Petter' s Latest Oil Tractor.
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In our issue of the 19th January, we illustrated one of the Petter latest-type oil tractors engaged in ploughing operations. Since that time, we have had an opportunity to submit one of these machines to a very-severe test on the roads in the vicinity of the maker's works at Yeovil, Somerset., and we are convinced that it is not only sufficiently powerful for any road-haulage work within the scope of a heavy motorcar, but that it is also the simplest oil-engirted tractor which we have so far examined, and that, once its qualities are thoroughly appreciated by users and potential users, Petters, Ltd., should be kept busy in the production of these machines, to which branch of its extensive business the company is about to allot a special department. Few companies have been 'so persistent in their endeavours to produce a really-satisfactory oil motor as has this Yeovil con-cern ; its first tractor was produced in the year 1900, and, after several experimental machines had been built, one was exhibited at the Royal Agricultural Society's Show, at Park Royal, London, three years later. Since that time, developments have been slow, but none the less thorough, and the tractor now under notice embodies many good features of tractor aaign, whilst the engine itself is the result of experience accumulated by its maker since the first Petter oil engine was produced in 1896. Briefly described, the tractor consists of a slow-running oil engine (the main features of which are almost identical with the company's stationary type of engine), which, together with the necessary clutch and change-speed gearing, is secured to a bed plate consisting of two deep-section longitudinal channels, these in turn being spring mounted over the leading and driving axles. The engine has but a single cylinder, of 101 in. in diameter, and a piston-stroke of 12 in., and, at its normal speed of 300 r.p.m., the engine gives continuously 30 b.h.p., and can do all the work of a 6 h.p. nominal steam engine.. One of Bridge's patent expanding clutches (a detailed view of which we reproduce herewith) is mounted on the near-side end of the crankshaft, the inner member of this clutch being secured to the crankshaft, whilst the outer member and the two change-speed pinions ride freely on the shaft. A couple of large-diameter sliding gearwheels are mounted on a secondary shaft, and through these the tractor may be driven at speeds of 2i and five miles an hour. A reverse speed equal to that of the lower speed forward is also provided, the necessary third pinion for which reverse gear is mounted on a small auxiliary crankshaft. The secondary shaft is carried in adjustable bearings below the crankshaft, and extends to the off side of the engine, and the drive thence to the live back axle is through a Hans Renold roller chain of 2f in. pitch.
A reference to the clutch view will show that a brake band partly encircles the outer shell, and this band may be tightened by means of the same lever -with which the clutch is actuated. This lever is a very-long one, and is hand operated, the _first part of its forward motion disengaging the brake, and, later, allowing the clutch to engage. When the engine is used as a portable power plant, for driving chaff cutters, threshing machines, etc., by belting, the clutch-shell brake band may be removed and the shell itself employed as a power-transmission pulley ; it is 28 in. in diameter and is 6 in, wide on the face. On the opposite end of the crank is a massive flywheel measuring 45 in. in diameter with a 41-in. face, the provision of such a large flywheel being necessary on account of the slow speed at which the engine is run. All the bearings are provided with ring lubrication, and a large aluminium dust cover is provided for the connecting-rod big end, and the open end of the cylinder.
The starting of an engine with a 101-in, cylinder, were it. to be effected by hand, would entail the exertion of considerable physical force on the part of the driver, but this is avoided by the provision of a self-starting arrangement, which consists of a cylinder of compressed air with suitable valves. Before starting the engine with compressed air, the flywheel should be pulled round until the crank is just past the dead centre on the normal firing stroke. Compressed air may then be admitted to the cylinder, and the pistori is forced downwards, the charge of air escaping during the return, or exhaust, stroke. The normal cycle of operations is then followed, air and fuel being drawn in, compressed and fired by hot tube in the ordinary manner of stationary gas and oil engines. With a pressure of between 60 lb. and 80 lb. per sq. in. in the compressed-air cylinder, the starting of the engine is a perfectlyeasy matter. The compressed-air cylinder may be recharged to the desired pressure by opening the valve while the engine is running, and at each explosion within the engine cylinder a small charge is forced into the air cylinder. Should the latter accidentally be discharged while the engine is stationary, the necessary pressure may be pumped up by means of a hand pump that is fixed to the footplate. The cooling arrangements, as may be seen from one of our photographs, are rather novel, and consist of a single helical coil of gilled tube inside which coil a centrifugal type of fan is driven at high speed by a chain and sprockets from the longitudinal camshaft ; an extra quantity of water is carried below the radiator, in a horizontally-disposed water tank which is slung between the side members of the main frame. The fuel tank, it may be noticed, is suspended below the driver's footplate. Ordinary crude petroleum may be consumed in the cylinder of this engine, but there appears to be some difficulty in obtaining that class of oil in some parts of this country. Ordinary paraffin, however, may be burnt with equal facility. The tank carries a sufficient quantity of fuel for about 10 hours of working.
