Mystery bind in wheel/rod motion

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Steamin
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Mystery bind in wheel/rod motion

Post by Steamin » Thu Jan 28, 2021 10:53 am

What started as a simple job to repair a broken petticoat and split waterline has turned into a complete frame-off teardown and overhaul. I am starting on repair and reassembly, only to be stopped by bind in the wheel/rod/frame assembly which prevents smooth rolling. While I can machine more ‘clearance’ into all bushings/bearings, I want to know the root cause of this bind. And I cannot figure it what is causing it.
This is a Little Engines 1” scale Pacific. Dated on the frame drawing: “Started 1973”.

Here is what I started with during the teardown:

The side rod bushings were worn 0.030 oval, most fell out of the rods when the rods were taken off. The rod bores were oval. The pins were worn oval. After pulling the wheels off the axles, five of the six bearings were shot, having rust pitting on the race and needles. The inner race on the engineers side #2 (main) axle was loose and had spun on the axle.
20210127_012653241_iOS.jpeg
Oval Bushings
Surprisingly, the axle boxes and pedestal openings in the frame were not worn. The tire profile is in good shape.

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Steamin
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Re: Mystery bind in wheel/rod motion

Post by Steamin » Thu Jan 28, 2021 11:01 am

What has been done:
  • Wheel pins pushed out and each minimum turned until round again. Since the wear on each pin was different, the finished size for each pin is now unique. The pins were pushed back into the wheels, aligning them to the original setscrew holes. All pins were a press fit in the holes.

    -New bearings of same specification were installed on all axles.

    -Wheels pressed onto axles.

    -All rods were bored round again. New bronze bushings for each rod were turned and pressed into the rods.

    -The rod bushings were then bored to size for each pin. Care was taken to reference all hole centers from the coupling rod and side rod connection, which was not worn or disturbed during the machining processes.
20210108_004856188_iOS.jpeg
Boring the rod bushings in place
  • The wheel and rods were re-assembled in the frame and a bind was found when rolling the wheels over in forward or backward direction.

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Steamin
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Re: Mystery bind in wheel/rod motion

Post by Steamin » Thu Jan 28, 2021 11:08 am

The diagnosis begins:

-The side rod was disconnected from axle #1, but left on axles #2 and #3, the wheels rolled freely with no binding. When axles #1 and #2, but not #3, were connected, the wheels had the bind. With some pressure, we could roll the wheels past the binding spot. It was observed that the frame bowed when pushing through the bind. During the bind, we cannot move the axle boxes on 1 and 2 in the pedestal openings, but axle 3 boxes remain free to move. Given axles 2 and 3 have no issue when coupled together, I focused on axle 1.

-The bind only occurs in one quadrant of the motion. On the Fireman’s side it is from 11:00 o’clock to 10:00 o’clock, which is Engineer’s side 4:00 o’clock to 5:00 o’clock.
20210127_010336643_iOS.jpeg
Degree of motion with binding, Engineer's side
-Much measurement, rechecking and head-scratching was performed on the rods, wheels and frame. All were found to be -+ 0.0015 of print.
20210118_210326674_iOS.jpeg
Measuring pedestal openings
-The pedestal openings on axle #1 and axle box widths had 0.005 clearance. While this made for a nice sliding fit, we felt it was too tight. The axle boxes were machined and 0.003 on each side removed. After re-assembly, this additional clearance reduced the binding, and the boxes now give a ‘clunk’ as they shift fore and aft through the rotation.

But there is still a bind in the motion.
It looks like the pins are trying to move 0.015 further than the rod length. At least, this is the gap between the pin and rod bushing at the point where it binds. If the rod centers were too short, I would expect binding 180 degrees opposite the current bind. But the bind only occurs in one position.
20210127_010855821_iOS.jpeg
Rod / Pin gap at bind
I want to know the root cause of this bind before I go randomly machining additional clearances. I've measured Rod bushing centers, pedestal openings, axles boxes and all seem to measure per print dimensions....

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Greg_Lewis
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Re: Mystery bind in wheel/rod motion

Post by Greg_Lewis » Thu Jan 28, 2021 11:44 am

Very curious that it's not twice at 180. Have you checked the quartering and the distance of the rod pins from the axles? And are the rod pins square to the plane of motion?
Greg Lewis, Prop.
Eyeball Engineering — Home of the dull toolbit.
Our motto: "That looks about right."
Celebrating 30 years of turning perfectly good metal into bits of useless scrap.

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Steamin
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Re: Mystery bind in wheel/rod motion

Post by Steamin » Thu Jan 28, 2021 1:39 pm

I did measure how perpendicular the pins are in the wheels; one main crankpin had 0.0007 taper from the wheel to the end (the pin measures round). These front pins measured less than half a thou, which is my limit of skills to detect.
20210118_210401147_iOS.jpeg
Pin measurement set up
I did not come up with a good method to measure the quartering. Searching around, I found lots of notes how to quarter axles, but not how to measure quartering on a finished/assembled set.

I tried to measure the pin distances from center in the mill, but my setup was not solid, so I did not trust the measurements.

