Wheel Pitting

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Wheel Pitting

Pitting of locomotive wheels is a problem that has to do both with the materials that the wheels are made from and the peak currents drawn by the locomotive. Some locos and some wheels seem to pit, other's don't. In some cases, I think that I have figured out why.

The short story is that if the current density at a wheel contact patch is high enough and the melting point of the wheel (and rail) material is low enough, serious micro heating at the contact patch can vaporize wheel and rail material leaving pits. That vaporized material instantly oxidizes and redeposits in and around the the pits forming a tough non-conductive layer that is hard to clean off.

A wheel in this condition has seriously reduced area available for conductivity. As the wheel rolls, the unpitted areas are exposed to heating and they pit as well. Soon, the wheel is nothing but pits filled with crud and the wheel stops working at all for the purposes of power pickup. Some of the crud can be cleaned off with the non-aggressive abrasion action of a wire wheel, but if the material is really dug in there, the wheel must be refaced. If the wheel is plated, then the plating will be removed in the resurfacing process, usually exposing the base material which is usually more susceptible to pitting than the plating was.

Pitting is more likely when most of these conditions are present.

Pitting can be mitigated by these conditions.

When the track and wheels are already covered with non-conductive materials, the probability of any given wheel loosing contact with the rail increases. This increases the current density in the remaining wheels. When only one wheel is making contact at any given instant, the current density in that wheel is far higher than normal. It is this condition that is really conducive to generating enough heat at the wheel to rail interface to melt metal. The more wheels there are contributing to power pickup, the less likely that any one wheel will have to shoulder the full load. Cleaner rail also reduces the chance of high individual wheel current densities.

USAT GP9 Issues

I have one USAT locomotive, a GP9. Many years ago this loco has had it's stock wheels replaced with NWSL stainless steel wheels because the stock wheels simply burnt up. I had initially replaced the stock wheels with semi-finescale nickel silver (an alloy of mostly nickel and copper, there is NO silver in it) wheels from NWSL because the original wheels needed replacing and they were too small, a little less than 33" where they should have been 40". These wheels didn't work out as the flanges were too small and the loco would not stay on the track. NWSL replaced them with a slightly smaller 36" stainless steel wheel with a larger flange. Initially, these wheels worked fine, however over time even the stainless steel wheels started to pit. Eventually the loco became unreliable and it got sidetracked.

Feb 19, 2009 I put the GP9 on the track again to see how it ran because I hadn't actually run it much in years. The GP9 ran badly. It sputtered and stopped. The DSX sound system continually reset. It was unusable. I flipped the loco over and looked at the wheels. They all had bands of pits around them and the sliders were packed with crud. I took the loco into the shop and used a Dremel tool with a brass wire wheel to try to clean the wheels and sliders. I got some of the crud off, but not all of it. I put it back on the track and it actually ran fairly well, the DSX at least wasn't resetting but after a few minutes, it degraded again.

Back in the shop, I turned the loco over and connected DCC power to the sliders, ran the motors, and used a Bright Boy to polish each wheel. After some considerable time on each wheel, they got better, but on some wheels, I had to use a grinding wheel to get through the pits. I then did some more extended running with the Bright Boy to polish the wheel surface again. These wheels are solid stainless steel and it is hard stuff.

Back out on the layout it ran for awhile and then I flipped it over again and the pits were back. Something had to give.

090219_girr_gp7_burnt_wheel_6458.jpgThe GP9 didn't get run very much because this continual wheel pitting even before DCC was installed. This photo is of an original USAT wheel that had been pitted when run from Aristo PWC only. That black band is not a traction tire, it is a collection of perhaps millions of micropits. This pitting is the main reason that the stock wheels were changed out in the first place. The nickel silver wheels didn't get run long enough due to derailments to develop a pitting problem. The stainless wheels still pit nearly as badly but in a much more severe environment.

Since others are not complaining about this problem, this extreme pitting has to be unique to my particular situation and now I think I know why. My situation is what is known as a "corner case." This means that my situation is way out in a multidimensional corner of the space of all possible conditions. I have extremely high current available combined with pulse power and with a loco that uses motors that want to draw extremely high currents. I also have track that is typically not very clean therefore it is likely that only a few of the available wheels are doing any useful work at any given instant there there are lots of times where the entire load of the loco is supplied by only one wheel on either side.

