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28th October 1919
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Page 10, 28th October 1919 — STEAM WAGON CONSTRUCTION.
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The Features of -Representative Types of British Five-ton Vehicles.

OWING TO the rise in the price of oil fuel of all kinds, the steam wagon has benme more popular and deserves the serious consideration of all who require transport for loads of three tons and over.

To many transport users the steam wagon is still an unknown quantity, and to a certain extent is regarded with suspicion, this is very largely due to a , lack of knowledge of the subject, and this article has been written with the purpose of very briefly outlin ing the various features of steam wagon design in the hope fhat it will lead to a better appreciation of the " points " of a steam wagon. This review is split up into three main parts: the • bailer, the engine, and the transmission and steering gear, but, before beginning, it is as well to consider the problems which confront the designer of any heavy road vehicle. , A commercial motor, whether steam or petrol driven, to be a profitable investment, must be : reli able, cheap to operate, easily controlled, and foolproof. In addition to this, it must possess a reasonable radius of action and be capable of working well beyond its normal power output for short periods without being overstrained in any way. The advocates .of,the steam wagon claim that it is at least the equal of the petrol wagon on all these points, and that in the first two mentioned a properly handled steam wagon is a long way ahead of the internalscombustion-engined lorry. In passing, it might be as well to deal with two very frequently urged objections to the steamer. First, that it has to stop frequently to pick up water.

A modern steam wagon will run from 20 to 30 miles on one filling of the tank, and the fact that regular steam wagon services are in operation between London and such places as Birmingham and Manchester, seems sufficient reply to this objection. Secondly, that there is a waste 0f time raising steam in the morning.

When the fire has been properly banked the night before, steam can, with a clean boiler, be raised in 20 to 304 minutes. This time can be occupied in rebunkering, filling the water tank and systematically going round the wagon, checking the tightness of the various nuts and attending to the lubrication. This regular attention each day means a gain duo bath to lessened time lost for repairs and a much smaller repair bill, which more than compensates tor thaptime lost.

The Boiler.

To meet the conditions mentioned above it is obvious that a steam wagon boiler must be capable of steaming freely and at the same time be economical in fuel. To meet the rough conditions of service, it should be strongly built and so mounted as to avoid the racking and straining inseparable from road work. It must also be easy to examine and clean, and, in this connection, it must be remembered that the average steam wagon driver is not highly skilled, and the wagon very rarely comes under the same skilled supervision as, say, a railway engine.

The types of boiler in common use may be, broadly speaking, divided intotwo classes.

The horizontal or locomotive type invariably associated with the " overtype wagon" and the vertical, almost always of the water tube type, used on ".undertype wagons."

The locomotive type is simply a small traction-engine boiler of the fire-tube type, that is the hot gasei from the firebox pass through a. number of tubes which traverse the—water space. These boilers allow of easy inspection of the tube plates and ends of the tubes, but it is somewhat difficult to examine thoroughly the inside of the boiler. On the " overtype " wagon the boiler usually forms part of the main frame and-carries the engine, so that it must be strong enough to supportpart of the weight of the wagon and resist the torque of the engine as well as the strains due to internal Pressure.

The vertical boiler is mounted at the forward end of the chassis and takes no weight except its own, as, on an undertype wagon, both the front axle and the engine are mounted directly on the chassis.

When the boiler is of the water-tube type; the tubes,which are usually of large -diameter, run across the combustion space in the firebox ant:I.-the heat of the fire causes..the water-to circulate rapidly through them. These boilers are usually so artanged that. the tubes and firebox can be completely. exposed_ for examination with comparatively little trouble. For example, on the Sentinel the firebox and tubes can be lowered right out without disturbing the boiler fittings, while on the Leyland the top half of the boiler shell can be lifted off, thus exposing the tubes.

The boiler fitted to the Yorkshire steam wagon is an example .of the vertical fire-tube type:. This boiler, at first glance, would appear to he a horizontal boiler set across the frame, hilt the firebox is. immediately below the funnel and is shaped like an inverted bucket, being closed at the top. The fire-tubes slope upwards slightly and run outwards towards the flat ends of the boiler, at each end of which there is a much shallower edition of a "bee-type smokebox; and thence by andther, set of tubes to the smokebox, which is immediately below the funnel. By removing the covers at. the ends of the boiler the tubes can be cleaned-easily and the outer tube-plates inspetted.

The boiler feed arrangements are the same on all wagons, consisting of a feed pump driven off the engine, and an injector. Where maximum economy is wanted, a feed-water heater, to extract, some of the heat. from the exhaust steam which would otherwise be wasted, and a superheater, through which the steam passes on its way to the engine, should be fitted.

The Engine.

Having considered the method of producing the steam, the next step is to see how it. is used to produce the power at the road wheels.. Everyone is familiar with the general outlines of the steam en gine, but it is interesting to see how different designers deal with the same problem.

The indicated horse-power of an engine is obtained P.L.A.N.

from the formula where P is the average 33,000

pre sure throughout the stroke, L the length of the stroke in feet, A the area of the piston (if P is in

pounds per square inch, A should be in square inches) and N the number of revolutions per minute. This only gives the power from one. end of one cylinder, so that, for instance, the horse-power obtained from a two-cylinder simple engine would be 4 x 33,000 provided, of course that the four Ps were the same.

