Accumate Proto:HO couplers

Dennis Storzek <dstorzek@...>


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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