Opinions on washers on loco axles

[mmagliaro]

This is in regard to my steam loco project, but it would be a general question about
locomotive design.

Consider a pair of wheels on an axle in an engine frame.  There are nice bearings in the frame.
The axle runs through the bearings.

Would it not be a good idea to put thin bronze thrust washers inside the drivers, between the drivers and
the frame, rather than just let the driver backs rub on the frame when they slide over?

I ask because, no matter how smooth the frame and the wheel back is, would they not always slide more easily against each other if there were a washer in there that was free to slip and spin however it wants between the two?
(assume it's something like a nice thin, smooth .005" thick phosphor bronze washer).

It seems to me that if you've got the room, this would always be a good idea, but I'd like to hear arguments
*against* this.  Is there a way it could make things worse and not better?

Thanks.

[nickelplate759]

Max,
The only downside I can think of is that it will reduce how much the axle can slide side-to-side, which might increase the minimum radius of the completed chassis.

George

[peteski]

Many (or even most) factory-made steam locos have a flange in the back of the wheel (around the axle) which acts like the washer you described.  But I'm sure you know that. 
As far as side-play clearance goes, the washer can be made quite thin.  If even more play is needed the frame could be made narrower.

[nkalanaga]

I would think it, or the shoulder, would help.  Even if the frame is nice and smooth, there would still be less bearing surface, and thus less friction, with the washer. 

As Peteski said, if side play, "lateral motion", is required, just make the frame narrower.

[mmagliaro]

The reason I posed this is that I will have enough lateral play to afford some .005" washers in there.
I think I'm going that way.

As far as the hub cast onto the backs of the drivers, yes, that helps, but it is not the same as a washer.  The washer
is not connected to either the driver or the frame so my gut feeling is that it would work better because
where there is any momentary friction on either side, the other side is still likely free to spin so the
friction doesn't bind anything.  With a hub cast into the driver, if there is a momentary friction
against the frame, there is not "free side" to give and the wheel will slow.

[peteski]

I agree Max - a separate washer (like a thrust washers on motor- or worm-shafts) will most likely work better than a shoulder around the axle on the back of a driver.  (Shoulder, as Nick described it, I think is a much better name than flange, which I originally used.) 

But I think the difference in amount of friction between those methods is negligible (as far as N scale steam locos are concerned).  However in your case (since the wheels are already made), installing a washer will be much easier than adding a shoulder/hub to the back of the wheels.

[narrowminded]

I hear you and it's certainly tough for me, with an industrial equipment design and build background, to not want to build it TOUGH, ROBUST, take ANYTHING.  Take a lickin' and keep on tickin'.  It's in the genes.  BUT, when you get into these little devices, the reality is that the biggest loads these things are likely to ever see in their service life is US, handling and working on them.  Once you set them free down the rails there is virtually NO load worth a mention on ANYTHING.  Think about it.  Bearing areas are almost always WAY beyond what's required and often with fits that in much of the machinery world would be laughed at as they failed in minutes of starting up. Plastic gears work fine, often better than their metal counterparts, and both of those things is because, for the service, they are GROSSLY overdone and therefore take all of that in stride.  And it's not because they are subject to different physics than the big machines but because all of those parts, for all of their faults in material, fit, and finish, are still PLENTY adequate, GROSSLY over designed for the actual service they see.  When we experience mechanical problems it's often when the fits were either too tight, the rotating parts weren't on center, wobbling, wiggling and binding, or somebody found a level of terrible that's beyond the terrible as we know it and have seen it. Or the biggest problem of all, when the power pickup was lost to an oil or corrosion film barrier that while miniscule in the scheme of all things machinery, was once again, GROSSLY beyond the capacity of milliamps at single digit voltages to break through.  I think I've shared much of your angst when I first took to designing my tiny mine loco but very quickly came to these realizations and still find myself occasionally having to slap my own wrist when the pencil and compass come out.  And even though the inclination, the seat of the pants feel suggests one thing, it all makes sense when you do a few of the numbers and reality sets in, just like on the big machines.

The friction will be what it is.  Adding a brass washer between two other brass pieces or even just one other brass piece won't alter that friction.  IF it was a highly loaded piece with wear potential it might be nice to have it as a replacement part for the wear that would come but in this service, with the small loads it will see, REALLY small loads, I doubt you will EVER have to replace anything for THAT reason.  IF you were still to want to do something, a hardened steel washer as you might be able to fabricate from a piece of feeler stock would be a more appropriate material choice for the one hard face against the one soft face.  But again, because there is no real load and for what load is there, and I suspect there is a HUGE relative bearing area, so you could probably put any material in there and do no harm.  That brings to mind though, if you did do something, you might consider a smaller diameter washer that REDUCED some of that bearing area and therefore a smaller major diameter, thereby reducing the leverage that whatever friction DOES exist, has over the driving force.  A .010"/ .015" face, nicely finished, would still fall into that area of being grossly oversized for the load and without knowing your flange dimensions I suspect would move the leverage closer to the axle C/L.  With a .010" face you probably would have about .005 square inches.  Doesn't sound like much, does it?  Take it times the materials and all of a sudden you've got some pretty good side thrust capacity, probably way more than you'll ever, ever, need.   Hope that helps and believe me when I say, working in this small machinery, I know where you're coming from.  Good luck and I'm anxious to see your finished results.  It will be quite an accomplishment.

