Aristo Prime Mover Basics

Overview:

I started this page, like most of my pages, as notes to myself. I had a custom built Northern locomotive bashed from two Aristo pacific locos.

The first time I ran it there was smoke coming from it! Not the stack, but the connecting rod on the first driver! I reasoned that there was a power pickup problem. I was not really familiar with how the entire system worked, so I decided to investigate and learn myself. Thus I began to understand the "inside story" on the Aristo modular drive system, which they call the "prime mover".

Thus this is both not only an explanation of how the system works, but also a real story of how I figured out what problems I had, and how to correct them. You will see that this page is written mostly in a narrative form.

The basic problem was that I found that there was no power pickup to several of the drivers in the loco. (Please remember this is built from 4 modular assemblies from 2 Pacific locos, i.e. a 4-8-4.)

Basic Design:

The basic modular component is a gearbox that has 2 half axles (for the drivers) and hex drive recesses on each end so they can be connected in a "string". Often the connections between gearboxes have a flywheel as part of the hex drive shaft. The motor likewise has a hex drive on it. The hex "recess" is in the actual worm gear, and is supported by ball bearings. There is no play or slop in the bearings, so the alignment of the worm gear with the driven gear stays constant.

In steam locos, there is a motor at one end, and then a gearbox, then a flywheel, then another gearbox.

In diesel locos, there is usually a motor between each gearbox module. One motor for a 2 axle truck, two motors for a 3 axle truck.

The gearboxes are sealed, and have ball bearings to support each of the two half axles. The half axles are screwed to a central plastic drive gear, thus the metal axles are insulated from each other. The axles have tapered ends, where the drive wheels are attached. Quartering of steam loco drivers by simple alignment, there are no keyways on the shafts.

Most drive wheels have a recess in the center, "outboard" of the taper.This is to hold a "star" type lockwasher. A screw goes through this washer and goes into the axle. The screw bears on the washer which bears on the recess in the wheel. The threads pull the tapered end of the axle into the taper in the wheel. The integrity of this connection relies on the taper fit, kept tight by the screw.

By inspection, it appears the tapers on the ends of the half axles are cast, not machined. More on this later.

Overall the construction of the gearbox is very robust, there are 4 screws holding the halves together. The fit of the halves is excellent, I detected no seepage of grease or oil at the seams on any gearbox. The gears are likewise large and appear very "tough". From conversations and observation of many posts on several forums, it's clear they can handle very heavy loads with no failures.

Power conduction:

The conduction of power from the wheel treads to the main circuit board is clever, but involved. I will explain how the power gets from the wheel tread to the contacts on the outside of the module later. This is also basically a narrative of how I troubleshot my power pickup problem.

The modules have metal "retainer clips" on each side, which perform several functions. One function is to conduct the power from the gearbox modules to the locomotive main board. Aristo locomotives normally have a metal strap inside of the truck or motor block. These clips bear against these straps.

Here's where steam and diesel differ.

On a diesel motor block, the block has 2 metal strips on top, with fingers that touch a circuit board in the chassis. There are metal straps that connect to these strips that then run inside of the motor block. The motors get power from engaging "forks" with contacts to the brushes.

Here is a shot that shows the metal straps inside a steam loco motor block: (it's the shiny vertical piece of metal that is on the side of the housing.)

These straps come out the top of the motor block and clip onto 2 wires that run the length of the block:

You can see where the power pickup "bus"wire goes into the housing of the motor on the left... look closely right across from the threaded screw holes and you can see the little curved finger of the end of the strap on top of the wire. This is the first place I thought I could have a problem. These little fingers have a small contact patch on the wire, only on 25% of the circumference.

In a meeting with Aristo, I understand that these clips are now soldered to the bus wires. If you have an older unit, I would recommend that you solder these. Be careful to not melt the surrounding plastic. I would recommend using some paste flux when you solder them.

I cannot comment on how often this is or is not a problem, but, Aristo has obviously seen this problem in the past and changed the assembly process to solder them. This is one of the first places to look if you have a pickup problem. My problem though, did not lie here, and thus the investigation continued.

The picture shows a longer view of a steam motor block. The two track pickup "bus" wires are not in the block at this time. What I want you to see is the circuit board on the left. The motor is in the larger part of the block next to the white connector.

