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The New Zealand Railways Magazine, Volume 12, Issue 8 (November 1, 1937)

New Zealand's Latest Locomotive — The “G” Class

page 57

New Zealand's Latest Locomotive
The “G” Class

(Rly. Publicity photo.) One of the new “G” Class Locomotives described on this page.

(Rly. Publicity photo.)
One of the new “G” Class Locomotives described on this page.

The three “Garratt” locomotives imported into New Zealand in 1929 were exceedingly powerful machines. Each locomotive comprised two independent sets of six driving wheels, each set driven by three cylinders and each set developing more tractive force than the largest engine in use at the time in New Zealand. Each set was carried on an independent swivelling frame similar to a bogie and the superstructure was carried on the centres or swivelling points of these bogies. This super-structure carried the large single boiler and the double tenders; these, to equalize the weight, being arranged one in front of and one behind the boiler.

These extremely powerful machines proved too cumbersome for conditions in New Zealand where traffic requirements and restricted marshalling yards and crossing loops prevented full use being made of the tractive force the engines could develop. Certain phases of the design also proved to be troublesome, particularly that of firing. The engines were too large for hand firing and the automatic stoker fitted proved unsuitable for use with New Zealand coal, since the coal mines in New Zealand, and Australia, are not equipped with machinery to grade coal to regular size as are mines in England and America. It was soon recognised that a locomotive of about half the power would be more suitable for New Zealand conditions, and as the Garratt type lent itself to a fairly cheap modification whereby such engines would be obtained this course was decided on.

The outcome was the six new “G” class locomotives, one of which is featured on this page. The first of these engines has just been turned out of Hillside shops, and the six will soon be working on the section between Springfield and Arthur's Pass, where heavy coal traffic has to be handled over heavy grades.

The sketch gives the main dimensions of the reconstructed engines, and from these and the photograph it can be seen that the locomotive is virtually an enlarged AB type. The three cylinders, which are set at crank angles of approximately 120°, are 16½” diameter by 24″ stroke, and develop a tractive force of 25,800 lbs. with the 4′-9″ driving wheels as against 20,000 lbs. of the AB and 30,815 lbs. of the K, both with 4′-6″ drivers. The cylinders are each fitted with 8½” piston valves, the two outside valves being moved directly by ordinary Walschaert gear, while the valve of the centre cylinder is driven by a system of levers worked off the tail rods attached to the outside valves. This system of valve gear for three cylinder engines is that invented by the well-known English locomotive engineer and is known, therefore, as the “Gresley” gear. The box shown in the photograph under the smokebox door is simply the protection cover for the lever system mentioned.

The other larger box on the side running board above the cylinder is a cover for the steam pipes from the smokebox to the cylinders.

The boiler is practically the AB boiler with a slightly larger firebox, and the superheater is the same volume as that used on the AB.

The driving wheels are 4′-9″ diameter and provide an adhesive weight of 7 tons each of 42 tons total. This gives a higher proportion of adhesive weight to tractive force than is provided in the AB type and the tendency to slip on greasy rails with heavy goods loads should be substantially reduced in the new engine.

The bogies are fitted with roller bearings, the advantage claimed being that the danger of hot boxes is entirely overcome.

The photograph, shows the long outside connecting rods and the side rods all fitted with “bushed” ends and grease lubrication.

An exhaust steam injector is fitted and the recovery of the heat in the steam which will be put back into the boiler instead of being wasted up the funnel should result in a substantial saving of fuel and water.

Locomotive men will realise that a very economical and satisfactory solution has been arrived at of the problem of the best use to make of the Garratt locomotives and render them more suitable for Dominion conditions.