Opinions on washers on loco axles

[peteski]

Interesting. Looks like the rear "truck" (wheelset in the rigid frame)  conducts electricity through the uninsulated half-axles and brass bearings, while the drivers have wipers on the backs of their wheels.

[narrowminded]

Sometimes my responses on these topics might seem as though I think any old sloppy and lousy fit will work best and is the answer to fine running equipment.  That's not the case at all.  In my loco project I can assure you that I am holding a few tenths (.000 THEN a number) on many key parts.  Some parts warrant that kind of tolerance for quality running and it's nice to have access to equipment capable of reliably producing those results.  But it's still up to the designer to know what, where, when, and finally, how.   That's actually a huge part of how I've achieved such extremely smooth low speed operation in such a small chassis with very little power at the motor shaft.  I very quickly discovered that it's ALL details because they are all you've got when a dimension that's give or take a few thousandths has that same few thousandths effecting a 20, 50, or even hundreds of percent change. It's not JUST how much torque the motor has nor how many poles it has (when the RPM's are high and the gearing extremely low), it's about efficiently using what you've got, not giving what little power you DO have up to mechanical loss.  What I AM trying to point out is where we DO have key features and also, how in fact we give away power unnecessarily and that when designing just seat of the pants, we can easily create or at least compound a perceived problem we are attempting to address, always with the best of intentions and most often with some theoretical validity. 

A broad face bearing (relative to the load) and running at very tolerant surface speeds, with a relative lot of surface area and a finish that fully engages that surface area, WAY beyond the area needed to support the load, then oiled with a uniform film of oil, can and I'll go so far as to say WILL, have measurably more friction than the same bearing placed dry.  It sure won't wear out from that but it will consume energy in friction, shearing the oil.  Put some oil on a smooth washer and put it on a smooth surface, tip it up, and see what happens.  What held it there or at least slowed its movement and if I wanted it to move, how would I do that?  I'd need to use some force, I guess.  Use some of my energy.  Crude example but some of the principle that's in play here.  It can further create a vacuum that creates an altogether new additional load that had nothing to do with the service load and aggravates the condition requiring me to shear the film strength of the oil, much more than was required to reasonably handle the load.  And it was a wholly created loss. 

So what can we do to fix this?  First, is there a condition that actually needs addressed? If it's determined that something is actually needed then get the surface area near appropriate for the condition being addressed.  Reduce the area to something appropriate and/ or break the vacuum.  One thing that will do that (cringing) is a poor finish/ poor fit.  Reduces the area and eliminates the condition that creates an oil film induced vacuum.   Another way (not cringing this time), on a thrust face that actually HAS a good finish is to introduce planned breaks and reductions of area.  Radially placed grooves in the face will do that and is a common feature in thrust bearings.  It's also done to a plan and is predictable and repeatable.  But all of this goes to why some very crude devices, like a Bachmann loco (Chris), or to be fair, MOST of these locos at some level and the real origin of my referencing those devices, pointing out the sloppy stuff we see but that works.  The molded bore gear with a taper mold draft, that sits on an as cast pin with its own release draft, flopping, wiggling, and the envy of any belly dancer worth her salt, still works.  Why is that?  Well apparently the width of that gear maybe .1" and at a line contact, VERY few square inches, IS enough for the load it's seeing.  It MUST be or it wouldn't work.  And wow, how cheaply can THAT be produced?!  While it's easy to see that as junky stuff, there's actually a beauty in it, too.  It's sloppy as could be, inexpensive to produce, but it still works.  Could it be better?  Sure!  Isn't what we're doing... if we don't miss the boat? So that same .1" wide gear bore, but this time with a true bearing fit, while pure logic would tell us MUST be much better, will last FOREVER, is true.  But what's not so intuitive, it will be accomplished at a measurably higher relative friction and all for no real functioning reason.  With a good fit and not just a line contact, then that .1" width could be 1/4 of that, 1/10 of that, even less?  See where that goes?  In the extreme it might be able to be so small that we literally won't be able to handle it to assemble it.  And we sure can't measure some of these loads nor reliably produce the parts in appropriate tolerances that take every advantage of ALL of this, but we can get a lot closer than we do if we keep throwing another .010" at something already over designed at maybe .003".  And for a lot of this, testing is sometimes the only way to know for sure if we have an adequate part or if we made an improvement or a problem with our best guessed plans.  On my test track I use a milliamp meter as a standard part of the track wiring and it's probably my most useful tool when trying to determine effects of changes.  Early in the process there were some surprises that weren't intuitive but each time, in hindsight, should have been.  I quickly got a reset from my big heavy industrial design approach background.

