Table of Contents
Current collection systems
Introduction
There are probably as many ways of current collection for model locomotives as there are wheel arrangements. All the methods available have their advocates and modellers should select a system that they find easiest to apply to a particular locomotive.
Pick-up systems fall roughly into three groups:
- Commercial plunger types where a spring-loaded contact bears on the back of the flange
- Wiper systems where a wire or strip bears on the wheel
- Contact is made through the axle to the bearing bush. This requires either metal wheels or plastic centered wheels to be fitted with a shorting strip.
Plunger pick-ups
There are several suppliers of plunger pick-ups and also some kits come with the manufacturer’s own design. As the name suggests, a plunger is pressed against the back of the wheel. A spring is used to apply the pressure (Figure 1).
The installation requires a hole in the frame to fit the body of the pick-up or to clear the plunger. The position of the hole can be important. If there is any form of suspension then the hole should be on the horizontal centreline of the axles, or nearly so. When using plunger pick-ups, it is important that the wire that connects the pick-up to the motor is flexible, so that it does not restrict the movement of the plunger. The current drawn by modern motors is small, and so a fine gauge can be used. The wire should be stranded rather than solid.
Figure 1. Typical plunger pick-up arrangements in a chassis under construction. (Photo Nick Baines).
Wiper pick-ups
Wiper pick-ups are probably the simplest and most adaptable type. They can be fitted in many places and can be almost invisible.
One of the best method of mounting them is to use a piece of copperclad attached to the chassis. This can be sleeper strip or cut from a sheet. If you do not wish to glue the strip to the chassis, double-sided copperclad can be used and soldered on. Take care not to bridge the gap and create a conducting path between the two sides of the copper conductors.
The wiping contact is a matter of choice. 0.45 mm diameter brass wire is a good choice and easily soldered to the copperclad strip. Nickel silver or phosphor bronze can equally be employed (a source of the former being 0.012 in. guitar E strings). The geometry of the wire will depend on the installation. In Figure 2 it is apparent that several bends are needed to clear the bottom of the chassis then reach over to the back of the wheel. The end in contact with the wheel can be either the end of the wire pressing into the flange or shaped as in the illustration to sweep the back of the wheel. This is a matter of personal choice, neither method having a great advantage over the other.
The contact pressure on the wheel should not be too heavy. Treat the wire as a cantilever spring. A simple rule of thumb is if the length of the wire from its fixed point is taken as ‘X’ units, ignoring any bends, then the deflection required to bear on the wheel is about 0.2X units (Figure 3).
Sometimes it may prove difficult to thread the pick-up wire through brake gear without it touching. A simple solution to this is to insulate it, using insulation stripped from some fine electrical connecting wire, or the thin insulated tubing used in electronic work. Such insulation is threaded over the pick-up wire.
Figure 2. Brass wire pick-up soldered to a copperclad strip bonded to the chassis. Several bends have had to be introduced to reach the back of the flanges. (Photo Bob Alderman).
Figure 3. Pick-up wiper deflection.
An alternative to rubbing on the back of the wheel is to have the wiper rub on the tread, the theory being that it is self-cleaning. This is not always easy from inside the chassis, and can lead to a convoluted pick-up wire. If possible try to mount it on the outside of the chassis. The illustration (Figure 4) shows a pick-up fixed to the head of an 8BA bolt threaded through an insulated bush in the chassis. The bush was made from the body of a plunger pick-up but styrene tube can serve equally well. In the example illustrated, the bolt is slightly free to move in the bush allowing one set of wheels to move on the compensation. The motor wire is soldered to the inside end of the bolt.
Figure 4. In this variation the pick-up is largely hidden from view by the locomotive body. (Photo Bob Alderman).
Another type of tread pick-up is shown in Figure 5. A wide strip of copperclad is mounted between the frames and scored to divide it into four electrical sections. The outer strips provide a means of soldering to the underframe and the inner strips provide a mounting point. The outer strips and the frame edges have an insulating layer to prevent the pick-ups shorting to the chassis.
Figure 5. Another variation of tread pick-ups. (Photo Ken Sheale).
Axle/bush contact
This system is used in conjunction with either split axle systems or the ‘American’ pick-up system, described below.
