Resistance Wheel sets

Characteristics:

Now that we have our detectors in place we will need some way to detect every car on the layout. (at least to do things up royally) As before, the requirements are simplicity and cost effectiveness. Remember even a modest layout will have hundreds of wheel sets to convert. Using commercial resistance wheel sets was just not an option for me. Besides, some of them have oversized axles that will not fit properly into my trucks. The most common metal wheel sets on my layout are Kadee. I'm not sure what metal they are made of, but it sure isn't easy (if even possible) to solder to them. A second problem that I noticed is that many of my freight cars have little if any clearance from the coupler pockets to the axles of the outer wheel sets. Under frames often limit the clearances on the inner wheel sets. My six wheel passenger trucks have even less clearance to the axles than the freight trucks do. The Kadee wheel set axles are apparently made of Delerin or some similar engineering plastic. This makes it hard to reliably glue anything to them.

Operating principles:

The design resistance I chose for my wheel sets was 10K ohm. This is low enough to cause reliable detection, but still only draws just over a milliamp per wheel set. Even a layout with a hundred cars will not draw as much as a single locomotive.

Solution:

I have used two different solutions. One is surface mount resistors, and the second is resistive paint. For wheel sets with plastic axles such as Kadee I prefer to use resistive paint. For metal axles and metal wheels the resistors may be simpler because there is no length of insulated axle to hold the resistive paint.

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.

Installation:

Once the resistance is in place using either method, be sure to buff the wheel treads. This not only makes for a much better looking model, but ensures good electrical contact from the wheels to the rails by removing the blackening. Many folks have found that polished metal wheels seem to remain much cleaner and do not pick up junk from the rails as easily. I do not bother to paint over the resistance paint, as it is a dark grey, almost black. However, you may do so if you want to. Be sure to test your paint on one wheel set first if you do decide to paint over the resistance paint. Some paint solvents might effect the resistance.

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.

Conclusion:

With about $30 worth of materials and a reasonably small investment of time it is possible to convert very large numbers of existing metal wheel sets into resistive wheel sets. (I can only guess at the numbers, as my 12oz. can of EMI-RFI shielding is showing no signs of depletion after about 10-15 years of use.) I have experimented with spray painting plastic wheel sets with the graphite paint to make them conductive. You are on your own in this area, as I have not followed up on the reliability of this method. I did this after finding that it was nearly impossible to remove the resistive paint from the metal wheeltreads where it oversprayed. I figured that if it would resist buffing it might also last for a while in normal operation.

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Last updated 12-31-99