The inflammation of the charge, as we have already stated, is effected by means of a hot tube. The initial heat for this tube is applied by means of a small Primus oil lamp, but, after the engine has been running for a short time, the tube is automatically kept hot by the heat of combustion of the gases within the cylinder. The necessary levers for regulating the governor, controlling the fuel supply, clutch, change-speed gear, etc., are all conveniently arranged for operation by one man, and, in order to prevent the accidental engagement of the reverse gear while either of the forward-speed gears are in mesh, the reverse-gear lever is placed on the opposite side of the tractor to that of the forward-speed levers. A simple but positive locking device prevents the reverse lever's being moved unless the forward-speed lever is in its central or neutral position. This locking device we illustrate.
The machine is very-well sprung, the forward part being .carried by a single transverse spring over a usual traction type of front axle, whilst the back part of the frame is suspended from longitudinal plr to
springs, which are clamped on top of the axle boxes. The under part of the tractor is quite clear of projecting parts, so that, if the leading
axle clears an obstacle on the road, the rest of the machine will safely pass over it. The leading wheels are 3 ft. 6 in. in diameter, with 6-in. treads, so that, allowing for a 3-in. square front axle, there is 191 in. clear space between the under side of the axle and the road. The back wheels are 5 ft. in diameter, and the standard width is 10 in.-, but extra-wide wheels may be fitted if the machine is to be used for ploughing ; this extra width may be given either permanently or as a detachable flange. H the wheels greatly exceed 10 in. in width, there is some danger that the unladen weight of the machine will be higher than that permitted by the Heavy Motor Car Order. The back axle is of the usual traction type, with bevel gear and winding drum, but, unlike that of many steam tractors, the winding drum is very accessible, a fact which was clearly demonstrated to our representative during the course of the trial, to which we have already referred.
During the first part of the trial, the trailer was loaded with five rough flywheel castings, the weight of each being just over 21 cwt., making a gross load behind the drawbar, including the trailer, of 6 tons 18 cwt., and this load was hauled from the works to Sher
borne (Dorset) and back at an average speed of just over four miles an hour. On the route taken, there are two very-severe hills, one of them being about a mile long, with a considerable stretch which has a gradient varying between 1 in 10 and 1 in 15; the other one, encountered just as the tractor entered Sherborne, is very steep and has a gradient of between 1 in 8 and 1 in 10. During the run both out from the works and back again, the engine worked with perfect regularity, pulling well at both low and high speeds with a remarkablyclean exhaust ; in fact, the exhaust was absolutely invisible except for short periods when the engine was running lightly, as when going down hill, or at the moment of changing the gears. A further test of the machine was subsequently made to prove the maximum capacity of the tractor, three more flywheels being loaded on to the trailer, thus increasing the load behind the drawbar to just over 10 tons. The driver then tried to climb a hill of about 1 in 15 quite close to the works, and, had the road been slightly softer, so as to give a better grip to the wheels, our representative is convinced that the tractor would have successfully mounted the hill without uncoupling the drawbar. As it was, the wheels failed to grip on the hard, bright surface of the road, and it was necessary, after going some distance up the hill, to chock the trailer wheels, uncouple the drawbar, and proceed ahead a distance of some 60 yards : the tractor wheels were then chocked and the trailer hauled up by cable. The trailer was successfully hauled to the crest of the hill in four stages. One of our photographs shows the cable in operation on the last stage of the climb..
We are convinced that in this machine Colonial visitors to the Royal Show will find much to interest them, whilst tractor users at home will find that, for cost of running on ordinary haulage work, the Petter tractor will compare most favourably with any other make of machine, whether propelled by steam or by internal-combustion engine. As we have already stated, it is in construction the simplest machine we have examined.