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DESIGN PARAMETERS FOR DEMOUNTABLE CONTAINER VEHICLES
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BY P. A. C. BROCKINGTON
AMI.Mech.E ILST a number of users of self-unloading, demountable container equipment transport materials of a given density and consistency, the range of materials carried overall covers widely differing densities; this also applies to many operators individually. Consequently, the maker of the equipment is faced with the difficulty of catering for manifold requirements, and at the same time of standardizing to the greatest possible extent to reduce production costs and to offer a good service in terms of the ready availability of spare containers.
According to the technical staff of Sheppard Fabrications Ltd., of Barnham, nr. Thetford, Norfolk the density of the more common materials varies from 3 to 4 cwt. per cu. yd. in the case of household and flat refuse up to 30 cwt, per cu. yd. for heavier types of metal scrap and waste chemical product. The lighter variants of these have a density of some 10 cwt. per cu. yd. in each case.
Industrial refuse varies in density from that of household refuse up to 15 cwt. per cu. yd., cullet (broken glass) from 15 to 21 cwt. per cu. yd., and materials derived from earth-moving and demolition work from 18 to 25 cwt. per cu. yd.
Obnoxious chemicals represent a disposal problem that promises to become increasingly difficult of solution and to necessitate greater use of demountable containers. Dumping in the North Sea and, more recently, in the Bay of Biscay has had to be discontinued and the transport of chemicals to processing plants, and later to suitable dumping sites after they have been neutralized, will gain in importance.
The problem of dumping generally has been exacerbated by anti-water-pollution regulations, and transport by road to distant sites is frequently the only alternative to discharging locally into a water course. The handling of obnoxious chemicals has to be strictly methodical to obviate splashing, and this necessitates avoidance of tilting.
Other developments and trends cited by Sheppard that will promote a growth of container handling include the proliferation of packaging, the decline of open fires in domestic premises in which part of the householder's refuse is normally burnt, the spread of controlled tipping as a result of the litter act, the closing down of inefficient refuse incinerators in compliance with the smoke abatement act and the limited latitude allowed by law before sand and gravel pits have to be back-filled.
To Sheppard, the many operational variables and mixed prospects are taken as a challenge to their ingenuity in providing matched chassis-equipment combinations for all industries that offer the optimum payload whilst making use of standard ranges of hydraulic units and rationalized containers. This requires, inter alia, that the angle of tip is suitable for discharging sticky materials, such as clay and a variety of chemicals. Also that the
tipping action is positive to cope with materials which tend to "bridge" across the container and others such as scrap iron, the discharge of which may be erratic.
It is also necessary to design the equipment so that the container discharges as far to the rear as can be arranged to minimize the possibility of damaging the chassis and to allow the vehicle to stand on firmer ground when material is being discharged over the edge of a tip.
To meet the need of a substantial number of operators, a flat mounting platform is provided that can be used for direct loading when the container is not being carried. All the standard containers are symmetrical (in both planes) and can therefore be picked up from either end by the hydraulically operated arms of the machine. A pallet can be used in place of a container if required, and non-standard units include tanks as well as special containers of various types. A large number of standard containers of all types is stocked At the company's depot.
Details of the ranges of Sheppard-Meiller equipment include hydraulic lifting units of five sizes—namely, the 4000, the 6000, the 8000, the 10/12000 and the 14000, which have approximate capacities in kilogrammes corresponding to the type number. The maximum capacities are, respectively: 4 tons 12 cwt., 7 tons 3 cwt., 8 tons 1 cwt., 11 tons 17 cwt., and 20 tons 9 cwt. The units cater for respective g.v.w. up to 8 tons 5 cwt., 12 tons 13 cwt., 15 tons 1 cwt., 20 tons and 30 tons 19 cwt.
In practice, the useful capacity varies according to the length of the arms and the location of the pivot points relative to the arms, which depend upon chassis-equipment matching requirements. In turn the arm length is determined by the need to position the container on the platform to give a centre of load that complies with the chassis maker's g.v.w. rating and axle loadings.
Also variable is the distance between the hydraulic stabilizing jacks and the rear shackles of the rear springs, the container being tipped about a line joining the two shackles. Normally the jacks are automatically retracted when the hydraulically operated hooks are raised to engage the cross-pins of the container prior to tipping, but an overriding control can be used to prevent retraction. The jacks are designed to fold forwards to enable the ground clearance to be increased for hperations on rough ground.
Apart from the use of heavy-duty springs of the type fitted to tipping vehicles, chassis modifications are not normally necessary, but in some cases the chassis members are truncated at the rear to provide for container lowering immediately behind the vehicle.
Imported from the German Meiller company, the no-loss hydraulic systems are rated to operate at 3,000 p.s.i., the pump being of the six-cylinder, horizontally-opposed type driven by a carden shaft from the gearbox p.t.o. Compared with rams operat ing at a pressure of, say, 1,500 p.s.i.. the Meiller rams are of rela tively small diameter and this advantage is exploited to increase the platform width by approximately 4 in. Employing a multi cylinder pump obviates surge in the delivery line, whilst the use of check valves prevents creeping when the control-valve lever is neutral.
