The Knox Four-wheel Tractor.
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A Machine which, Although It Can Carry No Load Itself, is Capable of Pulling Up To 40 Tons. It has Hydraulic Brake Gear.
In our issue of the 11th February last, we described in some detail the threewheeled Knox tractor. It may be remembered that this was a special type of machine, consisting of a substantial high-powered, petrol-engined chassis, provided with a fifth-wheel device which is arranged to support the front end of a two-wheeled trailer. Other trailers may be attached to the rear of this combined six-wheeled vehicle (subject to localauthority approval in the U.K.), and some noteworthy performances in the way of haulage have been attained in the course of the past seven years by this machine. Eventually, however, it has been decided to replace this by one having four road wheels and consequently only two tracks. The three-wheeled machine has proved itself to be suitable for good paved city and country roads. For rough and ready work, such as must be undertaken by a modern tractor, however, it was found that four wheels were really desirable.
The Result of Experience.
The present vehicle is therefore the outcome of some seven years of experience of this class of machine and it is believed to embody, so far as is possible, all the improvements that have been dictated by experience in that time. That this is so Co a considerable extent is made obvious on a close inspection of the chassis.
The tractor is of a type which should make an appeal to some users who consistently carry heavy loads, whether over short or long distances. With one trailer only, that is to say, as a complete sixwheeled vehicle, it can be loaded until the gross axle-loads reach the limits set by English law, that is to say, eight tons gross may be carried on the weight carrying driving axle, and four tons gross on the rearmost axle, under which classification we regard the unit as a heavy motorcar drawing a trailer. On the other hand, we learn that Mr. C. H. Fryer, of 11, Southampton Row, WU., has taken counsel's opinion as to whether the vehicle is to be regarded, not as a tractor with a separate trailer, but as a, six-wheeled vehicle. The limitation which then applies, under the existing motorcar law of the United Kingdom, is the laden weight of 12 tons gross for the heavy motorcar. Under this classification, it, will be observedwhilst it may be held, from the legal
point of. view, that the unit is entitled, in respect of speed, to classification varying with the maximum axle-weight, no advantage can be gained in respect of bolding, seeing that a redaction has to bI made for the weight, whatever it may be, upon the leading axle.
An Interesting Weight Problem.
The problem is an interesting one, although it does not affect the excellence of the design, considered as a type, with which we are dealing. If the unit is a single -heavy motorcar, As is contended, despite its three-axle construction, if its gross weight be kept within the 12-ton limit, and if the load on the pair of weight-carrying 'driving, wheels be • kept, down to six tons, then, with rubber tires on all wheels, the legal speed may be accepted as 12 m.p.h. -If,however, the load on the central axle exceeds six tons, but does not in approaching eight tons involve an excess weight (above 12 tons) for theunit as a, whole, the legal speed may be accepted as 8 m.p.h., again on rubber tires for all wheels. From the standpoint of constructional efficiency, this type is usually loaded in the United States with 15 tons net, and sometimes with loads in excess, going so high as 25 tons.
The Speed as a Trailer.
• The difficulty, under the present law of the United Kingdom, in the event of its proving that the correct classification for this type of combined unit is as a heavy motorcar drawing a single trailer, is found in the imposed maximum legal speed of 5 m.p.h. There is no doubt whatever that the more general. utilization of trailers would tend in a great many cases to lessen considerably the cost of haulage. It is a question whether the law relative to this matter of the utilization of trailers might not with some considerable advantage be revised. Conditions Differ in the States.
In the United States, where such limitations as are imposed on British users do not hold, the trailer is rapidly becoming an important feature of its transport. Here, of course, it may be
pointed out that certain special features of the traffic preclude the use of such methods to anything like the same extent. Naturally, any such matters must he kept in view in the event of any suggested alteration to the law as it stands. It should be borne in mind at tho same time that experiments in the way of road trains that have been made both in this country and on the Continent have not met with the success which theoretically would seem probable.
Turning now to consideration of the chassis itself, for the enormous loads which its three-wheeled prototype has proved itself capable of handling, it at first sight appears to be extremely light. After an examination of its peculiar construction, however, it is apparent that first appearances are in this case misleading. As in the case of the three-wheeler, the tractor is in two separate and distinct parts: an engine and transmissioncarrying portion which extends from the front end to just in front of the rear road wheels, and which is supported on the comparatively light springs which are all that is needed to carry these portions of the mechanism ; behind this, but connected to it by a special springing arrangement, which we illustrate, comes the fifth-wheel platform, which is carried by the driving wheels of the tractor, and is specially sprung on very substantial tractor springs that certainly appear easily able to sustain almost any load that one can reasonably apply.
Final Drive by Chains. Honeycomb Radiator.
The final drive to the hind wheels is, under the circumstances, by chains, the only method that would appear to be practicable.It is claimed for this peculiar method of suspension that the engine and transmission are thereby instared against having to sustain the considerable shocks and constant vibration which would necessarily follow if they were rigidly attached to the load-carry, ingplatform. One result of this is seen in the radiator, which, strange to relate, is of the honeycomb type. Naturally we
gave vent to our surprise at finding a cooler of this class on the machine. The company's engineer, with whom we had the pleasure of a long discussion on the chassis, told us that there was never any need, when taking into account experience gained with the predecessor, the three-wheeled tractor, to consider the substitution of any other type. His belief is that immunity from trouble in this respect is in the main due to the very light springing which is all that is necessary for the power unit of the tractor.