And finally, a specific example of how a thrust bearing face was addressed.  Keep in mind I'm working with REALLY small.  Early in the process and with my focus on several things that prevailing wisdom and even seat of the pants accurately suggested were where the real problems existed in making such a small device, I had to set a bearing face for the thrust of the worm shaft.  I knew it wasn't much of a load so without any real detailed thought I set a very small thrust face on that bearing.  "Very small" as my industrial design mind saw it at that time was a .010" face.  Then, when actually making the part and realizing how very tiny a 1/8" O.D. bushing actually looked when in your hands, on the fly I changed that .010" face to .015" face.  And boy it still looked small.  Never did a number on it but damn, it's small.  Much later in testing and by then knowing that YES, this chassis could be made to work (that wasn't always clear), I found that after a cleaning in an ultrasonic cleaner and a careful oiling on assembly, the milliamp draw was measurably higher but after several hours of steady running, steadily improving, it finally settled down and really ran good.  It wasn't running badly, just laboring more than it had and running a little slower.  Changing oil to every different oil and grease made by LaBelle and others, graphite, molybdenum dry film, etc, made no appreciable change in this (its own useful test).  This went on over about fifteen hundred hours of test running.  I knew there were improvements that could be made in this area but my real focus remained on the bigger issues.  Finally, somewhere in the process with the original prototype chassis and as the bigger problems were being reined in, I gave a cursory look at that bearing face.  WOW.  Just using round numbers, not as a bearing load but the material's strength number, and it was in the range of 40 to even 60 pounds of static force that it could withstand.  It sure was plenty when the running load was probably in the lower range of single digit grams.  Again, the worst potential load this part would ever see was me pushing it in and out of place and that would be more than covered at about 10 pounds.  So what I did on the next protoype was reduced the initial face to the original planned .010" and THEN, radiused THAT face so that it was crowned in its center which afforded generous support for my hammer hands pressing it in and out but reduced the running surface area to a virtual line.  And then, further, added two radial grooves about .005" deep in that face to break any vacuum effect and clear the errant oil that would inevitably find its way there.  That face could "seat in", if and as needed, and while the additional width potentially added is small in micrometer numbers, it can rapidly increase the surface area by HUNDREDS of percent for just a few thousandth or so wear. AND... that only occurs if it's actually even needed.  Another three thousand or so hours of testing on the new chassis with that new design and that face shows NO wear worth a mention.  And along with similar adjustments made elsewhere in the device it rapidly settles in to a predictable and repeatable amp draw.  Still slightly higher draw on a fresh lube but very rapidly, a few minutes, settling down and running hundreds of hours without a dismantling and cleaning, just a couple of drops of oil on the gear... which finds its way elsewhere even if not needed.

Here are a couple of links that a quick search found and might prove useful in these matters.

http://www.copper.org/publications/pub_list/pdf/sleevebearing_a1063_06.pdf

And Delrin Design Principles.  Look it over as there's a lot of good info worth understanding but specifically page 37 and the topic headings, Frictional Properties, and then, Wear. 

http://www.dupont.com/products-and-services/plastics-polymers-resins/thermoplastics/resins-technical-library.html



Hope that's helpful at some point, to someone.   

[Lemosteam]

 Narrowminded, I will agree with everything you wrote, but these things are mainly true and usually can only be achieved in single case handmade/machined scenarios.  These types of tolerances cannot be maintained across the variational spectrum.  THIS is why mass produced locomotives are required to have generous clearances, and what seem to be sloppy fit, are there to accommodate such tolerances.  To engineer a one off, perfect fit machine (let's face it that is what it would be) requires that the maker hit the mark only once as a sample, as each piece is custom made.  I know you know this cannot be achieved in a production setting where installation and fit of parts is always secondary to function.

Without such allowances, the average modeler would not be able to afford a loco.

It is the difference between a Rolex and a Timex, save that they both end in "X", lol.

This is the REAL beauty of what so few can accomplish with appropriate tools at one's disposal.  Think how much harder that is to achieve when one is using files and sandpaper to achieve a similar outcome.

[mmagliaro]

Hey, wait a minute... I'm using files and sandpaper..   

