It’s almost a cliche- the photo of a giant tyre dwarfing the man standing beside it, at some remote mining site somewhere in Western Australia. They always seem to populate the financial pages, and mining magazines.
They’re big alright- the wheels are large enough to accommodate the electric drive motors in the hubs- a 52 inch wheel rim is small fry. The dump trucks can carry loads over 250 ton, and they’re not exactly light themselves. All this is carried by 6 giant tyres.
They are shipped into the country in empty ore carriers from overseas, generally Japan. However, due to the mining boom, there is a worldwide shortage, and relatively few manufacturers tooled up to produce these tyres for use on extraordinarily large and expensive equipment.
The tyres are expensive, but downtime on the machines they equip is even more so. So mining sites maintain their haul roads in excellent condition, so that rocks or spoil cannot damage the tyres.
Thirty years ago, retreading these tyres was practised extensively, but eventually died out because of the economics of transporting them to a central factory and return, when the new tyres were delivered practically on the doorstep of the mines.
However, due to the shortage of tyres, and the skyrocketing cost of natural rubber, the economics of retreading (sometimes called relugging) are being examined again.
Rebuilding the tyres involves removing what is left of the old tread, preparing a surface to which new rubber will bond, applying new rubber, and sculpting it to a tread design compatible with the original tread. For this, massive machinery is required, so the investment is not undertaken lightly.
30 or more years ago, lugs of rubber prepared with a sticky base were heated, and hammered onto the prepared surface. Rebuilding a tyre could take two days by this method. The system has been mechanised to some extent. A continuous strip of hot sticky rubber is spirally wrapped around the tyre to a controlled profile. The result looks like “corrugated rubber”, and at this stage has no tread pattern.
A tread pattern-cutting machine, massively strong, is then used to cut out the surplus rubber between the tread lugs. This excess can then be used again, representing quite a saving in material.
Finally, the rebuilt tyre is loaded into a giant autoclave, bigger in diameter than the rebuilt tyre.
When I left the industry in 1989, these were 130 inches in diameter, 12 feet deep, and required a very large steam boiler and air compressor to fill them, taking three tyres at a time. Curing time was in the order of 14-16 hours. Vulcanisation was completed over 24 hours as the tyre cooled down. In fact, the internal tyre temperature continued to climb as the tyre was removed from the autoclave.
These tyres are BIG! 33.00 x 52 was the biggest 30 years ago. Bet they are bigger now!
In terms of conservation of resources, retreading these tyres makes sense. After all there are only so many ways that discarded giant tyres can be used to define a farm front gate, line a mining road, or impress a tyre service’s customers. Reliability of performance is still the prime requisite as far as the mines are concerned. It only requires one lug to fall off, or get torn off, and the machine is out of action.
One of my earliest memories of these tyres was at the bottom of the Avon Gorge in W.A., where a deep gorge was being cut to accommodate the then new Trans Continental railway. The gorge was so deep that at 2 o’clock in the afternoon, it was dark at the bottom at the work site. The Euclid scrapers had experimental Michelin Radial tyres on them, the tyres having been developed for use in the Sahara desert. When empty, the scrapers used to charge back at 30 m.p.h. with the driver hanging on for dear life, firmly belted to his seat, clad only in shorts and work boots, as the contrivance bucked and kangarooed along the haul road. That’s when I learnt that the tyres were the whole of the suspension system.
Another experience was at Dampier, W.A., when the salt mine there wanted to shift 12 million tons of salt in a hurry, away from the adjacent iron ore stockpile. Triple trucks (i.e. one more trailer than a double, weighing 101 tonne empty, (aluminium) were loaded with 160 tonne of salt, carried it 10 kilometres, dumped, and returned empty. Tyre size involved was a comparatively small 14.00-24, but the problem was that it was 52 degrees, and the tyres never had a chance to cool down. So the haul speed had to be restricted, and this cost money. So did blown tyres! At 68 km/h the tyres performed O.K., at 73 km/h they became unreliable. Quite a lesson there in “make haste, less speed”.