[Chris333]

I don't think I've seen the driver rub right on the frame. Usually the frame is black and rubbing would wear off the paint. Part of the wheel bushing must have a collar built in to stop it from touching the frame.

But I think there is way too much slop in Bachmann's bearings. There is slop between the axle and bearing. There is (a lot) slop between the bearings and the frame in all directions. If the bearings where mounted solid in the frame and there was less slop in the axles things would run much more smoothly.

[peteski]

The friction will be what it is.  Adding a brass washer between two other brass pieces or even just one other brass piece won't alter that friction.  IF it was a highly loaded piece with wear potential it might be nice to have it as a replacement part for the wear that would come but in this service, with the small loads it will see, REALLY small loads, I doubt you will EVER have to replace anything for THAT reason.  IF you were still to want to do something, a hardened steel washer as you might be able to fabricate from a piece of feeler stock would be a more appropriate material choice for the one hard face against the one soft face.  But again, because there is no real load and for what load is there, and I suspect there is a HUGE relative bearing area, so you could probably put any material in there and do no harm. 

The way I understood Max is that he is not worried about wear or having to replace parts - he simply wants to minimize the amount of friction wherever he can.  You also addressed that later in your post (decrease friction by minimizing the contact area). As you mentioned that can be done by making the washer as narrow as realistically possible.

[Lemosteam]

If less friction is desired, I would not use a metal to metal arrangement.  If you think about it bronze bushings and washers are usually only used in an oil or grease bath scenario\ where the bronze can absorb the lube.

Thin Delrin or nylon washers have a natural slipperiness to them and as narrowminded points out, will see virtually zero side-load and so should not wear unless there are burrs, which I'm sure you will polish away.

[mmagliaro]

Good point.  I've got some natty .005" PTFE (teflon) washers that are slipperier than
a greased hog. 

Chris, speaking of clearances...   These clock bearings I'm using are 1.5mm bore and they darn well mean it.
In fact, the 1.5mm drill rod axles "just fit" so perfectly that I needed to spin them in the mill and polish them
down ever so slightly with some 2000 grit strips so that they would spin free in the bearings.   They were more
of a light "press fit" before I did that.

[randgust]

I see about as many problems from not taking up the lateral motion as in allowing it.   It really depends on whether or not your driver(s) in the center are blind, and on what you really need for curve radius lateral travel.   Anything else needs taken out.

I think you have your answer in terms of friction issues, but there's a lot more benefit to taking out slop than just that.   I see crankpin screws banging into cylinders and crossheads, sometimes a main rod.... sometimes you realize that it isn't grinding against the wheel/frame, it's actually the spur gear on the axle grinding into the inside of the frame.  If you run a mechanism upside down in a cradle and push drivers around while it is running you'll see all kinds of things happening that probably shouldn't be.

I've made a practice of taking out all the slop I can out of the lead and trailing drivers and seeing if the remaining lateral motion of the center driver(s) is enough to still be reliable.    I don't tear things apart to put in washers, I tend to make my own "u" shims and also modify the delrin MT truck washers to 'clip' over an axle. 

The other thing that happens is that for some inexplicable reason, steam locomotive drivers are narrow in gauge more often than not.   And if you correct the gauge, now the lateral movement is worse, and so is the 'banging into things' problem.   That as much as anything is where I've had to get a lot better at shimming the travel out of these situations.

Our 'display window' HO layout has AHM/Rivarossi 2-8-0's as the only steam power that's survived the abuse of being on a layout that is controlled by the public with a pushbutton. They have an unbelievable amount of lateral travel in the two center drivers and almost none on the end drivers, and the main rod is segmented to allow everthing to 'bend' going around 18" curves.   They are absurdly quiet and smooth going through their paces, at realistic speed, hour after hour, week after week, most run for years.    They only fail when inevitably they wear out their brass crankpins against the nickel-silver rods, despite regularly soaking them in lubrication.  I've studied that design as it is really, really good approach so I've done my best to mimic than in N scale.

[Chris333]

Bachmann recently came out with a Skarloey model that is technically a 0-4-2 HOn30 Thomas the Tank Engine model.

The front and rear wheel are fixed and the center driver slides back and forth so it will go around 9" curves. The center driver has plastic washers on the axle.

[mmagliaro]

Randy,
YES.  This is so true.  Steam loco models are built to negotiate sharp curves because the customer base demands it.
But the lateral slop in the mechanisms required by this just makes is harder to tune the engines to work well.
Everything with gears and wheels in it works better when everything just stays put and runs in a true line.

Chris, very interesting.  They probably came to the same conclusion.  That center driver has to move over and rub on
the frame on curves, so a plastic washer provides less friction than metal on metal.

In my case, we are talking about an 0-6-0 here, so the drivers only need very minimal lateral movement to get
around curves, and I plan to have as little side-to-side play as possible.   That also lets me have as wide a frame as possible,
allowing all that much more room for the motor, gears, wiring, weight, etc.

[Chris333]

Also look what they did above. The front driver (right) has very small bushings that sit in the frame. The center driver sits right in the frame slot with no bushing. The rear wheel has bushings, but sits into a plastic piece that holds the pick-up strips. The pick-ups ride in the bushing groove.

And this is a toy. I hope they start using these bushings on N scale steam.