"Heavy Motor Vehicles." #
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Further Extracts from the recent paper.
We published the first portion of our abridgement of the paper which was read on Wednesday of last week before the I.A.E. by Messrs, Watson and Kennedy on pages 26 and 27 of our last issue. We continue our extracts by further reference to the authors' remarks upon double-reduction hind axles. They wrote of the Leyland design :— The easieg cuiraining the double reduction and differodial gearing, is supported by the dipped portion of the axle forging. and the differential shafts pass through the hollow ends of the forging and transmit the power through star cunplings to the road wheels, the wheels riding freely on turned portions of the solid axle forging. In the authors opinion there is objection to this form of construction, insomuch as there is practically no compression member in the loadsarrying axle, and no matter how stiffly it is made there is bound to be a certain amount of deflection, which, acting through the wheels and differential shafts, sets up periodic and severe side stresses upon the ball bearings which carry the inner ends of the differential shafts and their final drive geari ngs.
" A sketch shows the arrangement of the double-reduction hack axle adopted by Walker Bros., Ltd., of Wigan, for the vehicle which they are now constructing to meet, the requirements of the War Department subvention scheme. It will he seen that in this axle a solid forging carries the had, but id its central portion it is split and spread, so th.at there is both a tension and a compression member in the forging, and the w hole of the gearing is contained in casings which are securely bolted to the split portion of the axle, the resulting construction being clean, symmetrical, and sturdy, and all the parts are readily accessible for adjustments, renewals, or repairs.
"The Thornycroft subvention vehicle axle is another capital example of clever design. It has a solid forged axle of the double-banjo type, truly bored out at its central portion, and secured to this solid forging are a front and a back casing which house the double-reduction gear. In this case, however, the first reduction is through spur gearing, whilst the bevel gear is reserved for the final drive, the object being to reduce the tooth speed of the bevel gear, and thus ensure a more silent running axle.
Steam Vehicles.
A number of interesting points were in the section concerning steam vehicles, characteristics of which were dealt with by the authors in the following words :
" At the present day steam wagons may I e divided into two broad classes; the traction type, as exemplified in the Foden wagon; and the under-type, as exemplified in the Leyland and Sentinel wagons. The traction type of wagon may be compared to a steam horse harnessed to a twowheeled wagon, the frame of the wagon ta:ming the shafts which extend along Ile, sides of the boiler, harnessing the toiler and forecairiage to haul the load( arrvitg platform. The word haul ' used in tbi:: sense is not exaetly correct, but it -sill readily convey the author's meaning. " The authors have found that the traction type of wagon can be overhauled at less than two-thirds the cost of overhauling the under-type of machine. Repairs and adjustments are far more 'easily rarried out on the traction type, and beCallSe this is so, those in charge of the machines are more likely to look after them and not to neglect parts which, retidire constant attention.
" One distinct difference tetween the traCtion and the under-type of wagon is that the traction type is generally an open engine, and the under-type is en
closed, running in an oil bath; one exception in the traction type being the Wallis wagon, which is completely enclosed and rune in an oil bath. *rile Wallis arrangement is further interesting inasmuch as the cylinder casting and engine can be removed bodily front the boiler, rendering it quite an easy job to carry out an overhaul. An enclosed engine will stand far mnre puniAing thin an open one ; the latter type, if pushed to its utmost, will frequently fire the big-ends aiel eccentrics."
Steam-Wagon Change-speed Gears.
The majority of traction type wagons followtraction engine practice very closely, and the transmission is generally arranged on the three-shaft system—that is, crankshaft, countershaft and main axle. The crankshaft carries fast and slow pinions on one extremity, except in the case of the Mann wagon, the arrangement of which is shown in diagram below. These pinions engage with a double-speed wheel on the countershaft, and a chain pinion mounted on the countershaft transmits the power by means of a chain to the back axle.
" The arrangement of the change-speed mechanism on traction type wagons necessitates the stopping of the vehicles to effect a change of gear ; this is a most desirable feature, as it obviates the risk of a missed gear on a hill, with the usual dire consequences.
"As a ride, machine-cut gears are employed on the firstmotion shaft, but more than one maker has found a really good machine-moulded gear very satisfactory. The machinemoulded gears have a far longer life than those which are machine cut, because of the hard skin formed on the teeth.
