Brass Technical Tips
November Brass List 1997
"Diagnosing Shorts in Steam Locomotives"
Welcome back to this, the second in the series of Brass Technical Tips. This issue deals with the diagnosis of electrical shorts in brass steam locomotive models. This topic was covered in Tips # One at a general level, but this issue will address the diagnosis process in a bit more rigorous and formal manner. A Cause and Cure cross reference follows the diagnosis dialog.
At this point, it behooves us to take a moment and review a couple of definitions and conventions.
Polarity: The polarity of a two-rail DC (Direct Current) system refers to which rail is considered minus ( - ) or ground, and which is considered positive ( + ), in a given situation. Note that when you flip the direction or reversing switch on the power pack, what you are doing is reversing the polarity of the rails. For the forward direction of a locomotive, it is the normal convention for the right (engineer's side) rail to be positive ( + ) and the left (fireman's side) rail to be negative ( - ).
Loco and Tender Electrical Pickup:
It is a standard convention for the locomotive to pick up from the right rail. Hence, the right side drivers are un-insulated and pick up and pass the right rail polarity through their rims to the axles and hence to the frame (chassis). How it then gets to the motor depends, usually, on the type of motor. An Open Frame Motor will often be wired so that the frame of the motor is ground, hence if the motor is mounted to the frame with a metal screw, the motor receives it's ground polarity through this route. Can Motors, on the other hand, usually have an insulated housing. They must have a wire to the loco frame to receive the loco's ground polarity. Be aware, though, that some open frame motors may be mounted in a manner to insulate them from the frame; a wire from the motor to frame is then required.
Obviously, then, the left side drivers of the loco must be insulated. This is normally accomplished by the driver rims being insulated from the wheel centers by a thin strip of insulation between these elements. Pilot and trailing trucks will usually have at least the left wheels insulated. The safest way, though, is to have wheels on both sides of these trucks insulated.
Accordingly, it is a standard convention for the tender to pick up from the left rail. Hence, the left wheels are un-insulated and the right ones are insulated (usually by insulation between the axle and wheel itself, just like loco pilot and trailing truck wheels). The left rail polarity is thus picked up and passed through the axles and hence to the sideframes, bolsters and tender frame. It is then passed to the locomotive motor through the metal drawbar and the loco drawbar screw (which are insulated from the loco frame).
At this point, it is worth noting that every piece of the locomotive, except the left and possibly the right pilot and trailing truck wheel rims, the left driver rims, the motor, and the insulated drawbar assembly will be right rail polarity. Similarly, every piece of the tender, except the right wheels, will be left rail polarity.
So, you have coupled your loco and tender together, put them on the track, turned the throttle on and whiz goes the ammeter and click goes the overload circuit breaker. This is how to proceed:
First, determine whether the locomotive or the tender will produce the short condition, each by itself (i.e., only one piece on the track at a time). If this is the case, diagnosis is eased considerably.
If it is the tender that produces the short, there is a path within the tender that physically (and hence electrically) connects the left rail to the right rail. This can be caused by a part of the tender touching the right rail (e.g., a step, hose, sideframe, brakeshoe etc.). Remember, all parts of the tender, except the right wheels, are left rail polarity, so if a low hanging piece happens to touch the right rail, you have a short. Also, a short will exist if a right wheel touches a truck sideframe. Look close for this possibility; it may only occur at extreme lateral movement of one or more wheelsets.
But, the most common cause of a tender short is much simpler and stems from one tender truck being turned around during handling. Then too, when a loco is taken apart and put back together for painting, it sometimes happens that one wheelset gets installed backwards in a truck, and you have an instant short.
Okay, your track test has given the tender, by itself, a clean bill of health. Now do the same track test with the locomotive by itself.
If it is the locomotive that produces the short, there is a path within the locomotive itself that physically (and hence electrically) connects the right rail to the left rail. This can be caused by a part of the locomotive touching the left rail (e.g., the pilot, hose, brakeshoe, pilot or trailing truck sideframe, etc.). Remember, all parts of the locomotive, except the left drivers and 'little' wheels are right rail polarity, so if a part touches the left rail, you have a short. You will also have a short if a loco piece touches a left driver rim or a left rim of a 'little' wheel.
The classic case of this occurs with locomotives having metal brake hangers and shoes, which are not insulated from the frame attachment points (there are some exceptions to this, where an insulated bushing assembly attaches the hanger to the frame). If one of these makes contact with a left driver rim, you have a short. Again, you must look closely to ascertain this condition. It may only occur when one or more drivers is at extreme lateral position (i.e., as happens when going around a curve). The solution here is to very carefully center the hangers between the drivers and adjust them 'out' to clear maximum lateral driver movement (but not so much as to interfere with the siderods). In some models, though, fidelity of scale is so fine that there is virtually no adequate clearance possible, and even the slightest driver movement or rotational runout (wobble) will cause contact. The recommendation here is to remove (and save) the left brake hangers and shoes, and replace them with plastic ones.
