TOMORROW ' S by P. A. C. Brockington,
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A Suggested Design with Air Suspension, Air-cooled Underfloor Engine and Hub Reduction Gear to Overcome Operators' Current Difficulties
AHAULIER employing tippers for arduous site work combined with runs on the public roads is an outstanding example of a businessman who wants cream with his pudding without incurring the penalty of indigestion. In practice, digestive troubles are normal to his frustrated existence and he would pay a high price for a curative pill.
In his musings on the ideal tipper chassis he includes pleas to the makers to avoid non-standard features of doubtful reliability and modifications that would add to its unladen weight. He suffers from brainstorms which create the illusion that, with a little cunning, measures could be used to safeguard the vehicle from the results of misuse, without resort to the obvious cure of employing stronger parts designed to withstand a severe overload.
He knows that this would necessitate building a non-standard vehicle, and although he might be willing to pay the price, loss of payload on the road and spares-replacement difficulties, added to a higher fuel consumption, would, he believes, offset any gain in reliability, Such a vehicle might also have other disadvantages such as a greater loading height and a low top-gear ratio that was unsuitable for long hauls.
"Ideal "Tipper Chassis
Suggesting design features that should be incorporated in the " ideal " tipping-vehicle chassis is, therefore, a complicated mental exercise. It must be basically similar to the ideal lorry chassis, and apart from considering extras that can be fitted to accommodate the special requirements of users (and the facility with which the necessary modifications can be made), the exercise resolves into: (1) an analysis of operators' difficulties, and (2) an attempt to show the makers the value of certain contemplated design changes to the large proportion of hauliers who use vehicles for both road and site work.
The need for operators to reduce labour eharge6 per ton by carrying heavier loads and to run manceuvrable vehicles, which, if required, can be employed for general haulage, has created an increased demand for tipper chassis in the 7-ton class with a relatively long wheelbase. This type is more frequently subject to overloading than the short-wheelbase tipper. I will, therefore, concentrate on the design of a vehicle in this class.
• The tipper market is a valuable one, partly because the frequency of replacement is high, and if a new chassis is not acceptable to operators sales will suffer severely. Tipper applications can provide a yardstick of performance and economy, and there have been several cases of a haulier disposing of lorries used for normal goods delivery work, despite satisfactory results on the road, because of repeated trouble with similar vehicles employed for tipping operations. In many overseas countries this yardstick often has even greater value.
Costly Transmission Failures
To reduce transmission failures is generally the first concern of the transport manager. One contractor told me that the cost of maintenance (including loss of working time), of which replacement of transmission components formed the major part, often exceeded in 12 months the first cost of the vehicle. Another operator had had so much back-axle trouble that he was contemplating the purchase of a Continental make.
Limiting the torque load in the lower gears, in default of fitting lightweight components with supra-normal stress characteristics, would, therefore, be a great boon to many users. A well-known operator told me that he was seriously contemplating replacement of his oilengined vehicles by petrol-driven types because with the latter the engine inertia loads imposed on the transmission when the clutch was engaged were substantially
reduced and stresses were lower as a result of the maximum torque being developed at a higher speed.
If the transmission components of a standard lorry chassis were designed to withstand the torque provided by a low bottom gear, their weight and cost would represent an uneconomic concession to reliability and their use might start a vicious circle of weight increases. It is common practice, therefore, to base the design of the components on a torque value substantally below the maximum.
Torque limitation could be provided by a fully automatic gearbox giving a torque-overlap characteristic,
changes between the lower ratios being made at speeds appreciably below maximum torque revolutions.
After it has been fully proved in service automatic transmission would be gratefully accepted by the tipper operator, but as yet his interest is mainly academic. It was emphasized by a well-known commercial-vehicle designer—who considered that the automatic gearbox would eventually be fitted to all vehicles—that the tipper unit should preferably have six or more ratios.
A fully automatic torque-converter transmission with art auxiliary train of gears could also be designed to limit the torque, but its use in tippers would probably represent a costly and highly specialized application, because it would not be acceptable to the average lorry operator. Its relatively low efficiency would also be a deterrent to many potential users.
Torque limitation by a comparatively simple means should be possible if designers Made a serious attempt to evolve an appropriate unit. If it were reliable it would be immediately welcomed by the majority of tipper operators. At least one contractor has adopted the " do-it-yourself " policy and is developing a shearpin coupling to prevent overload, his intention being to mount the uMt in an accessible position where it can be reached by the driver, so that a replacement pin can be easily inserted.
Tests with a special clutch, designed to slip at a 1113 predetermined overload, have shown that transient variations in friction increase operational tolerance to such an extent that the slip torque is unpredictable within wide limits. According to an authority on the subject, the shear-pin device will be prone to the same defect.
