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South Island Gold Mining
 Introduction:
In the gold mining industry north of Dunedin, MacRae's
contractors excavate entire hills to be processed in the search
for the precious metal. After processing, the landscape is
restored to its original form. Very large earth-moving equipment
is used, in particular the Caterpillar model 777 dump truck
which has a designed loading capacity of 85 tonnes. In practice,
this translates to a minimum load of 95 tonnes, achieved by
3 scoops of the massive diggers (one scoop equals the volume
of 3 normal truck loads). Fully laden, the Cat 777 is a 200
tonne fast-moving object - often driven by experienced female
drivers. The trucks are equipped with on-board computer systems
so mechanics can plug in laptop computers and monitor particle
gain and other vital parameters.
Problem: The life-span of a rear axle is about 13,000
hours which is equivalent to about 2 1/2 years work. The axles
are expensive to replace and difficult to obtain, so the question
was - could their life-span be increased ?
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 Solution:
A specialist metal spray contactor from Dunedin arcsprayed
the rear axle. Due to the nature of the process, the base
material of the axle was not placed under any risk of metallurgical
change during the process. After surface preparation, a bond
coat of Ni - Al - Moly was arcsprayed giving a tensile bond
strength of over 7,000 psi. This bond coat was followed by
a customised alloy of one wire of 420SS and one wire of Ni35,
Cr20, Fe45. The marriage of these two wires produced a hard,
wear resistant coating. After the arcspray was complete, the
axle coating was machined to a fine finish and close tolerance.
Results: The first Cat 777 rear axle was arcsprayed
one and a half years ago and has now done over 7,000 hours
work. 18 axles have been repaired to date, at a cost of $1,500
each and a turnaround time of 13 hours. No losses of coating
materials have been recorded on the oil monitoring systems.
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