With the emphasis on cost and reliability, I thought that I ought to describe what I look for in determining if a loco will be reliable in the future. The future is always hard to predict, but there are some things observable in the present that that can be indicators of a possible or likely future outcome.
My background is in spacecraft electronics design, manufacture and troubleshooting. In space, there is often little chance for repair so that reliability is the single most important characteristic. The equipment can be made in such as way as to be not likely to fail, usually at some significant cost and guided by a ton of experience with stuff that did fail. Then this highly reliable equipment is made redundant with identical equipment that can be substituted remotely to cover for equipment that fails anyway.
On a model locomotive, I look for features that my experience shows are inherently reliable (and those that are not) and how redundancy is implemented.
Now to the details of what is specifically important to look for.
Power Pick Up. Track powered model railroads use redundancy with good results in the process of collecting power from the track. What is typically redundant is the power pickup. A single wheel contacting the rail is not very reliable for a lot of reasons. There are usually many wheels contacting the track and the more the better. This is observation #1. How many are there? Observation #2 is to determine what the wheels are made of, if possible. Wheel material has a lot to due with the reliability of any given wheel's contact. Observation #3 is the type of connection made between the rotating wheel and the fixed structure of the locomotive. Observation #4 is the current drawn by the locomotive. Observation #5 is the presence or absence of flywheels. Headlight flickering is #6. All of these are important as indicators of how reliably a locomotive will run.
Number of wheels. Many older HO locos, especially brass ones, do not pick up power from every wheel. This is a problem. Since the reliability of a given wheel contact is often marginal to poor, many are required to assure that at least one wheel on each side is making contact at any given time. 8 working wheels is good, more is better, 6 is marginal and 4 is poor.
Wheel Material. It is not often easy to determine exactly what a wheel is made of, but sometimes it is obvious. Copper is bad. This includes brass, bronze and any other copper alloys including nickel silver (which has no silver but is loaded with copper). High melting point materials are good, pure nickel, good nickel plate and stainless steel melt 500°C higher than copper. Copper is bad because it melts at lower temperatures and it oxidizes easily, especially when heated to incandescence. Arcing at the wheels makes enough heat to micro-pit wheels and then the material that was just excavated by the arc instantly oxidizes in the heated air and redeposits non conductive oxides back into and around the pits. See the link to micro-pitting for more details.
Wheel Condition. Older locos have had to time to accumulate lots of miles and this can literally wear the wheels out. The wheel tread should be shiny and clean. Wheels that have a hazed, pitted, or grooved surface will be unreliable. Wheels with a yellowish surface have had all their nickel plating worn off exposing the brass base material of the wheel. The excess wheel roughness scrapes crud from the track as the wheel slips (they all slip at times) and this material packs into the rough surface. Light contamination can be cleaned with alcohol, but heavy contamination needs to be abraded off. If the wheel surface cannot be restored by resurfacing with a grinding block (a Brite Boy is not sufficient and usually doesn't have a good enough edge to get up next to the flange) then the wheels should be replaced.
Athearn and Life-Like diesel wheelsets are interchangeable. Note that Athearn wheels come in both 40" (0.458" diameter) and 42" (0.478" diameter). The whole set must match or the loco will derail and have other difficulties.
Atlas and Kato diesel wheelsets are interchangeable but again, watch out for the wheel diameter if you don't change a whole set.
Contact Type. HO models sometimes use some kind of wiper against the back side of a wheel for contact. These things are often flakey and they do wear out. A better method is via a bushing at the axle. Both methods require maintenance. A very small amount of "conductive" oil on each contact area once in a while can help. Conductive oil isn't actually conductive, it is formulated to aid in conduction of electro-mechanical joints by excluding oxygen and reducing corrosion. It also tends to provide a little cooling and in the case of a micro-arc, it can help exclude the oxygen that results in oxidation.
