretty much at any power. However I think you can make one gear on aluminium and pair it harder or sofer material and you should be fine. Smaller gear is usually made out of stronger material.

There are may different grades of aluminium, fairly pure is not that strong and it's not nice to machine. Many more alloyed grades probably would offer success.

If you pair aluminium alloy with plastics, you ususally don't need any lubricant, which might be plus.

Pekka

We have worm drive gears on our Theodolites, the gear itself is anti backlash and both the worm and gears are anodised. So obviously they are Aluminium.

They seem to work OK and are able to turn the Theodolite and also the Telescope.

Clive

I made an aluminium changewheel for my mini-lathe. It runs very happily with the standard nylon ones.

Neil

Hmmmm

Well it obviously works for both Neil and Clive's applications. Anodising ? How does this respond to a rubbing action - will it, in time, be removed ? The thing is that a clock runs for a long time ( no pun here). I recently overhauled an English Dial clock which was last repaired over 100 years ago. No sign of any wear to the wheels. I think material costs are not really an issue here and surely brass is just as easy ( easier?) to machine as aluminium ?

Norman

I repaired the small cog on my backgear on my Drummond Mseries over a year ago with T6 aluminium

It's done some pretty serious work over the period without any problems

Edited By Ady1 on 06/05/2012 00:11:06

Many years ago I was in Kenya and I had a breakdown with my Opel Kapitan when the camshaft gear stripped its teeth, it was made from Tufnel. (Linen Resin composition)

The replacement gear was made from Aluminium and I had no more trouble from it at all.

Clive

Please be aware that as far as anodising goes, for maximum wear resistance "Hard Anodising" is normally specified as opposed to the normal decorative (coloured) anodising we see everywhere.

I have not seen it used for gears but I have used it for 16 mm static film guides with the film travelling through the guides at a very fast lick.(old engineering expression)

It has the drawback of increasing the component dimensions.

For example you can specify a thickness of up to 5-6 thou and this would result in a dimension growth of half this amount (i.e. 50% in and 50% out) and this would be taken into account at the design stage.

Also the finished coating is porous and non-conductive, in the dim distant past I have used it as an insulator on diode holders in Doppler Radar systems.

Most hard anodised components are subsequently sealed (closing the pores) and this sealer could be one of many things including lubricants, ptfe etc.

Whether or not Anodising companies nowadays are prepared to accept very small orders is debatable.

Clive,

There are other metals that can be anodised including Titanium and Zinc.

Len. P.

Thank you len, i was aware of the anodising alternatives having for many years done small instrument parts in alu. and other exotics all requiring durable finishes.

Regards the anodising, the hard condition is AA25 for things like window frames and perhaps extreme exposure to weather.

Normal cosmetic anodising is AA5 and for medium conditions AA15.

There is another type of anodising treatment that gives a very shiny finish but the name eludes me at the moment, I know its German name, Eloxiert.(which is of course anodising) but it has a further treatment.

Clive

I don't see why aluminium wouldn't work, but one would have to choose the grade carefully. As far as I can see 7075 is about the same hardness as CZ120 brass, so should be ok, and machines well.

Ideally for a really hard and resistant finish one would use the Keronite process:

http://www.keronite.com/

But, as always, the problem would be actually getting it done.

Regards,

Andrew

Thanks all for your comments and apologies for taking so long to reply. There dont seem to be any major risks so I will give it a try.

As for the anodising, I thought I may try to do it myself - there was an article on it in MEW. Keeping the current low gives a fine finish. I dont plan to use any lubricant in the finished product and hope that the anodic coating will prevent scuffing and keep the friction down. But as one of you said, I need to avoid small machining ridges on the teeth or they will create high localised stresses.

Regards

Gerry

All this talk of friction between the gears is confusing me. Perhaps I am missing something but as I understand it with correctly depthed cycloidal gears there is a rolling motion on the teeth and no rubbing so there can be very little friction. Any wear is caused by dirt? The only wheel with rubbing on the teeth should be the escape wheel.

In the 1720s John Harrison famously made a turret clock from oak with lignum vitae bearings and it is still running. I'm sure aluminium alloy should be more durable!

Russell.

My local anodiser does Hard anodising but only on pure aluminium. ie not alloys. Alloys also affect the colour you obtain if not just clear. Small orders not a problem.

Russell,

You are going into clock making ,good. The first thing you have to remember is FRICTION, friction, friction.

Forget all about the proper fits between gear wheels as given by calculating the PCDs etc fitting the gears is done by hand using a Geneva tool and make them a nice light fit. Back lash is never a problem. The pivots (shafts ) are a rattling good fit in their bearings . Remember clocks are very highly geared items. Ask yourself how many times will the thing beat in say 30 days and how many times the winding drum (or spring) will rotate in that time?

Aluminium can be intrinsically a very sticky metal, hard brasses used in clocks are not.

As a well known clock maker remarked “If it rattles it will run”.

Old John Harrison was a ‘downy old bird’. He knew the properties of lignum vitae. It is a very hard, dense and oily material. It was (and may be still is) used as part of the main of the thrust blocks which transfer the push of the propellers to the ships. Harrison’s lignum bearings make your aluminium seem like soft cheese and they are self lubricating.

Rdgs

Dick

Hi Dick

Absolutely but, ignoring elastic distortion, friction is caused by rubbing. The teeth of properly designed and meshed clock gears have a rolling action against each other without rubbing. By far the greatest source of friction in a clock should be the escape wheel rubbing on the pallets.

I used to use hard rubber in the cutless bearings of my boats but, as you say, lignum vitae is excellent. A good source is old lawn bowls.

Russell.

I have a comercially made French three train wall clock from the 1930's that has aluminium plates and gear wheels along with the spring barrel as well.

The pinions are steel as per a normal brass wheeled clock, it still runs OK so obviously the use of aluminium is not as new as you might think.

Here's another one for you to think about. Have you decided to use cycloidal or involute teeth ?

I know of one local company (Sinclair Harding) that currently produces some remarkable clocks using involute gears, but there are also many people who will also tell you that they will never work in a clock.

Phil

Nothing wrong with involute teeth in a clock, despite what "they" will tell you, and the cutters are a good deal cheaper than the cycloidal ones from Thorntons. And of course, if you can hob them, you only need one cutter.

It is an unending source of amusement to me that today's clockmakers are, for the most part, such a conservative bunch - if it issn't how it was done in 1785 then it can't be right. On the other hand, the old clockmakers, such as Harrison, were among the innovative scientists and engineers of the day - they would probably have given their eye teeth to have had access to the kinds of tools, techniques an materials that we have available to us and wouldn't have given a second's thought to using them if it would have resulted in a better instrument.

Regards,

Tony

Yes, quartz clocks and watches use involute gearing. This is because, with cycloidal gearing, you shouldn't let the pinion drive the wheel. The disadvantage though comes when you need small tooth counts on the pinions.

I agree about conservatism. Much of clockmaking practice has been derived from trial and error using what was available at the time. It still works well but could probably be improved using modern theory and technique.

Russell.