Re: Accumate Proto:HO couplers

David Ball

Dennis (or anyone else),

Has it been determined the practical minimum radius the PROTO:HO couple can
be used for effective operation? I'm thinking literally how tight can the
radius be before it may cause problems for the coupler, not just that it
cope with typical tight radius like 18"


David Ball


From: STMFC@... [mailto:STMFC@...] On Behalf Of
Dennis Storzek
Sent: Friday, 14 October 2005 5:39 a.m.
To: STMFC@...
Subject: [STMFC] Accumate Proto:HO couplers


Word of the recent discussion of near scale size couplers and draft gear
boxes finally prompted me to join this forum. I spent some time last night
reading the entire thread, until it was way too late to compose a reply.
I'll try to make comment on all the points raised in this one message.

Both the Accumate PROTO:HO couplers and boxes are made from acetal
co-polymer engineering resin; either Celcon or its equivelant. This
material was chosen for its high strength, high stiffness, and low
coefficient of friction. While the most important property for the couplers
is strength, the most important property for the box is stiffness. The
problems that some have reported with the standard size Accumate couplers
"jamming open" is due specifically to boxes that are either too deep (too
much space above or below the coupler shank) or boxes with covers that are
too flexible. The amount that the twin shanks of Accumate couplers can
offset to open the knuckle is limited by a tiny interlock between the
adjacent surfaces of the shanks, and too much clearance allows the
interlock peg to come out of its slot and jam against the opposing surface.
The parts won't be damaged, but the situation is inconvenient. The standard
size Accumates exhibit more problems in this regard than the PROTO version
simply because we have no control over the dimensions of the boxes other
manufacturers build into their floors.

While both couplers and boxes are made of the same material, they are not
made in the same mold; each is in a different tool, one pair of coupler
shanks in one and a pair of boxes and covers in another. This is
"runnerless" molding, every molecule of plastic we shoot is shipped to you.
There are no scrap sprues to be reground and used again, degrading the
material properties further with each pass through the molding machine. By
using only virgin material in these parts we guarantee that you get all the
physical properties the resin manufacturer designed into the material.

While it may appear convenient to simply cement the draft gear boxes in
place, this will ultimately not have the strength to ensure that the
coupler shanks can't spread and jam, therefore we don't recommend any
mounting that doesn't have a screw through the boss that the couplers pivot
on. As Dr. Anspach pointed out, a 00-90 flathead screw can be used in this
location after first lightly countersinking the hole. We chose to go with
the 0-80 screw because many people are uncomfortable with working with the
smaller size drills, taps, and screws. I did consider incorporating the
countersink in the mold and allowing it to be covered by the larger 0-80
pan head, but doing so would only give the 0-80 head bearing outside the
area supported by the boss, making it easy to pinch the coupler shanks by
over tightening the screw. The box is easy enough to coutersink by hand for
those special cars.

Design, Engineering, and Execution:
Dr. Anspach is correct that the PROTO:HO coupler and its draft gear were
"engineered" as a system, although since I don't have an engineering
degree, I prefer the term designed. And yes, during the building of the
tool the core pins that make the mounting holes ended up slightly off
center in the bosses. As they say, stuff happens. Since this really doesn't
affect the utility of the parts, the decision was made not to do an
extensive re-work to the pins. However, the fact that one side of the boss
is thinner makes this the first area on the part that doesn't fill, a
"short shot". We consider these short shots a defective part, and like any
defective Accurail part, we will replace any that are sent to us.

Dimensions and Car Spacing:
The PROTO:HO draft gear was designed to appear to be the continuation of
the center sills through the body bolster. The box is designed to match the
common 20 1/2" width of center sill cover plates that was common during the
late steam / early Diesel era. The detailing on the underside of the box
represents the commonly used friction draft gears of the day, contained by
the sills, draft lugs, coupler yoke, and yoke support plate. The coupler
head itself is a dimensionally accurate model of the AAR Type E coupler,
with the knuckle shape modified to allow coupling with the other common
magnetic couplers. Since the coupler head is positioned to match the common
3" striker horn to striker plate spacing used with these draft gears , if
the end of the draft gear box is positioned where the prototype striker
plate is located, the models will have the proper distance between cars.
This isn't necessarily where the oversize box provided on some models ends,
as none of the manufacturers has really paid much attention to these
dimensions in the past, because all the available couplers were oversize.
Keep in mind, when modeling more modern prototypes with extended draft
gear, long shank couplers, or end of car cushioning devices, that the
position of the PROTO:HO box may have to be adjusted to put the coupler
pulling face at the proper location.

Mounting on cars without molded on boxes:
Dr. Anspach has given us an excellent summary of how the PROTO:HO box lends
itself to mounting on most commercially available equipment, but someone
asked about mounting on models where there is no coupler box provided, such
as resin kits. The maybe not so obvious answer is to simply use both parts
of the box that are provided with the couplers. While the unique design of
the Accumate PROTO:HO coupler allows it to be mounted in the box alone,
using the surface of the existing box as the cover, press-on covers are
provided that when used, yield the same mounting surface dimension as the
other popular couplers. These covers are also useful for modeling the
extended draft gears on modern cars.

I hope this answers any questions that anyone had.

Dennis Storzek

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