" The usual meafts of transmission of power from the second-motion shaft of a three-shaft engine to the rear axle is by a single roller chain, but in some makes of wagons two side chains transmit the power, one to each rear wheel, the differential gear being then mounted on the countershaft instead of on the axle. The latter system is generally employed on 'under-type' vehicles. There are now only two exceptions to the chain drive; these are the StewartThornycroft 'under-type' wagon and Mann's patent tipping cart, both of which are gear-driven."
Steam Boilers.
The authors, in the section dealing with steam boilers, quoted freely from a series of articles written by one of them in THE COMMERCIAL MOTOR
(" Boilers for Steam Wagons and Tractors," see issues for January and February, 1910), but when dealing with the feed-water arrangements, the authors raised one or two points which are of undoubted interest to all steam wagon and tractor users.
Feed-water Fittings.
"The choice of the feed-water fittings varies, some makers preferring to use two injectors, whilst others fit one injector and a pump, which may either be driven by the engine or be of the steam-driven donkey type. It may generally be taken that two independent forms of water supply are essential, but the final selection is a matter of individual choice. The reason why some makers prefer the use of two injector; is that it is quite safe and practicable to fit a stop cock between each injector clack valve and the boiler, so that if one injector goes -wrong its valves may be completely isolated from the boiler, the other injector meanwhile being in action, so that the faulty valves may be taken out, cleaned, reground and replaced, without inconvenience. " On the other hand, if an engine-driven feed pump is fitted in place of one of the injectors there are certain objections to the provision of a stop cock between its clack ' valve and the boiler, because the driver may inadvertently start up the pump with the stop cock closed; the water delivered from the pump barrel is then driven up into a blind alley,' and, having no outlet, may lead to the burstiiig of the pump barrel, the blowing out of the glands, or fracture of the driving mechanism. Many makers do fit such a stop cock on the pump delivery, but the authors arc opposed to the provision of such a fitting. " The independent type of feed pump has practically disappeared, owing to its not being absolutely dependable—a fatal fault on steam wagons. It is probable that a great deal of trouble with this type of pump has arisen through its small proportions and minute valve gear, which is quite incapable of withstanding a healthy blow with a spanner or hammer, the usual treatment givers to start it going."
Boiler Fittings.
"It is essential that two check valves should be fitted between each injector and the boiler : if a small amount of steam or hot water works back to the injector, it will invariably cause it to refuse to work. It is not a pleasant experience on the road to have both injectors hot, with little or no water showing in the gauge glass. " Great attention should be paid to the provision of suitable and accessible strainers on the pump and injector suction pipes from the feed water tanks. Mud and grit are sure to be present in injurious quantities when water is taken from roadside ponds and ditches.
" The authors unhesitatingly recommend that neither injectors nor pumps should be attached to the boiler. All check valves should be fixed to the boiler barrel, well away from the fire-box. It is a mistake to inject a stream of cold water into the space round the fire-box, as the water, impinging against the fire-box plates, undoubtedly cauSes harm ; the barrel is the best place for the introduction of feed water, and we cannot do better than follow the practice of locomotive engineers in this respect. " Careful provision should be made for the draining off of all water from the suction and feed pipes of the pumps and injectors in ease of frost. It is far less expensive to arrange the details of a feed-water system so that it is self-draining than to replace some pounds worth of parts after a frost,"
Steam-Wagon Driving Axles.
" Steam-wagon axles are usually of the traction-engine type, and there is no special point to mention in connection with them excepting the methods of attaching the road-wheels. More than one leading maker presses the wheels into position in a. hydraulic press, but from the user's point of view this method has serious disadvantages; it is impossible for the driver with ordinary appliances to remove wheels which have been mounted in that manner, and this is a serious matter when it becomes necessary to change the wheels quickly in case of failure or wintry weather. Several attempts have been made to overcome this difficulty, one commendable, arrangement being that designed by Bauly, and shown below. In this arrangement the wheels are a push-on' fit on the driving axle. A sleeve is firmly keyed to one end a the axle, whilst another sleeve is secured to one of the differential bevels. The drive is transmitted from each. driving sleeve to a road wheel by means of a pin, which may be passed through the hub of the wheel or made integral therewith. The Bauly axle is an adaptation of and improvement on an arrangement long employed by Wallis and Steevens, but although the latter makers provided sleeves, thewheels were not interchangeable. In Bauly's design the wheels will lit either side without modification—a most corn
mendable feature from a user's point of view. The authors hope that makers will consider the merits of this type of axle. 'The differential gear is usually of the bevel-gear ' using three four pinions, and the chain ring is bolted to
the differential centre, as also, on some wagons, is the brake drum. Some makers provide ler the enclosing of the differential gear—certainly a more workmanlike-looking job—but it is an open question whether the casing offers any practical advantage in the long run, owing to the massive design and the usual long life of the working parts.''
