Hi all,

As my motor bogie design slowly progresses with the arrival of the wheelsets, I'm now thinking about the geartrain itself. My supplier has told me that they can't do POM (aka Acetal/Delrin) gears but that brass is no problem - and indeed the M0.3 brass gears on the test axles are beautifully made.

So before I go shopping around for a supplier for POM gears, what are your thoughts about modern all brass mechanisms? Is it anachronistic, or is it viable as a quality mechanism, with precision machining and modern lubricants?

Interested in your thoughts on this, thanks all .

Will.

Nothing wrong with a brass geartrain in my view, but perceived wisdom has it that brass gears meshing with a brass worm can lead to bad wear on the gear in contact with the worm.

I guess it just depends how much you thrash it. My Minitrix Dock tank suffered horrendous gear wear that way.

Bob G

I know you asked the same question on FB, so this is the same answer I gave there

Having replaced so many split plastic gears on various brands of model (not just Farish but also Minitrix, Arnold, Roco, Bemo) I'll try and replace with brass whenever I can get hold of a matching gear.  I had a favourite 3rd party German firm who sold sets for specific European N locos, but not sure if they are still actively producing.

I've only ever come across worn brass where the mechanism's clearly never been serviced and given a little lube, and has been running totally dry for a long period.   I have properly looked after all brass mechs from the 70s still running beautifully smoothly with no wear.

I use Labelle 106 PTFE grease for gear teeth and worm threads.

@Bob G  yeah the old Minitrix 0-6-0T chassis are notorious for wear - not necessarily the gear in the first instance but the axle bearings open out until the axle is floppy and that the causes the gear to wear.

Quote from: Bob G on Today at 02:35:47 PMNothing wrong with a brass geartrain in my view, but perceived wisdom has it that brass gears meshing with a brass worm can lead to bad wear on the gear in contact with the worm.

I guess it just depends how much you thrash it. My Minitrix Dock tank suffered horrendous gear wear that way.

Bob G

Brass worm driving a brass pinion gear is certainly not a great idea, as you say, the worm will typically wear over time. I had a number of Farish locos with the "new" integral motor chassis used from circa 1976 which initially used this arrangement. As they wore you could see  brass shavings and a distinct arc in the worm develop, to the point where they would no longer engage with the pinion gear. Of course lubrication helped, but did not fully cure the issue, that eventually happened when Farish switched to plastic/nylon gears from about 1983/4 (but brought about other issues, especially on diesel models - the notorious split gear).

The 2mmSA I believe uses some kind of aluminium to be strong and avoid the brass wear problem. Possibly someone who knows more can fill in ?

For plastics the big problem is the wrong plastics. Nylon gears are near indestructible but they cost pennies more so we got garbage gears made worse by then pressure fitting the wrong plastics as well. Good quality nylon or similar gears can also be made self lubricating which is a huge win.

Quote from: Roy L S on Today at 03:59:19 PMOf course lubrication helped, but did not fully cure the issue, that eventually happened when Farish switched to plastic/nylon gears from about 1983/4 (but brought about other issues, especially on diesel models - the notorious split gear).

All sorts of theories have been put about for split gears, including these:

• Excess oil attacking and weakening the plastic gears
• Thermal expansion and contraction (regardless of whether this is correct or not, storing locos in an even temperature is recommended)
• Stress fracturing caused by the interference fit on the drive axles

You certainly should not run your new toy when the postman has just delivered it, whether in cold winter or hot summer, until it has equilibrated with the railway room temperature.

Having rewritten and reinvestigated the split gear issue when I re-wrote the split gears page on the NGS website many moons ago, I seem to remember that the earlier nylon gears never seemed to split. They were, however, significantly more expensive than the later plastic ones. The worst of all were the smallest gears fitted to the class 20 bogies. They would fail even as you looked at them (they must have had the highest interference fit stresses), followed by IIRC the largest gears used in the double ended class 101 DMU drives to drop from the motor axle to the worm axle (this problem could have been engineered out using universal joints).

Steam locomotives were not completely safe, either. The gear that meshed with the worm in the steam locomotives was also a potential failure point.

I know this is off-topic, but the OP might not be aware of all the problems this community has faced over the years.

Bob G

Quote from: Bob G on Today at 04:46:19 PMYou certainly should not run your new toy when the postman has just delivered it, whether in cold winter or hot summer, until it has equilibrated with the railway room temperature.