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Greg_Lewis
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Re: Mystery bind in wheel/rod motion

Post by Greg_Lewis » Thu Jan 28, 2021 2:02 pm

Was this problem there before you pulled the drivers off the axles?
Greg Lewis, Prop.
Eyeball Engineering — Home of the dull toolbit.
Our motto: "That looks about right."
Celebrating 30 years of turning perfectly good metal into bits of useless scrap.

Mike Walsh
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Re: Mystery bind in wheel/rod motion

Post by Mike Walsh » Thu Jan 28, 2021 4:29 pm

Hi Denis,

Looking good. I have some pics to share with you about a new project, link up with me on FB.

If you find you are in need of parts, let me know. I have spare 1" pacific castings at my shop, for now...

Mike

Pontiacguy1
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Re: Mystery bind in wheel/rod motion

Post by Pontiacguy1 » Thu Jan 28, 2021 5:17 pm

I don't know if you're a novice or a seasoned vet here, so I'll just say what I'm thinking: Rod length. If the center to center on your side rods doesn't correspond to the center to center on your frame, then it will bind. If it's only off 5 or 10 thousandths or something, make yourself another bushing and make the center hole offset to one side by .010 or .015 or so. Mark where the thin spot is. Then put it in and keep rotating it until it rolls freely. You're basically correcting the center to center with that offset bushing. Once you find where it rolls smooth, then you can bolt or loctite it in that position.

I would not have opened up the axle box openings any at all, as 0.005" is plenty. It may roll more freely, but now you've taken some of the binding and given it room in another area. The axle boxes aren't supposed to move around in the frame, other than up and down as the suspension works.

If you've already tried that and/or measured the center-to-center and found them the same, then I would probably look for something out-of-square, or check your crankpin location. A crankpin that is farther out on the wheel than the others will do something similar to what you describe. One other thing to consider which will do the same thing: if one or more of your axle boxes wasn't bored exactly in the center. That will throw off the center distance as well. You may have already checked some of this, but those are always the top items on my list to check. I Hope this helps. Hate having a problem I can't identify!

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Dick_Morris
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Re: Mystery bind in wheel/rod motion

Post by Dick_Morris » Thu Jan 28, 2021 6:56 pm

Also remember that all those small inaccuracies can either cancel each other out or add up to cause a problem.

10 Wheeler Rob
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Re: Mystery bind in wheel/rod motion

Post by 10 Wheeler Rob » Thu Jan 28, 2021 8:46 pm

Just some observations. One or more of the bushings may have been bored non concentric to oliviate the bind in the original build. The frame may have suffered some distortion due to derailments and during tramsport.

Due all of the ovalness of the worn bushings and bores in the rods, the remaching may have lost the true center location by a few thousands of an inch. Which is the cause of the binding

I would make some adjustable rods and find the fit that runs smooth enough to suit. Use there these center to center dimentions to bore the rods or bushings in the rods.

Rob

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Steamin
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Re: Mystery bind in wheel/rod motion

Post by Steamin » Thu Jan 28, 2021 10:16 pm

@Greg_Lewis - Before I took it apart, the rods and pins had so much wear on them, nearly 0.040 cumulative, that it is unknown if there was a preexisting binding issue. I suspect there was, but it is only a guess.

@Pontiacguy1 - It was probably premature to machine the boxes before finding the true cause, which I why I stopped again. Maybe I can put one of the old bushings back in to diagnose.

@Dick_Morris - I was beginning to worry there are multiple small errors (quartering, bushings, rods, pedestal jaws) all adding up to the error.

@10 Wheeler - you might be on to something, I have not measured the frame for a twist or warp. I know it can flex, it bows up in the middle when I push it past the bind.

I think next steps are to diagnose further maybe with an old bushing in place, and build an adjustable rod to get the necessary true dimensions.
Steamin

RET
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Re: Mystery bind in wheel/rod motion

Post by RET » Fri Jan 29, 2021 6:16 am

Hi Steamin,

I agree with Pontiacguy1 that you shouldn't have opened up the pedestals, they were fine the way they were. What you might try is to remove one of the side rods between #1 and #2 axles (since #2 and #3 axles seem OK). From what you say, that should allow everything to roll easily.

Next, every 30 degrees of rotation, measure and record the distance between the two open crank pins and see what you get. I'm guessing that the quartering is off, but it could also be that one of the crank pin holes is at the wrong radius.

To check the crank pin radius, make a gauge that is a close fit on the crank pin and see if the hole you bored in it at the crank pin radius matches the axle diameter location where it is pressed on the wheel for all the wheels.

If the quartering is off, the measuring should show a sine wave difference in the "phantom" coupling rod length as the wheels rotate through one revolution. If the crank pin radius is wrong, the phantom coupling rod length measurements should be a max and min at the front and back dead centers. You might have to do this measuring on both sides by removing the coupling rod on one side at a time.

Hope this shows you where the problem is. Please let us know what you find.

Richard Trounce.
Last edited by RET on Fri Jan 29, 2021 10:03 am, edited 1 time in total.

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