THROUGH THE NIGHT WI] [ A SCAMMELL 1 3-TONNER 0
Page 54
Page 55
Page 56
Page 57
If you've noticed an error in this article please click here to report it so we can fix it.
F the commercial-vehicle chassis that have been introduced during the past twelve months, it is probably safe to surmise that the Scammell 13tonner embodies more novel features than any other. When it first appeared at the Commercial Show last year it aroused great interest, which was not diminished by the long period of testing that subsequently elapsed before its manufacturer was fully satisfied that the new model was ready to go into production. Appreciative of the avidity with which our readers were awaiting a more detailed description of this model than could be published at the time of its introduction, as well as a report of its performance, we made every effort to obtain this information at the earliest moment, and now publish. an account of the main particulars of a journey of considerable length (during which thorough tests were conducted) that we recently undertook in one of the first of these new vehicles to be completed.
Special interest attaches to this road test because it • was largely carried out while the Scammell was in use on regular service. We are indebted to C. D. and T. (Contracts), Ltd., the owner of the vehicle, and to Whitbread and Co., Ltd., the concern for which it is in operation, for affording us the facilities for carrying it out.
Constant Speed Maintained.
We set out from Chiswick some hours before midnight, travelled all through the night and arrived at Cardiff at about half-past seven in the morning. The actual distance is 157 miles, but our journey was interrupted .for the making of observations, whilst a long break was taken for rest and refreshment for driver and
passengers. Although the legal limit was never exceeded throughout the journey, the average speed for the running time was not greatly below 20 m.p.h.
On the face of it, the schedule observed would seem to make no exacting demands upon the capabilities of the machine, and to offer poor scope for a demonstration of its performance, but the margin between average and maximum speeds could hardly have been kept so small without certain of the excellent characteristics revealed in this vehicle. The route is one of the most exacting of the big arteries radiating from London, and an important contributory factory to the constancy of our speed was undoubtedly the Scammell six-speed gearbox.
This is prominent among many noteworthy feature of the machine, and in order that the interest we found in the journey may be shared, to the extent it deserves, by our readers, the salient poir.ts of the chassis should first be described.
The unique rubber bogie-suspension system deserves first mention. An accompanying illustration shows the principal parts. Considering one side, a balance beam is pivoted at its mid-point, below the welded box-section bogie cross-member. Its front end is attached by a balljointed link to a bracket at the back of the driving axle and its rear end to the lower plunger plate of the cushioning columns. These consist of a number of rubber discs placed, one upon the other, in two piles. An upper plate carries the two plungers which move axially in the columns, and the rubber discs are compressed between the upper plate and their base plate. The last named is free to rock on the inner extension of the trailing-wheel stub axle, itself rigidly mounted on the rear end of a radius arm, also hinged to the bogie crossmember.
Ample Springing Effect.
Thus the pair of columns float, and any upward motion of either axle that is not compensated by downward motion of the other, compresses the discs, thus reducing the length of the column, which varies by a maximum of 2/ ins. The rubber columns act in a similar manner to springs, but afford an elasticity more nearly approaching perfection, and are very much lighter.
Lateral movement is prevented by a transverse radius rod anchored to the back of the driving axle and the near side of the cross-member at the front.
The base plate, supporting the rubber columns, is carried on the stub-axle extension on a roller bearing to prevent binding of the plungers; the axle-arm bearings are bronze bushed. The transverse radius rod has -rubber-bushed attachments, and the remaining joints have hardened steel bearing surfaoes—best calculated to withstand neglect.
It is something of an accomplishment to have produced a gearbox with six speeds and only eight gearwheels. The drive is at all times indirect. All wheels are in constant mesh and those not fast on their shafts run on needle-roller bearings. Engine-type lubrication is employed, a gear-type pump in the sump at the base of the box feeding oil through tubes in the hollow shafts, to the selector collars, needle bearings and other parts, holes being drilled at appropriate points in the shafts.
Oil is fed from the pump through a Tecalemit felt filter, whilst ferrous particles are separated from the lubricant by a magnetic filter. For the sake of simplicity no gauge or tell-tale is provided. We are tempted to question the wisdom of this, for an electric indicator of some type would necessitate little additional complication and would constitute a valuable safeguard against the serious consequences of failure of the circulation.
The four sliding dogs are operated by an ingenious selector gear, which necessitates changes to be made in the correct order. The gate is appropriately shaped and the lever must be moved through a zig-zag course in running through the gears, either upwards or downwards.
It is not possible to change from " top" to " first " without going through the intermediate gears (with the exception of " fifth " and "second "). This might be regarded as a disadvantage, but not a serious one, and the driver soon accustoms himself to the new conditions.
An epicyclic reduction gear in the back axle is definitely unusual, but, mechanically, very sound. The system is to drive the relatively small crown wheel with a bevel pinion in the usual manner, and the differential is accommodated within the crown wheel in the orthodox way. Each differential wheel, however, has a short shaft with a small sun wheel on the outer end. This meshes with planet wheels carried on a spider on the inner end of each
axle shaft. These., planets engage with an internal gear formed on the inner periphery of the axle casing. The small bevel reduction and large epicyclic reduction are noteworthy (see accompanying table).
