Brass gears: what are your thoughts?

[Will_J]

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.

[Bob G]

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

[ntpntpntp]

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.

[Roy L S]

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

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).

[EtchedPixels]

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.

[Bob G]

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).

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