Brass Technical Tips
February Brass List 1998
"Diesels: Diagnosing Shorts and other Electrical/Mechanical Tidbits"
For the last four issues, we have focused mainly on the subject of steam locomotive models, even though many of the techniques and recommendations are equally applicable to any type of locomotive model. However, this issue will address the topic of diesel models, with an eye towards the several common types of electrical pickup techniques and their associated maladies. We will also address tune-up tips where appropriate.
Any diagnosis of electrical problems in diesel models, is made much easier if one understands the particular type of electrical pickup system used on the model in hand. For this reason, we will catagorize and discuss these in four main groupings. One of these four should most likely cover the most common models you will encounter. The 'Type' groupings, that follow, are of my own choosing, and are for convenience of discussion here only. So, let us begin.
Models Imported by Oriental, Alco, Hallmark and others. They are usually typified by the presence of Samhongsa (S.H.S.) or KMT Gearboxes.
These models utilize un-insulated wheels on one side of each truck and insulated wheels (at the axle) on the other side. The insulated / un-insulated wheel placement is opposite on each truck, so each truck picks up one rail's polarity. The un-insulated wheels, hereafter referred to simply as the pickup wheels, pass the current through the axles to the gearboxes and sideframes, and hence to the truck mounting screw, which passes through the chassis floor via an insulated bushing. Thus, each truck assembly is 'hot' with the polarity of one rail. The connections to the motor are via a wire from the top of each mounting screw; the wire connector and retaining nut being insulated from the chassis floor with an insulating washer. The chassis floor is normally not 'hot' with either rail polarity.
Be careful, though, if the model has been re-motored. Normally, the motor frame, even in Open Frame Motors, is not 'hot', as both wire/brush connections are insulated from the motor frame (unlike some Open Frame Motors in steam locomotives, as discussed in Tips #2). If a replacement motor has a 'hot' motor frame, due to one un-insulated wire/brush connection, the polarity of this wire will be passed to the floor chassis via the motor mount screw. This will hold true unless the motor and mounting screw is completely insulated from the chassis floor. This concern generally applies to open frame motors only. Most motor housings on can type motors are insulated from the motor and brush assemblies themselves, and hence can be attached to the chassis with metal screws with no electrical worries.
Type I Short Possibilities:
* Insulated - side wheel touching sideframe or gearbox.
* Both wheels on an axle simultaneously touch the chassis floor or other body part (e.g. pilot).
* Truck mounting screw and insulated bushing/washer assembly not installed correctly, or with missing pieces, such that a screw/nut or wire connector touches chassis floor.
* Wires pinched between chassis floor and body during assembly, causing insulation to be pierced.
* Other bare wires or connections touching chassis or body. One wire touching may not cause a short by itself, but may energize a body or chassis part such that a wheel rubbing on this part may cause a short in turn.
Type I Conductivity Tips:
All parts involved in the electrical pickup process do a better job if they are clean. This applies to the wheel treads, axles, sideframes and even the gearbox housing itself. Obviously, lubrication is required, but not to excess. The wire connector, attached to the truck mounting screw, is usually held in contact via a coil spring. A clean connector, screw, and spring promote reliable current passage to the motor.
OMI, early 'tank drive' diesels, where drive shafts are through the fuel tank.
Most of these vintage of models use the same or similar method of electrical pickup as that described in the previous Type I Section. There may be some transition models which use wipers to pick up current from both sides of a wheel set and convey it to the motor via a wire for each side. Obviously, the pickup/wiper assembly has to be insulated from the truck assembly itself, in this case. Some models use the Type I approach, but augment it with a pair of supplementary wipers for the pickup wheels. In these cases, the wipers are not insulated. They are in fact are attached directly to the truck, usually on the sideframe crossbar. Thus, the sideframes and gearboxes are still 'hot' with the pickup wheels' polarity.
Type II Short Possibilities:
Generally are the same as for Type I.
Type II Conductivity Tips:
Generally are the same as for Type I.
Late OMI, Challenger and possibly others.