Current. The amount of current drawn by a locomotive is actually quite important. When the condition occurs that only one wheel on one side is making contact, ALL of the current drawn by the locomotive will be going through that one wheel. The current density at that instant at that one wheel will be highest and cause the most likelihood of a resistance initiated arc and therefore a possible pit. Less current means less available heat and less likelihood that sufficient temperature will be reached to actually evaporate wheel or rail material.
Flywheels. Flywheels are good for two reasons. The first is that the provide mechanical momentum that allows a loco that is having power pickup difficulty to run past the bad spots. Second is that the continued rotation of the motor helps keep it's BEMF (Back Electromotive Force) higher so that when a wheel makes contact again, there is less of a tendency to draw a spike of partial stall current. This keeps the current density down.
Headlight Flickering. Headlight flickering is an indicator of intermittent power pickup and a demonstration that the loco has low reliability. This is hard on the wheels because the flickering demonstrates that at some times NONE of the wheels are picking up power and it is likely that at other times only one on a given side is working. This means that the current density in some of the wheels is high some of the time and those wheels will pit and eventually, the loco will run poorly enough to require heavy wheel cleaning. An alcohol wipe will not clean up wheel oxidation.
DCC Command Reliability. Intermittent power pickup can also impact the reliability of receipt of DCC commands, especially if the interruptions are short and rapidly reoccur. If an occasional power dropout is long enough to cause the decoder to drop out and reset, it may not even be noticed except for perhaps a deep headlight dropout. However, if the intermittent power is in the form of "noise" due to many short dropouts then DCC packets sent at that time may be rejected for corruption. The dropouts are often short enough so that the decoder does not drop out and the loco continues to run smoothly but perhaps at a slower than expected speed because the decoder is not getting power all the time and the average power that it does get is lower than it should be. The decoder will continue to operate in it's last commanded state even if it has been commanded to do something else.
The noise may cause valid packets to be missed. The loco may not change speed when initially commanded and then accept the change later when the command station sends the commands again. Command stations tend to continually repeat commands to refresh decoders that may not have received the message the first time. Headlights may go on or off unexpectedly or even show a reverse headlight indication while running forward. This also tends to be self correcting after a few seconds due to the command station repeating commands.
All this is another indicator of bad wheels. Dirty wheels tend to cause complete dropouts or outright stalls. Clean, but worn out wheels, tend to show these DCC issues (for DCC systems) or just slow speed for DC systems. The dropouts may be so quick that the headlights (especially incandescent ones) are not materially impacted. The only real fix here is to simply replace the offending wheels. A visual inspection will likely show that the base material of the wheels is showing on the wheel tread where the plating has simply worn off.
Motor Type and Condition. The type and condition of a model locomotive motor is very important. Older open frame motors can run reliably, but they tend to be poorer than newer enclosed can motors. This is not so much due to the open construction but to the fact that over the years, the motor manufacturers have learned how to make more reliable motors. In the process, they have also learned how to make less expensive motors by using can motor construction. These two learning processes tended to run in parallel so that the less expensive can motors tend to be more reliable, not because of any inherent superiority of a can but because that is they way that most modern motors (the ones with the learning curve built in) are made. Can motors, however, are not designed to be serviced. They are too inexpensive for that. They are designed to be replaced when the fail. Except for brush replacement even open frame motors should be replaced if they misbehave.
Motor Current. I bring up motor current again, but this time in the context of motor reliability. High current implies high stress on the brushes and high overall heating of the motor. Heat kills motors. I pay most attention to motor current when the loco is running light. If the light current is high, this is a problem. Typically, HO locos have safe motor current running in the range of 100 to 500 mA. Motor current that is higher than "normal" is typically caused by a few things.
High Loads. Motors draw higher current when they are loaded. This causes them to sag in speed which reduces the BEMF that they generate. Then the motor draws more current. The main problem here is the heat generated by the higher current. Heat is bad.
Friction or Binding. A motor can be loaded running light by friction or binding in the mechanism therefore causing an increase in current from what it should be, even for a healthy motor.
Overload. An overloaded motor could be the result of a poor motor or gearing selection by the manufacturer or simply because the locomotive's load is too high. However, running light, a motor should not be overloaded so high current indicates other problems.