[Lemosteam]

Hey, wait a minute... I'm using files and sandpaper..   

My point exactly, Max! 

[Chris333]

Interesting. Looks like the rear "truck" (wheelset in the rigid frame)  conducts electricity through the uninsulated half-axles and brass bearings, while the drivers have wipers on the backs of their wheels.

Yep exactly that.


While there is side to side play. There is no gyration like I get with other N scale steam. And this is the body that sits on it:
http://shop.bachmanntrains.com/images/HO_Scale/58601.jpg
 

[narrowminded]

Hey, wait a minute... I'm using files and sandpaper..   

Staples of the trade.   Even with the best of everything the 1200 paper is never too far away.   And when putzing at home it's back to the Dremel, the .010" saw, some jewelers files, and a good right arm and an operating back door for those few things that really just need to be launched into orbit.  Sputnik was actually an N scale chassis that had too much bearing area and refused to pickup track power... EVER.  Not a lot of people know that.

[narrowminded]

Yep exactly that.


While there is side to side play. There is no gyration like I get with other N scale steam. And this is the body that sits on it:
http://shop.bachmanntrains.com/images/HO_Scale/58601.jpg
 

I hope that guy got a bonus.   For what they're doing and the budget they're doing it with, that's pretty clever.

[mmagliaro]

My point exactly, Max! 

"I am endeavouring, ma'am, to construct a mnemonic memory circuit using stone knives and bearskins."

                          -- First Officer Spock, Starship Enterprise

[peteski]

Here is an example of how Fleischmann (IMO, one of the top N scale model steam loco manufacturers) designed their wheels. They minimized the contact area (which also minimizes the friction) with the frame by molding a thin ring on the outside of the axle hub.  It is plastic (unknown kind) contacting the diecast metal frame. But like it has been said, there won't be much pressure applied to that contact area, so there won't be much wear to the ring.

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[narrowminded]

Here is an example of how Fleischmann (IMO, one of the top N scale model steam loco manufacturers) designed their wheels. They minimized the contact area (which also minimizes the friction) with the frame by molding a thin ring on the outside of the axle hub.  It is plastic (unknown kind) contacting the diecast metal frame. But like it has been said, there won't be much pressure applied to that contact area, so there won't be much wear to the ring.

(Attachment Link)

That's a perfect visual of what I was describing.   And it isn't just thin in overall structure which would address the bearing requirement but may not survive our handling of it.  It starts pretty wide at the base then tapers towards the bearing face, reducing surface area that will eventually engage at the bearing face, and finally ends at that face narrowed appreciably, but THEN... the area is FURTHER reduced by having a crowned face resulting in little more than a line contact (notice it's not a square, sharp transition corner).  And that's all of the bearing face needed for the small load as can be logically deduced or guessed going in, some sense of the reality confirmed with a few reasonableness calculations of area vs: load even when those numbers aren't known to the last fraction of a gram, and then confirmed by test, the fact that those faces don't wear away and in fact, retain their crown shape across the bearing face and over a long service life. But if in service, it proved to need a little more area, the face would wear a little, and in that action would increase the bearing area exponentially for each thousandth that it wore, eventually finding a nice equilibrium.  And the claim about the worst loads these see is us handling them assembling, disassembling, etc, is addressed in the broader basic structure including the crowned bearing face, giving it ample structural support for those errant loads that are caused by US.  All of what I was trying to describe. 

Now, taking it to the extreme, which could be done because there's no good reason not to, there are two things I would still consider doing in that wheel design.  One would be to make the hub even smaller, getting what load it ever sees closer yet to the C/L, reducing the leverage that friction has over the driving force.  Any benefit at this point with so much in the right direction already would be minor or, when not engaged, non-existent.  But hey, it's free and couldn't hurt.   The second thing which may actually afford some real benefit, especially if the unit ever gets over oiled, would be cutting two or three small breaks radially in that bearing face.  It will break and clear excess oil that may try to accumulate and create a vacuum or at least need to be sheared in operation.  It's also a path for miniscule foreign matter to escape.  Also at little to no cost.  And while these things may be passed over as not necessary for a particular service which may be 100% correct, they are exactly the kinds of features that could be looked at if amp draws fluctuated after oiling, for example, or the forward reverse operating characteristics are appreciably different.  Hope that's helpful from a general principles standpoint if not specifically helpful in this narrow instance.