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78 TYRE MANAGEMENT

I

n October 1985, Goldsworthy Mining,which operated the Goldsworthy and

Shay Gap iron-ore mines in thePilbara region of Western Australia,undertook a pilot ore-haulage pro-gramme of 20,000t over a 45km cyclefrom the Nimingarra pit to the Shay Gapcrusher using Wabco 110t haul trucks.

Two years later in November 1987,routine production began fromNimingarra. The ore was again hauled tothe Shay Gap crusher however, thistime over a shorter, realigned haul road(32km cycle) that eliminated most of thedips and curves of the road used in1985.

These hauls are among the longestever undertaken using rear-dump haultrucks of 100t or greater capacity, andtherefore provide an excellent opportu-

nity to examine the effect of the improvedhaul road conditions on haul-truckproductivity and tyre workload.

THE EFFECTS OF CHANGE

Improving the condition of a haul roadcan result in a substantial improvementin haul-truck productivity. It can alsoreduce tyre workload considerably,thereby reducing the incidence oflow-life tyre heat failures for mine sitesthat are operating haul-truck tyres close

to their workload limit.The work-shift average speed isdefined as the distance travelled by ahaul truck during a mine shift, divided bythe shift duration. For example, if a haultruck covers a total of 120km in aneight-hour shift, then its work-shiftaverage speed is 15km per hour.

Work-shift average speed is one ofthe two determinants of tyre workloador tonne kilometres per hour (TKPH); theother is tyre average load. Generally,work-shift average speed increases asthe haul-cycle distance increases. This is

because, on a longer haul, a truckspends more time travelling relative toits periods at rest while being loadedand while dumping its load.

Let us look at two scenarios: scenarioone is a short haul where the work-shiftaverage speed for a haul truck with anaverage payload of 100t operating on a4km cycle is 12km/h. Scenario two is along haul. If that same truck wereoperating on an 8km cycle, then its workshift average speed would typically behigher in the order of 16km/h.

For the 4km haul cycle, the productiv-ity of each haul truck is 2,400t during aneight-hour shift (12k/h x 8hr / 4km x

100t), while for the 8km haul cycle,productivity drops by 33% to 1,600t pertruck per shift (16k/h x 8hr / 8km x 100t).

Although the work-shift averagespeed in scenario two is higher byone-third, this has been more than offsetby a doubling of haulage distance,causing an overall reduction in truckproductivity. We can therefore concludethat a higher work-shift average speed istypically associated with longer hauldistances, with a resultant increase intyre workload (operational TKPH) and areduction in haul-truck productivity.

UNEXPECTED RESULTS

The 1985 pilot haulage at Nimingarrawas on a relatively poorly formed road

Rules of the road

Tony Cutler of OtracoInternational presents a casestudy from the Nimingarra minesite in Australia that examinesthe effect of good haul-roadconditions on truck productivityand tyre workload

Hauling at theCollahuasi

copper minein Chile

Improving

the

condition of

a haul road

can result in

a substan-

tial improve-

ment in

haul-truck

productivity

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July / August 2013www. .com

TYRE MANAGEMENT

that followed the contours of the terrain rising, dipping and curving betweenthe Nimingarra pit and the Shay Gapcrusher.

The road was extensively reworkedprior to the start of full productionhaulage in 1987. Cut and fill wasundertaken to straighten the road and toeliminate many rises and dips. Thisrealignment of the road shortened itslength from 22.5km to 16km areduction of 29%.

As we have seen, shorter hauldistances generally result in a reducedwork shift average speed which:

partially offsets the truck productivityincrease associated with the shorterhaul cycle, and

reduces tyre workload (operationalTKPH).

However, what occurred at Nimingarrawas quite different and unexpected. Thehighest work-shift average speedrecorded for the haul trucks operating atNimingarra in 1985 had been 22.5km/h

for four complete haulage cycles over aneight-hour shift. The average shift speednoted during the first month ofoperation, November 1987, on therealigned and shortened haul road was24km/h, which allowed six completehaulage cycles over an eight-hour shift.

Contrary to expectations, the shorterhaul cycle resulted in a higher work-shiftaverage speed, providing much higherproductivity than had been budgeted.

