The Design of Modern Steam Tractors.
Page 9
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(Concluded from p. 136), By " Hephcestus."
A special point in connection
ith the valve gear is the casehardening ; if this be not properly eseried out the parts soon become sla.ek, resulting in -a badly-knocking engine. Proper 'allowance must be made for taking up wear in the moving parts, but in a well-designed engine the aim should be to provide ample wearing surface and proper lubrication, so that adjustments are not often necessary.
The best type of cylinder lubricator is a positively-driven oil pump feeding into the passage from the regulator valve to the high-pressure steam chest, so that. it may be examined under steam. Oil boxes of ample size should be fitted to the bearings, especially the crankpin journals of the connecting rods.
Some tractors are arranged with a row of oil boxes over the crankshaft, delivering by drips to the crankpins and eccentric-straps, and this is a great advantage. Forced lubrication to the crankshaft and motion work would be the ideal type for an engine running at such high speeds as the.:e at which tractor engines are often driven.
The Three Types of Drive.
To transmit power from the crankshaft to the driving wheels, three systems of gearing are employed. (1) the three-shaft system ; (2) the fotheshaft system ;and (3) the 'double-geared system. In the three-shaft method all the gear wheels are outside the hornplates, but the fewer working parts yield little internal friction and also a very low repair bill. With the four-shaft system, the first-motion gear can be. placed inside the hornplate and the gear wheels are smaller in diameter ; but the internal friction is, of course, greater, as is also the weight.
The double-drive system in which each hind wheel is driven separately from the eon ntershaft, has the advantage that the hind axle and the wheel spokes are relieved from the driving strain, and can therefore be made lighter. The axle is usually fixed, but as the bushes wear, the wheels tip in at the top, and the gears consequently become crossed. It is better to let the axle revolve inn the bearings, being driven by one wheel only, in order to allow the compensating gear to act. Practice has also shown that with this system, unless the compensating gear be locked, the drive, is mainly on one side, with consequent gieiter wear on one pair of gears.
The first motion of fast-running gear wheels have machine-cut teeth to' reduce the friction.
Some tractors arealso fitted with a third speed, to use when travel
ling light, but it is questionable if the advantage gained is worth the extra gear-wheels.
The change-speed gear should be arranged So that only one pair of wheels can be put into mesh at any moment. Many large 'gear wheels are now made with the tooth ring separate and bolted to a spider. This makes it cheaper to supply wearing parts, but the rings are liable to become loose, due to the crushing strain on the bolts.
Springing Methods.
Both coil springs and laminated springs are used, but the latter give the best results ; the coil spring is too quick in action, and there is no friction to absorb the energy. The method of placing the. springs outside the hornplates, in locomotive style, is being adopted by many make's. [Foster's loco-type of laminated-spring suspension is particularly to be commended. We have described its effect and illustrated it fully in our columns.--E le] The outside suspension gives greater stability, and more resilient springs can be used.
Two methods are in use to allow for the spring movement of the hind axle. In one, the countershaft remains stationary, and the gear wheels are provided with specially long teeth to allow for the " in and out " movement. Very little movement is obtained, j1 in. being about the maximum, consequently the axle bearings are constantly knocking against the stops, unless the springs. are very stiff. In the other method the countershaft and hind axle are connected by links to keep the gear wheels in constant mesh, and a universal joint is arranged on the countershaft. The spring movement ca.n be made as much as 2 ins: with some types, and this is a great advantage to both the tractor and the driver.
Wheels and Tires.
The usual diameter for the driving wheels is now 5 ft., the width being 10 ins, for town work and 12 ins, for country work. Both wheels are usually loose on the axle and driven by steel pins. Sometimes the wheel on the compensating-gear side is bolted direct to the outside bevel wheel, but this depends on the winding-drum arrangement. The straked steel wheel is still predominant, but treads of wood are often used for town work, especially on stone setts. Wheels with wooden. spokes and treads of specially-prepared fibre and rubber blocks are also being used with good results. The front wheels should be about 3 ft. 6 ins. diameter by 5 ins, or 6 ins, wide, and bushed witl phosphor bronze, as cast iron bushes are liable to heat and cut.
The steerage chains should spritig shackles to relieve the boilei of shocks which would othemisn cause leakage, and the worm are wheel must be cased in to protee them from dust and dirt.
All levers and fittings should bt conveniently placed so that on men can drive and steer, and stall cient room allowed for stokiie without too much manoeuvring or the driver's pare The drawbx should be connected to the hind axle bearing guides, to relieve thi tender of all pulling strains, and ii should be bell-mouthed to feeilitat( coupling up.
Superheating and its Advantages The advantages to be gained b3 fitting a superheater are evident13 not fully understood by etean tractor users, hut, judging lieorr railway experience, the time Iva. probably come when superheateri will be almost. universally used. Apart from the decreased fuel bill, the radius of action is increased, a very important point, and the life of the boiler is prolonged. ThE type which is u-holly contained ir the smokebox is the must suitable. as it is durable and safe tc use. A. steam tractor has to work under such varying conditions that any type which is subjected to very great heat; and which requires an arrangement Of dampers, such as is used on railway locos, would necessitate very skilful attention, both as regards skilful driving and maintenance.
The economy in fuel consumption, with a properly designed superheater, should be at least 30 per cent., and the reduction in the coal bill alone would be, sufficient to wipe but the extra cost of the tractor in about three years. [We have recorded great economies.—En.) The compensating gear ought to have at least three pinions, to prevent rocking of the centre plate, and it should he provided with means for locking, operated from the footplate for preference. A slip type of winding drum is now standard practice.
All bearings should be capable cf being rehushed and adjusted for wear, the bushes flanged' where a gear Wheel adjoins, and the hindaxle bearings fitted with phosphorbronze rubbing washers outside the hornplates to take the side thrust of the driving wheels and to allow for the spring movement. Stout gear covers, easily detachable but firmly fixed to prevent rattling, and with inspection doors for oiling: must be provided.