Semi-Scale Wheelsets for IMRC 70-Ton Trucks


tmolsen@...
 

Andy and List members,

While attending last weekend's Timonium trainshow, I stopped by the Reboxx table to obtain semi-scale .088 wheelsets for the 24 pair of IMRC 70-Ton truck sideframes that I bought from Andy.

Andy had said that he could not supply wheelsets as Brian was no longer carrying the IMRC semi-scale wheelsets. When the Reboxx people measured the sideframes to get the proper axle length, they came up with the startling information that the two ends of the sideframes had DIFFERENT axle lengths!

Therefore, it was necessary to buy 2 different sizes to accommodate the sideframes that I have. The Reboxx part numbers are:

33-1-1.015
33-1-1.010

So you will have two different wheelsets in the same truck sideframes. I bought four packages of each to wheel the 24 pair of trucks. There is always a possibility that this may have been an anomaly that occurred to the sets that I took down to be measured, but I will not know until I mount all the sets.

Even so, I thought that I should pass this on for those who are mounting wheelsets to these new truck sideframes.

Regards,

Tom Olsen
7 Boundary Road, West Branch
Newark, Delaware, 19711-7479
(302) 738-4292
tmolsen@udel.edu


jerryglow2
 

Why not "ream" them out with "the tool" so one size axle would fit?

JErry Glow

--- In STMFC@yahoogroups.com, <tmolsen@...> wrote:

Andy and List members,

While attending last weekend's Timonium trainshow, I stopped by the Reboxx table to obtain semi-scale .088 wheelsets for the 24 pair of IMRC 70-Ton truck sideframes that I bought from Andy.

Andy had said that he could not supply wheelsets as Brian was no longer carrying the IMRC semi-scale wheelsets. When the Reboxx people measured the sideframes to get the proper axle length, they came up with the startling information that the two ends of the sideframes had DIFFERENT axle lengths!

Therefore, it was necessary to buy 2 different sizes to accommodate the sideframes that I have. The Reboxx part numbers are:

33-1-1.015
33-1-1.010

So you will have two different wheelsets in the same truck sideframes. I bought four packages of each to wheel the 24 pair of trucks. There is always a possibility that this may have been an anomaly that occurred to the sets that I took down to be measured, but I will not know until I mount all the sets.

Even so, I thought that I should pass this on for those who are mounting wheelsets to these new truck sideframes.

Regards,

Tom Olsen
7 Boundary Road, West Branch
Newark, Delaware, 19711-7479
(302) 738-4292
tmolsen@...


Brian J Carlson <brian@...>
 

Tom: I've had a fair number of trucks where this has happened. It may be the
way the trucks were removed from the mold, or some excessive pressure on my
part with the rebox tool for truing the conical hole.

Brian J Carlson P.E.
Cheektowaga NY


Jon Miller <atsf@...>
 

When the Reboxx people measured the sideframes to get the proper axle length, they came up with the startling information that the two ends of the sideframes had DIFFERENT axle lengths!<
I noted this with a RTR car I bought a few weeks ago. The molds apparently go together wrong and the sideframes are skewed.

Jon Miller
AT&SF
For me time has stopped in 1941
Digitrax, Chief/Zephyr systems, JMRI user
NMRA Life member #2623
Member SFRH&MS


Denny Anspach <danspach@...>
 

It is not at all unusual that any given molded styrene truck may optimally require two different axle lengths for best operation. This disparity can vary from one molding machine to the next, and in fact can vary from one batch of trucks to next from the same machine. These trucks are not precision instruments, and if they were manufactured to be so, no one of us could afford to purchase them!

Usually, the difference between the required lengths is slight, and one can simply choose a compromise axle length that will work with no discernible decrease in rollability, or perhaps even more importantly, any excess end play.

Occasionally, a tiny metal sliver at the point of an axle (a common byproduct of being made on screw machines) can create the false effect of an axle of given length being "too long". This can be felt by the pad of a finger, and very easily removed by several gentle swipes with fine grit sanding board (I use a 1000 grit).

Although I use the Reboxx reaming tool (sparingly), using it to alter required axle length can be pretty chancy, actually making things worse.

BTW, all the same issues hold for cast metal and brass trucks. They too are not "precision instruments".

Denny


Schuyler Larrabee
 

Andy and List members,

While attending last weekend's Timonium trainshow, I stopped by the Reboxx table to obtain
semi-scale .088 wheelsets
for the 24 pair of IMRC 70-Ton truck sideframes that I bought from Andy.

Andy had said that he could not supply wheelsets as Brian was no longer carrying the IMRC
semi-scale wheelsets. When
the Reboxx people measured the sideframes to get the proper axle length, they came up with the
startling information
that the two ends of the sideframes had DIFFERENT axle lengths!

Therefore, it was necessary to buy 2 different sizes to accommodate the sideframes that I have.
The Reboxx part numbers
are:

33-1-1.015
33-1-1.010

So you will have two different wheelsets in the same truck sideframes. I bought four packages of
each to wheel the 24
pair of trucks. There is always a possibility that this may have been an anomaly that occurred to
the sets that I took down
to be measured, but I will not know until I mount all the sets.