A less frequent cause of locomotive short, but a bit more mysterious to cure, is the dreaded 'insulation failure', between a left driver rim and driver center. In all likelihood, actual insulation failures are quite rare. If you have a direct short across both sides of a driver set, a more likely cause is that either the driver rim has moved in relation to the driver center, or a metallic particle or burr is bridging the very fine gap between the outside of the rim and wheel center. In the latter case, it is sometimes productive to run the tip of a #11 (or similar) blade around the gap to remove the offending particle. If this does not help, and you are not experienced in playing with driver rims, it may be wise to seek assistance at this point. In any case, this gets into non-trivial subject matter, which cannot be adequately treated in a paragraph, and hence may be food for a future article.
Well, you are still reading, so evidently the cases of the short being exclusive to the locomotive or tender do not apply. This means that the short only occurs when the locomotive and tender are coupled together. (Bear in mind though, that if the independent locomotive / tender analysis described above is skipped, those kinds of shorts will also show up when the locomotive and tender are coupled together).
A good way to think about what may be happening, in this case, is to consider what has transpired when you have hooked the tender to the locomotive via the drawbar. It is this; you are now feeding left rail polarity into the locomotive, whose intended destination is the motor and the motor only. There are now two basic groups of physical shorting possibilities:
Case 1. The right rail locomotive polarity is getting into the left rail polarity tender or ...
Case 2. The left rail polarity of the tender is getting into the right rail polarity locomotive.
Let us rid ourselves of the most obvious and heinous possibility right off the bat. This occurs when insulation of the drawbar into the locomotive is lost or somehow negated. This can occur if the complete insulation assembly for the drawbar screw going up under the cab is broken or missing. This assembly usually consists of a screw, spring, lower frame bushing, top insulating washer, tab for the motor wire and possibly a nut. If the screw touches the frame, you have a short, so these parts have to all be there and be installed correctly. A more common cause of a short in this area is due to the drawbar screw being tightened and moving the upper tab for the motor wire such that it touches the frame. Also, the drawbar screw can sometimes be a tad long and actually touch the bottom of the cab structure. This happens, for instance, on certain Sn3 locos, due to the amount and fidelity of detail packed in confined areas and the resulting tight clearances.
The likelihood of a pure Case 1 occurrence is rare. If it does happen, it is almost always attributable to a wire for tender lights, from the locomotive, shorting on the tender body. This is likely to occur where the wire goes through a hole or opening and rubs through the insulation. A Case 2 occurrence is more likely. The drawbar scenarios described above are real.
Other common causes here are:
1) Locomotive lighting wire, with left rail (tender) polarity, shorting on the body.
2) Left rail motor wire or motor wiring tab touching body.
3) Constant lighting module contact tabs touching body.
A good approach to trouble shooting shorts of this nature is to remove the locomotive body from the chassis and see if the short still occurs. If not, the short is a result of the assembly of the chassis to the body, i.e., something happens physically to connect two parts to produce the short. This could produce any of the three types of shorts just listed previously. Also, if lighting wiring is suspected, try disconnecting the wires from their source (motor) and see if the short goes away.
One particularly pesky short was finally traced as follows:
1) Short occurs with everything assembled and connected.
2) Short does not occur with tender connected to loco and body of loco removed from chassis. 3) Short does not occur with loco body placed on chassis without screw attachments.
4) Short does not occur when chassis to cab support screws are installed.
5) Short does occur when the screw through the cylinder saddle into the smokebox is installed, but goes away as the screw is backed out.
Cause: The long screw reaches up into smokebox packed with constant lighting stuff and over time, finally rubs it's way through a wire. Simple cure: Replace screw with one just long enough to securely reach the smokebox threads.
The search for a short can sometimes be an aggravating process. It is eased a bit, though, if you think through the paths where the two polarities of electricity should be and where their paths may inadvertently cross. To help you better understand this has been the purpose here.
One last little technique to locate an intermittent short is this: Turn out the lights and run the locomotive. Very often, you will be able to see the arc or spark produced at the location of the short (obviously this does not work for internal shorts). It can be fruitful, though, for wheels and such touching other parts they are not supposed to. Additionally, most shorts due to a wheel touching a sideframe or locomotive part will leave tracks on the wheel. These will be tiny little burn marks and their presence is a give-away to an intermittent short.
Good luck in your hunt, and may your short search be short!
Shorts - Quick Reference Guide:
CAUSES and CURES
* Body part touches right rail - Adjust.
* Right wheel touches sideframe or body - Adjust, shim or otherwise increase clearance.
* Tender truck reversed - Correct installation (right wheels insulated).
* Wheel set reversed - Correct installation (right wheels insulated).
* Wheel set insulation failure - Replace wheelset.
* Body part touches left rail - Adjust.
* Left pilot or trailing truck wheel touches frame or truck assembly - Adjust, shim or otherwise increase clearance.
* Metal brake shoe touches left driver - Adjust.
* Left driver rim hits chassis or body part - Adjust.
* Left driver insulation failure - Adjust, repair or replace insulation.
Locomotive and tender coupled:
* Insulation failure - loco drawbar assembly - Replace parts or correct installation
* Drawbar motor wire tab touches body - Adjust or correct installation
* Drawbar screw too long - end touches body shorten or insulate body area.
* ( - ) wire for tender lights shorts on tender body - Correct insulation or routing.
* (+) wire to motor or motor tab touches loco body - Adjust or insulate.
* (+) wire for loco lighting shorted on loco body - Correct insulation or routing.
* Lighting module tabs touch body - Reposition or insulate.
* Loco assembly screw interferes with wiring - Eliminate interference.