THREE YEARS OF ST AGON DEVELOPMENT.
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Modern Tendencies and the Reasons for Transmissions Becoming Spheres of the Two Types. Enclosed any Boiler Improvements.
NO PERIOD of time since the inception of the steam wagon has shown such great improvement in this type of vehicle as has occurred during the past three years, and it is of great interest to trace out the reasons for this sudden increase in activity on the part of makers. it is also of importance to note that, with the exception of certain overt4rpes which are improved editions of former models, the whole of the new wagons produced in the period in question have been of undertype construction, if we include in this category those vehicles such as the Clarkson, Yorkshire and Fowler, in which the engines are not of the horizontal type, but in which the crankshaft line is below the level of the frame.
When steam wagons first came into prominence, the majority were of the undertype, but the overtype soon made such headway that only two or three undertypes, including the Sentinel and the Leyland, remained on the market. This was chiefly due to the fact that the overtype can be considered as a development from the traction engine, and was the result of many years' experience with ti-us type of vehicle, whereas, at that time, the undertype was something in the nature of an experiment, and although obviously possessing great advantages, such as increased manoeuvrability and a satisfactory proportion of body length to overall length, its disadvantages of low steaming capacity, inaccessibility, and the impossibility of using it over rough roads, and even off the road, as was often done with the overtype, outweighed the first considerations, so that for the time being it fell into disrepute.
Despite this condition of affairs, two or three makers continued to pin their faith to the Undertype, and subsequent events have gone far to prove that they were correct in so doing, and the present situation appears to show that there are two useful fields in steam road transport, the overtype being particularly suitable for comparatively rough jobbing work where the mileage is not very great, and where the surfaces over which the vehicles have to operate are of such a nature that high speed would be more deleterious than otherwise, whilst the undertype is generally considered to be an ideal vehicle for fast transport over good roads, with or without a trailer, and is the more direct competitor of the petrol vehicle.
As a matter of fact, the dividing line between the two classes is somewhat obscure. Recent improvements in overtype vehicles, such as the Foden, Mann And Ransome, have not only increased their load capacity, but have also made them more suitable for higher speeds, this, in some instances, being assisted by the use of Ackerman steering, and in the provision of better comfort for the driver and his mate.
Similarly, the undertype has been so improved as regards its steaming capabilities and general strength and design, that it often tackles with success work for which the overtype was formerly almost exclusively employed, but that the distinction is recognized by the makers, is proved by the fact that the two types are now, in some instances, built side by side,
and the undertype is usually referred to as that built for speed. We also find that the makers of what is perhaps the best-known of the overtypes have just produced an undertype model, although they will continue to market the former type.
Whether the overtype will eventually be superseded altogether cannot yet be ascertained, but there are indications which show this to be a possibility. The essentials for any road vehicles are ease of maintenance, the reduction of non-paying weight, ease of control combined with a high degree of manceuvrability and a good appearance. In most of these points the undertype scores heavily, particularly where it has to be used in congested traffic, for the driver is afforded a much better view of the road right up to the front of the vehicle, and the disposition of the various parts enables more comfort to be provided, and there is no splashing of oil as occurs in some of the overtypes ; also the drive line can be almost straight to the rear axle instead of being taken at right angles, as has to be done in the overtyPe.
It is also significant that the last three wagons to be produced all follow petrol-vehicle practice in having enclosed change-speed gears and the final drive by enclosed double-reduction or worm gearing, combined with differential gears, all enclosed in axle casings resembling those used in petrol vehicles.
This tendency towards petrol-vehicle nractiee cannot be considered entirely new; in fact, in the Clarkson steam wagon which was exhibited at Olympia in 1921, this inclination was apparent to an even greater degree than is now the case, for not only did this wagon have a clutch, three-speed gearbox and worm drive, but the resemblance to the petrol vehicle was carried still farther by the provision of a condenser closely resembling the radiator of a petrol vehicle and mounted in the position normally occupied by the radiator in that type, and the buyer was given a choice of having the boiler equipped with a short chimney or two long pipes by which the products of combustion could be carried under the chassis towards the rear.
The vertical boiler of the Clarkson was carried behind the radiator under a bonnet, and behind the boiler was a 90 degree IT twin-engine, of which one of the cylinders was the H.P., and the other the
L.P. The boiler employed was the now famous thimble-tube type, which is being fitted at the option of the purchaser on several wagons, including the Leyland. The Clarkson wagon was really some years in advance of its time, and did not receive the support of users to the extent which its novelty merited.
