Re: Kadee Scale Coupler Operational Reliablity

railwayman <stevelucas3@...>

I think that it's time that we had a look at prototype couplers, in keeping with the STMFC list's mandate.

My 1995 AAR Field Manual gives coupler dimensions for Type E couplers. Rule 16, paragraph 8 gives a distance of 21" (.241" in HO) from back of the head to the end of the shank on a BE60AHT coupler, as well as several other Type E's. My belief is that this is a standard coupler length found on STMFC's. Here's a sketch of a Type E coupler--

This coupler is condemned if the back of the draft keyslot is less than 3 3/4" from the end of the shank. This gives a rear key slot distance of 17 1/4" (.198" in HO) from the rear of the coupler head. An AAR standard draft key is 6" long on its flat side. Assuming a keyslot of the same length on a Type E coupler, we deduce a length of 11 1/4" (.129" in HO) from front of keyslot to the back of the coupler head.

So in draft, the radius that determines coupler swing on a car with a standard length Type E coupler is 17 1/4, and in buff, 11 1/4".

Think about how much swing a prototype coupler can achieve on an 11 1/4" radius for a moment. It becomes clear that a standard Type E coupler is very tolerant of sharp track radii. Such as here, on the Bronx Terminal--

Notice the boxcar on a less than 100' radius curve next to the curved freight shed in both photos.

Tim Warris of FastTracks ( is building an excellent model of the Bronx Terminal in HO, using cars fitted with what appear to be standard NMRA draft gear on curves of about 12" radius--

And here is where I think that we get tripped up in modelling scale draft gear boxes--NMRA RP-22 (original Ken Mortimer drawing date of August, 1958) gives a distance of .265" in HO from the centre of a .060" diameter coupler pivot to the face of the cover plate (ie. where the head of the coupler is shoved against in buff).

A Kadee standard coupler pocket, #232, used on many of their couplers, including likely the most popular HO couplers used by STMFC modellers, #'s 5 and 58, (yes, I know that many on this board are using Sergent couplers now) has a measurement of .254" (maybe my measurements are a bit off--I'm thinking that this is a nominal 1/4" actual distance) from end of cover plate to centre of coupler pivot stem (which the coupler pivots on).

With a Kadee #5 or #58 coupler's depth from back of coupler head to centre of pivot being about .270", we have about twice the distance between pivot point and back of coupler head on the Kadee coupler compared with the Type E's HO scale measurement of .129" in buff between pivot (the front of the draft key) and the back of the coupler head.

So the Kadee coupler box HAS to be wider than scale to accommodate "sharp model radii"--but remember the real Bronx Terminal? They were able to take 40' and longer cars with standard AAR draft gear around curves of less than 100' radius--less than 14" radius HO curves. And their train crews were able to couple and uncouple cars on these curves.

The culprit is the distance between coupler head and pivot point (or if you want to be even more accurate, the distance between pulling face of coupler and pivot point) on the model couplers that we use now. Shorten it, and we can have scale width coupler boxes. But then of course, we "need" those coupler centreing spring that mostly weren't used on STMFC's, barring Cardwell draft gear...

But it'll take some subtle or not-so-subtle pressure on the major manufacturers to have working scale width draft gear in something other than a niche product. It CAN perform well on those "sharp model railroad curves".

Steve Lucas.

--- In STMFC@..., Jim Betz <jimbetz@...> wrote:

Denny, et al,

While I agree that the standard length of the shanks on
the Kadee product line (and all other mfgrs as well - with the
possible exception of the Sergents) is longer than it should be -
it does mean that we can operate our model trains on track with
significantly more curvature than the prototype. The same can
be said of the width of the coupler box - and that measurement
is directly related to the same thing.

I have never seen a layout that has truly prototypical
curvature standards. And even those that have truly generous
standards (such as "nothing smaller than 36" on the mainline")
still have several areas on the layout that have smaller
curves and lower numbered turnouts. If I remember correctly
a #10 turnout -approaches- the typical smallest radius of the
prototype (and I'm not talking about mainline). Yes, of course,
these are generalizations/standard practices and you can go
out and find examples where the real RRs had more curvature
than these statements. But they are pretty close to what
the real RRs use as their standards (which they may or may
not break depending upon the situation).
On our layouts we 'fudge' even our own standards - not once
in a while but often/always. The phrase "I can make it fit if
I use a #4 turnout here for this industry track" is one I hear
a lot. The other thing you hear are statements such as "we
have 30-inch radius every where on our mainline - except for
the areas at _____ and _____" ... and those statements are made
proudly/as a brag about how generous that particular layout is.
And there is a real reason why the time-saver is based upon
all #4 turnouts.
If I remember correctly I think that an HO layout would
have to adopt s curvature standard of something approaching
that used on O-scale in order to be in the same ball park as
the real RRs.
I can't remember ever seeing a commercially available
turnout number larger than a #10. Even Fast Tracks only
goes up to a #12 (and I'll predict that they don't sell very
many of them!) - in HO and N ... and they only go up to a #6
in O-scale.

If you can get Kadee (or any other coupler mfgr) to answer
you I'm certain that you will find that the size of the coupler
box and the length of the shank has been chosen in order to
provide for operation of normal length equipment on tight
radius curves (where tight is defined as 22" or larger - at
least). The other thing that you will be told is that they
are using the sizes/measurements they are using due to the
NMRA standards/RPs.

Until we start to have layouts that do not compromise on
the curvature and turnout numbers in use I'm afraid we are
going to have to compromise on the coupler boxes and shanks.
I don't see any way around it.
I'll even go further and state that due to the sheer
number of layouts and existing model trains that are
already out there already you are going to be hard pressed
to figure out how to influence/change this reality. We may
be 'committed' to this state "for ever". *Sigh*
On the other side - I'm not sure any of us would ever
attempt to build a layout based upon true prototype
curvature standards ... we just don't have the space.

One last thing - all of the above relates directly to
the operational characteristics of our 'typical' trains and
is equally applicable to all scales and eras. If your layout
is early enough in the STMFC era that you have only 40'
or shorter freight cars then you might be able to use
smaller boxes and shorter shafts. Even a train of all
50' cars, going around a 40" radius curve - will look 'funny'
when compared to the real RRs. By 'funny' I mean that the
cars will be hanging out over the rail in the middle of the
car further than you will see them do on a real RR.
- Jim

P.S. It certainly -seems- to be true that having a coupler
box available that has a 'built-in shim' across the
opening to prevent coupler droop would be a good idea.
But I highly doubt we'll ever see it from Kadee - if
for no other reason than that it would prevent that
box from being used for a #5 with the existing copper
centering spring.
A good argument can be made for the idea that a
coupler "needs" to be able to 'droop' during certain
loading situations. I have certainly seen couplers
between two cars that were "pulled down" from the
normal orientation when going thru a vertical curve.
Less possible movement in that direction under these
conditions would translate into more frequent break-in-
twos (but eliminate others).
And I'm sure that Kadee will tell you that if your
couplers are drooping that you need to look at how
they are installed and fix the problem that way. And
their argument is technically correct. However, it is
also true that installing a shim may be a much quicker
and easier way to fix the problem on a particular
One thing that I've seen done (and done myself) is
to reverse the copper spring in the box - to put the
'face' of it below instead of above the coupler. This
is one "quick and dirty" way to correct droop.

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