An, engine cut-out actuated electrically by contacts in a flexible torque-sensitive coupling might appear to offer an easy solution, but in practice it could not limit inertia loading when the clutch is harshly engaged, which in extreme cases may be a multiple of the rated output torque, because of the inevitable operational lag.
Limitation of torque to an exact amount is a feature of the nut-runners used on factory assembly lines, and the application of a similar principle in the form of a spring-loaded face cam would probably be practicable. The effect of friction changes would be reduced by using low-angle cam faces and a high spring tension.
Variations of the face-cam unit could include one designed for mounting immediately behind the clutch thrust block to limit inertia loading by releasing the clutch at a predetermined torque. Alternatively a unit for fitting behind the gearbox could be of the " selfslipping " type, the location of the device at the rear of the box having the advantage that output torque in the lower gears could also be limited.
The latter unit would be subjected to wear and tear by impact loading of the oscillating cam plate when it was slipping, but the period would normally be very brief. The clapping noise created by the slipping cam would act as a warning to the driver.
With both types of unit, an electrical contact could be employed to actuate a solenoid which closed the throttle or limited the opening, an appreciable lag in operation being acceptable if the clutch response were immediate. .Momentary " revving-up " of the engine would be a warning to the driver that the limiting torque had been exceeded.
Apart from improvements in suspension characteristics and spring life, which will be discussed in some detail later in this article, operators' criticism of other components mainly related to frame breakages caused by the severe torsional stresses inseparable from rough site work.
Avoiding Local Stress The basic characteristic required is that the loading should be evenly distributed throughout the length of the chassis to avoid local stress concentrations. A number of users criticized makers for giving too little study to this aspect of design. Reinforcing the chassis is often a routine requirement and some operators appealed for closer co-operation between vehicle and tipping-gear manufacturers in the examination of stress problems created by local stiffening and in the production of suitable members. Users who have found a way of reinforcing the chassis satisfactorily, after many years of trial and error, hesitated to disclose the methods employed.
Among future chassis developments that might aid the tipper operator can be included the use of hub-reduction gears in conjunction with a step-up gear between the clutch and the gearbox, or preferably between the engine and clutch if the chassis were completely redesigned.
This layout would enable much lighter transmission units to be employed (possibly of the private-car type), and some designers are confident that the system would provide an overall saving in weight, in addition to reduc a 14 ing inertia stresses. The use of step-up gearing .could be avoided by employing a higher back-axle ratio in combination with the hub reduction gears, but this arrangement would not reduce the loading on the gearbox or propeller shafts. Concentric spur hub reduction is advocated, but a straight helical spur type would be preferable if it could. be accommodatedin the space available.
An obviousdrawback of both systems is the introduction of additional friction losses, which could amount to more than 5 per cent., but might be reduced to around 24 per cent. with a straight helical spur. A gain in payload would offset this disadvantage, and the Suitability of transmission units of the car type would be of great benefit, particularly when the fully automatic gearboxes now being developed become available.
The cost of providing an anxiliary gearbox in a " stepped-up " system would be less .than that of a unit transmitting the full engine torque, and greater latitude with regard to choice of units could foster improvements in performance and gear changing that might more than compensate the friction losses. Civil engineering contractors who employ heavy vehicle equipment incorporating hub gears report favourably on their reliability. Of even greater significance is the well-founded rumour that a well-known manufacturer is developing a system of this type as a possible standard feature of a future range of quality-produced cars.
Important projects being considered by vehicle makers include the development of air-cooled engines and the general use of underfloor engines for goods vehicles. It is pertinent that Midland " Red" technicians intend to use an air-cooled engine in this location. Moreover, they are confident that a thermostatically controlled variable-speed fan drive will ensure even cooling and reduce losses to a minimum.
Forward control is. favoured by operators on account of improved vision for the driver, but is often criticized because of the relatively high front-wheel loading associated with its use, the loss of rear-wheel traction and the discomfort (often acute) to the driver caused by the stiff front springing. An underfloor unit amid'ships would transfer weight to the rear, but could reduce ground clearance and increase the danger of engine damage, If the vehicle were equipped with a fixed lorry body, accessibility might be reduced.
The difficulty of mounting certain types of tipping gear resulting from placing the engine in the centre is another possible disadvantage. As a compromise the engine could be accommodated with its forward end across the front axle, which would afford greater space for underbody tipping gear and improve accessibility from the cab, in addition to removing the groundclearance disadvantage.
With an emphasis again on lack of reliability, and inadequate performance an important secondary consideration, suspension Sysfems take second place to transmissions in the list of components of which operators are generally the most Critical. Spring breakages account for a high proportion of breakdowns. When running empty the rigidity of conventional leaf springs reduces tyre life, increases torsional vibration stresses in the transmission (on account of the bouncing wheels) and may damage the cab and bodywork, as well as causing discomfort to the driver.