It therefore follows that provided the product P.L.A.N. is the same, the engine dimensions and speed can be varied to produce the same power, i.e., a wagon may be fitted with a small, relatively high speed engine, driving first through reduction gears and then by chain to the rear axle, or with a larger slow-speed engine, fitted with a direct chain drive to the back axle, sufficient reduction, being obtained without the use of intermediate gearing.

Steam wagon engines may be divided into two classes, simple and compound, the latter being used on the " overtype" wagons and the former on the " undertype."

In the compound engine, which is usually also of the high-speed type, the two cylinders are of the same stroke' but one is of larger diameter than the other. The steam, after first doing work in the smaller cylinder, is exhausted at a sufficiently high pressure to do still More work in the larger one, the engine being so designed -that in normal working the product P X A in each cylinder is the same ; thus the work is evenly divided between them.

These engines are usually so designed that; it is possible to work them as " simples " for short periods when under heavy loads such as stairting or ascending

hills, but this "double as pressure gear" should only be used Sparingly, as the abnormal demand for steam puts a heavy strain on the boiler if continued for long periods. The valves are operated by sonic sort of link motion to allow for expansive working, and this usually necessitates having two eccentrics • for each cylinder.

If superheated steam is used, piston valves should he fitted and special arrangements must be made for the lubrication of cylinders and valves, as steam, when superheated, does not possess the slight lubricating qualities of saturated or wet steam, and the ordinary type of D slide valve is liable to be damaged ' in consequence. The " simple " engine used on " undertypes " has both cylinders of the same size and the steam does its work in one stage only instead of two, and, as this type uses high-pressure superheated steam, the valve gear consists of a camshaft driven at engine speed and operating valves which are returned to their Beatings, by springs. These camshafts are made to slide laterally, and there are four cams for each valve, giving, respectively, a late cut-off for extra heavy work and an earlier cut-off for normal economical running, a stop position and a reverse position. It may be argued that this arrangement is not likely to give such economical results as the gradual variation of cut-off, obtainable with a link motion, but it has been conclusively proved in actual tests that, when using high-pressure superheated steam, the most economical results are obtained by cutting off as early as possible and controlling the speed of the engine by means of the throttle. In practice, this engine is quite as economical as the compound, and as it is usually of the slow-speed type, friction losses are considerably less. The " undertype" engines are completely enclosed and are lubricated On the splash system and, although this method is fast becoming obsolete for in ternalcombustion engines, it is eminently satisfactory on The slower speed steam engine where the temperature is very., much lower. Having now followed the power from the boiler to the cylinders, and thence to the crankshaft, the next step is the transmission betiveen the crankshaft and road wheels.

On wagons fitted with small high-speed engines, whether simple or compound, it is necessary to have a two-step transmission, for two reasons ; first, that a sufficient gear reduction could not be obtained with direct chain drive, and the chain would be much/too lung (the shorter the chain, within limits, the higher is the efficiency of the drive), and, secondly, because it 'is necessary to fit a two or three speed gear to enable the small engine to run sufficiently fast on hills, etc. to produce the necessary power at a, low road speed. In this connection it must be noted that the gear cannot be changed while the engine is running, as on a petrol wagon, but the wagon must be stopped and the wheels scotched before doing so. Some " undertype " wagons fitted with large slowspeed engines are sufficiently powerful to dispense with change-speed gears and drive direct, and the large slow-speed engine is no heavier than the smaller engine plus the necessary reduction gears.

Some makers still give a choice of either steel or rubber-tyred wagons, though most have altogether stopped the manufacture of the former, so convincing . is the superiority of the rubber tyre. Rubber tyres undoubtedly cost more in the first place, but, for this extra cost, the user obtains the following advantages (1) More mileage per day and therefore greater earning power, as the legal limit is 5 m.p.h. on steel and 8 m.p.h. or 12 m.p.h.•according to the weight of the wagon when on rubber tyres.

(2) A lessened bill for fuel.

(3) Fewer repairs and longer life for the wagon, whilst the work of driving the wagon is rendered easier and more comfortable.

Steering Gears.

These are of two kinds, the pivoted front axle or traction engine type and the Ackermann or motorcar type. In the first, the wheels are rigidly connected by the front axle, which is swung about its centre by two chains wrapped round a 'drum, rotated by a worm gear on the end of the steering column. This gear, though simple, is slow in action and more tiring to handle, due to the fact that the wheels have to slide over the ground sideways when the front axle swings.

Snatching of the Chain also Occurs.

The Ackermann type is the ordinary motorcar method, where the wheels are pivoted on the ends of the axle and are swung together, their steering arms being connected by a rod parallel with the axle. Where the pivots are immediately over the point of contact with the ground, a very easy steering gear is the result, and although this is naturally desirable on all wagons, it is absolutely essential on the lighter 3-4-ton types -with a road speed of 12 miles an bqur. In choosing a wagon, a prospective owner should carefully consider the following points:— Suitability for the work, And, for the sake of the driver in whose hands will lie the care of the wagon:— Handiness of manipulation. Comfort and weather protection.

All controls should be within reach of one man.

Reliability is placed before cost of running intentionally, as although one wagon may have higher operating charges than another, due to greater first cost, if it is more reliable it is the cheaper wagon in the end, as a broken-down wagon may easily lose in a day more than it earns in a week, apart from the inconvenience caused by delaying its load.

The driver should be well looked after: it should bie seen that he has time to attend to his wagon properly, and that he does it ; if he does not, another driver should be got, but he must be looked after too.

• A good driver is the best investment the owner of a steam wagon can make, and it pays to get a, good man and pay him well so long as he really takes an interest in his wagon. E. H. W.


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