Points in Goods-vehicle By .
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Chassis Selection A. W. Haigh, N the first, part of 'this article, which Was published in 0 The Commercial-Motor " for last week, the writer dealt with features of engine design, and in this instal
ment other components of the chassis are briefly commented upon.
Engine-cooling arrangements on a chassis would seem hardly worthy Of Mention,' as all systems can be said to be efficient now that forced circulation is almost universal. The experience of our war 'transport _overseas, however, has shown that the usual system, operating at atmospheric pressure: can be improved upon by-sealing the radiator -and building up internal pressure, controlled by, a calibrated blow-Off valve.
The usual method of.specifying-a radiatOr is to stipulate what is known as a " temperature difference," that is, the minimum difference in temperature between the atmosphere, and the boiling point of the coolant at which the radiator will operate efficiently without loss through evaporation. AS an example, a teinperature difference of, say, 110 degrees F. (a usual figure) ensures that the system will give satisfactory results tip to an atmospheric temperature of 102 degrees F. (212 degrees F.-110 degrees F.).
In many, of the theatres of war 102 degrees F. is often exceeded, so that, in order to, be certain -of cooling efficiency, either the radiator 'must be enlarged or the boiling point of the water increased' beyond212 degrees F The latter cm be accomplished by increasing the internal preS7 sure of the tooling system., 5 lb. per sq. in. laising it by approximately 20 degrees F. Because of this simple expedient no excessive 'weight of water is necessary in hot climates and, 'furthermore, in temperate climates like our own, the weight of water. required in an atmospheric-type radiator can be reduced, which is quite an important factor wider present licensing
conditions. • , '
A further point in favour of pressure cobling is provided by the Carnot cycle, Which shows that -the higher the temperature. at which:heat is delivered to an engine-the greater its efficiency. Pressure cooling increases operating temperature and, 'therefore,'increases, engine -efficiency. -The actual increase may be sniall, lint it is, nevertheless, one of which advantage should be taken if possible.
Gall for Full Synchromesh Gearbox
Dealing with gearboxes, most of those employed on commercial vehicles are of the crash " type, with normal spur-toothed gears, o1 either. involute or stub prOfile. The difficuity of changing gear, which 'is sometimes experienced with this type of box, and the whirring noise .which is almOst invariably associated with it, should tend. to give A 1;as in favour Of those vehicles fittict with the synchromesh typeof gear, although the synchronizing mechanism be hued' on only the lower ratios.
Apart from the ease of changing gear with the latter type of box, the helical-gear teeth are stronger, than the slur variety and, consequently, can withstand a greater
anment of rough handling. Before the war, there was ' little chance of making a choice between grarboxes, as they were all much of a muchness, but following the cessation of hostilities, it is to be hoped that commercial-vehicle manufacturers will take beed of the experience of private-car makers and give operators a full _synchromesh box. We now come. to. -propeller shafts, of which there are two main types. One is the purely mechanical in which Hooke-type joints are employed, and which mast be fitted with a sliding spline device to compensate for longitudinal movement of the live axle where Hotchkiss drive is
favoured, but which can be of fixed length if the vehicle has . torque-tube drive. The other is the'cushioned type of shaft' which relies on rubber discs, or bushes, to provide universaljoint action and to take up any varia.tiOn in .length.
Elimination of shock loads and' complete freedom from lubrication problems are two claims made for the latter type of coupling ,which has seen extensive use on passenger' vehicles. There have been cases where it has nbt stood up too well under the rougher conditions of operation of the goods vehicles, but this does not mean that the shaft is unsuitable RN' commercial work, for the failures which have occurred may have been due to any number . of causes, such as the joints being under-size or the rubber being affected by oil or • abrasives. :The advantages of such a coupling are of undoubted value on goods vehicles, as the relief of the shock loads on the chassis would prove beneficial in prolonging its life. • The mechanical type of universal joint is still extensively used, failures being extremely rare. It takes up but little space in the chassis, is cOnvenient to fit, and the provision of sliding splines permits easy removal when servicing the transmission, There is • little to choose between the two types, each having its advantages and disadvantages, so tha't the final decision may be left to the designer, with every confidence that adequate service will be obtained whichever type be employed.
Axles and Final Drives .Surveyed Of the .orthodox types of axle offered by manufacturers, all the heavier ones—above 10 cwt.—are fully floating and, consequently, designed art the strongest possible lines. Below,10 cwt, the three-quarter floating type is sometimes used, but here the pay-load is so small that the end thrust in the half-shafts, the one detrimental feature of the layout, can be safely. 'accommodated, so that insistence on the stronger, fully floating type, is not as necessary as it would be on a heavier vehicle.