Tipper operators would be the first to welcome a selflevelling constant-periodicity air-suspension system and to express doubts of its reliability. It should be noted
that in endurance tests of the rubber air-bellows of the Pneuride suspension system produced by the Dunlop company, up to 41m. cycles have • been successfully completed. Reports from America show that a service life of the pneumatic elements far exceeding the normal mileage life of the vehicle can be expected. Moreover, in the unlikely event of a "puncture," air is exhausted from the bellows on both sides, so that the axle weight is supported on the bump rubbers, and the vehicle may be driven to base without removal of the load.
Another claim made by the Dunlop technicians is important with regard to first cost. They emphasize that the chassis should be designed for air suspension in preference to attachment of the units to a standard chassis, and they state that it should then be possible to reduce the cost of the vehicle. The suspension also offers a valuable saving in weight.
The advantage provided by a self-levelling system giving constant-periodicity spring characteristics for all loads is proportionate to the ratio of laden to unladen weight. Tippers may carry a load of double the unladen weight, but run empty for half their mileage life.
Self-levelling would be of particular benefit when the vehicle was being loaded on uneven ground, and the reduced maximum deflection of the axles, combined with a constant frame height, would enable the loading height to be reduced.
For site work, the near-side and off-side bellows could be interconnected (by incorporating valves in the control system). This would greatly enhance. the ability of the vehicle to traverse rough ground and would aid rearwheel traction at the expense of roll resistance. By fitting each bellows with a two-compartment expansion tank, equipped with an interconnecting control valve, a two-rate system could be provided.
A Joss of roll resistance, if the spring elements are fitted in the normal position, is a functional disadvantage of air-bellows suspension that may be overcome by locating the,units well outboard of the frame members at the rear (an even greater distance than that indicated in the illustration). Lack of roll resistance at the front is less important. Nevertheless, self-levelling would favour the use of independent front-wheel suspension, in that it
would virtually eliminate excessive scrubbing action of the tyres.
Many chassis designers would welcome independent suspension at the front because of the facility it would give for mounting the power unit. I have incorporated it in my tipper design to enable the underfloor engine of the forward-control chassis to be located across the front axle and to provide greater roll resistance.
Independent suspension for the rear axle should also be given serious consideration, as it would provide a means for mounting the gearbox in unit with the differential and final drive, and, if required, the engine could be incorporated in the same assembly. This would increase the rear-wheel loading and would be favoured for forward-control vehicles if the engine did not occupy space required for the tipping gear.
Damage to bodywork by the impact of dropped loads represents a severe financial loss to many contractors, and the use of light-alloy bodies is often regarded as undesirable on this account, despite the higher payload they provide. Reports from operators employing bodies of this type confirm, however, that a suitably reinforced structure can withstand severe impact loads.
A sectioned dumper body recently completed by G. E Neville and Sons, Ltd., Forest Road, Mansfield, Notts, for a Foden chassis (to carry a payload of 10 cu. yd. in place of the normal 9 cu. yd. by :virtue of the lower weight of the body) is equipped with full-length side capping rails mounted on rubber strip, and the pivot assembly also incorporates a rubber cushion. The capping rails are subjected to the highest impact loading of any part of the body, and the rails of the Neville body are designed for easy removal for repair or replacement. The sub-frame is of steel and multiple pads of large section support the body in the lowered position.
Figures supplied by the Northern Aluminium Co., Ltd., show that„ in the case of a typical oilengined 7-ton chassis equipped with a 6-cu.-yd. body, the total vehicle cost is increased from £1,950 to £2,080 when a light-alloy body is fitted. The gain in payload for an extra £130 is 10 cwt., and the annual tax is reduced by £.7 10s., so that the higher initial cost would appear to be justified.
The British Aluminium. Co., Ltd., substantiate the ability of a light-alloy body to withstand impact loading by pointing out that a 16-cu.-yd. all-aluminium dumper was employed 24 hours a day for a six-day week over a period of 15 months in the building of the Boulder Dam in America, and that it carried about 800,000 tons of rock loaded from excavators, rocks up to 7 tons often being dropped into the • body from a height of over 6 ft.
Stricter enforcement in this country of the weight regulations by Ministry of Transport examiners will enhance the value of a high legal payload. A high unladen weight penalizes the British operator in terms of payload, but this restriction may well be a spur to British designers to build vehicles that weigh half that of their foreign counterparts and carry a similar load with improved economy but without sacrifice of reliability.
Air cooling and air suspension, combined with lightweight transmission components driving through hubred uction units, may well be •the basic requirements upon which the success of the lightweight chassis will depend.