Even so, I thought that I should pass this on for those who are mounting wheelsets to these new
truck sideframes.

Regards,

Tom Olsen

John "Reboxx" Burroughs is a good friend of mine. This phenomenon of the truck frames not being
truly square is something they have been aware of for a long time. Before they set up their table
on their webpage showing the recommended axle lengths for various trucks they tested many pairs of
each type of truck. While the recommended length is probably the best, there is some significant
variation truck to truck, and as you found, end to end of individual trucks. That is a second
reason to have one of their testing sets, which contain one each of every length they carry. You
may well find that the "right length" for a particular truck is .005 or even .010 longer or shorter
than what the chart says.

I doubt, Brian Carlson, that your " some excessive pressure on my part with the Reboxx tool" has
anything to do with it. The Tool is designed to clear out extraneous, uh, "stuff" in the hole, but
not to cut any deeper. I have found small balls of plastic in the journals, maybe .020"-.030" in
diameter in some trucks, which the Tool cleans out. That's the reason to use the Tool.

The annoying thing about the skew in the trucks is that it's very hard to do anything about it.
The reasons cited may be at fault. Another may be that the tooling is not uniformly hot when the
specific truck frame was molded. I don't think anybody really knows all of the reason(s). Dennis,
do you have any comment here?

SGL





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Gene Green <bierglaeser@...>
 

HOW did they make the measurements? Some special instrument? Inside calipers?

Gene Green

--- In STMFC@yahoogroups.com, <tmolsen@...> wrote:
<snip>
When the Reboxx people measured the sideframes to get the proper axle length, they came up with the startling information that the two ends of the sideframes had DIFFERENT axle lengths! <snip>


Schuyler Larrabee
 

Exactly so. Another mutual friend is a machinist. He added some extensions to a set of calipers
and recalibrated them so that they provided a measurement between the bottom of the journal cones.

I wondered if someone would ask. Shows you thought about it, Gene.

SGL

HOW did they make the measurements? Some special instrument? Inside calipers?

Gene Green

--- In STMFC@yahoogroups.com <mailto:STMFC%40yahoogroups.com> , <tmolsen@...> wrote:
<snip>
When the Reboxx people measured the sideframes to get the proper axle length, they came up with
the startling
information that the two ends of the sideframes had DIFFERENT axle lengths! <snip>




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Mark
 

Years ago I worked in a machine shop, some of the machinists had inside calipers and micrometers. The only ones in my collection are outside!

The tool for clearing out the burrs can be interesting, my friend ran it through one side.

Mark Morgan

--- On Sat, 4/11/09, Schuyler Larrabee <schuyler.larrabee@verizon.net> wrote:

From: Schuyler Larrabee <schuyler.larrabee@verizon.net>
Subject: RE: [STMFC] Re: Semi-Scale Wheelsets for IMRC 70-Ton Trucks
To: STMFC@yahoogroups.com
Date: Saturday, April 11, 2009, 4:59 PM

















Exactly so. Another mutual friend is a machinist. He added some extensions to a set of calipers

and recalibrated them so that they provided a measurement between the bottom of the journal cones.



I wondered if someone would ask. Shows you thought about it, Gene.



SGL



HOW did they make the measurements? Some special instrument? Inside calipers?
Gene Green
--- In STMFC@yahoogroups. com <mailto:STMFC% 40yahoogroups. com> , <tmolsen@... > wrote:
<snip>
When the Reboxx people measured the sideframes to get the proper axle length, they came up with
the startling

information that the two ends of the sideframes had DIFFERENT axle lengths! <snip>


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[Non-text portions of this message have been removed]


Schuyler Larrabee
 

The tool for clearing out the burrs can be interesting, my friend ran it through one side.

Mark Morgan

You're not supposed to press it that hard!!

SGL





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Dennis Storzek
 

--- In STMFC@yahoogroups.com, "Schuyler Larrabee"
The reasons cited may be at fault. Another may be that the tooling is not uniformly hot when the
specific truck frame was molded. I don't think anybody really knows all of the reason(s). Dennis,
do you have any comment here?

SGL
It's "differential shrinkage" in the plastic molding. Almost all plastic trucks are molded in acetal, either DuPont Delrin or one of the competing brands.

Acetal has a rather high rate of shrinkage. The shrinkage starts as soon as the material solidifies. In a one piece truck where the part is gated in the center, the gate must be, by necessity, on one side of the bolster, and thus on one side of the hole. When the plastic enters the mold, it immediately runs around to the car side of the hole and stops flowing. As soon as it stops, it starts to cool and solidify. Meanwhile, the plastic on the side of the hole with the gate is still flowing to fill the rest of the part, so it's still being fed under pressure while the first side is solidified. The final packing pressure normally stuffs a few more molecules on the gate side of the hole, and it becomes longer, because more material in the same space means less shrinkage.

We QC sample parts while they are hot to an oversize "pre-shrink" dimension, and you can actually see one side pull up tight on the gauge before the other, but since the amount is minuscule, about .005" or less, and within the amount of variation that naturally comes with wear of the moving mold components that make the axle cones, it is not deemed a problem.

