First my materials list:
1. Resistive paint. GC Electronics EMI-RFI Shield 10-4807
2. Conductive Paint. Automotive rear window defogger repair kit.
3. ACC Use the kind that is plastic compatible.
Resistive Paint Method:
First prepare the wheel sets. Using a Dremel or similar tool with a small wire brush (use eye protection) remove the blackening from an area of the back of each wheel. This blackening seems to be an insulator, and we need to remove it to get a good contact to the wheels. Next check that the wheel sets are in gauge, and then run a fillet of ACC between the wheels and the axles. There is normally a crack where they meet, and this will be difficult to bridge reliably with paint unless it is filled first. Once the ACC sets up you may paint the axles. Actually I prepare 20-40 wheel sets at one time, and then paint them all at the same time. Set them all in a row on some scrap cardboard with the buffed areas facing up. You can hold them in place with strips of masking tape, but be sure to not cover any part of the axles, nor the insides of the wheels. Make two fairly rapid even passes with the EMI-RFI paint. The first pass aiming toward the inside of one group of wheels, and the other aiming more toward the inside of the opposite wheels. The idea is to not only cover the width of the axle (just on the top side) but to get some resistance paint up onto the wheels and cover the buffed areas.
When you are done with the spray paint, BE SURE to turn the can upside down and clear the nozzle. If you do not, particles of the powdered graphite may lodge in the nozzle valve and prevent the nozzle from shutting off properly. (voice of experience) Let the resistance paint dry. It does not become conductive until it has dried. Check the resistance of each wheel set with an ohmmeter. If they are too high, give them another pass with the paint. If they are too low you have two choices. You can either leave them as is with a lower resistance, or you can scrape some of the paint off to raise the resistance. If you choose to fine tune the resistance just clip your ohmmeter to the wheels, and adjust the resistance. You can actually lower the resistance somewhat by burnishing the paint. When scraping paint off, do it the long way on the axle, not around. It is much easier to control that way.
If a wheel set has no connection, check with your ohmmeter to see where the break is located. I found that it was usually at the junction between the wheel and the axle. If you find such a break try applying a little drop of the automotive conductive paint just to the break area. Let it dry prior to testing again.
Surface Mount Resistor Method:
The second method of adding resistance that I have used is with surface mounting resistors. The first steps are the same. Buff the blackening from the wheels, check the wheel spacing, and fill any cracks with ACC. Next choose a suitable location for the resistor. Pick a spot that will not hit any other running gear or car body mounted parts. This location should also be either on or bridging over an insulated section. Mount the resistor with either ACC or a dab of epoxy. It is important to make sure that the underside of the resistor has no open gaps that would allow the conductive paint to flow from one side to the other and thus short out your wheel set. Also be sure that the mounting material does not cover the conductive end caps of the resistors. If it does, wait till it cures, then gently scrape away enough to expose some of the conductive area again. When the chip mounting glue has cured you may then connect the ends of the chip to the wheels. Draw a narrow line of your defogger repair paint from the buffed area of one wheel to one end of the resistor. From the other end continue along the axle to the opposite wheel and up to the buffed area. After letting the paint dry, check for conductivity, and repair any breaks that may have formed.
I put one conductive wheel set at each end of a freight car, and from experience, place two conductive wheel sets on each three axle passenger truck. The reason for this is that usually one out of the three axles in the typical passenger truck is not carrying any weight, and that is the axle I seem to pick for the conductive wheel set. By using two one is always carrying a load. (unless the track is really uneven) Using 100% resistance wheel sets would probably improve reliability slightly, especially if a single car is just barely pushed over a block boundary, but the extra expense is not compatible with my budget. Maybe once I have all my existing cars converted I can go back to add in the rest as they get pulled for maintenance.
Back to the RR-CirKits Home Page