Wheel requirements
For these systems the pick-up via the axle/bush contact requires an uninsulated wheel. All-metal wheels are available in both insulated and uninsulated versions. For the split axle system the uninsulated version is used throughout. For the ‘American’ system half the wheels are uninsulated and half insulated. For plastic centred wheels like Slater’s it is necessary to fit a shorting strip between the rim and the boss. Figure 6 shows a Slater’s driving wheel shorted from tyre to centre bush. The metal parts are drilled to accept a copper wire or, as in the illustration, a phosphor bronze strip. This can be readily soldered if it is tinned before fitting and a large hot soldering iron is used. The soldering should only take moments, and must be done quickly to avoid softening and distorting the plastic centre of the wheel. If desired, the spokes can be relieved to accept the strip, as shown in the illustration. To ensure smooth running, the shorting wire should not protrude above the back face of the wheel at the tyre or the centre.
Figure 6. Slater’s driving wheel shorted using a phosphor bronze strip. (Photo Fred Lewis).
In this type of system, current passes from the axle through the bearing. For plain bearings this is not usually a problem because the bearings are not flooded with lubricant so that it forms a continuous insulating film. There is sufficient, continuous, metal-to-metal contact to carry the necessary current. However, with ball races, the area of contact between the ball and the race is very small, and even a small current can cause sparking, oxidation, and erosion that can destroy the bearing in a short time. The solution in such cases is to insulate the bearings from the axle (the inner race rotates with the axle so no rubbing is involved), and current is collected from a wiper acting in a groove on the axle.
Split axle system
A number of methods of producing split axles using epoxy resin adhesive as the securing compound have appeared in the model press. Most require the use of a lathe and an assembly jig to hold the components in position while the resin cures. There are two methods that do not require the use of a lathe and cover most requirements. A method suitable for axles not required to carry gearwheels is shown in Figure 7.
Drill two holes in line with each other through the axle and join with a saw cut. Pack the slot so formed with epoxy resin and allow to set. When it is hard, saw across the axle from opposite sides into each hole. Pack the cuts with more epoxy resin and allow it to set. Care needs to be taken to ensure that no metal swarf is trapped in the cuts to cause a short circuit. The outer sleeve shown is optional.
The axle carrying the final drive from the motor needs to be treated differently. There are commercial axles having double insulation leaving the centre electrically dead. These allow the final gearwheel to be mounted directly on the axle. The design as shown in Figure 7 is only suitable if the final drive gear is bored out to fit over the insulating sleeve or if the gearwheel is made from a load bearing plastic which is electrically non-conducting.
Figure 7. Split axle system suitable for squared and shouldered driving axles.
A conventional drive can be installed if the final gearwheel is offset and the insulation cuts are limited in length. The arrangement is shown in Figure 8. The amount that the motor and gearing can be offset depends on the space available in the locomotive body and the method of mounting. Most final gears are about 10 mm wide including the boss, so it should be just possible to insert an insulation break in the space available. If the final drive gear forms part of a motor-gearbox design it is still possible to use the system providing the gearbox final drive bearings, which will most likely bridge the split, are of insulated material.
A further method that is only suitable for tender or bogie axles, which do not require quartering, is shown in Figure 9. The axle is sawn in half, and the two halves are glued together with epoxy resin, ensuring that a continuous film of resin occupies the space left by the saw cut. A plastic sleeve holds the two halves together in a jig at the correct back-to-back dimension. The small hole, approximately 1 mm diameter, is to allow excess epoxy resin to escape when the two half-axles are pushed into the sleeve.
Figure 8. Final drive gear offset to allow room for an offset axle spilt while retaining the squared and shouldered axle.
Figure 9. Split axle system suitable for bogie and tender wheels.
Frames for split axle locomotives
Split axle arrangements using metal bearings or hornblocks require that the frames be electrically isolated from each other. This requires insulated spacers and any other features that cross the frames such as brake rodding must also be insulated. Insulated hornblocks (Slater’s or similar – see Figure 10) make insulated frames unnecessary and are easier to use in many cases.
If it is decided to insulate the frames, a solution shown in Figure 11 makes use of printed circuit board (PCB). Modern locomotive kits using underframes etched in relatively thin brass can easily be modified for split axle pick-up. The frames should first be assembled using the kit spacers. These are only to hold the frames in position, and will later be removed, so solder them only lightly or, if possible, screw them in place. Fix mounting nuts to the footplate. Use a steel screw well oiled to hold the nut in place when soldering it, to ensure that the screw comes out again. Cut PCB spacers and groove them for insulation gaps and secure them to the footplate, using the mounting nuts. Put the frame assembly on to the PCB spacers and align it by eye.