The USAT motors have a stall resistance of about 1 ohm each. There are two motors in parallel. The low motor resistance combined with the very high current D808 decoder, the NCE PB110 booster and low resistance track wiring, allows these motors to draw just about as much current as they want.

DCC (and Aristo PWC) puts pulses on the motor. The pulses are about 20 volts. When the motor is not turning, the motor acts like a resistor, in this case about 1 ohm per motor. This means that the motors want to draw 20 amps EACH until they get moving. There are parasitic resistances and inductance that limit this current to lower values, but by measuring the voltage drop on my track when the loco is running and knowing the actual resistance back to the booster from the point of measurement, I can estimate that the peak current drawn by the loco is on the order of 20 amps in the worst case. The peak current is less than that much more often but it still reaches 20 amps at times. This is just plain crazy.

When a wheel finds some finite resistance in the track to wheel contact and the value of that resistance is not high enough to actually limit the current much, the loco will try to draw current through that resistance especially if that wheel is the only one making contact at that instant. The wheel contact patch is very small so the current density can be very high. Any current drawn though the resistance of the wheel contact will immediately produce heating in the very small contact area proportional to the square of the current density in that area. The heating can be enough to actually vaproize a little rail or wheel material and spit it out, hence the sparks that can be seen when running at night. The sparks are really the trails of incandescent particles being ejected from the contact patch. The temperature of the superheated area can be estimated based on the color of the ejected material. Bright orange and white "sparks" indicate that the material temperature exceeds 1100°C, hot enough to melt brass. In the area of the superheated contact patch, some of the metal that is evaporated from the rail forms a plasma and oxidizes instantly in the air. Some of it leaves via the sparks, some of it deposits itself right back on the wheels and rail as oxides of the metals involved. The wheels get the worst of it because each spot on the wheel gets this treatment every wheel turn instead of just when a train comes by.

Some of the sources of those extreme temperatures are resistance heating due to an imperfect contact, an actual arc across a very small gap and the heat of oxidation of materials that can be initially melted and converted to a plasma by the heat that is already there. "Oxidation" is the term that describes combining a material and oxygen. This is an exothermic (released heat) reaction also known as "burning." Fire is the term used for rapid and self sustained oxidation. Rusting (for iron anyway) is the term used for slow oxidation. Iron and steel will burn, ignite some steel wool and see what happens (in a fire safe enclosure please). Copper and aluminum don't burn with a self sustaining reaction like iron does without the presence of chemical oxidizers. Aluminum perchlorate is the stuff in the space shuttle solid rocket boosters.

Eventually, the metal that was evaporated from the wheels leaves pits behind. These tend to fill with the oxide crud which results in a band of oxide filled micropits all around the wheel. This pretty much messes up the wheel and makes it MORE likely to have resistance at the wheel contact patch which causes more pits and more oxide. Further, since the oxide is hiding down in the pits, it is hard to remove. When the pits are bad enough, the only way to get it off is to remove metal from the surface of the wheel to eliminate the pits.

The reason why the GP9 was more susceptible to this problem than any other loco I have is that it can draw very high current, especially when starting. I have beefed everything up so much that the peak currents can be so high that even stainless steel wheels cannot take the abuse.

What to do?

I needed to change something or this loco would never run reliably again. I considered using a decoder that would limit the current that the motor can draw and still not blow up. I know that RCS's RC system can drive USAT locos without failing. I also know that RCS uses a very high quality motor driver IC, the LM18200, that provides current limiting (at a little more than 3 amps) and protects itself. I don't know of a DCC decoder that is "safe" to use on USAT locos other than the D808. But the D808 is part of the problem just because it is so beefy. It is designed to source all the current that these motors want and they want lots of it. I wasn't ready to cut and try other decoders and possibly turn them into slag heaps if they couldn't deal with the current spikes. The MRC AD322 decoder isn't rated for these locos, but I know of people using them. But the MRC AD322 is a junk decoder and I didn't want to put one of these in that loco, besides, it may not actually solve the problem.

090219_girr_gp7_resistors_6454.jpgHowever, it occurred to me that if I wanted current limiting, there is any easy, if not crude, way to get it. I put two 0.5 ohm 15 watt sandstone resistors in series with the decoder output to the motors. This increased the stall resistance that the decoder sees by a factor of 3 and brings the peak stall currents down to 13 amps maximum. In practice, the peak current is less than half of what it was before. The resistors also causes voltage drop when the engine is running, but only about 3 volts worth. The loco still has plenty of torque to slip it's drivers and runs at an acceptable maximum speed, at least I find it acceptable. Now the current is lower and there is less energy available to burn pits in the wheels.

