Design of the MÁV Class 327 steam locomotive model

#5 Part, Details making Troubles

During the design I encountered several hard to solve problems. There were a few that have been discovered during the construction phase and needed to redesign sometimes already finished parts. Other troubles were already visible during the design. Here is a small collection of them.

Center of Gravity

According to the valuable book of R. Guy Williams the pony trucks must be constructed as a free sviveling small "car", without carrying load. His arguments sound reasonable: this is the easiest way to ensure that the suspension does not interfere with the twisting and rocking movements of the truck. This also means the model is only supported by its driven and connected wheelsets, without support of the truck. The solution is even more complex if the model is suspended by a compensation rod. In this case the center of gravity of the ready made model - with cylinder blocks, boiler and cab - must be not farer to the front than the compensation rod center, or even better, behind the central connected wheelset axle. As at the 4-6-0 locomotives the firebox is supported by the drivers, the connected wheels are close to the back of the loco, and both the frame and the boiler hang over the unsupported front.

This also means the loco front, the smokebox and the cylinder block must be constructed lightweight, from the thinnest sheets. The rear part, however, needs to be weighted. Lead should be placed around the firebox, the back of the frame and even in the cab roof - even if this sounds not as a good idea concerning the model stability.


Class 327 model frame drawing minipic When designing steam loco models the most delicate area is the tracking. The track radii usual for railway models are considerably smaller than the corresponding prototype ones. As a consequence the wheel geometry for the usual (not Proto'87) models are wider. This also means the frame of a model loco must be narrower than proportional to the prototype.

For shorter locos, like switchers without pony trucks this makes less troubles. The wheelsets must be designed to negotiate sideways movements and the distance between the loco and the tender should be increased. This way the front corners of the tender can not touch the loco in curves.

Construction of longer locos makes more troubles. The frame must be cut out or designed much narrower to give place for the leading wheelsets. If the prototype also shows such a cutout, the model probably requires to enlarge this to clear its higher flanges.

For the Class 327 models several design approaches were made on PC. It was soon clear the usual solution, to fix the front and rear drivers in the frame and allow only the second drivers to move sideways, would not work. The front wheelset of the pony truck would touch the cylinder block, or it would require a large cutout to avoid this contact. The rear drivers must be fixed, however, as they are driven by the gearbox. Thus the only solution is when both the first and the second drivers can move sideways. In this case the model might show a "sneaky" movement, as only one wheelset determines the frame position. This can be avoided by fixing the pony truck position by the kingpin. In this case again, the drivers' sideways movement would be too wide. This could be done only by designing a very narrow frame.

The compromise was a special solution. The pony truck can move sideways, too, but its position will be limited by two springs that push the truck back into central position. This was made in a similar way in the prototype lovomotives, too. The spring parameters must be set up cautiously. If they are too strong, the pony truck can not really move sideways. If they are too loose, they do not protect against "sneaky" movement.

The isolation

This was no problem for the earlier models. The V40 electric, the first Class 342 and the Class 326s all were equipped with Piko drivers which have a plastic body, steel flanges and brass hub. The cranks are molded into the plastic, well isolated. The second Class 342 model had Romford wheels, where the tires are isolated from the wheel star. The cranks are also isolated from the rails.

The Class 327s, however, get drivers from Fleisschmann. These are made from nickel silver, only the hub is plastic. Thus the cranks are not isolated from the rails, nor the connecting rods. The main rod goes to the cylinder block wich would short both sides.

One solution could be something similar to the "split axle". The framesides could be connected with an isolating material, both frameside electrically connected to the rail of the same side. In this case there are many problem areas, like the cab, the boiler supports and the smokebox saddle with the steam pipe cover. These all should be isolated from the frame they sit on. Another problem could be the gearbox - very hard to isolate.

Here the compromise was to isolate the main rods from the crosshead and the eccentric link from the reverse link. In both cases the holes must be made bigger and a plastic insert can isolate the rod from the crosshead and the reverse link. This can be delicate considering the small size of the reverse link.

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This page was changed last time on August 26th, 2012
© János Erô