The standard range of Sheppard containers comprises open units of 5, 6, 7, 8, 10 and 12 cu. yd. capacity, closed units of 8 and 10 cu. yd. and a drop-ended type of 6 cu. yd. For bulk loads, containers with capacities up to 16 cu. yd. can be accommodated by fitting a fly-boom extension to the beave blocks of the lifting arms in place of the connecting tube which allows the height of the container to be increased.
Given that the g.v.w. of the chassis are suitable, standard containers are interchangeable, and in a typical case this enables the operator to employ containers of differing capacity in conjunction with a single motive unit; this is desirable because of the large variation in the densities of the materials carried.
It is pertinent that the number of hauliers who cater under contract for a cross-section of industry is increasing, and to such operators interchangeability is particularly valuable.
The bed frame is of all-welded fabrication and is based on two L-section longitudinals which are interlaced with channel-section cross-members, the ram side plates being welded to the ends of the members. The method of mounting the bed frame enables the frame and chassis members to flex individually and thus obviates high local stress concentrations.
T-shaped mounting brackets are employed, the lower flanges of which are bolted to the chassis members, the weight of the load being carried by pads sandwiched between the bed and chassis. The pipework attached to the outside of the frame longitudinals is readily accessible, and its reasonable proximity to the neutral axis of the frame reduces flexing strain. The British-made flexible hose connections are of 0.5 in. bore and have a pressure rating . of 4,000 p.s.i. Two men can remove the bed frame from a vehicle in about two hours.
The latest model to be equipped in the Sheppard workshops, a Commer CCGW841, is rated at 28,000 lb. g.v.w. (12 tons 10 cwt.) and the load lifted by the 6,000 system is 6 tons 10.5 cwt.
Carrying a 12 cu. yd. container, the vehicle has a payload of 5 tons 14.5 cwt., the weight of the container being 16 cwt. When fully laden, the front axle carries a load of approximately 3 tons 17 cwt., whilst the load on the rear axle is 8 tons 12 cwt. Unladen, the front axle carries 2 tons 13 cwt. and the rear axle about 3 tons 6.75 cwt. The wheelbase of the vehicle is 141 in.
This vehicle is one• of three Commer-based demountable containers employed by A. King and Co. Ltd., Hall Road, Norwich, who are scrap-metal dealers and steel stockists, the remaining two vehicles being 26,516 lb. g.v.w. machines.
Two of the vehicles are employed in conjunction with 25 Norwich-based containers, four in Ipswich and one in Spalding (the remaining vehicle operates from another depot) and collect scrap of varying types and densities in 6 cwt., 10 cwt. and 12 cwt. open skips. pre-loaded at customers' premises.
After cutting-up or processing, selected scrap is supplied to the same or other customers, empty units being exchanged for full ones or vice versa. Additional containers will be acquired in due course and it is probable that the system will be extended in the not-too-distant future with the use of larger multi-wheeled vehicles for operations over longer distances.
Compared with collections .and deliveries in articulated outfits which are loaded or unloaded on customers' premises, one demountable-container motive unit does the work of three machines in the Norwich area, and in comparison with smaller 6-ton artics that, like the containers, are left at premises for filling "as required", the saving on maintenance costs is very substantial.
The cost of maintaining one of these small artics is frequently as much as £50 a month, which is partly attributable to their susceptibility to damage and partly to the punishment the tyres receive at the depot. In contrast, the lower-cost containers stand up to abuse and there is no running gear to go wrong. Moreover, they take up less space and the system is greatly preferred by customers.
The larger artics may obstruct work at a factory or foundry not only by the space they occupy but by the time taken to manoeuvre in confined spaces and by the need to release labour for loading or unloading, which can occupy up to four hours. About 150 tons of scrap are moved in containers by one prime mover in a five-day week and the target is 250 tons. On average, eight miles is covered on a round trip which is completed in one to one and a half hours.
Accompanying the driver on a trip in the Norwich area, the author covered just over five miles on a route through the centre of the city and called at the premises of two customers. After delivering a 6-ton load of railway-line chairs at a foundry, the empty container was exchanged for a full load of off-cuts at an electrical engineering works and the vehicle was back on the depot site in 1 hr. 4 min., despite a total of nearly 20 min. being spent by the driver in weighbridge offices and so on.
After weighing and checking, the container was finally offloaded at Hall Road about half an hour after returning to the depot. Containers are returned and collected on average two to three times a week following a phone call from the customer.
Although a distance of about 40 miles is considered the maximum for the economic operation of container vehicles in the 12-ton-gross category carrying low-grade scrap, a longer run may be justified for "special reasons" and, as mentioned earlier, larger vehicles would have a greater mileage latitude. Moreover, the standard vehicles are ideal for the carriage of heavy unit loads such as transformers, and in this case distance is virtually no object.
Prospective plans include the use of demountable containers for conveying scrap to a quay in Norwich for export in preference to employing a hired artic for the job. The container would be loaded the previous night at the depot and 12 or more would be ready for tipping into the ship's hold by crane at 7 a.m. Because it is hired, it is not feasible to use the artic for overnight loading.