makers, at the present day, only three methods of effecting such
a drive: (1) by means of spur or double-helical gear to a live
back axle, as illustrated in figs. 6 and 7; (2) with two side chains
transmitting the drive from a differential intermediate countershaft
to the wheels on a fixed back axle, as shown in fig. 8; and
(3) by means of a single chain transmitting the drive from an
intermediate shaft to a differential gear on a live back axle, as
depicted in fig. 9.
The transmission on the first type of vehicle (figs. 6 and 7) is by means of gearing throughout, and is completely enclosed, thus ensuring protection from dust, and more perfect lubrication. Change gears to give two speeds are provided. This form of drive necessitates a special disposition of the spring mounting, in order to ensure that there shall be no sliding motion taking place between the teeth of the reducing gear, due to the distance between the centres of the countershaft and the back axle varying with the changes in the spring deflexion. The gear drive offers advantages which, for heavy loads, are not offered by any other form of drive.
The features which are common to all steam wagons of the second of these types are: vertical fire-tube or water-tube boiler; horizontal compound engine; two-speed gear; differential countershaft; and two sprockets which transmit the final drive, through chains, to the rear road wheels. The exact form which the boiler takes is a point which has considerably exercised the ingenuity of individual designers, in order to arrive at one that can be cheaply produced,
Fig. 10.—The Unique Type of Steam Wagon made by the Yorkshire Patent Steam Wagon Co.
efficient, simple of construction, easy to clean and repair, strong and reliable, and one which will not prime. The vertical type of boiler lends itself admirably to the general design of a steam wagon, because it takes up so little of the total length of the vehicle, whilst the fittings can be mounted in much more accessible positions than are possible with the majority of loco-type boilers. The efficiency is not, however, so high as is the case with the latter type. It may be generally stated that boilers of the vertical class, as used on 5-ton or 6-ton steam wagons, have a total heating surface of about 90 to 95 sq. ft., with about 4 sq. ft. of grate area, and the working pressure is from 200 ℔ to 225 ℔ per square inch. The usual and necessary fittings include: pressure-gauge; two safety-valves, two check-valves, and a blow-off cock. The feed water is normally supplied to the boiler by a plunger pump driven from the crank-shaft. The compound engine has all the valve motion completely enclosed and running in an oil-bath, and is provided with means whereby high-pressure steam can be supplied to the low-pressure cylinder, in order that extra power may momentarily be obtained. The change-speed gear pinions, cut from solid steel, are most often mounted on an extension of the crank-shaft, and mesh with machine-cut gear wheels which are mounted on the intermediate shaft, on which the differential gear is also mounted. The gears provide for two ratios of reduction from engine to road wheels; the higher one is for all ordinary running, and the lower one is for steep hills or for very bad roads. The outer ends of the differential shafts are fitted with chain sprockets, from which the drive is taken to the back wheels by means of chains.
The third type (fig. 9) of steam wagon is particularly businesslike in appearance, and sound in construction and design. It is the outcome of many years’ experience in the design of road locomotives, on the lines of which it is constructed. The loco-type of boiler is very economical in fuel consumption, and is a very efficient steamer. Comparing this type of boiler with the vertical type, the former can be made of a much lighter construction for a given rate of evaporation, and the smoke-box door at the forward end offers a most simple and easy means of access to the smoke tubes for cleaning purposes. No vertical boiler offers such ready means of effecting this operation. The engine, which is mounted over the boiler in this type, is in full view of the driver, and, should it become necessary to make any repair or adjustment when on the road, this can easily be effected without grovelling under the wagon or removing any of the load. Objection may be raised to the position of the engine, on the score of its necessitating such a long chain drive to the back axle; this objection has not been sustained in practice, as many wagons employing this form of drive have been running for lengthy periods without giving any trouble on that score. The engine, which is a compound one, is close up to the source from whence it is supplied with steam, and consequently receives the steam in a much drier condition; there is less condensation in the main steam-pipe, because the length of this is reduced to a minimum. The short steam-pipes should tend to lessen the risk of their being broken from the sagging or twisting of the frame, a fault which is not unknown with vehicles having the engine a considerable distance from the boiler. This type, like types 1 and 2, also has two change-speed pinions, mounted on an intermediate shaft. These pinions may be of the sliding type, or may be operated by means of dog-clutches. The pinions mesh with two wheels that are mounted on a countershaft, on which is also fixed a chain-sprocket, from which the drive is transmitted, through a long chain, to a chain-wheel surrounding the differential gear mounted on the back axle. Traction-engine type of steering gear, with revolving fore-carriage, is most generally adopted, and is rendered sufficiently rapid in its movement by suitable gear, operated by a diagonal shaft and wheel, from the driver’s footplate.
Fig. 10 shows the general disposition of the main parts of a “Yorkshire” steam-wagon. This machine follows the general lines of those of type 3, so far as transmission gearing is concerned, but its boiler is of very distinctive construction, as may be seen from the sectional view in fig. 11; its engine is one of the vertical compound type, and is mounted directly behind the driver.
Fig. 11.—The Yorkshire Steam Wagon Co.’s ingenious Loco-type Transverse Boiler.
The Sheppee steam-wagon, or “steam-gas” vehicle as it is sometimes termed, on account of the high degree of superheat to which the steam is raised, and which superheat gives to the steam many of the characteristics of gas, is shown in fig. 24, and, it may be seen, this wagon is entirely dissimilar to any other machine with which this article deals. The generator and paraffin burner are housed within a “bonnet,” and the temperature of the steam is controlled by a very simple form of thermostat. After leaving the engine, some of the heat in the exhaust steam is utilized to heat up the feed water before it is passed into the generator; the steam then passes in series through two condensers—one in front and one underneath the vehicle.
Another vehicle which embodies many novel and practical
features is the new Leyland steamer, the construction of which includes one of the well-known Leyland fire-tube boilers, as shown