WANTED Tippers Designec SITE WORK
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A Midlands Civil Engineering Contractor Gives His Views on the ideal Vehicle. for Off-the-road Operations: Design Problems Recognized by
P. A. C. Brockington,
A.M.I. Mech. E.
TIPPERS in the 5-7-ton class are probably the most hard-worked vehicles in this or any other country, and manufacturers are faced with the, difficult problem of striking an acceptable balance between first cost, toughness and carrying capacity. Whereas the requirements of most hauliers operating on regular runs are not subject to wide variations, the conditions governing the use of tippers are relatively more diverse, and a chassis that is suitable for one type of job may be unreliable and costly to operate on other work.
Could a more favourable balance be achieved? In the opinion of Mr. R. I. Atkinson, the plant manager of the Turriff Construction Corporation, Ltd., civil engineering contractors, of Warwick, the attempt must be made, but he cannot envisage an ideal road vehicle that would also be ideal for rough site work. Manufacturers have no alternative, he suggests, to building two types of light tipper, a road model and a site model.
After obtaining details of fleet operations on a recent visit to the main Turriff depot in Warwick I had a long discussion with Mr. Atkinson on the design of tippers in this category. Later Mr. L. G. Wildey, of the Eagle Engineering Co., Ltd., gave the opinion of a specialist bodybuilder on some of the comments made. The Eagle company are producing scow-ended bodies and lifting gear for a number of the Turriff 7-tonners.
Of the 136 -commercial vehicles in the Turriff fleet, 41 arc Thames tippers of 4-5-ton capacity •and 11 are B.M.C. 7-tonners. Like many other contractors, the company employ mainly short-wheelbase vehicles because relatively small tippers are necessary on most of the sites to operate in conjunction with excavators in confined spaces.
The great value of high manceuvrability is shown by the inclusion in the fleet of 36 Ferguson industrial oilengined tractors, which haul 3-ton tipping trailers. These are employed both on the movement of material from the excavator to the dumping ground and for the carriage of plant on the sites and sometimes on the roads. Extra trailers are used on a number of sites where the rate of filling is slow and the tractor can deliver a load in one trailer while the second trailer is being filled. The low floor height of the trailers facilitates hand loading. Current operations include work at some 60 sites in all parts of the country, including South Wales, the Midlands and London areas, Oxfordshire and the south of .England, Newcastle upon Tyne and a number of
centres in south Yorkshfre. Concrete-mixing plants have been established at Ryton-on-Dunsmore and Bedford, near Coventry, and in Birmingham.
Heavier vehicles in the fleet include a Foden 20tanner, five A.E.C. and Maudslay 10-tanners or 15-tonners and three Bedford low-loaders. The remainder comprises 5-cwt. to 30-cwt. vans of Austin and Ford manufacture. The Foden chassis and a Maudslay, together with A.E.C. and B.MC. chassis are employed to carry truck-mixers of 21-cu.-yd. to 61-.-Cu.-yd. capacity, the total number of mixers being 18.
Mr. Atkinson pointed out that the efficiency of site work was largely dependent upon the continuity of excavator operations and that a tipper breakdown could have a serious effect on the total output. The first cost of the tipper is normally, therefore, secondary to reliability and ease of repair, and this is indicated by an analysis of the yearly cost of breakdowns debited to vehicles individually. In some cases the loss associated with one vehicle in 12 months is greatly in excess of the first cost.
If two chassis were produced, one for road work and a second for site operations, it would not be necessary, said Mr. Atkinson, to apply the policy of "building down to a cost" to the manufacture of the site model, but he admitted that the designers would still be faced with a vicious-circle problem, particularly with regard to the transmission. 4
Site conditions are often so severe that they cause shock loading of the transmission components, and designing for the worst conditions is almost impossible. To attempt it might involve such an increase in weight that a higher power would be required for propulsion. This could necessitate a heavier chassis and larger tyres, and shock loading would be aggravated.
The provision of a device for safeguarding'. the transmission from severe 'Shock loading is, in Mr. Atkinson's opinion, a development to which manufaCturers should give their objective consideration. He disagrees with Mr. G..Braithwaite, a director of Latham Haulage, Ltd., that it would be practicable to fit a coupling having a shear-pin. This view receives support' from Mr. Wildey, who has employed such a coupling experimentally in an attempt to safeguard the mechanism of tar spreaders.
The shear characteristics. of the pins of various types of material showed large variations and eventually a
face-cam type of slipping clutch was fitted, in preference to the shear-pin coupling, and this has proved highly successful. The clutch can be 'adjusted to slip at any predetermined torque, but is not favoured by Mr. Wildey for incorporation in vehicle transmissions on the score that it would not eliminate inertia loading.
A more urgent requirement is, in his opinion, a limited-slip differential, which, in addition to giving improved adhesion on slippery ground, would prevent ono wheel spinning freely at a high speed. The most severe shock loading can be caused by a fast-rotating wheel striking a hard roughsurface.