Hydraulic Side-brakes. .
The most striking departure from customary practice, and the chassis itself is full of unusual features, is the provision of hydraulically-operated brakes on the rear wheels. The side-brake lever performs the function of a pump handle, and it is possible to lock the road wheels of the vehicle under any likely conditions of loading and on the steepest hill, which up to the present time have been encorintered, by three or four strokes of the side-brake lever. The pressure may be instantaneously released and can be graduated to any desired degree.
A Very Heavy Test.
We were -told that, as a test On one occasion, the machine was brought to a stop by means of this brake on a hill having an inclination of roughly 1 in 10, with a special load of 70 tons, it was left in that position for four days without there having been a loss of pressure great enough to affect the braking. All showing the size of brake which the adoption of this principle renders possible, it may be pointed out that the rear brake drums are 20 in. diameter and the shoes are no less than fa in. wide. Almost instantaneous adjustment of these brakes is possible, and it is perhaps unnecessary to point out that the hydraulic principle ensures
theoretically correct balancing.
The foot brake on the chassis, although operating in the usual manner, is nevertheless of a size consistent with the duty which it may be called upon to perform. The drums, of which there are two, are 14 in. diameter and 4 in. face. They are provided with cooling ribs, are readily adjustable, and the shoes are very accessible indeed, when there is any need for renewal of the linings.
A differential lock is provided to enable the tractor to pull itself out, if by any chance it be driven over places where it is only possible for one of the road wheels to grip. A feature of this differential lock, as showing the careful attention which has been paid to rendering the chassis fool-proof, is that it is impossible for the driver to put it in gear without first of all putting his changespeed lever into the neutral position. The need for some such provision will be readily appreciated if one considers the effect of suddenly locking an exception ally heavy wheel, such as is of necessity provided on such a chassis as this, spinning at a high speed to the other one which is likely at the same time to be held in contact with a good road surface. Another special feature is the improvement of the engine. All the bear. ings, including those for the pump and
camshaft, are fitted under pressure of 60 lb. per sq. in. With the exception of one short pipe from the crankcase to the water-pump bearing, all this oil is carried in steel piping embedded in the casting. The oil pump is carried inside the ease, but attached to the top half, so if it be desired to remove the lower half of the crankcase this can be done without in any way disturbing the arrangement for lubrication. A very accessible filter is provided, and lifting the hinged cover of this opens at the same time a level cock, thus preventing the possibility of over supply of lubrication, and at the same time ensuring that the engine is receiving its proper supply. An oil pressure of 50 lb. per sq. in. is obtained with the engine revolving at a very slow speed, so that a good head of oil is available when labouring under full throttle and pulling a heavy load. This pressure is prevented from any increase by a safety valve, so that the oil pressure is constant at all engine speeds.
Over.-head.valve Engine. Self-governing.
The specification embodies a 5 in. by in. bore and stroke four-cylinder
engine. This, although the same size as that on the three-wheeled tractor, differs in many ways from its pre decessor. In the first place, the cylinders are pair cast, instead of being
separate as on the original engine. The overhead type of valve, however, has been retained, as also the arrangement of detachable cylinder heads, No governor is provided on this engine, the valve sizes have been revised so that the maximum horse-power available is developed at 1000 revolutions, and after this the power curve rapidly drops. The engine is three-point suspended, being carried from drop-forged I-section cross-members firmly bolted at either end to the main frame. The engine is, of course, water-cooled, the circulating water passing through a pump of the ceritrifugal type. • The crankcase is of aluminium, the lower half being simply an oil reservoir ; this is detachable without disturbing any of the mechanism. Ignition is by high-tension Mea magneto. A complete starting and lighting outfit is provided, embodying storage batteries, generator, and motor.
The engine control is by means of accelerator and usual pair of control levers above the steering wheel. The clutch is a multiple disc, with cork inserts. The clutch transmission is by way of the usual clutch shaft to a threespeed-and-reverse gearbox, thence by means of bevel and cross-shaft to take 2 in. pitch. /eller chain. The speeds are engaged by the medium of the usual gate change, the operating lever for which is placed in the centre of the chassis. With the engine running at 1000 r.p.m., the speeds obtainable are 9.4, 3.9, and 1.6 M.p.h. The steering column is situated on the left-hand side of the chassis, the gear teing the worm and sector type operating on the steering wheels in the usual manner. Remarkable Turning Possibilities.
The lock of the chassis in itself is remarkable, this being obtained by setting the steering wheels well away from the frame, the axle being stiff enough to allow of this and also the arrangement of the steering pivot which is brought very closely up to the wheel itself. The possibilities in the way of steering in confined spaces are perhaps better shown by our illustrations. The overall dimensions of the tractor itself are as follow :—Wheelbase, 9 ft. 44 ins. ; front track, 4 ft. 98 ins.; rear track, 4 ft. 74. ins. The minimum clearance which obtains under the front axle is
10iins. The front tires are 36 in. by 4 in. singles, and the rear ones 38 in. by 6 in. • twins. The price is £975 with tires.