Weak Magnets. Another possible cause of high running current is a degraded magnet. Weak magnets result in reduced magnetic field strength in the motor. This causes the motor to sag in speed more easily and results in the same issues as an overloaded motor.
Short Circuits. Some motors develop intermittent short circuits due to commutation problems. These motors can draw huge current spikes (many amps) at particular times or rotational positions. Motors like these are really hard on DCC decoders, often killing them on the spot. These motors could be declared scrap but some of them can be saved.
Intermittent motor shorts are not easy to detect on a loco running on DC as the short circuits occur for only very short periods of time. They can look like bad power pickup by causing the headlights to flicker. However, if your power pack has meters look for unstable current. If the current tends to spike upwards (or the track voltage spikes downwards) then suspect a motor with serious problems.
A better way to check for motor issues is to monitor the locomotive current on an oscilloscope with a current shunt while a loco is running unloaded on the bench with power supplied to the loco via clip leads so that any issues with power pickup from the wheels do not have any impact on the measurement. Many motors will draw spikes of an amp or so for a few microseconds, this is not usually a serious problem. If the spike is larger or longer, then it is a serious problem. A small resistor (3 to 5 ohms) wired in series with such a motor can protect a decoder at the cost of reduced top speed.
If the the loco has already been converted to DCC and it hasn't eaten the decoder, then it is probably ok. However, with an oscilloscope, the DCC running current can be checked as well.
Commutator Sparking. The most common cause of commutator sparking is short circuit or partial short circuits between commutator segments. This is only visible on open frame motors. If a can motor does this, it is toast. This is most common on 3 pole motors, but it can happen on motors with more poles. Usually, brush material wears off and particles deposit themselves in the gaps between segments. When this happens, the segments can be shorted together. It's often not a hard short, but enough to cause spikes of current. As one brush crosses a gap and connects to two segments, the other brush is in the center of another segment. The current is supposed to flow through the motor windings, but instead, some of it jumps a gap and goes directly to the other brush. This causes high brush heating. Sometimes they will actually glow, get hot enough to issue smoke, or leave a trail of fire around the commutator as it turns. These are all very bad things.
When the problem is not too severe, you may notice that only one brush sparks. Reverse the motor and the other one sparks.
The commutator material is pretty soft. Never touch it with anything hard. Use a sharply pointed toothpick to scrape the crud out from between each segment. Remove the brushes and clean their faces. Then reassemble the motor and run it at medium speed. Use a toothpick again to abrade the surface of the commutator. This will tend to scrape off burnt stuff. A VERY SMALL amount of "conductive oil" will also soften this stuff and allow it to be cleaned off. NEVER use a swab on a motor commutator, the fibers will mess up the motor big time. This treatment will materially reduce the tendency of the brushes to spark.
Open Circuits. Especially with older motors, there can be open circuits at the brush to commutator connection. This can cause an immediate motor stall if the motor is running at low speed. If the headlights are still on and the loco just stops, suspect this problem. Commutator stalls can be confirmed by slightly tweaking the motor shaft in the suspected condition. If the motor picks up and runs again, a brush or commutator problem is probably at fault. A commutator can be dressed or cleaned by gently rubbing it while it is running with the end of a wooden toothpick. This will knock off burnt lubricant and allow the brushes to work better without serious sparking. A very small application of conductive oil can also loosen dried on crud on the commutator, but it should be brushed off again with a toothpick. Do not use anything that will shred on a commutator, the shredded material will pack up under the brushes and really mess up the works.
Type. Most modern HO diesel locomotives use the "central motor/flywheel/U-joint/gear tower" construction. Athearn (I believe) first introduced this combination and it was good. Others have copied it because it is modular, inexpensive and reliable. There are many other drive train configurations out there, none as good. Steam locos typically don't lend themselves to this kind of construction but can contain some of these features.
Slow Speed Performance. The ability of locomotive to crawl very slowly is a good indicator of the overall health of the drive train. Locos that won't run slowly either need service or they are dying. A good loco will run slowly enough so that you actually have to look at the wheels to see that they really are turning.