While the 29% reduction in hauldistance had been expected to result ina 3% drop in work-shift average speed

from 22.5km/h to 21.8km/h, it in factincreased by 6% to 24km/h. Work-shiftaverage speed on the new shortened,realigned road was 10% higher than

anticipated with an equivalent 10%increase in actual haul-truck productivitycompared with what had beenbudgeted.

MANAGING TYRE WORKLOAD

While the higher than expectedwork-shift average speed on theimproved Nimingarra haul roadproduced an equally unexpectedwindfall in productivity, it immediatelyset off alarm bells in relation to tyre

workload.The longest haul in the Shay Gap mine

proper at the time was from Sunset 7 pitto the crusher with a work shift averagespeed of 20.6km/h. A heat studyconducted in March 1987 had shownthat tyre temperatures on this haul wereapproaching maximum allowable levels.So a work-shift average speed of 24km/hon the new Nimingarra haul had the

potential to create a spate of expensive(in terms of tyre usage cost andtyre-related haul-truck downtime),low-life tyre heat separation failures.

Otraco immediately recommendedthat measures be implemented to

minimise the risk of tyre heat damage.These included:

tyre pressure and temperaturemonitoring at the end of each haulcycle, with maximum allowable limitsset for inflation pressure andtemperature build-up, and

swapping trucks between Nimingarraand the shorter Shay Gap hauls asrequired.

While high tyre air-chamber tempera-tures and inflation pressure build-upswere recorded over the duration of

Nimingarra haulage from November1987 regular chamber temperatures of90C and as much as 100C, andpressure build-ups of 30psi (2.07bar) andoccasionally considerably higher theincidence of heat-related tyre failureswas exceptionally low.

Based on theoretical TKPH calcula-tions, the operational TKPH, at awork-shift average speed of 24km/h,exceeded the tyre manufacturers TKPHratings by between 10% and 15%. Thetyres used on this haul should have beensuffering an exceedingly high incidence

of low-life heat-separation damage.However, they were not, even when thetrucks were on occasion left operatingon the long Nimingarra haul rather thanbeing swapped to shorter Shay Gaphauls.

This highlights a major shortcoming ofthe TKPH system of calculating tyreworkload in that it does not take intoaccount haul-road condition.

Truck tyressustain a heavyworkload on longmine haul routes

Inhospitableconditions anddusty roadsurfaces cancause heavywear on truck

tyres

While the

higher than

expected

work-shift

average

speed

on theimproved

Nimingarra

haul road

produced a

windfall in

productivity,

it set off

alarm bells

in relation

to tyreworkload

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82 TYRE MANAGEMENT

LESSONS LEARNTThe lesson from haulage at Nimingarra isthat improved haul-road conditions canlead to a substantial increase in a haultrucks productivity while simultaneouslyreducing the workload of its tyres.

There are four main elements tomaking haul roads more productive andtyre-friendly. They are:1. Reducing haul distance and unneces-

sary gradients, through:

horizontal curve minimisation; and

vertical curve minimisation.2. Recucing rolling resistance and

tyre/truck stress, through:

pothole and undulation elimination;

road sub-base material selection andcompaction; and

selection of appropriate road surfacematerial and smoothing.

3. Reducing tyre/truck lateral stress,through:

road curve radius optimisation;

road elevation optimisation; and

Improving theconditions andlayout of minehaul roads can

make a positivecontribution to

tyre efficiency

Maintaining the conditionof haul-truck tyres is critical

to the safe and efficientoperation of mines

road camber minimisation andcamber profile optimisation.

4. Reducing haulage bottlenecks,through:

road width optimisation.

The improvement in haul-roadconditions at Nimingarra between the1985 pilot haulage programme and1987 production haulage and thesignificant benefits gained in terms oftruck productivity and tyre workload were achieved mainly through the firstof these elements (reducing unnecessary

curves).However, all four elements were

incorporated into the upgradedNimingarra haul road, and they allcontributed to improved productivity andthe ability of tyres to sustain anoperational TKPH that significantlyexceeded their TKPH rating. Apart fromproductivity and tyre workload capability,overall tyre life and truck component lifewill also benefit substantially fromimproved haul-road conditions.

Overall

tyre life

and truck

component

life will also

benefit

substan-tially from

improved

haul-road

conditions