In spite of this tendency towards the enclosed transmission, and the use of reduction gearing, we must not forget that one of the most successful under type wagons on the market still retains the utmost • simplicity in its construction, the policy of its builders being to eliminate all complication and as many wearing parts as possible, and to provide a vehicle which would be powerful enough without reduction gearing, even in emergencies, and at the
same time very cheap to maintain—we refer to the Super-Sentinel which, produced in 1923, was the result of many years' experience with a previous model.
The Atkinson was another vehicle employing a single speed, but comparatively recently the makers ceded to the desire of certain users, and now provide as an optional fitment an unusual form of epicyclic reduction gear which can be mounted on the crankshaft.
Following the Clarkson, the first real effort to design a successful wagon with enclosed drive resulted in the 7-ton Yorkshire, which is manufactured side by side with the chain-driven model. It is unnecessary for us to do more than deal with its leading features. The boiler is of the double-ended loco type, similar to that employed on the chaindriven model. The twin-cylinder vertical engine is also retained, as this has proved satisfactory in every way, and has a high degree of accessibility, whilst the difficulty of balancing the engine with the cylinders side by side is not so noticeable if any vibration can be damped out by the springs ; as a matter of fact, with its unit construction of threespeed gearbox and engine and enclosed shaft drive to a double-reduction rear axle, the vehicle will run with wonderful smoothness and silence at comparatively high speeds. The one difficulty with a construction of this type is to obtain a satisfactory degree of accessibility for the gearbox and the universal joint within its spherical housing at the front end of the torque tube.
The next vehicle with enclosed drive was the Mann Express wagon, which has many unusual characteristics, the chief of which is the high degree of accessibility afforded by placing the two-cylindered engine across the frame so that all the valve gearing, cylinders, pistons, etc., can be examined from the near side, and the engine bearings at the off side, a large cover plate being employed in the crankcase to facilitate this. In this vehicle only one low speed is provided by the gearing, but the provision of two points of cut-off from the engine gives the effect of a four-speed gear, and enables a medium-sized engine to be employed, as a low cut-off provides a reserve for surmounting short lengths of extra steep gradients, whilst the -top gear can be arranged for extra fast running on the level or on down grades.
in actual practice, starting on hills, which would be taken on top gear if the vehicle were running, but which would, if one point of cut-off only were employed, necessitate the use of the low gear, can, in the Mann, by the use of the late cut-off, be effected on top gear, and once in motion the early cut-off can again be employed.
The drive from the gearbox is taken through an open propeller shaft with enclosed universal joints to a bevel-and-spur double-reduction axle which is provided with a stout torque rod. The brakes consist of contracting bands on the transmission and internal shoes in the rear drums, a novelty of the latter cal being that they are actuated through the medium. of steam pressure, a cylinder being bolted to each side member, the system providing a compensating effect.
Not long after the appearance of the Mann came the new Fowler. Like the Clarkson, it has a 90-degree V-twin compound.engine, the arrangement being such that the cylinder heads come immediately under the seats, the fuel bunker being disposed between them.
The accessibility is satisfactory, and the engine in this position makes use of what would otherwise be wasted space. Here also the gearbox is mounted as a unit with the engine. Three speeds are provided and the drive is taken through a propeller shaft with a universal joint at the front end and carried in a torque tube the forward end of which is spherically mounted, the final drive being through worm gearing of the latest enveloping type designed by Bostock and Bramley.
A feature of the engine is that the crankshaft is divided to permit the use of roller bearings for the big-ends. 1
The latest undertype wagon produced is the Foden, to which we can only briefly refer at the moment. In some respects this resembles the Mann, as the engine is mounted with its cylinders across the frame, and so being accessible from both sides of the vehicle. The final drive is by worm gearing.
Many of the recent advances in steam-wagon design generally have been due to the increased use of alloy steels and ball or roller bearings. These have given vehicles a long wearing life, combined with ease of maintenance and a lightness of construction which would, a few years ago, have been considered dangerous, if not impossible.
In referring to vehicles .produced during the past three years, we must certainly not forget the Garrett undertype, as this may be considered as a mean between extreme simplicity and the new types which we have just described. The power unit is situated longitudinally and has Joy valve gear for its two H.P. cylinders, the crankshaft is supported in Timken bearings and there is a differential countershaft and gearing giving two speeds, the final drive being by roller chains to the rear wheels, which also are carried on Timken bearings. Balanced piston valves are em
ployed, and the vehicle is capable of 25 m p.h.