Current. The current drawn by a loco running light, even better on the bench under no load at all, is an indicator of the health of the motor and drive train. A good HO loco will draw less than 150 mA at running voltages. Ones that draw much more, 0.5 amp or more, are likely having problems, either with friction load in the drive train or with a weak motor magnetic field. A loco that draws higher current may indeed run for a long time but it will run hotter when it does run and heat kills all things electrical and electronic. The cause for the higher current should be investigated to see if can be mitigated or not.
Slop. Slop is an indication of wear. If there is lots of slop, either the drive train has worn badly or it was never manufactured well in the first place. Locos with more gears in cascade tend to have more slop. Slop can be a leading indicator of impending failure.
Noise. Noise tends to be a leading indicator of impending failure. Noise can an indication of very bad wear or it could be normal for any given loco. It is the TYPE of noise that is important. A loco that needs lubrication probably was run without sufficient lubrication for some period of time and it's mechanical wear has probably increased significantly.
Screech or Howl. Dry or bad bearings tend to screech or howl. The shaft, usually a motor shaft or worm shaft at the head of a gear tower, wobbles around in the bearing beating it into a non-circular shape after awhile. These need IMMEDIATE attention or the bearing can be ruined permanently. Unless the bearing is badly worn, some oil can usually "fix" it for awhile.
Grinding Noises. Dry gears tend to make a grinding sound. If the gears are not too badly worn, gear grease will help.
Loud Gear Noise. Straight cut gears normally whine some when they run but if it is really loud, the problem is more serious. Gears that have badly worn teeth can slip. This most often occurs on a gear driven by a worm. If the motor mount is not secure, the thrusts under load will try to lift the worm and cause it to skip over teeth on the driven gear producing a popping sound. Each time the worm rides over a gear tooth, it wears a little off the tooth. Eventually, the teeth will wear down and the worm will start to slip. Slipping is indicated by a loud buzz.
Clunking Sound. Gears with missing teeth or cracks tend to make a repeating clunking sound or actually jam. A common problem with Athearn, Life-Like and Bachmann locos that use half axles and a plastic axle gear is that the gear splits on the axle. The loco will tend to clunk at it runs, but NOT ALWAYS. Sometimes, the pressure of the weight on the loco will keep the axles in place and the clunking noise will not be obvious. However, if you hold the loco up so that the wheels drop by the force of gravity, any tendency to clunk will often become much more obvious. Either lift each end of the loco off the track, one end at a time, or use clip leads or a Kadee brush to apply power to the loco while it is hanging in the air, wheels down.
Athearn and Life-Like locos use nearly identical truck assemblies, the wheelsets are identical. When the axle gear cracks, which happens quite often, the tooth spacing changes a little causing a poor mesh and a once-per-revolution clunk. The split gear will also not hold the wheels in gauge and they can move closer together and short INSIDE the gear. A split gear is easy to locate, just use your thumbs to rotate each wheel set against the gearing. If the wheel turns, the gear is split.
Binding. Very old locos can have dried grease embedded between the gear teeth that can sound like bad gears or cause binding. This can be picked out, but the simple fact that the grease was dry is not a good thing.
Acoustic Impacts. Some locos amplify their natural noise via acoustic coupling to the shell. This is a cosmetic problem that can usually be fixed, either by finding the coupling mechanism or by reattaching loose parts that vibrate. If a loco makes more noise with the shell on than it does with the shell off, then it's time to look for acoustic issues.
Internal Drag. Another source of noise is when wires drag on the mechanism, usually a flywheel. The sound can vary, but it is generally a mixture of a hissing and whining sound. This materially increases the load on the loco, slows it down, increases its current, will eventually cause the insulation of the wire to fail and is generally a bad deal. This usually occurs in new DCC installations were the new and repositioned wires are pushed around during reattachment of the shell. Sometimes structures have to be added to the loco to hold wiring off the flywheels.
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© 2009-2011 George Schreyer
Created 9 Dec 09
Last Updated November 10, 2011