Since the motor solders to this circuit board, and the 2 track pickup wires come out and solder to this board, then the white 4 pin connector is all that is needed to feed the 2 track pickup connections and the 2 connections to the motor. All of this is a nice, solid arrangement, although to remove the board you need a high wattage iron, or a way to remove the solder. My recommendation is if you need to remove the board, is to unsolder the 2 "bus" wires, and pull them out IF the straps are not soldered to the bus wires. Then you can heat the motor tabs and rock the board off. If the clips are soldered to the bus wires, you need to get all the solder off the motor terminals and the bus wires before you can remove the board. Clean all the solder away from these connections when re-attaching. You should never have to pull this apart unless you need to replace the motor or the bus wires. (There is enough flexibility in the board to allow the removal of the gearboxes without disturbing the soldered connections. All of these connections are robust, and obviously, my problem was not here, so I again continued my investigation.

OK, so by the process of elimination, my problem lies between the wheel tread and the metal straps that connect to the "bus" wires.

Time to take a gearbox out, this must be where my problems lie.

Here is the second part in the Aristo design that you could encounter a problem. Here is a contact area that depends cleanliness and on spring contact on the gearbox. The gearboxes are held loosely in the motor block, and move up and down and also twist in relation to the long dimension. Here's where dirt could be a problem, but especially weak spring tension on the  "retainer clips" on the gearbox module. I believe that motion over time can take the "spring" out of the clips and cause a problem. I have had a few people confirm this, but I do not believe it is a common problem.

Lets look at the gearbox and view the metal retaining clip on the each side. When the gearbox is put in the frame, these gearbox clips press against straps on the frame. (These are the curved bits below the axle in this shot).

The amount of flexing (rotationally along the axis of the motor block) concerns me that this can become a problem area with extended use, or poor repairs, or defective "clips". These "clips" are not spring steel, or phosphor bronze, nor any other material normally known for retaining spring tension over time. They are easily deformed when handling.

So my recommendation is to be sure these curved ends have good shape and are springy enough to have good contact to the straps on the motor block. A bit of grease here will keep oxidation and corrosion at bay. Any good quality grease will be fine. If you have disassembled a loco to this stage, make sure they have a nice snug fit into the motor block housing.

Since the contact area is large, I am not worried about normal dirt or corrosion, but losing spring tension from motion over time.

All right, I checked each gearbox, and the power conduction between each gearbox and the straps in the power block were OK. Several clips were a somewhat loose fit, and would break contact on occasion when I "rotated" the individual gearbox, so I bent them outwards a bit.

But this was only intermittent on a couple of gearboxes, and I had a "hard open" on several drivers, so the problem must be between the wheel tread and the metal "clips" on each side of the gearboxes.

It's clear that the power from the wheel tread goes to the wheel itself, and thus to the axle.

So how does the power get from the axle to these clips on each side of the gearbox?

Well, at first I figured the power came through the ball bearings on each axle. If you remove the clip, you can see that the clip has a second function, it retains the ball bearing for "it's" axle.

Below, you can see the ball bearing that is in the gearbox and the axle rides in, the "ball bearing drive" in this picture.

So it seemed impossible that I was having a lack of conductivity between the wheel and the clip. It appeared to be all metal to metal. But I had no continuity on 3 axles! How could this be? I was perplexed.

I was sitting there staring at the gearbox, and said, "what the heck is that spring I can see back there? Why is that there?"

So, I removed "axle" ball bearing on one of the non-conducting gearboxes. Surprise! There was no conductivity through the ball bearing that supports the axle. Wow, could not believe it. Well, that confirms why the obvious did not work, the ball bearings that support the axles are not in the electrical path!

Update: Bob Grosh reports that the non-conductivity of the ball bearing is probably because the balls in the bearing are probably ceramic. This seems to be a common situation, apparently many manufacturers are changing to ceramic balls because of rust resistance and the ability to handle heat better. Some ball bearings were also steel (not Aristo) and they rusted (typical in some LGB). This is often called a "hybrid" ball bearing, part metal and part ceramic.

So, now seeing the ball bearing does not conduct, I was just sitting and staring at the gearbox.

Ahh, wait!, with the ball bearing removed that spring and single ball bearing riding on the axle MUST be there for a reason. No way Aristo added this just for fun.

So, it was clear the single ball bearing is pressed against the axle with the spring, so it has great conductivity to the axle, and thus the wheel tread.

(As an aside, be CAREFUL, many people have reported losing this spring and ball bearing. Being forwarned I did not, although seeing it caused me to order 2 spares! If you remove the axle ball bearing, take this spring and single ball bearing out immediately and put them aside. Put them back in AFTER you replace the axle ball bearing)

Again, reported by Bob, some of the early RS-3 gearboxes were of a similar design, and the tab was bent up at right angles, so that the spring seated flat on it. But you can see that trying to keep the spring compressed while installing the clip is virtually impossible. I might try doing this myself, but an assembly nightmare.

So reading between the lines, if this was the original design, it would work better, but current costs to assemble would be too high.

