Re: Kadee Scale Coupler Operational Reliablity
I think that it's time that we had a look at prototype couplers, in keeping with the STMFC list's mandate.toggle quoted messageShow quoted text
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 (http://www.handlaidtrack.com/index-2.php) 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".
--- In STMFC@..., Jim Betz <jimbetz@...> wrote: