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A BRITISH APPLICATH ? FOUR-WHEEL DRIVE.

11th November 1924
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Page 16, 11th November 1924 — A BRITISH APPLICATH ? FOUR-WHEEL DRIVE.
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Complete Technical Description of " Hathi in Undeveloped Countries. Designed by Thornycrofts for Haulage Work vel Features of Construction.

" WHAT a magnificent job! " was the exclamation which inveluntarily escaped the writer after a survey of the technical details of the Thornycroft four-wheel-drive tractor, the design of which has been carried through to perfection by this famous concern, although initiated by military experts in transport matters. Robust and powerful in appearance, massive of shoulder, and yet docile to control, one hopes that the Army name of -" Hathi " (the elephant) will be retained as a commercial appellation because it is so appropriate. In short, from end to end, it is a production of which the famous firm of Thornycroft may well feel proud. The wonderful performance of its prototypes was, of course, fully described in our issue dated October 21st.

Referring to the technical details of the construction, the most interesting features are, perhaps, the arrangement of the front-wheel drive, the six-forward speed gearbox, the winch and cable gear, and the brakes. To preserve clearness in the description, however, we will start with the power unit and work thence through the transmission, brakes and suspension system.

The engine is Of the six-cylinder type with a bore and stroke of 41 ins. and 61 ins, respectively, giving a cubic capacity of 695 Cu. ins. (111 litres) and an R.A.C. rating of 54.2 h.p. the actual power output is, however, 90 b.h.p. at 1,200 r.p.m., and the engine has shown itself capable of turning over at speeds in excess of 2,000 r.p.m. The use of six cylinders, of course, gives a balance and uniformity of turning effort not obtainable with the more ordinary fourcylinder power unit.

The cylinders are cast in two blocks of three each, mounted on a massive crankcase which, in turn, is three-point supported. It is r.ot placed centrally, but is off-set towards the near side of the chassis. The water piping, water pump and fan are mounted. on the off side, and the off-setting of the engine is sufficient to bring the fan practically central with respect to the radiator. Cowling assists the flow of air through the gilled tubes. The magneto is also placed on the off side and embodies a trip gear which greatly facilitates starting. Incidentally, the starting handle is mounted above the near side front dumb iron and drives the engine through a two-to one reduction provided by a roller chain and sproc kets. Two safety devices are incor porated, the se

being a springcontrolled cam on

t h e crankshaft and a spiral-faced cam on the starting handle, which is detachable. The valves are situated on the near side, those controlling th3 exhaust being placed beside the cylinder block, whilst the inlet valves are above them and inverted. One set is driven directly by tappets, and the other is controlled by overhead rockers and push rods. The induction pipe is designed on modern lines with abrupt right-angle bends and extended ends, these features having been found to assist equable distribution, and, therefore, to promote efficiency. A point of interest is the provision of a dynamo on the near side, positively driven, which gives current for electric lighting.

The lubrication system also embodies the most modern practice, being of the dry sump variety. Two low-pressure gear pumps withdraw oil from the sump continually and deliver it to a five-gallon tank mounted on the dash, (sufficient for 500 miles). From this tank a supply is drawn by a high-pressure pump, which delivers it through a hollow crankshaft to the bearings. We were particularly impressed with the smooth running of this engine, and, considering its size, with the ease with which it could be started from cold. The earburettet, incidentally, is supplied from a large two-part dash tank, the main section of which holds 30 gallons, whilst an emergency compartment contains . a further six gallons. Readily detachable filler caps are fitted to the tanks and to the radiator, of such a size that no funnels are necessary. A variable hot-air supply is provided for the carburetter.

The engine transmits power to a cone clutch of ample proportions, from which the drive is taken through a large fabric joint to a

separately mounted gearbox. This Fearbox, by the way, is carried by three supports, being on one side secured to the side member and on the other to a sub-frame member.

The principal feature is the manner in which six forward speeds and two reverse speeds have been obtained. At the top of the box is situated the mainshaft to which the drive is conveyed by the clutob. To the right of this shaft (viewing it from the back) and slightly below it, is a layshaft, and below this again, still further to the right, is the transposing shaft.

The mainshaf I and layshaft are provided with sliding gears of the normal type, by which three forward ratios and one reverse ratio (all of them indirect) can be obtained. These gears are controlled by an ordinary gate-change lever mounted on the box. The layshaft can be coupled to the transposing shaft through either of two pairs of gears, giving two independent final ratios. These gears are controlled by a lever working in a quadrant which is marked " low," " neutral," " high." This lever is set by the driver according to the nature of the work to be undertaken, and he then uses the normal threespeed and reverse control. He has, however, four more ratios available by shifting the second lever which controls the transposing shaft ratios. The design of the gearbox is such that the shafts are extremely short and stiff, and they are all mounted upon ball bearings. Provision is made for driving a mechanical tyre pump from the lay shaft • and, in addition, there is a coupling for driving the winch from the mainshaft, to which we will refer again later. The tyre pump delivery line is provided with an oil separator. Each end of the transposing shaft carries a universal joint of the mechanical type, to which the propeller shafts are attached, one running forward to the front axle, whilst the other is taken to the back axle. It will, therefore, be seen that there is no differential between the two axles. It is interesting to note that, after exhaustive tests, the makers have convinced themselves that not only is such a differential unnecessary, but it definitely impairs the performance of the vehicle when on rough ground cr loose, marshy surfaces. So far, they have not been able to detect any disadvantages in doing without the central differential, but, of course, differen tiaI gears are fitted between the wheels of each axle.

The propeller shafts are open, the torque and drive being taken through the front and rear springs. At one time it was thought that radius rods would have to be fitted, but this proved unnecessary.

Each axle is of the well-known Thornycroft pot type, built up of three sections and containing an overhead worm-and-wheel drive at the centre. This drive gives a ratio of 8.25 to 1, and the six overall gear ratios are tabulated below. To give a better idea of what these figures mean, we have also shown the nominal road speeds to which they correspond when the engine is running at 1,200 r.p.m., the diameter of the wheels being 40 ins.

There is nothing unusual about the back axle but for the fact that the design is exceptionally clean owing to the entire absence of brakes. Precautions have been taken to prevent the entry of grit or moisture, and each rear wheel is mounted on three bearings of the ball and roller type. The innermost bearing is of the deep-groove variety, with no filling slot, so that it is well adapted to withstand side thrust.

The front axle is similar to the back axle as regards the central portion, but the ends differ in order to provide for front-wheel steering and frontwheel drive. At each end of the casing is bolted a casing of semi-circular section to which the steering pivot is secured. This pivot, in addition to supporting the stub axle, carries at the bottom a horizontal bevel wheel which rotates upon a ball bearing and -a roller bearing. This bevel is an idler which meshes with a second bevel wheel mounted on the cross shaft transmitting the drive from the central differential. Finally, there is a third bevel wheel carried in bearings on the stub axle meshing with the idler wheel and bolted to the central hub of the road wheel.

It will, therefore' be seen that the drive goes through three bevels, which are so arranged that they do not interfere with the necessary movement of the wheel when steering the vehicle. It might be mentioned that the stub axle part of the gear is enclosed by a second semi-circular casing which is a close fit in the casing fitted to the axle, so that dirt or moisture is completely excluded and the parts work in an oil-tight box, which, by the way, is fitted with an oil filler. The ratio of the bevel drive is simply I to 1. The differentials in each axle are of very robust construction andare of the spur-wheel type.

Detachable disc wheels are employed, secured by no fewer than eight studs and carrying 40-in, by 8-in. straight-sided pneumatic tyres on spring-flanged detachable rims. The makers desire to emphasize the fact that for vehicles of this type the large pneumatic tyre is infinitely preferable to the solid tyre, owing to the way in which it obtains a grip on all sorts of surfaces. Twin tyres have been tried, but have not been found to be any better than single tyres.

The brakes are particularly interesting, being both of the transmission type, this principle entirely avoiding the use of drums on the wheels, which is a very desirable feature in a vehicle which may often work up to its axles in mud or sand. At each end the transposing shaft carries a large drum which, incidentally, encloses the universal joint connection to the propeller shaft. Both the brakes therefore operate on all four wheels.

The two brakes being the same, one description will serve for both. The drum is-surrounded by a fabriclined steel band terminating at the top ends in a pair of plates set at an angle when viewed from above. Mushroom-shaped pieces of hardened steel engage with the ends of the brake band, and when these are pulled in a fore and aft direction they draw the ends of the band together which, at the same time, causes it to rise and contra,ct round the drum. A very powerful brake is so obtained, the band being simply anchored to a steel tray which is fitted to the bottom of the gearbox. The bands are, of course, lined with asbestos fabric. One of them is actuated by the pedal whilst the other is controlled by a long hand lever. Either brake will hold the tractor and a five-ton trailing load on a slope of I in 5.