OK, so hows the heck do we get contact to this spring? It looked like it was just a dead end, but something had to take the power from the axle to the metal clip. Take a look at that clip again, wait! there is a little "finger" that goes inwards. Dang! That finger looks like it should touch the spring! OK, that makes sense.

So this clip has 3 functions, hold the bearing in place, conduct power from the gearbox to the "straps" in the motor block, and conduct power from the axle to itself. A very important little piece of metal to be sure.

So the conductivity path is:

wheel tread > wheel center
wheel center > axle
axle > single ball bearing riding on axle
single ball bearing to > spring
spring > "finger" on clip
clip > strap in frame of motor block
strap > "bus" wire
bus wire > circuit board
circuit board > 4 pin connector and thence on to the main circuit board

Now that finally makes sense. It is also where my problems were. Unfortunately these clips are inconsistently formed, they vary all over the place in contour, in the way they fit to the module and the angle of the "finger". The critical part is that the "finger" MUST touch the spring to make conduction. The way the finger contacts the spring is a poor design in my opinion.

The contact area is small, at different angles, and there is no way to "line up" a coil of the spring with the fingers contact surface. Pressing the finger in too far can interfere with the spring tension, not enough (as my case) and you don't touch the spring at all. I am considering a way to improve this, but all you can do right now is try bending the finger to see if you get it working.

So that was the problem, checked the rest of the wheels, yup, the wheels that pick up power fine have that little finger touching the spring.

On the wheels where there is no power pickup, the little "finger" was not touching the spring. Push on the little finger a bit, whammo, conductivity. Note: BE GENTLE, a little at a time.

My advice to Aristo is find a way to improve this. I know that no one has reported this problem before me, and after talking to a lot of people, I know why, I only found TWO other people in the hundreds that have talked to, that knew about this "finger".

Consider this: A person has a Mikado. Suppose 2 drivers have this issue. The locomotive exhibits poor performance on dirty track. The owner cleans the track pristine and the locomotive runs better. His conclusion would be that the locomotive is sensitive to dirty track. I have only encountered a few people who have checked for this problem.

Note well: on steam locomotives, the metal side rods can also "fool" you into thinking the power pickup is ok. The power would flow from a driver to the connecting rod and then to a "good" wheelset.

This is EXACTLY what happened to me. Luckily there was oil in the connecting rod journal, and since so many drivers were not picking up power, all this current through the connecting rod journal vaporized the oil, alerting me to the fact that power was being conducted through the connecting rods!

Therefore, this problem may exist in many locos, but the owner is unaware, and may blame poor operation on dirty track, or lack of pickup in the pilot or tender wheels.

So, the lesson: CHECK THIS when you do not have good power pickup. I have checked several new out of the box steamers and found up to 50% of the drivers had this problem. I won't embarass Aristo any more with the full story. (Unless they threaten me again!).

There are also other common mechanical issues with this drive system. I'll list the ones I have experienced or read about and solutions.

Axle Slippage - glued on, not properly tightened, junk in threads

The most common and frustrating problem is a steam loco driver coming loose on the axle. When this happens, it almost always involves damage, gears stripped, axles ripped from the main gear, bent or broken connecting rods.

As stated earlier, the friction fit relies on the taper on the axle and drive wheel, and the screw clamping them together.

Historically, Aristo has tried various applications of Loctite red to secure this, often glopping it all over the screw, the washer and the recess in the drive wheel.

One common problem is when the factory apparently has applied the Loctite and it has set before the screw has been tightened correctly. In this case the wheel WILL come loose eventually. Loctite is not epoxy glue, and has little strength unless in a very thin coat. Solve this problem by removing the screw, washer and wheel, clean everything up and reassemble.

I went a step further, and used a 3.00 - 0.5 mm tap and chased the threads in the axles, cleaned out all the loctite red in there. What a mess. You normally destroy or damage the screw head (cheap metal) removing them, so getting some spare screws and lock washers is a good idea.

Properly done, no loctite, or a dab of loctite blue (designed for vibration) is what you want. Loctite red is for stuff never to be disassembled. It is unreasonable to use it in this application. Do not put it on the axle ends or the lock washer.

Axle Slippage - axle protrudes too far into wheel

This one also happens fairly often and is harder to solve. The problem is that the axle enters the wheel so deeply that the screw and washer bears on the axle end, not the wheel. So tightening the screw only fixes it to the axle, no pressure is applied to the wheel. The cure is normally to grind off the end of the axle so it is recessed properly. Be aware that this might be related to another problem, that the gauge of the wheelset might be wrong. It might be worth considering purchasing another wheel. It's not clear what the problem is here, the wheel recess too deep, or the axle taper too skinny.