Figure 10. Slater’s insulated hornblocks. (Photo Ken Sheale).
Put the wheels in the frames and spin them to ensure clearance at the splasher cutouts. When satisfied, solder the frames to the PCB spacers. Either remove the original metal frame spacers, or split them and support them with an insulated backing material. Check that the frames are electrically isolated from each other.
A similar method using double-sided PCB spacers set between the frames is shown in the scrap view at the bottom left of Figure 11. Because of the close proximity of the footplate, it is useful to put a thin layer of isolation along the top edge of the frame or on the underside of the footplate to prevent electrical contact. Other methods include using frame spacers made from Tufnol, perspex or similar materials.
Figure 11. Insulating the frames.
When using insulated frames, it is also important to ensure that any other components that bridge the frames, such as inside cylinders and motion brackets, or brake beams, include insulating gaps to avoid short circuits occurring that way. These methods can also be used for locomotives with ordinary axles and pick-ups to ensure that the body is electrically neutral. Hence, when double heading, operators are not greeted with the sight of two stationary locomotives with couplings glowing red hot.
‘American’ wiring system
This system is designed to eliminate the use of pick-ups rubbing against the wheels of a locomotive but is only applicable to tender engines and some tank engines. The locomotive wheels are insulated on one side only and power is picked up from the rail on the uninsulated side. The tender wheels are insulated on the opposite side and the uninsulated side acts as the return to the other rail. One terminal on the motor is connected to the locomotive chassis and the other is taken to the tender chassis via a connection between the engine and tender. Examples are shown Figures 12 and 13.
Figure 12. The connection system fitted to an old model converted from three rail. The tops of the chassis members have a thin insulating layer attached to ensure that the locomotive and tender bodies are electrically isolated. The mounting screw holes in the frame stretchers have been enlarged to allow an insulating sleeve to be installed. Insulating washers are fitted under the nuts. The locomotive and tender are semi-permanently connected by the wire, which just uses a simple ‘chocolate-block’ connector to link the two. The wheels are cast iron and therefore conducting. (Photo Ken Sheale).
Figure 13. The system can be adapted for tank engines having a bogie. An insulated block has been fitted into the frame at the rear and it carries the bogie pivot post. This makes contact with the bogie frame. The bogie wheels are insulated on the opposite hand to the drivers and provide the return path. The bogie sideplay is slightly restricted to prevent the leading bogie wheels from coming into contact with the steps below the cab and creating a short circuit when traversing curves down to 4ft radius. The wheels are cast iron. (Photo Ken Sheale).
Although the system is simple it has certain drawbacks. For example, unless the locomotive and tender chassis are insulated from the bodies, shorting will occur if the bodies make contact on a sharp curve. Locomotives with long overhanging cab roofs may come in contact with high-sided tenders under similar circumstances. If the fall plate between the locomotive and tender is modelled, it must be insulated. A thin layer of styrene sheet or insulating tape on the underside is adequate for the purpose. Unless the locomotive couplings are insulated, double heading can also cause a short circuit if both locomotives are wired the same way round.
In this system, current is transferred between the locomotive and tender using a wire as shown in Figure 12 (which might be disguised as a water hose) or by means of an insulated drawbar connector. The simple arrangement shown in Figure 14 that is used by some locomotive builders makes use of an insulated drawbar as a built-in connector. The tender carries a pivot pin fixed firmly to, and making electrical contact with, the body. The locomotive has a similar pin located in an insulated block carrying the drawbar. Two holes may be provided as shown, one for the normal connection and a longer one to allow for use on tight radius curves, although if building one’s own locomotive for track curves of known radius, this may be unnecessary. The light wire spring fits a groove in the pin and prevents the drawbar sliding off the pin, although on good track the tender should not bounce around and this should not be necessary. The act of coupling automatically makes the electrical connection. If combined with insulated chassis, the system provides a simple pick-up system that does not require checking and adjustment.
Figure 14. An insulated drawbar.
This article was written by Bob Alderman and Ken Sheale for the Gauge O Guild Manual. It was adapted for the GOGWiki by Nick Baines.