The frame is of very deep and Strong construe

tion, adequate strength against torsion being provided by rigid gusseted cross members. It is supported by four semi-elliptic springs, a point of interest being that those at the front are shackled to the front dumb-irons and carried on pivots at their rear ends. The spring seatings are of the Thornycroft pattern, which is curved, so that deflections of the springs do not tend to lessen the anchorage. A pair of inclined TT 'bolts connect the spring bracket to lugs on the axle, and the master leaf of the spring is provided with projections which embrace the bracket. at each'side, so giving a direct point of application for the drive. . The ends of the master leaves are formed •with solid eyes bushed with bronze. Incidentally, a rebound leaf is fitted to the top, and in each spring the plates are held by six massive clips. No centre bolts are employed, so that the plates are not weakened by drilling. The steering gear has an unusually high ratio, which, of course, is necessary to reduce the labour involved in manoeuvring so heavy a vehicle. Bracketed to the frame is a steering box containing a worm and nut mechanism, from which a drop arm depends at an angle and works to and fro across the chassis. It is connected with a transverse steering rod, ball-jointed to the near-side steering arm, from which a track rod is taken to the off-side steering arm. The track rod passes through a slotted bracket fixed to the axle at the centre, the object of this being to prevent vibration, which is apt to fracture the rod through fatigue. All chassis parts are provided with nipples for grease-gun lubrication.

As regards the winch, this is fitted to enable the vehicle to pull itself out of an impasse or to pull a trailer up into an otherwise inaccessible position. It is not of much service to give full details of the mechanism because the design is about to be altered in several respects. At present the winding drum is of relatively small diameter and has a long horizontal axis set athwart the chassis, This, although working satisfactorily, involves the use of a mechanical laying-on gear for the cable, which is highly expensive to construct. It is intended, in future, to use a larger horizontal drum with a vertical axis, which, of course, will reduce the number of turns of cable required. In any case, 50 ft. of cable will be carried.

The winch is fitted to the frame behind the gearbox and is driven by means of a joint from a shaft coupled to the main shaft by a pair of sliding gears brought into motion when required by a lever controlled by the driver ; the driver then starts and stops the winch by means. of the ordinary engine clutch, the gear lever, of course, i

being n neutral. The necessary speed reduction is obtained by means of a 28-to-1 worm and wheel drive fitted -to the winch itself. The cable is normally taken back between a pair of guide pulleys mounted at the rear end of the chassis, but can, if desired, be taken forward round a third pulley and through a ram's horn fitted on the near-side front dumb-iron, so that by means of the winch the vehicle can actually be Made to pull itself forward. A pull of five tons can easily be obtained on the cable.

To ensure against movement of the tractor when the winch is being used for haulage purposes, a pair of skid pans is provided, which are placed beneath the front wheels and anchored by cables to hooks secured to the side members. Another point is that when the tractor is being used over rough, loose surfaces non-skid chains can be passed round the tyres and through slots in the disc wheels. The capabilities of this tractor were so fully described in a recent issue that but little need be said here as regards its performance. The following particulars are, however, of interest.

Weighing nearly 5 tons, it has preyed capable of exerting a steady pull of 9,000 lb., as recorded by a Watson dynamometer. This means that, taking the road resistance RS 60 lb. per ton, it should theoretically be possible to haul a gross load of 150 tons on the level, apa'rt from considerations of inertia and internal friction of working parts. With a trailer loaded to make a total weight of 15 tons, including the tractor, the vehicle has proved capable of ascending a gradient of 1 in 10 at 7 m.p.h. With a total weight of 10 tons, including the tractor, it has successfully been stopped and restarted on a slope of 1 in 5. Hauling a trailing vehicle of a gross weight of 5 tons on the Thornycroft 28-mile test circuit, a petrol consumption of one gallon per 4.15 miles was obtained, which can be considered very creditable in view of the fact that, under such conditions, the engine was running on a small throttle opening ;

clearly, a much larger gross weight could be hauled on this circuit with but a very slight increase in consumption.

At 28 m.p.h., which is the nominal maximum, the engine speed is 2,000 r.p.m., but we are informed that the tractor we have described has actually reached, 30 m.p.h. on the road. At the lowest point the ground clearance is 10 ins., but there is more than 2 ft. to spare over the central portion of the chassis, so that very large obstacles can be cleared. The overall dimensions are : wheelbase 11 ft. 6 ins., track 6 ft.

We understand that the makers will be ready to produce this tractor on a commercial basis in the near future, and we believe that it will have a great field of useful work in the Colonies, or anywhere where roads are few or absent.

It would be possible to fit the tractor with a turntable and trailer to make a six-wheeler, which should have considerable scope in this country for heavy duty in hilly districts where high average speeds are required. It would also be possible to use the winch drive from the gearbox for actuating a fire pump, thereby obtaining a fire engine capable of running across country and gaining access to natural sources of water supply which could not readily otherwise be reached.

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Organisations: Army

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