I have an old United Scale Models (imported my Pacific Fast Mail) HOn3 Cowichan RR 25-ton Shay that is not "DCC ready". As with many brass locomotives that pre-date DCC the motor is not isolated from the frame.
The drive shaft is attached to the frame by a metal bracket. For DC the frame acts as pickup from one rail and one side of the wheel axles picks up from the other rail. For DCC the motor must be totally isolated from the frame. This can be very tricky to accomplish, especially in smaller locomotives where there isn't much room to work.
Bill Payne, fellow member of our NMR club is far more knowledgeable than me about solving these kinds of problems.
He came up with a way to to do the engine remotoring and isolation. His ideas may help you with your engine remotoring and decoder installations.
We began this project early in 2013. It took awhile because we had to source parts from Northwest Short Line (NWSL) and do some major surgery on the drive shaft to shorten it and connect it to the new motor. Using calipers we determined that we could fit in a single shaft 1.5 x 9 mm DC motor (1627S-9). This is a smooth running permag miniature motor that is 16 mm wide x 27 mm long. It is an excellent choice for engine remotoring of small HO, HOn3 and large N scale locomotives. After checking the shaft sizes we settled on a set of NWSL U-joints (482-6). This set has 1.5, 2.0 and 2.4 mm balls, cups and dogbones. It is a versatile set to match up motors and the shaft to the gears. By trial and error and some measuring we determined that we could fit the motor if the shafts from both motor and gears could be trimmed to allow a bare minimum of play once the couplings were installed.
We floated the motor in silicon caulk to isolate the metal from the shay's frame and let the caulk set up.
We used a thin piece of styrene and cut a barrier that could be placed between the motor and the frame. For some reason the connection between the shafts works better if one shaft is slightly offset from the other. I don't understand the physics as to why this is. You can see in the photo how much the shafts had to be shortened to fit inside the shay's boiler and cab. Clearance was very tight.
Bill Payne used the original motor bracket for the engine remotoring and reshaped it to support the shaft from the gears. The old shaft went directly between the gears and the motor and would have no support after it was cut. By using the original bracket Bill could keep the holes that were already drilled to attach the bracket to the frame and maintain the original height of the drive shaft. The plastic couplings isolate the gear set up from the motor shaft and the silicon isolates the motor from the frame. We didn't have to put any styrene under the bracket or use nylon screws. The other task was to find a way to route power from the other rail to the decoder.
We did this by gluing a small piece of thin circuit board to the isolated side of the bolsters on the trucks, ie, the side that wasn't connected to the frame. This allowed us to still use the frame for one power lead to the decoder and the contact plate for the other lead. Thin, flexible phosphor bronze wire made the contact from the pad to the wheels. I hope these engine remotoring tips help you with your projects.
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