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FOUR-WHEEL-DRIVE
TRACTOR-LORRY
A' HEAVY haulage unit, which has achieved a considerable degree of success in America, but which has not yet found its way on to the British market, is the _Walter tractor-lorry. Among the many striking features of the desig-n are a six-cylindered engine, a five speed gearbox, automatically locking differentials to prevent wheelspin and loss' of traction, driving and braking on all four wheels, and the employment of cardan shaft drive to internal gears in all wheels, the differ-entials being attached to the sprung portion of the chassis.
Two main types are marketed ; in one the motive unit itself has a carrying capacity of 6f tons to 7i tons, and will handle a 15-ton semi-trailer or pull 50 tons on trailers; five different wheelbases are listed. The second type has a carrying capacity of 4 tons to 5 tons, and will haul a 10-ton semi-trailer or 50 tons on trailers; this model can be had in three different wheelbases.
In the main both patterns of 'Walter vehicle follow the same general principles of construction. The engine has six Lheaded cylinders, cast in pairs, the heads being detachable and of the Ricardo turbulent type. The bore is 4i ins, and the stroke 51 ins., giviug a rated horsepower of 48.6, but
the maximum output is stated to be approximately 100 b.h.p.
Circulation of the engine-cooling water is effected by pump through an oval-tube radiator, behind which is a 20-in. fan. A vacuum feed is employed for the Zenith carburetter. The power is transmitted through a 14-in, singleplate clutch to the five-speed gearbox, a single control lever being mounted above the box. The range of gear ratios available is very wide, top being 8.5 to 1 and first speed 85 to 1.
One of the most remarkable features of the design is the use of automatically locking differentials, of which there are three—one in each axle-drive set and one in the gearbox, from which emanate the propeller shafts.. The principle used is that of the irreversible worm ; that is, the worm turns the wheel but
the opposite action is prevented. ILL this case the worms are on the axle shafts and the worm wheels on the driving shafts. When rounding a corz ner the outer road wheel can over-run its fellow but, on bad surfaces, one wheel cannot absorb all the power to the detriment of the other, as the worm wheels • must drive the pinions equally; thus wheel-spin is entirely obviated.
Great stress is laid upon this feature of self-locking differential mechanism by the makers. Undoubtedly this is fully justified as the lot of the tractor is usually a hard one involving the crossing of many surfaces which a lorry would never be called upon to tackle. If it were necessary for the driver to stop the vehicle, dismount from his cab and then proceed to lock the differentials by manual means, the vehicle might well sink a few inches, thus increasing many times the necessary sthrting effort. When the tractor is hauling a train of trailers the need to avoid a stop becomes the greater. It often happens that a halt, even on a good surface, is followed by consider able wheel-spin when getting away again, which causes undue tyre wear and sometimes results in ditching when a wheel " bites" suddenly before the steering can be corrected.
Internal gear drive is used, a doublereduction being provided ; the brake drums are on the inner ends of the jack shafts. This gives four-wheel braking, but allows the drums to be kept well out of the way of mud or boulders on bad terrain.
Semieelliptie springs are fitted to both axles, and the necessary road ad Lesion of the front wheels is give t by allowing considerable forward overhang, the dead front axle being below the lateral centre of the gearbox. A 14-in, clearance below each axle is allowed and the track is 5 ft. 6 ins.
The makers are seeking a distributor for Great Britain. Any of our readers who may be interested in the Walker tractor-lorry from this point of view should apply to the, TTnited States Departaent of Commerce, Office of the Commercial Attache, Bush House, London, W.C2.