Final drives take three forms, these being the worm and wheel, the crown wheel and pinion, and the hypoid gear and pinion. The worm and wheel is practically obsolete for goOds vehicles for, although it is stronger and quieter than the crown wheel and pinion, it is far more expensive. It is still used quite extensively on passenger vehicles, the worm being mounted below the axle so that a low floor height may be obtained.
The spiral-bevel crown-wheel-and-pinion type is, comparatively, the weakest and noisiest of the three types, but it is still the most popular because of its relatively low cost. Furthermore, although it is the weakest, it is still amply capable'of transmitting all torque pit through it.
The hypoid gear differs from the crown wheel and pinion in that, whilst still 'retaining the spiral-bevel form, the axis of the pinion does not pass through the centre of the gear, so that the teeth are of special shape and have been found fairly difficult to cut. Reports have become -current, of late, •of new machining methods which bring the cost of the hypoid gear almost as low as that of the ordinary spiral bevel, . and, for this reason, considerable interest is being shown in it. It is claimed that hypoid ,gears are quieter and stronger, which means that, in plate Of the somewhat bulky andoften noisy units housing spiral-bevel crown wheels and pinions, more silent and smaller axles
can be used, The greatest drawback of the modern, live rear axle Is that, if the connection to one be broken or if a wheel be allowed to slip, all drive is lost. Differentials have been patented to obviate this fault, but they suffer, mostly, from the same complaint, that of high. cost due to the number of additional parts required.
The simplest and least expensive cure is the, differential lock, but, unfortunately, this is subject to abuse, often being left in operation when not i-equired so that differential action is eliminated,-_and this results in exceSsive tyre wear. Some form of lock, which can operate only when one wheet is freed, either as a result 'of a broken shaft . or having . become bogged, may be pOssible. If, however; it is decided . that the nuisance of not being able to proceed when an •
axle shaft breaks is to be 'overcome, then a normal type of differential lock must be incorporated in the rear axle. Special instriictiOns, together with a conspicuous notice on the axle itself, would indicate that the lock was • to be used only in cage of a breakdown. The foregoing would nut apply if the additional cost Of a specidl differential be not objected to. ' • • . .
There is one big point in the choice of a rear axle, which most mainifaaurers' appear to-lave realized for their-postwar vehicles,. namely, the vast superiority of the .banjotype over the Split-case unit with reg-afd to ease of servieing. If any parts in the pot" itself, differential case, gears or bearings-,.or the crown wheel and Pinion, needatterition on the latter assembly, it is necessary to dismantle the axle completely. to get at them. On the other hand, with the banjo axle, all the items Mentioned above al-pi-mounted. on a Separate carrier which can be removed from the casing by merely disconnecting the propeller shaft and removing a• ring of nuts or setscrews.. If 'inspection is all that is necessary, a sheet-metal cover -may be removed from the casing to reveal all the units with the exception of the pinion.
Frame Design is Quite Important
A prospective purchaser too often gives but little thought to the frame of his vehicle, more or less taking it for granted.If the best is to be obtained from the. chassis, every item in it must be suited to the work which it has to pertorm, and, therefbre, the frame. must 'be given the • same consideration as the Other units. A vehicle having an ordinary platform body, or a drop-sided or fixed-sided one, in 'which body distortion has no ill-effects, is best supported by a frame made up of the usual channel-section side-members with either channel-or 'box-section crossmembers, Such a construction is comparatively weak in torsion, so that any undue road shocks are partially accommodated by frame distortion. A van, on the other hand, particularly one of the Luton type, can be seriously affected by distortion passed on from a twisting frame.
The light construction of modernvan bodies, made necesSary. by our licensing system, is in no way capable of withstanding deformation without fesuhant damage, either immediately, or-ntime. It is imperative, therefore, that a frame to, support such a body should be as rigid as possible: This object is achieved by the use, wherever possible,of tubular cross-members.