The crown wheel runs in roller bearings housed in plates between the main casing and the and casings, whilst the short shafts are spigoted in the ends of the long ones, bronze bushes being used here.
With regard to the Dewandre compressed-air braking system, this resembles in layout the vacuum type. It is used in conjunction with an air tank, said to store sufficient air for more than 20 applications. The pressure is normally about 80 lb. per sq. in. and is created by a neat twin-cylindered, water-cooled pump—itself a good piece of c34
workmanship—driven from the end of the dynamo shaft. Toggle expansion is employed for the shoes. An arm, swinging on a vertical axis, moves two push rods (one above and one, below it) out of or into a vertical position. " Click " adjustment is provided on one arm, a movable cup-ended centrepiece enabling the shoes to centralize themselves after adjustment.
An ingenious detail is the method by which the shoe return spring can be detached; one end is secured by a cranked roller which has only to be rotated half a turn (with a tommy-bar) to release the spring tension.
Mention of the employment of Davies detachable radiator tubes completes this outline of the points of chief interest of the chassis.
To return now to our experiences of the machine on the road, our first impressions as we made our way out of London along the T.Jxbridge Road over the uneven and slippery surface of the tram tracks (it was raining at the time) were the smooth motion of the vehicle and the silence of the transmission. We had on board a 13-ton load of bottled beer, and our driver told us he had already made many such journeys with similar cargo, butylaad never had a broken bottle nor yet a skid. On a subsequent occasion we travelled on the platform of another model (fully laden) and were greatly impressed b,,r the almost complete absorption of road shocks by the
rubber suspension. The motion might be described as akin to gliding.
Through the somewhat congested streets, fifth speed was mainly used; for the conditions obtaining at the time it was almost ideal. As we entered the more open country, however, sixth speed was more constantly employed. Changes, incidentally, from one to the other of these two —the latter affords an upwards ratio—can be made with remarkable rapidity and ease. In sixth gear at 15-20 m.p.h. the Gardner engine was running, without fuss or vibration, at a speed that suggested effortless motion. Shortly after entering Buckinghamshire the-road began to rise fairly steeply to Beaconsfield. The gradient brought us down to third gear. At High Wycombe one is in the Chiltera Hills and the road beyond rises to over 800 ft. The slope, however, is less severe, and fourth gear sufficed. It seemed that for every gradient the most suitable ratio was available.
Running down the western side of the Chilterns into Oxfordshire, we gained an idea of the efficiency and smoothness of the brakes, but steeper hills lay ahead. As the night advanced we progressed through Oxford and Witney, skirting 13in-ford, to the fret of the Cotswolds in Gloucestershire. At Andoversford, on the high ground, a halt was made for supper.
Thence we dropped down to Cheltenham and the plain
beyond. The level road afforded an opportunity of carrying out some Of the acceleration tests; being anxious' to avoid exceeding 20 m.p.h-, however, we postponed the_ main tests. The sabsequently obtained results are shown graphically. A feature of the graph is the smoothly rising curve obtained through the gears. It is almost that which would be shown by an infinitely variable transmission
The brakes, too, we were unwilling to test in view of the nature of the load: we accordingly conducted them on a later occasion on a machine more suitably laden. The figures are excellent, but speak for themselves. The ease of application and smoothness of retardation are marked.
When we took use vehicle ever ourselves during the night run, we found that a moderate pedal pressure produced powerful retarding eftea, but sufficient resistance was offered to the foot to enable thapplication to be easily. regulated. During this period we found also that the clutch was beyond criticism, the 'steering lighter than earlier Scammell rigid six-wheelers, with less self-centring • action, but certain and stable. The gearchange was slightly disconcerting at tirst,• bat easy of accomplishment after a little practice, and possessing a singular fascination.
Beyond Gloucester lie the South Herefordshire Hills, and, of these, two demanded second gear. The first is known as Ben Craig with a maximum gradient of about 1 in 9. Our speed fell to approximately 5 m.p.h., with power in hand. Then followed ‘Vhitchurch Hill of about the same severity. At the summit, reached after a 1.5-min. spelt of second gear, the cooling water was only 130 degrees F. (air temperature 40 degrees F.). This is much too cool for the Gardner engine to rim at full efficiency. The third hill at any note, following shortly after Whitchurch, was climbed in "third" at 8 m.p.h.
Thenceforward, except for a little earlymorning mist, the conditions were easy, and we made our way through Monmouth and Newport without event.
On arrival at Cardiff we ascertained that 15 gallons of fuel had been consumed over the 157 miles traversed. This gives a rate of 10.5 m.p.g., a tolerably satisfactory figure, but one that we feel confident could be bettered by running at a higher temperature.