Commentary on the GMM.SML.5 Road Locomotive
by Simon Johnson
The GMM SML Road Locomotive is one of those smaller models which are satisfying to build without taking hundreds of hours to complete, essential if you haven’t reached retirement age and have a garden that is reverting to nature. The model was designed by P.W. Bradley who was responsible for other models in the GMM SML series. The plans are easy to follow. Dating from the late 1960s, they lack the colour the colour photographs of recent Modelplans but more than make up for this with clear line drawings which cover every aspect of construction.
One of the best known Traction Engine models is, of course SML 22. This pre-war model set the pattern of using Hub Discs and Spoked Wheels as a basis for scaling and was allegedly able to tow it’s builder on a specially constructed trolley. I have my doubts about this. For a start the trolley was built from Meccano which means the Binns Road Axles were carrying the weight of the boy without the benefit of wheel bearings to reduce friction. Besides, the old E6 motor would hardly have been up to the job.
Another problem with the E6 was its size. Housed in the rear of the model it imposed a width of six holes, an inch wider than the boiler.
The GMM Road Locomotive avoids this problem by building around a PDU motor which keeps the firebox the same width as the boiler - the correct proportions for an engine of this type. It also has a compact differential built into one of the rear wheels which means that the model can actually be steered. How the boy (Frank Hornby Jnr?) navigated around the family home on his trolley is hard to imagine since SML 22 had no differential.
Some adjustment is required to the differential to ensure smooth running. The plan shows the two 1/2“ Bevel Gears making up the spider spaced from the central Coupling by Collars. This arrangement doesn’t work since the heads of the Bolts securing the 11/2“ Bevel to the wheel will foul the Collars even when it is attached with Set Screws. Some sort of thin walled spacer must be used which is little more than a diameter of the rod it is fitted on. I used z section of drinking straw with an electrical washer.
Bert Love’s book Model Building in Meccano and Allied Construction Sets has an excellent section on Traction Engines. A similar model to GMM SML5 is illustrated and I have borrowed some of the improvements.
These include a working governor driven by a Driving Band from the crankshaft. It is extremely compact, with a small Contrate Gear running in a Coupling, the Contrate’s boss acting as a Pulley. Narrow Strips are used to improve the scale of the front axle and a belt made from a strip of rubber used for the dynamo drive from the flywheel. GMM SML5 uses Sprocket Wheels and Chain, an arrangement which would never have been used on a Showman Engine. Perhaps Mr. Bradley believed - as I did - that a rubber belt would never stay on the smooth rim of the flywheel. Yet this is the way the prototype engine powered dynamos and other equipment in the days when there were no health and safety standards to worry about. I bought the rubber strip for the belt from Para Rubber which sells a variety of rubber strips and tubing which could be useful for modelling purposes. The ends were joined with Ados contact adhesive and left to cure for twenty-four hours. The front and rear tyres were made from Centurylon, an adhesive backed rubber strip also bought from Para. This was only a partial success. The material is quite soft and when it has been stretched around the 51/2“ x 11/2“ Flexible Plates forming the rim of the rear wheels, the join is clearly visible. Bert Love used washing machine driving belts to great effect. However, these were presumably belts used on British appliances from the 1960s and 70s. Has anyone managed a neater solution to the problem?
Two other modifications are worth mentioning. Instead of using Hub Discs for each rear wheel. I bought a set of Jack Parsisson’s narrow strip inserts and fitted them to 51/2“ Circular Girders which were staggered in the usual way. These make for much neater spokes which are closer to scale. Since my PDU is presently fitted to another model, I tried out one of Andrew Well’s 320 rpm geared motors from Meccparts. Rated at 6 to 12 volts, it ran well on a small 9 volt rechargeable battery tucked in the coal bunker.
