Heathfield dump truck
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By R. D. Cater, AMInstBE
PICTURES BY DICK ROSS
N the next 10 years development programmes for roads, power stations, factory and housing estates will escalate to a degree never known before in this country. Along with projects of this kind go the requirements for vast movements of spoil from sites. And as most of these are situated in locations which necessitate transportation over public roads, vehicles engaged in such operations must comply with C and U Regulations which excludes the use of all but a few of the purpose-built dump trucks.
Thus operators are invariably forced to utilize strengthened tippers which, while they do an excellent job, often cost quite a lot of money in terms of repairs and lost time when bogged down in soft ground.
There is also a trend in the quarry industry towards the use of dump trucks in place of the narrow gauge railways popular in the past.
Recognizing these points Heathfield Engineering Ltd., of Newton Abbot, Devonshire, has spent the last two years develop
ing the Heathfield DF22, a robust on/off road dump truck with a capacity of 8 cu.yd. and 10 tons which will adequately perform all duties found in these applications.
I tested the vehicle in an area around Newton Abbot, putting it through its paces in a granite quarry, a clay pit and on roads. This is the only vehicle I have tested with such fierce braking that it was impossible to complete a full programme of brake tests. A combination of extremely short wheelbase, high centre of gravity, rubber suspension and high adhesion between tyres and road created a situation such as I have never before experienced on a large vehicle. With maximum pressure applied through the pedal, the rear wheels came clear of the ground and the fronts locked alternately to make the machine uncontrollable and release of the brakes essential for safety.
Other than this one failure to satisfy my demands, the machine proved more than adequate throughout the rest of the test. It produced excellent work-productivity figures, good fuel consumption, adequate performance on the road and my efforts to bog it down were completely defeated.
The Heathfield employs seamless rolled high-tensile steel hollow rectangular-section side-members and has its axles located by an A-frame at the front unit and by three torque rods at the rear. The construction is extremely robust throughout with the transmission able to accept far more torque input than is in fact developed by the Perkins 6.354 power unit. In this way there is a vast transmission safety factor when working in bad conditions. The available power is adequate to propel the machine through the worst conceivable conditions but, come what may, insufficient to break any part of the transmission line, which comprises a David Brown 552 five-speed overdrive gearbox and Centrax hub-reduction axle.
Suspension is by Aeon double-convoluted rubber units which provide 4.2 Sin, of deflection and completely overcome the problem of spring breakage so prevalent in the tipping vehicle world. An idea of the complete adequacy of the springing units is given by the bump-load capacity at the rear axle—where four units are used—of 132,000lb, around 59 tons.
I found a considerable degree of wander on the steering when the vehicle was fitted with its standard tyre equipment. This I was told is largely due to the design of the steering box which is completely non-reversible and does not therefore transmit very much feel to the steering wheel. It allows the power steering to take over to some degree causing the driver to oversteer quite a bit until the extremely tender feel is acquired.
Power-assisted steering On the second day of the test the machine was fitted with high flotation tyre equipment which produced entirely different characteristics that were completely acceptable. Working off the road, oversteer is not noticed but the lack of kick from the steering gear is, making the task of handling the machine on rough ground much easier than would be the case with a reversible steering gear. Under all conditions met with, the power assistance to the steering proved completely adequate. This was particularly so when negotiating soft ground, only the slightest touch being needed to change direction.
On the road the machine produced an average sort of performance and, as it is not designed to do long journeys, I give full marks to the designer's choice of gear ratios. Acceleration is adequate while the vehicle gives a maximum speed of 43 mph with the axle ratio of 13.32 to 1 as in the test vehicle—when on standard tyres. With the high flotation tyre equipment the actual road speed goes up by 3 to 4 mph, but it would normally then be supplied with the lower differential ratio of 16.65 to 1—an alternative that is much more suitable to soft ground work.
Top gear acceleration, despite the fact that the gearbox has an overdrive of 0.756 to 1, is extremely good, although, as is to be expected, there is a considerable degree of grunting and growling going on while the speed creeps up from 10 to 20 mph. Fuel consumption tests were carried out on a good-class undulating secondary road and these also proved the wisdom of the designer's choice of ratios. At around 30 mph, which will represent the majority of the type of running the vehicle will encounter, the vehicle produced a consumption of 10 mpg at an average speed of 24.8 mph. At this speed it is exceptionally flexible and comfortable to drive and in my opinion would not become outrageously tiring. Full results of the fuel and acceleration tests can be seen in the panel.