Judging by talks with tipper operators, this type of differential would be welcomed enthusiastically by tipper users throughout the country. It is significant that at least one gear manufacturer is contemplating the production of such a unit.
Mr. Atkinson asks why the fluid coupling or torque converter has never been applied to tipping vehicles. In normal times a relatively small increase in fuel consumption would not be important, as it would represent a negligible cost increment, but the gains derived from fluid transmission might be substantial. These could include reduced transmission troubles (particularly gearbox and back-axle failures), easier handling and a higher working speed.
The potential virtues of a fully automatic transmission are endorsed by Mr. Atkinson, the ideal transmission characteristic in his Opinion being one that approaches that of a steam engine, which provides a high static ' torque but operates in such a way that unpredictable inertia loadings are avoided. He agrees that if a gas-turbine unit of the two-axle type could be produced at an acceptable cost it would be operationally ideal as a unit for tippers on site work.
In practice the elimination of the clutch would obviate inertia loading (apart from that associated with a spinning rear wheel) and it might be possible to reduce the stress safety -factor in the design of the rear axle with a resultant gain in weight-saving and cost.
Dealing with Mr. Atkinson's recommendations with regard to those measures which an operator can apply to existing types of tipper chassis, it is appropriate to outline the modifications which have been made to a number of B.M.C. 7-tonners that have given excellent service in the past 12 months. The decision was made before the British Motor Corporation introduced the short-wheelbase version of the 7-tonner at the 1956 Commercial Motor Show to reduce the chassis length by 2 ft. and the wheelbase from the original 12 ft. 6 in. to 10 ft. In addition to this modification, which was carried out by Baico, 9.20-in. by 20-in. 12-ply tyres were fitted in place of 8.50-in. by 20-in. tyres, and the chassis was equipped with heavy-duty springs.
Choice of Lifting, Gear
The lifting gear is an Anthony Hoist single-ram slant type, which Mr. Atkinson prefers to other types on account of its high lift, speed in operation, long wearing properties and reliability. It also provides adequate support for the body and does not reduce the ground clearance.
The conventional steel body has scooped sides, which tend to prevent the adhesion of sticky materials (a high lift.is a valuable contributory factor) and the tailboard is mounted 9 in. above the sides to increase the clearance when material is being discharged and thus reduce the incidence of blockage by large lumps. The ram is mounted on a sub-frame extending the full cl0 length of the chassis, and this distributes the load and prevents local stressing of the chassis frame. The body has a capacity of 6 cu. yd.
Features of the chassis which have promoted ease of operation on site work include an Eaton two-speed axle and power steering. The axle is electrically controlled and the virtual elimination of delay when making a change of ratio is a particular benefit when travelling over bad surfaces and on gradients, because it promotes continuity of traction.
Power steeringreduces driver fatigue and the time taken to manceuvre the vehicle in confined spaces. Repeated damage to tailboards led to the decision to equip new B.M.C. short-wheelbase 7-tonners with bodies having 20° scow ends, which are being produced by Eagle.
To discharge the material from the body it will be necessary to fit a ram giving an angle of tip of at least 55', and although on full tip the centre of gravity of the body is some inches on the inside of the pivot points, gravity return of the body would not be certain under some conditions. The Eagle slant-type single rain will, therefore, be equipped internally with a coil spring, which will be compressed during the last few inches of ram movement and will return the body the requisite amount to ensure further lowering by gravity. Steel coil springs are used as an alternative to Aeon hollowrubber springs with which some municipal tipping vehicles have been successfully equipped.
Improved Suspension Required Although the conventional type of leaf-spring suspension is satisfactory both with regard to performance and cost, an improved type of suspension would be welcomed if it reduced replacements and gave better adhesion. Mr. Atkinson recommends that site tippers should be designed with single rear wheels to give flotation on soft or slippery surfaces, which would necessitate tyre sizes being increased to 12.00 in. by 20 in., as well as a modification of the chassis frame, unless a forward-sloping frame were acceptable. Single wheels would have the added advantage of eliminating tyre damage by stones becoming lodged between the two covers.
Mr. Atkinson does not consider that the plate type of guard could satisfactorily prevent the entry and retention of stones and suggests that steel wire could be employed as an alternative. The wire could be stretched between the wheels and a pulley mounted on the chassis in front of the axle. The flexibility of the wire would ensure that stones entering the gap were immediately flung clear.
The tippers average about 30,000 miles a year and are normally overhauled completely twice in that period, for which they are returned to the depot in Warwick. It is not normally necessary to strip the engine it 15,000mile intervals, the main repairs being those to the chassis components and the body.
Most of the tippers on site work are handled by local drivers who take little pride in the condition of their vehicles. A contract is given to a local garage to inspect the trucks and perform any necessary routine repairs, for which half a .day a week is allocated.
Although such frequent inspections are costly, both in terms of repair bills and lost time. experience has shown that the practice greatly reduces running costs. Admittedly they cause impatience among the drivers, who are paid a bonus based on vehicle output, but they. tend to inctilca a greater sense of responsibility. The driver is present in the garage during the inspection.