Twisted sideframes are. Injection molds are made of precision ground parts, so if the parts are assembled crooked, the mold won't close, or it will only close once :-( The times I've seen twisted sideframes was the old Front Range trucks. That mold didn't have movable cores for the cones; Fred was trying to have the parts flex to snap out of the mold, and depending upon the material temperature at the time they ere ejected, they could twist and take a set. It kept the tooling cheap, but was definitely a poor way to make trucks

Dennis


Jon Miller <atsf@...>
 

We QC sample parts while they are hot to an oversize "pre-shrink" dimension, and you can actually see one side pull up tight on the gauge before the other, but since the amount is minuscule, about .005" or less, and within the amount of variation that naturally comes with wear of the moving mold components that make the axle cones, it is not deemed a problem.<
And I don't deem .005 a problem either. The problem with the trucks I had was about .030 or so. As I replaced the trucks I don't remember the brand and I don't have the old trucks. So I would say .005 is good but .010 or more is bad.

Jon Miller
AT&SF
For me time has stopped in 1941
Digitrax, Chief/Zephyr systems, JMRI user
NMRA Life member #2623
Member SFRH&MS


Gene Green <bierglaeser@...>
 

--- In STMFC@yahoogroups.com, "Schuyler Larrabee" <schuyler.larrabee@...> wrote:

Exactly so. Another mutual friend is a machinist. He added some extensions to a set of calipers
and recalibrated them so that they provided a measurement between the bottom of the journal cones.

I wondered if someone would ask. Shows you thought about it, Gene.

SGL
Schuyler,
Thanks. Doesn't sound like the sort of thing I could do for myself. It probably means that trial-and-error will remain my method of choice.
Gene Green


Denny Anspach <danspach@...>
 

Like many other aspects of our hobby, this is just another instance where we are again faced with givens and druthers.

Measuring the inside diameter between the points of the conical bearing holes is an effort that is better in the thinking than in the doing. The thinking presumes that the bearing holes actually have conical points, and that such an accurate measurement between them would then accurately predict what the ideal axle length would likely be. It doesn't quite work out this way inasmuch as the truck bearing "cones" are never perfect, and in fact are commonly either actually flat, of flattened curvature, quite assymetric, or any combination of the three. So quite (most!) often, an axle whose length theoretically seems to be ideally fitted in any given instance, while in real time it does not roll worth a hoot.


The most objective means of selecting ideal axle length is with some type of measurable actual roll testing against a standard; which in turn has to be balanced against the thoroughly subjective judgement of axle end play, i.e. "enough" but " not too much".

As a personal note, I have found over the six or more years that I have been testing and installing axle/wheel sets routinely "to order", this has been one of my most rewarding efforts, resulting in my current fair sized fleet of very free rolling cars that, in addition, do not sway or swagger their way down the track. The latter good behavior in turn also enabled me to make practical the use of semi scale couplers as a standard.

Denny


Kurt Laughlin <fleeta@...>
 

Not to mention that each conical hole has its own axial center, and the presumption in the whole scenario is that the the opposing axial centers are perfectly coincident, perfectly parallel to the adjacent axle's centerline, and located properly relative to the kingpin axis. Or, if not perfect, at least significantly closer than the error caused/tolerated by a .005 increment in axle length. Otherwise there'd be no point in the exercise in the first place.

KL

----- Original Message -----
From: Denny Anspach

Measuring the inside diameter between the points of the conical
bearing holes is an effort that is better in the thinking than in the
doing. The thinking presumes that the bearing holes actually have
conical points, and that such an accurate measurement between them
would then accurately predict what the ideal axle length would likely
be. It doesn't quite work out this way inasmuch as the truck bearing
"cones" are never perfect, and in fact are commonly either actually
flat, of flattened curvature, quite assymetric, or any combination of
the three. So quite (most!) often, an axle whose length theoretically
seems to be ideally fitted in any given instance, while in real time
it does not roll worth a hoot.


tmolsen@...
 

My note regarding the disparity between the axle lengths on the IMRC 70-Ton trucks has generated a large number of opinions. It has been quite interesting to read the many posts regarding this problem and the many ideas as to how to correct the problem and also what has caused this situation to occur.

The best trucks for roll-ability right out of the box with standard wheelsets that I have found were the Kato A3 trucks which the market a number of years ago. I found it amusing that some people complained that they could not get good trucks that would roll freely, then when they tried these, they complained that they rolled to freely (ie. put a car in a siding with a little bit of slope to it and the car would roll out after being uncoupled. I guess they never heard of wheel chocks!).

My thanks to all that have responded with a wealth of information regarding this situation and your personal experiences. I think Denny has summed it all up in his comment regarding the replacement of sub-standard wheelsets and in doing so gave him a lot of satisfaction in trouble free operation.

Thanks guys for your comments and opinions.

Best regards,

Tom Olsen
7 Boundary Road, West Branch
Newark, Delaware, 19711-7479
(302) 738-4292
tmolsen@udel.edu