In the Teign Valley granite quarry of the Roads Reconstruction Group at Trusham near Chudleigh, the machine showed its paces working over a 0.2 mile haul from rock face to crushing plant. A little over half of this distance comprised a gradient of 1 in 10 with several short stretches of 1 in 7 and a hump which I recorded as being 1 in 5. Two points showed up here as detrimental to the speed with which the vehicle covered the course. I found it impossible to change from first to second gear without the vehicle coming to a standstill and because of this was forced to drive far greater distances in first gear than were really necessary from the power point of view. The inclusion of a clutch stop would be a distinct advantage here as this would allow one to make slip changes without the vehicle stopping. The second point that showed up here was continuous front-end pitching after hitting a bump. And although the initial shock may not have been sufficient to slow one down, the subsequent build up in the pitching certainly did. I felt that the inclusion of some sort of dampers at the front axle would be the answer to this one and I was informed that, in fact, development tests were already being carried out with telescopic dampers. The actual discomfort to the driver is considerably reduced by the Bostrom sprung seat, and I do not think that I would have been able to keep the speed up as high as I did had this not been fitted.
Excellent fuel economy
An average cycle time of 9min 43sec was recorded for this operation and was made up in the following manner: Spot under shovel, 22.3sec, load (30RB face shovel) 3min 9sec, laden trip 2min 2Isec, spot on crusher and tip, 22.7sec and return to face lmin 24sec. As the average load carried on these tests was 10.1 tons the maximum productivity the machine could produce over the distance would be 60.6 tons per hour, or a comfortable 600 tons per 10-hour day. Fuel consumption proved to be 0.97 gal per hour, 0.28ga1 better than the makers had told me it would be. Despite a fairly gruelling climb, at no time did the vehicle appear to be pressed beyond its capability and considering that the axle ratio fitted was designed for road and site work and not quarrying, I feel that these results are excellent because they represent 0.016gal per ton moved.
On the 1 in 5 section of the haul I carried out stop and start tests and hand-brake tests. Standing starts were a simple proposition but I found it almost too easy to make the machine rear up on its back wheels. This was not through any fierceness in the clutch but rather because the short wheelbase causes the centre of gravity to come very close to the rear axle centre line on a 1 in 5 gradient. In fact the vehicle reaches its point of balance when evenly loaded on a gradient of 1 in 3. The spring parking-brakes proved capable of holding the vehicle with ease facing both up and down the 1 in 5 gradient but one point showed up when by this time the floor of the single-man cab had become covered with dust and grit. Each time I released the air pressure which holds off the spring units I was almost blinded by a cloud of dust thrown up by the blast of air coming from the exhaust port on the hand reaction valve. This I feel should have been piped away beneath the cab floor.
Between the two days of tests Heathfield Engineering changed the wheel and tyre equipment from standard type to high flotation tyres. The second morning saw us carrying out unladen fuel tests over the same route used for the laden tests on the previous day. At an average speed of exactly 30 mph the machine returned a fuel consumption of 13.9 mpg so an overall average for roadgoing of about 11.5 mpg should be easy to obtain.
The next part of the test had the following object: to search out the wort possible conditions and get the machine bogged down to such an extent that it had to be towed out.
We proceeded to the clay pit of Watts Blake Bearne at Preston Manor near Chudleigh where we were told that conditions were at their best. I shudder to think what they must look like at their
worst for the test was carried out only four days after the blizzard of December 9, and as I drove onto the site I was mentally con
gratulating myself on securing conditions with which no single
drive machine was likely to cope. As it turned out I could not have been more wrong, the Heathfield waded through thick glutinous blue clay on which photographer Dick Ross was having difficulty keeping his feet, and despite the fact that I finished up bulldozing my way through freshly tipped spoil nearly 3ft deep, I failed completely to bring the machine to a halt.
I found it possible to stop with a couple of tons of spoil banked
up in front of the vehicle and, as simple as moving off with a private car, literally bulldoze the heap to the level of the front axle with hardly so much as a sigh of relief when we were through. I drove around for 45 minutes in these conditions and although on several occasions I managed to lose steerage when the front wheels lost their directional grip, these were the only times I was forced to stop.
Reasonable payload One of the main aims in the design of the fleathfield was to produce a ruggedly effective machine which at the same time would be capable of carrying a reasonable payload when operating under C and U Regulations on the highways. To achieve this end much thought was given to constructional details of the main frame which, as I have already said, has hollow rectangular-section main rails. It also has only three fabricated cross-members welded into position. The main rails are of 4 s.w.g. material (10in. by 6in, section) and incorporate all the necessary bracketry.
The 8 cu,yd. scow-ended body is fabricated from +in. thick steel plate while the floor plate is of in. high-tensile steel plate. The body is tipped to an angle of 65deg by twin, double-acting single-stage hydraulic rams powered from a 16 g.p.m. pto-driven hydraulic pump. This unit runs continuously, being controlled by a four-position spool-type valve. The hollow centre cross-member is utilized to carry hydraulic oil for the tipping gear while the control valve is mounted directly on the reservoir in order to keep pipe runs to the minimum. The test vehicle scaled 6 tons lcwt and so at its plated weight of 14 tons would legally carry 7 ton 13cwt. Despite the fact that it is plated for 14 tons its axle ratings are 6 and 10 tons so the axle loading problem is not critical. As a result of this test, Heathfield is modifying the braking system. When this work has been carried out I will retest the braking performance.