TheRailwire
General Discussion => N and Z Scales => Topic started by: mmagliaro on January 13, 2016, 10:56:45 PM
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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.
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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
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
(https://lh3.googleusercontent.com/-gXu3CaWvf-Q/VoGrPjEN-5I/AAAAAAAAOm8/D1dNaeg991g/s800-Ic42/IMG_5619.JPG)
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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.
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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.
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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.
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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. :D
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. 8)
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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.
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Hey, wait a minute... I'm using files and sandpaper.. ;)
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Hey, wait a minute... I'm using files and sandpaper.. ;)
My point exactly, Max! :D
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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.
(https://lh3.googleusercontent.com/-zEbasuSmVxk/VoGrPOqO1II/AAAAAAAAOm4/Y06sd5gjg5Q/s800-Ic42/IMG_5621.JPG)
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
:D
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Hey, wait a minute... I'm using files and sandpaper.. ;)
Staples of the trade. 8) Even with the best of everything the 1200 paper is never too far away. :D 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. ;)
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Yep exactly that.
(https://lh3.googleusercontent.com/-zEbasuSmVxk/VoGrPOqO1II/AAAAAAAAOm4/Y06sd5gjg5Q/s800-Ic42/IMG_5621.JPG)
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
:D
I hope that guy got a bonus. 8) For what they're doing and the budget they're doing it with, that's pretty clever.
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My point exactly, Max! :D
"I am endeavouring, ma'am, to construct a mnemonic memory circuit using stone knives and bearskins."
-- First Officer Spock, Starship Enterprise
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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.
[attachimg=1]
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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. 8) 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.