I have seen many examples of
trucks being loaded perfectly in two and a half or three and a half
passes. As I said in the last blog, for many mines the issue of matching
truck capacity to loader capacity is problematic and more often than not
results in a majority of trucks being under-loaded. As trucks and loading
units increase in size the number of passes required to fill the truck is
decreasing and the difficulty in attaining the match is becoming more
difficult.
Mines generally use one of five
methods for selecting equipment size/capacity.
1. Allow the supplier to decide. Suppliers love this
because they can sell the mine the same as someone else has received which cuts
down their costs significantly. However, if the mine abrogates their
responsibility to run their mine they get what they deserve. Remember back
last year when I discussed the 62.7 CuM rope shovel. The calculation had
fill factors and all sorts of multipliers to arrive at the correct
answer. However, you don’t need to be as cynical as me to be struck by
the fact that it was exactly the same dipper being used on exactly the same
make and model shovel at a mine about 150km away. Were they digging the
same spoil? No. Were they using the same bench heights? No. Surely
they were at least loading the same trucks? No. A completely different
operation and yet (quite by chance?) the supplier came up with the same dipper
as being the right size. Mining with a computer is really easy but it rarely
provides the answer which will help the mine optimise what they are
doing. Understand this – if you allow the supplier to specify the size of
the equipment you will get the capacity which is best for their profit, not
yours. It saves them much design, engineering and fabrication cost if a
supplier can simply sell you the same capacity that someone else has.
A
quick example from the coal mines on suppliers providing the same product when
something different was needed. A mine ordered a dragline bucket from the
dominant supplier. In this case the supplier has about 75% market share
and the mine was justified in choosing them. After doing some computer
mining the bucket supplier arrived at 57 CuM capacity. Once it went to
work the mine was very unhappy with its performance as the average payload was
about eight tonnes below what they previously achieved and the operators were
complaining about it not digging. We were called in to investigate.
We found the geometry of the bucket was not matched to the geometry of the pit
being dug. I found the exact same bucket had been built for another mine
about 9 months earlier and they were very happy with it. This operation
had an average pit depth of 50 metres and the design matched perfectly.
The second 57 CuM bucket was exactly the same as the first but the
digging depth rarely exceeded 20 metres. End result – the mine lost substantial
production and potential profitability. Anyway, back to the other methods
of selecting equipment capacity.
2. Guess. There are a number of
forms which this takes. Most people in the selection process will create
the “truck-loader” matching spreadsheet but will make a number of guesses about
key factors on density, fill factors, etc. Often this process is aimed at
justifying a particular capacity to management.
3. Existing Data. This is an extension on
guessing. Data is collected on existing performance and this is
extrapolated to new equipment. This is certainly a quantum leap up from
options 1 and 2 but can fall down when data is sketchy or non-existent or when
different equipment is ordered.
4. Computer modelling. This is an extension on point
1. Some suppliers have flow models for simulating material flow into
their equipment but while being good for research and development, they are of
minimal value for commercial decision-making. This is due to the models
not being far enough advanced to simulate specific spoil (as opposed to generic
spoils). Now I might get howls of opposition from highly intelligent
researchers but I have never seen one good enough for commercial
decision-making.
5. Physical Modelling. In 1977, D.J. Schuring,
released “Scale Models in Engineering: Fundamentals and Applications”, Pergamon
Press, New York, N. Y. In this book, he devoted a section to earthmoving
in general, (eg. Bulldozers, excavators, etc), in which he confirmed the
accuracy of physical modelling in earthmoving applications. Scale models
have been used successfully on dragline buckets and rigging since 1985.
Similar techniques have been applied to rope shovels since 2000, truck bodies
since 2002 and excavators since 2005. Schuring (1977) found that the key
to accurate results from scale models in earthmoving was that the behaviour of
the spoil was accurately simulated.
In my next blog I will carry
this discussion on and look at the flawed standard being used to determine
truck nominal capacity.
Graham Lumley
BE(Min)Hons, MBA, DBA, FAUSIMM(CP), MMICA, MAICD, RPEQ