Steam Wagon Brakes.
"The study of brake gears is interesting at the moment,. as the Local Government Board is taking steps to recognize the steam reversing gear as a legal brake; it seems difficult, to the lay mind, to realize why the reversing gear has ii'ut been recognized as legal before now. One has buttG ask the driver of a steam wagon, to find out that the reversing gear is the only real brake. In the new regulations which it is. proposed to issue, an independent brake, acting on both rear wheels, will have to be provided. Tip till now many makers have adopted the system of putting one independent brake drum on the axle, and no doubt there will he much heartburning when it is found that this form of brake is illegal.
Band brakes seem to hold the field, but the double-shoe brake, of the loco-type, is a better job. It is to be hoped that makers will see their way to adopt this design in future vehicles, since. although they are more expensive than band brakes, they do not rattle, nor bind on the drum. Whatever braking arrangements are made on a wagon there must of necessity he one which will efficiently retard the vehicle, and bring it to a stand-still on any gradient up which it can be driven with its load, because, if a chain were to break, there would be all the elements of a serious smash-up. followed by
a pile of claims upon the unfortunate owner. This prompts the authors to suggest that a simple form of sprag be fitted on all steam wagons."
• Wheels for Steam Wagons.
"There appears to be a divergence of opinion as regards road wheels. On some steam wagons built-up wheels, with cast-iron centres. wrought-iron spokes. and T rings, with riveted slabs. are fitted ; this is an excellent type of wheel for country roads, but such wheels shake themselves to pieces on hard town roads and setts. Cast-steel slabbed wheels have been tried, but they have not proved a success, the great amount of vibration causing them to fracture. One or two types of composite wheels have been evolved, with satisfactory results; Hindley's composite wheels for steam wagons are good examples of this class. This make is built up of cast centres, dished steel plates in place of spokes, connected to the wood feline, on which is fixed the band or tire. The authors understand that a well-known firm of brewers have used these wheels with great success, the repair bill for several years, with the exception of new tires, being nil. " Taking it all round, it is open to doubt whether the wooden wheel, with a cast-steel nave and plain steel tire, can be beaten for use on steam wagons; such a wheel is elastic, with beneficial results to the wagon, and if they are not neglected, i.e., allowed to get loose and otherwise nut of repair, their cost of maintenance will bear favourable comparison with that for metal wheels. Wooden wheels are far the best for running over hard roads and setts, and it is noticeable that a number of Lancashire users stick to then,.
" In connection with wood wheels, it is interesting to note that rubber tires have 15 per cent. more life on a steam wagon when fitted on a wood wheel than on an all-metal wheel ; one of the authors has proved this to his sorrow, as he had hoped that cast-steel wheels shod with rubber tires would be a success on steam -wagons, thus eliminating the cost of wheel maintenance. The combination of the all-metal wheel and solid rubber tires may be used with complete snecYss on most r.trol vehicles, on account of their lighter axl• . • " Steel-tired wheels fail miserably in wintry weather; several attempts have been made to get over this difficulty, and more than one device has been brought out which may be attached to the wheels when necessity arises. The Goodwin snow shoe is a typical example. The majority of designers have pinned their faith to separate wheels for winter
work.''
General Arrangement.
" Whatever special features there may be in the design of a heavy petrol vehicle, the general arrangement should be one tljat will permit the removal of any unit without disturbing any other parts, and for this reason the combinedunit form of construction, such as is now employed in many lighter vehicles, is not permissible. Every working part should be enclosed and protected from dust and grit. and between clutch and gearbox, and between gearbox and countershaft (if one is fitted, or between gearbox and back axle), there should be suitable universally-jointed shafts which will eliminate all possibility of the imposition on the driving shafts of undue stresses which tend to throw the shaft out of ali.1.-tnment. A mistake is made in trying to secure a rigid chassis construction, whereas the design should be so arranged that tile chassis may wotk.' Everything must be so mounted that distortion, due to uneven road surfaces and bad loading, will not subject any part to undue strain."
We have not the space in this issue to include a digest of the discussion which followed the reading of the above paper. We hope. however, to publish the interesting portions of it in our next number.