If it breaks because it's cold or warm but in normal household range it's not fit for purpose. There is no excuse for the awful historic design choices Graham Farish made and Bachmann repeated. Bachmann particularly have no excuse as they'd already had the same fiasco in the USA earlier and were widely known as Botchmann for many years due to it.

If it were an actual issue it would warn you in the instructions, spoiler alert - it never did.

I spent some time with someone who works on plastics and gears and the like when looking at doing some drive chains for Etched Pixels (I gave up as the wheel problem was too hard) 

- Mineral oils attack some plastics so over time are not good for them. That bit is true. Mineral oils are also often carcinogenic so don't belong on your fingers either.
- Thermal and stress fracturing are caused by the wrong plastics for the application. Nylon doesn't have that problem.

There are ways to use other plastics than nylon. Amusingly Bachmann USA had this right for years when their UK arm was pushing rubbish. If you've ever taken one of the US designs apart they have a central plastic gear moulding that takes two half axles and there is no pressure on the central critical gear area, and only a light grip on the axle halves.

The newer stuff does seem a lot better all round but they clearly had serious issues before that even on newer models where the 57s in particular appear to be built with a batch of very bad gears indeed. I imagine what it cost in warranty returns far exceeded the cost of using proper gears, and what it cost in reputation definitely did.

Alan

Quote from: Bob G on Today at 04:46:19 PMI know this is off-topic, but the OP might not be aware of all the problems this community has faced over the years.

Bob G

That's exactly right Bob. I can design and make things well enough, but I haven't got decades of immersion in the hobby behind me to know what pitfalls to avoid, so I rely on the collective wisdom of the community and am very grateful to receive it !

@Nick thanks for your answers both here and on Facebook. It's clear that there are definitely some mechanical considerations to it, but it's also great to get a feel for people's perceptions and whether I have any "bad BR" from previous manufacturing eras to overcome with it.

I am pretty sure I can find a good quality supply of M0.3 worms in nylon or acetal, and that should mitigate the possible wear issue there.

Thanks again everyone I really do appreciate the perspectives here.

Will.

Quote from: Will_J on Today at 05:07:53 PMThanks again everyone I really do appreciate the perspectives here.

Do reach out to the 2mm scale association wheel people for their experiences on the replacement drive axles they did. I am sure they'll be happy to share their experiences and possible supplier info and the like.

I think others have covered the salient points, so I will only add the following thought.

Plastic gears are the better engineering solution for a number of reasons, however I have a theory (and it is only that), that if presented with two identical mechanisms, one with brass gears and one with plastic, most people would judge the one with brass gears to be 'better' or 'higher' quality.

Therefore, a product might be more commercially viable with brass gears rather than plastic, due to perception of quality / value, despite it being a worse technical solution.

Just my personal opinion on this - I have never had a brass gear split!
Kev E.

Quote from: me00rjb on Today at 05:30:43 PMI think others have covered the salient points, so I will only add the following thought.

Plastic gears are the better engineering solution for a number of reasons, however I have a theory (and it is only that), that if presented with two identical mechanisms, one with brass gears and one with plastic, most people would judge the one with brass gears to be 'better' or 'higher' quality.

Therefore, a product might be more commercially viable with brass gears rather than plastic, due to perception of quality / value, despite it being a worse technical solution.

I was told by someone more expert than me that plastic worm onto plastic pinion is not optimal as there is more "stiction" (the inability to overcome inherent friction between the surfaces) than using a brass worm onto plastic or nylon pinion gears.

Yes...wot he says! 

Brass worm onto nylon gears every time.

Quote from: EtchedPixels on Today at 05:10:21 PM

Quote from: Will_J on Today at 05:07:53 PMThanks again everyone I really do appreciate the perspectives here.

Do reach out to the 2mm scale association wheel people for their experiences on the replacement drive axles they did. I am sure they'll be happy to share their experiences and possible supplier info and the like.

Asking The Model Railway Club might be an idea as well. Locos on Copenhagen Fields cover many miles (real, not just scale!) and I don't recall hearing many reports about worn gears. One of the recent posts on RMWeb from Tim Watson is about overhauling a 25 year old A4 due to worn rods and axleboxes - no mention of drivetrain problems.

Quote from: Roy L S on Today at 07:25:03 PMI was told by someone more expert than me that plastic worm onto plastic pinion is not optimal as there is more "stiction" (the inability to overcome inherent friction between the surfaces) than using a brass worm onto plastic or nylon pinion gears.

This is probably achievable. All I would need is a plastic duplex gear, which I could potentially have moulded and could be used in multiple future mechanisms. Brass worms in M0.3 are easy to source.

Will.