EFFICIENT TYRE UTILIZATION AND COST SAVING:
Five axle lines with a total of twenty tyres transfer the vehicle weight onto the road. The axles are oscillating 10 degrees left and right which always compensate the load between the tyres. ETF suspension spring rate is softer than the average tyre, this minimize tyre deflection and prevents heat buildup. During empty hauls eight tyres are lifted in order to save costs. All the wheels are steering which is also a benefit for tyre wear. ETF trucks are equipped with a Central Tyre Inflation System. This CTIS keeps the exact tyre pressure under real-time conditions. In case of a flat tyre the axle is automatically lifted to prevent a total loss of the flat tyre.
Wheel alignment and tyre scrubbing, how has ETF solved these issues?
In any on-road vehicle, be it a humble family saloon up to large class 8 road truck/trailer combination, it’s universally recognized that accurate alignment is essential to extended tire life, road handling, ease of operation, as well as achieving optimum fuel economy. When discussing mining trucks wheel alignment is hardly ever mentioned, while the cost of tyres often tops the list of haulage expenditure. Misalignment of front and rear axles due to road damage and or suspension/steering component wear has a dramatic impact on operating outlay with the majority of costs attributed directly to high priced mining tyres. Studies have proven that 80% of haul trucks operating today have some axle misalignment.
Generally, in any moving vehicle both driven and steered axles need to roll in parallel with the chassis centre line, with adjustments provided by threaded links between axle lines. Scrubbing, the term for this acceleration of tyre, is caused by this misalignment and can also be attributed to the rear axles. But other than the problem of ‘torque steer’ experienced when a powered axle is forced out of this parallel alignment because of the failure of components that support it, or the very nature of its design, there is normally little or no rear axle adjustment available.
Tyre behaviour comparison between a conventional 240 ton large haul truck a ETF MT-240 truck. Straight forward drive generates no scrubbing.
In curves the dual tyres of conventional trucks are scrubbing on the total surface. Compared with the total surface on the road on six 40.00R57 and the twenty 24.00 R35 tyres of ETF trucks the difference in scrubbing is 62%. This generates heat and results in excessive tyre wear.
Difference in tyre size for conventional 240 ton large haul trucks and ETF's 240 ton mining truck.
With the mining industry demanding greater productivity and the resulting push for larger and larger payloads, manufacturers responded in the most cost effective way by simply ‘up-scaling’ smaller capacity versions of the same design, with no apparent consideration to addressing some fundamental operational issues associated with this misguided philosophy. One associated problem with enlarging capacity is the increasing width of the truck and the increasing length of the rear axle causing the distance between the duel rear wheels to expand. While modern conventional front steering systems incorporating two independent wheels/tyres across the axle line, were designed to take full account of the ‘Ackerman principal’ whereby in a left turn the inside tyre turns at a greater angle than the outer. Rear axles are always supported in parallel with the frame, so with this conventional approach driving in a straight line there’s no real issue. However, things change rapidly for the worse when negotiating a bend. While each front wheel happily follows the differing path, at the rear it’s a very different story promoted by the distances measured between each duel wheel set and the affect this has on tyre scrubbing and ultimately vehicle stability. For example, during a powered right hand turn as the inside right rear wheel enters the bend it’s forced to travel at the same ‘faster’ speed as the adjacent duel wheel.
The very same can be said of the outside duel wheel set, the outside wheel travels a greater distance than its adjacent wheel forcing it to turn faster, when simple geometry advocates that each individual tyre should in fact travel a different distance and therefore differing speed compared with the others on the same axle line. (Fig.2) The result felt by the operator would be vibration as each wheel shuffles for wheel speed causing some road surface damage. And even with the differential on a mechanical and torque control on modern electric drive trucks, both still induce understeer as the drive wheels tend to push the truck forward rather than around the curve causing unnecessary tyre scrub to both front and rear tyres.
When discussing a rigid truck that possesses both, all wheel drive and all wheel steer, clearly attributes describing the ultimate mining machine; attempting to maintain all those wheels and their respective steering angles during every second of every operating day would be mechanically challenging to set up and almost impossible to maintain. So, how could this ever be achieved today?
Well, considering the largest ETF truck (Fig 1) has 10 wheel groups, 8 of which supports two independently powered wheels.
Let’s just say that ETF has achieved the almost impossible by incorporating fast microprocessors, some leading edge hydraulics, advanced angle sensors and innovative controlling software. Each wheel angle is controlled independently of all others, that’s 20 independent geometrically correct circles traced in the ground offering the very best in handling with a turning circle to rival a rigid trucks half its length while at the same time almost totally eliminating tyre scrub. But it goes further than that, powered and steered front wheels reduces the tendency to understeer so powering and steering all axles provides the ultimate steering and driving capability of any mine truck on the market today. (Fig. 3)
Automatic wheel alignment
The steering control system automatically adjusts all wheel-alignment angles on-the-fly. On a conventional two axle vehicle the technical term used by the automotive industry is ‘four wheel alignment’ so when describing the ETF this must now be changed to ‘ten wheel alignment’ simply because as mentioned there are 10 independent duel tyre wheel groups that need to be correctly aligned for perfect straight line tracking. Compared with current vehicles the ETF trucks steering and oscillating axles, soft long travel suspension, lifting axles on empty hauls and the Central Tyre Inflation System equate to tyre costs savings in excess of 45 %.
Last axle line lifted during empty haulage
Automatic axle lifting after tyre problem is detected
Tyre change within 15 minutes!
ETF developed a fast tyre-change system. A wheel can be exchanged within 15 minutes. Technical availability will be higher because of this exceptional short tyre change.
Two wheel handlers are being used to change efficient a flat tyre
Damaged tyre is removed and the new tyre is mounted
Wheel change in 15 minutes; no competitor can offer this feature!!
Superb load-spread over all tyres
With ETF's axle configuration all the tyres bear the same load. This reduces the heat build-up in the tyre and also has an important impact on the construction of the roads.
No more landslides when the operator goes a little bit too far!!
Single versus Dual tyres
Oscillating versus Rigid axles
The oscillating axles in combination with the unique suspensions system prevents also spillage on the haul roads.
The economic importance of the correct tyre pressure:
Increased fuel consumption Reduction in service life
Source: WABCO / MICHELIN
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