As regards the actual construction of the frame, riveting --is the most popular, but the writer has had experience ot welded passenger-vehicle frames, and can state that every _confidence can he placed in this type of construction, no complaint of 'frame trouble haviog been experienced in a fleet of more than 80 coaches. , Semi-elliptic springing has proved itself on all types of vehicle, but a choice of springs Must be made to suit the work of the vehicle. For instance, a refuse collector, with a comparatively high Centre of gravity, and on.'which it . is. necessary that deflection from light to fully laden shall be as small as possible, requires short, .stiff springs, which • have both a. high rate and high torsional resistance. The choice of springs for a Luton:type van, which may .carry valuable and-often delicate household goods over long distances, is, perhaps, the most difficult to make. The centre of gravity is high, which demands stiff springs to counteract roll, whilstthe nature of the load is such that stiff springs, with their high rate of.periodicity; would be . unsuitable. To suit the load, the springs should be. long and of low .periodicity, but, unhappily', long, slowly vibrating Springs have, a very low resistance • to roll, so that a compromise is necessary whereby. medium-length springs, fitted with some form of anti-roll device, such as Fox anchor slips, are the most serviceable. It can be appreciated from the foregoing that each individual chassis has its own springing problems which, for maximum operating efficiency, must be met. It is up to the „operator, then, realizing the conditions under which his vehicle is to work and the loads it has to carry, to ensure that his springs are those most suited to his require. merits, remembering that short springs and those of high rate give a bouncing ride, but have good anti-roll properties, and that the longer the spring the slower the period
but the lower, is its resistance to roll. ' • The brakes of any vehicle are amongSt the most important item and it is imperative, therefore, that-the most efficient
arid the most economical 'units be obtained. There are three main .types of brake proper, and these are the fixed
anchor the floating anchor duo servo and that known as . , the two-leading shoe. .
The 'fixed-anchor brake is perhaps the least economical. Its leading 'shoe:does three times the work of the trailing. shoe and, therefore; is subject to •far heavier wear; so that its . facing . reqiiires replacement longbefore that _ of the trailing shbe, with consequent wastage of material. '• The duo-servo type, whilst being, thestrongest, maY be • fierce.in action. This is due to the 'Manual effort, which is put into the leading shoe, being magnified -by the 'rotation of the brake drum and passed through the floating anchor to the trailing shoe,' where:it. is Magnified once more by the action of 'the drum.. This form of brake is little better than the fixed-anchor tyPe -as regards economy, the fault of uneven distribution of braking effort being displayed in exactly the-opposite w-ay to the former 'brake, that is,.more work is done by the trailing shoe-than by the
leading shoe. ' •
The two-leading-shoe type of brake is undoubtedly highly efficient. Soe pressure is evenly distributed over :the entire contact area of facing" and brake drum, servo. action is equal on both, shoes, so that facing wear is -equalized, and braking is uniform, there being.ho one-Sided distorting effect on the drum and no extra stress applied to
the wheel studs. .
Methods of Brake Operation • Actual brake operation can take many forms, three of which are, all rod, rod and cable, and hydraulic. The first named introduces difficulties in locating the rods, it being essential to give them a straight run without interference with other chassis -members. Of the other two methods, the reliability of the hydraulic system is still held in some doubt in this country, despite the fact that it is almost universally used in America.
It is assumed that brakes with hydraulic operatibn are spongy, and that the pipes are prone to fracture, with consequent loss of control. 'The supposed trouble of sponginess may have been a bad feature when.lead tubes to the brake units were of unarmoured rubber, which expanded and gave that insecure feeling, but, since, the advent of steel-sheathed flexible pipe, it is impossible for any pedal motion to be lost, as the fluid is incompressible. As regards the possibility of pipe fracture, this is no more likely than, say, the breaking of a rod or cable, hut even if it were to occur, the use of a double master cylinder would prevent complete loss of braking conlool.
Only one remark is necessary on tyres, namely, that it is equally bad to over tyre a vehicle as to under tyre it. The Manufacturer guards . against the latter, but many operators have out-size tyres fitted to their vehicles in the hopethat more mileage will be obtained from them and expenses reduced. More mileage may be obtained, but the additional rolling resistance of oversize tyres will increase fuel consumption which, added to their extra cost, will more than counterbalance the little saving which may be effected in increased mileage.
The final item, which must be given careful consideration when selecting the make of vehicle to be purchased, is the comfort designed into the cab. Comfort does not eno with the provision of a luxuriously upholstered adjustable seat, but is inherent in the disposition of the controls, the provision of' adequate. ventilation, the insulation of the engine from the driver, a simple signalling device, and all the small extras which contribute to the easing of the task of the driver. • It .rnust be remembered that the driver spends one-third Of his working life in the tab of his vehicle, and it is -the duty of the operator to see that the time spent in his service causes as little fatigue as possible.