Another vehicle which is of particular interest, in that it was designed in the endeavour to create a demand in a field which had not previously been exploited to any considerable degree, is the Atkinson wagon to carry loads of 50 cwt. This follows closely the lines of its prototype, the 6-tonner, and has an exceptionally short wheelbase, enabling it to make a complete turn in a roadway 20 ft. wide without reversing. The engine is the well-known Uniflow, with steel-ball inlet valves, operated by push-rods1 and central ports for the exhaust Valves in the pistons relieve the compression due to the Uniflow system.
Roth cylinders are high pressure, and the final drive is by a single chain.
There appears to be a considerable divergence of opinion as to the merits of the different types of valve. The ordinary slide valve is seldom employed and the fight is now between the piston and poppet types, and amongst the last-named we must include the ball type, such aa is used on the Atkinson. The poppet valve appears to be particularly good where high speeds are desired, as it gives a much more rapid opening and closing, and, consequently, enables a more rapid entry of steam, but piston valves have proved very reliable and are being employed with complete success on some of the latest models.
In spite of the importance of engines, the arrangement of the gearing and other points, the boiler must always be considered as one of the most vital factors in the efficient running of any steam wagon. In the overtype, during the period under review, there have been few changes, apart from increases in. size, but when we consider the undertype we find that developments of considerable interest have occurred.
The designers of the vertical boiler have had to overcome more difficulties than lie in the path of those concerned in the production of boilers of the loco type. Their size is necessarily restricted by considerations of height, road clearance and comfort, and at their best the majority cannot be. considered to possess the same amount of accessibility as the loco type, although partially successful efforts are being made to reduce this disadvantage.
Some types are inclined to become dirty far too rapidly, others show a tendency to distortion, with the result that troubles due to leakage sometimes occur.
In -view of the efforts which are being made, it is somewhat remarkable that the Clarkson boiler, which was the first developed in the period under review, has won a considerable measure of success. This boiler is of simple construction, the chief feature being the provision of thimble tubes in place of the crosstubes usually employed, thus giving freedom from internal stresses, as the tubes are secured at one end only. These tubes have proved to possess the property of rapid steam formation, whilst being remarkably easy to clean, tighten or replace.
In the Garrett vertical boiler the effort to obtain accessibility is clearly demonstrated, access to the tubes for cleaning or repair being provided by a large cover at the front and a hand hole at the rear. The centre of the 'firebox is squared and sloped towards the smokebox, in which is situated a waved tubular superheater. The water tubes themselves are also
• sloped,, thus assisting the circulation through them. Considerable success has been achieved by the double-ended loco-type boiler employed on both the old and new Yorkshire wagons. -The accessibility of
this type is most pronounced and the circulation of the gases is excellent. From the firebox they pass through small rising tubes leading to the two smokeboxes, and are then forced through horizontal tubes to the chimney. The arrangement is such that there is a uniform draught through each tube, and by splitting up the exhaust from the engine through a number of nozzles very silent working is obtained. Another feature of this boiler is that the tubes can never he uncovered when working on hills if the water be kept at its proper level.
The boiler used odthe small Atkinson vehicle closely resembles that employed on the larger model. It also has a squared firebox for crossed sloping water tubes, which ean easily be dropped for cleaning or repair. It has a large double-coil superheater in the smokebox.
The advent of the special type of boiler used on the Super-Sentinel was received with great interest, in view of the curious arrangement of the tubes. A. tapered, circular firebox is employed, and in this are -pressed a number of sloping landings, thus enabling the tubes to be arranged tangentially to the centre stoking hole and at a sharp angle. The slope of the tubes provides good circulation of the water, and dirt is less inclined to lodge on them, whilst the fact that there are no large flat surfaces makes the boiler of exceptional strength. A double-coil superheater is mounted in the narrow portion of the smokebox, and the firebox can be dropped for inspection and cleaning.
Another unusual construction ia that employed in the new Mann wagon. The centre portion of the firebox is pressed out to form deep longitudinal ribs, which carry the tube plates in tension. The plates and ribs are, of course, formed in one, and the arrangement can be seen by referring to the drawing. The tubes are well separated and are inclined to a fair extent and they fit closely to the stoking chute, so that the gases are forced to travel between them. Still more unusual is the construction of the boiler bn the new Fowler, and it is of particular interest in that fire tubes are employed in lieu of the more general water tubes. The water is contained in a space between the domed firebox and the concave lower portion of the smokebox, and the fire tubes are curved, so that heat cannot pass direct through them to the smokebox.
Riveted to the outer shoulder of the boiler is a circular chamber, which surrounds the smokebox and which is in communication with the rest of the water spade. This enables complete submersion of the fire tubes to be obtained, and, in addition, the expansion chamber thus formed obviates priming. The smokebox dome is hinged on the exhaust pipe and the whole top can be swung over to expose the superheater coils and to give access to the tubes.