I noticed a lot of manufacturers are switching to plastic drive gears. They use something like Delrin or nylon which has a low friction factor. Plastic gears also help reduce gear noise, and are cheaper to make than metal gears.
However a number of manufacturers have had some very severe problems with these types of gears. They wear out, they crack, they split, and they chip a lot faster than metal ones. (I won’t name names, but all of us who have been modeling that last 10 years know the mfgs and particular models very well)
I definitely prefer metal over plastic any day. I have two G-scale Bachmann Consolidations with cracked nylon drive gears, which makes them inoperative, and the only replacement offered by Bachmann is to buy a complete axle/gear/wheel set with another nylon gear.
Switching to plastic gears? Heck, when was the last time a plastic manufacturer used metal gears? Other than the worms in Athearn models, they’ve always been all-plastic since the rubber band days. Atlas has always used all-plastic gears. Stewart, too. BLI, Walthers, P2K, P1K, Kato, etc. They all use plastic and always have. In fact, they have to for driver axles as they almost all use the split axle for electrical pick up. If they used a metal gear, it’d short out.
Brass companies like Overland, Key, NJ/Custom Brass, Alco, Westside, etc. have used brass gears and continue to do so…but even then some use at least one plastic gear to lessen the noise.
The problem with brass gears is the noise. Some are so noisy that you can’t even add sound to them because you can’t hear the sound over the gear noise. If you want sound locos, then you want plastic gears.
Plastic gears can be made so that they don’t split. The problem with that is that they are more expensive.
My guess is that the problem with the split or cracked gears is due to the fact that the compression fit between the axle and gear is too tight. Having done some manual and CNC machining in my former job, a “proper” compression fit fits into a VERY narrow window of tolerance*.
*Tolerance = The minimum and maximum a particular dimension can be in order to achieve a proper fit
So, mating pieces have to be VERY consistent in their relationship with one another in order to get a proper fit. Too loose and the parts slip; too tight and the parts will eventually split.
And this is compounded upon when using different materials with one another. Metal expands and contracts with temperature changes. Plastics do not but do become more brittle when they are cold.
Say, for example, you have a Delrin gear on a metal axle and the temperature is rising. The metal axle wants to expand both axially (along the axis) and circumferentially (along the circumference) because that’s the nature of metal. However, Delrin - being a plastic - doesn’t respond to temperature changes the same way and the inside diameter (ID) of the Delrin gear’s center axle hole stays the same.
Delrin or Nylon is “forgiving” up to a certain point of stress from another object. (In this case, stress coming outwardly, along the ID of its center axle hole.) However, beyond that point, the stress has to be relieved somehow and…< CRACK! >…you have a split gear.
Again, “proper” compression fitting should eliminate this. The problem lays in:
How well the tolerances between with axle OD and the gear ID are maintained
How good the QC dept. of the manufacture is in checking those tolerances
Oftentimes, a QC person will only check a very small percentage of a lot fo
It depends on the scale. On HO and smaller plastic gears are the norm and I have only had problems with early P1Kand P2K axle gears splitting on HO locomotives. On larger scales stress factors may become an issue. My 1:1 scale automobile and yours too probably has several “plastic” gears internal in the engine.
I think NWSL has it right for their gear systems. They typically use a steel worm driving a plastic gear made from aged Delrin, and then each additional gear is brass. This way, the drive is still practically silent, but it’s also extremely reliable.
The problem with common Delrin gears is that the manufacturers don’t use a properly aged Delrin, and then the stress and aging will cause it to split. When plastic gears are made right, they can last as long or longer than metal.
The forces in model railroading are not sufficient to warrant concern over material selection. Being concerned about mesh and quality of the allignment should be of much more concern.
Usually the motor worm is metal. By the way, the spiral gear on the motor shaft is a worm, Not a worm gear.
The rest are plastic. Some plastic gears are not the best quality or the hole the metal shat fits in is to smaller a diameter than is should be that can crack the gear eventually.
I really like the NWSL plastic worm gears, spur gears. Excellent quality. The NWSL worms are always metal.
I agree, provided gear tooth loads are kept within the capabilities of the material, that plastic gears are quiet and perform well. I’m not sure that the tooth loads or compression fit loads can be kept within plastic’s capabilities in scales larger than HO. And I question how long even good plastic gears would last if we returned to the days of single die-cast locomotives pulling 60 car trains in HO down at the club.
The worm tooth has the highest loading in a normal gear train, which is why that is normally made from metal in most gear trains. The matching worm gear or idler gear must be made from a softer material to avoid excessive wear on the worm teeth. Brass worms and brass worm/idler gears have just as many wear problems as plastic worms/plastic worm/idler gears. Steel worms seem to work well with brass gears, and brass worms seem to work well with plastic gears.
Metal gears can be made to perform quietly. The gear teeth have to match each other, and be clean of any flash or irregularities. And gear mesh has to be right. With all metal gears, the noise is an indication those conditions do not exist. The noise is also an indication that excessive wear is/will be taking place.
Improper gear alignment with plastic gears doesn’t produce noise the way metal gears do, hence there is little indication that the gears will wear each other faster than if properly matched, mated, and aligned.
The gearbox plays an important role in the alignment and mesh of the gears. To me, the true test of a gearbox/gears is the overall friction level. Remove the gearbox, chuck the level worm shaft into a drill, and run up the RPMs. If the gearbox stays steady at the 10 o’clock position or lower (the lower the better), you are in pretty good shape. If the gearbox rotates, you ha
While I was installing a new gearbox on an older brass engine, I noticed that NWSL, the manufacturer of the gearbox, stated that gears of the same material should not mesh with each other. When set up of the new gearing was complete I had a steel worm driving a plastic idler which in turn drove a brass driving axle gear. The gear train I was replacing had teeth that had crescent shaped wear patterns on them. I’m guessing that was from the gear lash being too tight? has anybody else ever seen this wear pattern?
In order for gears to work properly, they have to be aligned on four planes. Vertically, horizontally, in yaw, and in pitch.Not only that, but the geometry of their working surfaces must be compatible, and designed to deal with forces over a range of their working surfaces thereby.
Crescent-like wear patterns are common. They also appear on the sprockets on bicycles, for examples, and also on well-worn, and especially on aggressively driven, motorcycles with chain drives.
I believe that nylon on nylon should work well with the correct engineering and assembly, and provided the assembly includes a suitable grease. However, just as metal gears and engines need breaking-in, and then an exchange of lubes to remove break-in bits worn off, plastic gears could probably stand a similarly respectful treatment. It would seem that, with the modest engineering and materials in our engines, just running them until they break, or until we remember to service them a year or more later, is not doing them much good.
Lastly, I believe that nylon and delrin gears would stand up well to model usage, even in O gauge, if they were placed properly on wide enough shafts, probably press fit onto starred shafts and not relying on a really tight fit on smooth round ones. If the shafts were increased in diameter by about 50%, and starred, and the gears cut/ground/cast the same way, a press fit would be much less onerous, and would not be likely to cause splitting. It may be that the gears, themselves, would also need to have mor mass and have larger dimensions commensurately, but I don’t know. Means redesign of many transmissions/gear towers, but it would be worth it…think of the un-green shipping and wastage of materials from parts that break. Hardly economical. Would any of us pony up another $15 per newer model of a given engine knowing we would not have to send it back because it has the