I cleaned up the wheels again and gave it an extended test run. SUCCESS. After a couple of hours dragging my track cleaning car on track that was already in good condition, the wheels didn't pit again. The loco was drawing about 2.5 amps average from the track. With the resistors in place, the full slip current is just under 4 amps. When I press the loco into the track to try to stall it, the current increases to just over 6 amps. This isn't stall current because the motors don't actually stall, it's just a very highly loaded slip condition. The loco still has plenty of torque and can easily slip it's drivers when restrained by the coupler.

I'm not looking too favorably at purchasing more USAT locos unless they make some serious changes in their motor specifications.

Over a period of heavy use, the GP9 wheels still accumulate oxides that must be cleaned off occasionally, but the wheels are not pitting. The oxides appear to be all from the brass rail material and they come off easily with the aid of a brass wire wheel and a Dremel tool. If I was using stainless steel rail, even this oxidation may not occur. I may add even more resistance the next time I crack this loco open.

For a time, I was using this loco to pull my track cleaning car so the when it ran it was usually on dirty track. I've stopped using it for that service and use an RDC instead. The RDC has motors that draw much less peak current so that it has never suffered from wheel pitting. Now when the GP9 runs, the track is usually in better condition and even the accumulation of track oxides on the wheels is less of a problem. Running on reasonably clean track and the use of the current limiting resistors seems to have mitigated most of the wheel cleaning that this loco required.

Issues with Other Wheels

Other locos that don't draw nearly as much current as the GP9 can suffer from wheel pitting as well due to having wheel material that is much more likely to pit than stainless steel. These locos usually have bare brass wheels, some kind of bronze casting alloy wheels or wheels with poor plating.

Brass and brass based alloys tend to pit badly as brass has a fairly low melting point (around 1100°C) so that it doesn't take as much current density to produce the heat needed to vaporize it. Often brass wheels are plated, usually with nickel, to prevent pitting as nickel has a melting point of about 1450°C. Stainless steel also has a high melting point, depending on the alloy it is in the range of 1400 to 1500°C.

The resistance of a plated wheel against pitting varies greatly depending on the quality of the plating. LGB loco wheels seem resistant to pitting while older Aristo plated wheels pit badly when the plating is either worn off or abraded off. Steel wheels will resist pitting as well, but they are only as useful as long as their plating (to prevent rust) holds up. If the plating fails, the wheel will oxidize quickly and electrical pickup will degrade immediately.

The rails can pit as well, but since there is a lot more rail area on a layout that wheel area, the pits tend to be spread out a lot more and don't cause as much of a problem. Further, if the rail pits but the wheel does not, then any deposited oxide will be on the surface of the wheel instead of inside recessed pits. Normal running will often abrade surface material off before it ever causes a problem. In every turn, one wheel or the other slips on the rails which is pretty effective at cleaning off the surface of the wheels and rails.

Stainless steel track should pit less than brass. Aluminum should be worse than brass.

Track sliders are usually made of stainless steel. These don't tend to pit and the continual sliding action will usually abrade them clean. Any pits that form in them are soon abraded away anyway. They can, however, pack up with crud scraped up from the track.

Lionel seemed to like to use bare brass wheels on some of their locos. Thomas and James both have bare brass wheels and keeping those wheels clean enough to run properly was a real challenge. I converted Thomas to battery power just because it would not run on outdoor track well enough. James does a little better on indoor track but it has been modified to also pick up power on the center wheelset and a pair of home made sliders. The Lionel handcar also has unplated brass wheels and it wouldn't even run reliably on my indoor track until I added some sliders.

Older Aristo diesels, those that predate the ball bearing trucks, typically have poor plating which wears or flakes off revealing the base metal underneath. I'm not sure what the base material is but it pits easily.

Older Aristo steamers, such as the original Pacific, the older 0-4-0 and Rogers also use a wheel base material that seems susceptible to pitting. The material has been changed in the newer locos and they seem to hold up much better.

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© 2008 George Schreyer
Created 20 Feb 09
Last Updated September 23, 2009