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• Locomotive pick ups (2) – steam locomotives only picked up from one rail and the tender only picked up from the other. The locomotive and tender bodies were live to their respective rails. As well as poor contact effects described above, the tender making contact with the locomotive would provide a short.
couplers both underset and overset couplers which raise or lower the coupler relative to the coupler pocket. These are useful in that they can accommodate up to 3mm variation in coupler pocket height:
• Trucks were metal but with only one wheel of each axle
insulated. Side frames were thus connected to one rail and if 9.9mm clearance between rail and side frame was limited, the side
frame could short to other rails on points.
• Couplers were metal and had metal coupler pockets. Shorts
could occur when cars coupled if the body of a car was
connected to one rail and the body of the adjacent vehicle
was connected to the other rail. Even worse, the vehicles did
not need to be contiguous as the short circuit current could 9.9mm pass from car to car through the couplers and frame of
9.9mm
12.7mm
Underset coupler
Overset coupler
Standard or centreset coupler
0.8mm
intermediate cars.
• Open-frame motors had relatively weak permanent magnets and so the efficiency was low and the current draw high. Any high resistance problems produced considerable voltage drop and hence even more erratic performance.
• Wheel flanges were sharp. RP25 wheel contours were still in the future. Wheels had flanges that were quite sharp so they could easily climb and derail if they found an irregular rail edge.
• Zamak castings – this was a commonly used alloy of zinc, aluminium, magnesium and copper. It was used for side frames and bolster of trucks. It tended to oxidise and become crumbly leading to trucks and bolsters falling apart. Some British Hornby models suffer from this even today.
Couplers
If you have operating sessions on your layout, then couplers become very important. In the course of an operating session, any vehicle may be required to couple to any other vehicle. Having couplers set to the correct height and correctly centred saves a lot of unintended uncoupling or failures to couple.
There are a number of points to remember about couplers. The following refers specifically to HO Kadee couplers but many points apply to other manufacture and scales.
Firstly the track: avoid sharp changes of gradient. The exaggerated diagram below shows what happens when two vehicles with correctly adjusted couplers meet a sudden change of gradient.
It would be unusual that correctly adjusted couplers would uncouple with gradient changes normally encountered, but if one or both couplers are at incorrect heights, a change of gradient may be all that is needed to lead to uncoupling. Watch for dips and humps in trackwork and eradicate if possible.
A Kadee height gauge is useful for ensuring that coupler heights are correct. With modern rolling stock, the coupler pocket will usually be at the correct height relative to the rail top but with older stock, the insertion of thin washers between the car and the bolster can provide fine correction. This cannot be done on locomotives and this is why Kadee include in their range of
9.9mm
0.8mm
11.5mm
0.8mm
In some cases where the coupler pocket is not the one supplied by Kadee, excessive vertical play can allow the coupler to move up or down relative to the vehicle, with the result that the coupler can be low because it droops.
The phosphor-bronze spring in the coupler pocket may seem to provide a simple way of raising or lowering the coupler by moving it from above the coupler to a position below it, thereby lifting the coupler. This should NOT be done. The spring should always be above the coupler as it provides different return forces for clockwise and anti-clockwise motion. Viewed from above, the force required to rotate the coupler clockwise should be less than the force required to rotate it anti-clockwise. Washers or shims are available to address this.
An uncoupled vehicle pushed over a permanent magnet uncoupler should cause the coupler to pivot from within the coupler pocket rather than at the knuckle (i.e. the coupler centring spring is weaker than the knuckle spring). This will ensure that the “delayed action” uncoupling will work reliably.
Again, problems can arise where the coupler pocket is not to the specification of the one provided by Kadee. If the pivot pin is of smaller diameter, it allows the coupler to move laterally within the coupler pocket which can lead to poor self-centring and poor delayed action uncoupling.
The coupler pin should clear the railhead by almost 1 mm. Much higher and it will not uncouple magnetically with reliability, but too low and it may catch on rails at points and crossings. The same applies to ploughs on locos – metal ones will short the track.
I use permanent magnets rather than electro-magnets for uncoupling on my layout. One problem I have encountered is with vehicles fitted with plastic wheels and steel axles. As a vehicle passes over the magnet, the axle is attracted to the centre of the magnet. If this vehicle is close to the end of the train, the vehicle can tend to catch up the preceding vehicle and take the tension off the couplers, allowing unintended uncoupling to take place. The answer seems to be to remove the steel axles in service and
April 2019 - ROUNDHOUSE 17