Somewhere around the mid-1980's, Overland Models Inc. converted to an entirely new drive, and hence a new electrical pickup system. These models dispensed with the 'tank drive' approach and used a system with drive shafts from both ends of the motor to gearbox 'towers' (actually very similar to the drive line design used in the Type I models). Where OMI improved over Type I, though, is in the electrical pickup design. These models have one or both wheels of each wheel set insulated at the axles, and pickup is performed by a wiper (wire or ribbon) rubbing on either the back or top of the wheel tread to pick up current. On four wheel trucks, wipers pick up current from all four wheels, via one insulated wiper set per truck side. On most six wheel trucks, the wipers pick up from only the two outboard wheel sets, even though all six wheels are powered. The key element of this approach is that the wipers are all mounted in a black plastic truck bolster, which provides the side-to-side insulation for the wipers sets. There are two motor wires connected to each truck, one to each side's pickup wire set. The pickup wipers and connecting motor wires are all insulated from any metal parts of the trucks, chassis or body. This design offers eight wheel pickup on all B-B and C-C diesel models, and at least that many on models with more wheels.
On these models, the gearboxes will be 'hot' with the polarity of the un-insulated side wheels. Even though the sideframes are mounted to the insulating plastic bolster, the sideframe on the side of the un-insulated wheels will receive that side's polarity via the axle ends riding in the sideframe journals. This polarity will be passed to the opposite sideframe via the axle. (In those cases where you may encounter all wheels insulated on a truck, the gearbox and sideframes should not be 'hot' with any polarity.) In most cases, though the wiper assemblies are completely insulated from the metal parts of the truck, the gearboxes and sideframes are 'hot' with the polarity of the un-insulated wheels on that truck. In fact, it is theoretically possible for the gearboxes and sideframes of both trucks to be 'hot' with the same polarity, since the trucks have the insulating plastic bolster as the savior. This is a significant difference from the Type I design where the trucks must be of opposite polarity. In the OMI design (a-la AJIN), the polarity of the truck gearboxes and sideframes is incidental. The wheel wipers and attaching motor wires are the only essential pickup parts. The same holds true for the similar pickup methodology used by Challenger Models in their diesels. However, as you shall see below, these incidental parts can play an important and hence irritating part in the production of life's little short circuits.
Type III Short Possibilities:
* A Sideframe part, including brake and sander hose detail, rub on insulated wheel rim.
* Wheel wiper wire touches gearbox. May only manifest itself when wheel set moves laterally in the gearbox, as on a curve or through a turnout.
* Wheel wiper wire touches a chassis part or sideframe part (insulated wheel side wipers only).
* Motor wire connection to wiper touches a metal truck or chassis part, or any bare part of wire touches chassis or body.
* Wires pinched between chassis and body during assembly, piercing insulation (a low probability, though, as the chassis body should be electrically neutral).
Type III Conductivity Tips:
* The wire wheel wipers are prone to picking up dirt, dust and grease (grease comes from the gearbox and spreads out on the axle, then down the wheel back). Clean regularly with a liquid cleaner.
* Watch out for an occasional wiper wire that is overly long, and sticks out past the end of the truck. These can catch on pilot or fuel tank detail and cause derailments on curves.
All those models that are not Type I, II or III.
One of the joys of writing a column like this is that there are very few true standards in the arena of imported brass models. Most of the 'standard' approaches one encounters are usually design practices peculiar to a specific builder and are therefore actually encountered practices, rather than defined standards.
It is the wise man, therefore, that ascertains the kind of electrical pickup design a particular newly acquired model has, before it has a problem. This can be readily done with the model upside down in your foam cradle (you do have one of these, I hope) and the test probes from your bench power pack ready to stick at key places on the model to find out what lights its fire. A careful scrutiny of the trucks will also reveal critical wipers, insulated wheels and other relevant parts. What are the potential relevant parts, you ask? Go back and re-read everything about Types I, II and III again, I answer. Most, if not all, of the clues are there.
One final piece of advice:
Once you have figured out a model's electrical pickup secrets, write them down on an index card for that model, so you will have it as a reference when you have a problem in the future.
See you next time.