Axle Slippage - poor fit between wheel and axle

This is really, in my experience, the primary cause of the slippage, along with loctite red that cured before the tapered fit was snugged.

The axles are chrome plated. If you look at the taper on the ends of the axles, it is not smooth or even. Now, I cannot tell if this is from the plating or that the axle is cast and the taper not machined.

The bottom line is that this design relies on these 2 surfaces mating well, and in my opinion, they do not.

I would have to say that Aristo agrees, otherwise why is loctite red (made for permanent assembly) used all over the place?

(after improving my "fit" I have put NO locking compound on my axles or screws and have not had anything loosen)

After the chrome plating, the bottom line is that it's just not good enough, in my opinion . I took some fine valve grinding compound (available from auto parts stores) and lapped the 2 parts together. You take some small dabs of the compound and put it on the axle tip. Spin the wheel on it with moderate pressure. Hold the gearbox so that the axle you are working with protrudes as far as it can from the gearbox, helps keep the stuff away from the bearing. You can tell by the drag between the axle and wheel when you've lapped it enough. You will wear off most of the chrome plating. Keep the wheels with their mated axles from now on. You will see when you put the screw back on this will never slip. I clean the axles and the insides of the wheels thoroughly.

Wheel gauge

A disadvantage of this design is that you have no way to adjust the wheel gauge. If the gauge is too wide, you could open up the taper in the wheel to have it sit further on the axle. This could be done easily with some valve lapping compound. If you do this make sure you do not cause the axle to protrude from the wheel, or you have the problem above.

If the gauge is too tight, you might be able to use some very thin brass shim stock between the wheel and the axle, but this is not proven yet, you might have slippage from the extra shim. Use Loctite for sure.

I purchased some custom-made shims from Grant Kerr of Outback Turnouts in Australia. (email: This e-mail address is being protected from spambots. You need JavaScript enabled to view it , web page http://mywebpage.netscape.com/soundgk/page18.html) They are precision made, and about .009 thick. You may use between 1 and three of them per axle, split them between sides. Thanks Grant!

One problem is that the Aristo flanges are often WAY too thick. This makes it problematic to set the back to back without exceeding the gauge width. To compound matters, the Aristo SS track I have is right at the NMRA minimum. So I shimmed the wheelsets to make the wheel gauge meet the same number, and that helped the back to back.

Unfortunately this made the gauge of the wheelset too wide in some cases. I think my only resort is to turn down the flanges. (Remember I am using a synthesis of the NMRA and G1MRA standards on my track and switches)

Most of my work has been on Aristo steam locomotives so far. I will report back on the diesels in the future.

Power pickup / troubleshooting

The areas of potential loss of power pickup have been extensively described above. Be sure to "wiggle" the axles around the axis of the motor block, and also slide the axle in and out (side to side) while checking. The best way is to use a precision ohmeter to test, i.e. be able to read the difference between .1 ohms and 8 ohms, for example. You should get darn close to zero ohms between any wheel and the corresponding "bus" wire (outer 2 pins in the power connector).

I read 15-50 ohms on half of the drivers on a new from the box Mikado. (Again, I won't embarrass Aristo as to the venue),

If you do not have access to this type of meter, you can remove the side rods when testing.

The most common problems are the little finger on the metal retaining clip not touching the spring inside, and the metal straps not soldered to the bus wires as outlined above.

Tracking problems

I am currently researching this in greater detail, but at least one of the gearbox assemblies in a truck or motor block needs to be held rigidly in place.

The design of the Aristo modular system allows great up and down motion for each axle. But if all axles were unrestricted, the locomotive could "list" to port or starboard at quite an unsightly angle.

It appears that all steam locomotives come with a rubber pad under the leading gearbox to perform this function. The rest of the axles float freely to follow the track. This seems to work very well, I have never encountered a tracking complaint, and all my Aristo steam locos track very well.

(I do however, want to experiment in the future with placing this at the last axle, just to see what difference, if any, it makes)

On the diesels, I am still researching I need to fill this in more, but the 3 axle ones originally came with the pad on the middle axle. Now, they apparently come with the pad on the unit nearest the coupler. It appears the pads used in the diesels are different that the one used in steam locomotives.

Recently all Aristo diesels come with the motor contacts soldered to the pickup straps. This locks the motor in place and can often cause binding. When someone mentioned that his diesel drivetrain sounded like it was binding, a highly respected Aristo forum member responded that he never had that problem. Turns out that he unsolders these leads when he first gets his locos. 'nuff said (he has about 100 Aristo locos).

Applying pressure to the motor housing and melting the solder on each contact has solved the problem in most cases.