Over the last few weeks I have
systematically pulled apart the issue of nominal truck capacities to
demonstrate why big mining trucks achieve 5-15% below what the manufacturer
says they should get on average. I don’t believe this is an issue that
too many truck manufacturers’ want to address and the cynical side of me
suggests that this article won’t help. Maybe a single voice in the
wilderness can gain support to force change.
My focus is on mines moving
more for less and apart from the engineering design work to increase the
capacity of trucks from the 150 tonne maximum size 25 years ago to the 360
tonne maximum size now I don’t think that the truck suppliers have helped the
“move more for less” equation too much. Even the notion of bigger trucks
being a great innovation and assistance in efficiency enhancement is
questionable. I will repeat something from a previous blog. On the
whole bigger trucks are less efficient than smaller trucks. They carry
less payload (as a percentage of nominal capacity) and work less hours.
However, this is not a consistent picture between OEM’s. In terms of
nominal capacity the 360 ton trucks are 50% bigger than a 240 ton truck.
however, in terms of actual annual capacity, average 360 ton trucks move just
20% more than 240 ton trucks. I am not pointing the finger at one
supplier.
Figure 1 shows the 2010 median
performance for each major mining truck make and model. Some of the older
and newer models are not included due to lack of data. Mining truck
performance is presented in this analysis as annual tonnes (normalised for full
year operation) * km travelled per tonne of nominal tray carrying capacity.
Trucks with different
designations (usually A, B, etc used by Cat and Liebherr) have not been
separated in this analysis. The capacities for these “sub-models” are
generally similar as is the output. It is important to note that
this plot does not attempt to say whether the make and model results actually
reflect better trucks or the operating characteristics of the sites at which
they are used. The trends with increasing size of mining trucks are
mixed. The Liebherr trucks become more efficient with increasing size
while the Cat trucks become less efficient with increasing size. The
Hitachi, Komatsu and Terex trucks achieve peak efficiency with the 240 ton (218
metric tonne) capacity size EH4500, 830E and 4400 respectively. The
larger capacity trucks are not as efficient with these OEM’s. Of the
larger trucks the Liebherr T282 is the highest performer with Terex and Komatsu
both achieving 20% less annual tkm/t and Cat 23% less annual tkm/t. It is
not without precedent for larger equipment to have lower unit production (ie.
draglines) however, the exceptional performance of the Liebherr T282 range
demonstrates that this is not a necessary outcome. Another clear finding
from this plot is that the performance of the smaller Cat trucks (777 and 785)
was, and continues to be, relatively high. They however, are not suitable
for loading with the larger loaders.
This industry has lived in a
world where bigger is better. But frequently when bigger equipment is
released it just doesn’t perform well. Those of us who remember the
release of 240 ton trucks would remember that they had real problems. It
seems too easy for a poorly performing mine to just get bigger equipment and
that is what they tend to do. They waste more millions of dollars when
the improvements they need are available by just operating more efficiently and
would actually cost very little.
To demonstrate this point I
will set up a scenario of a PC8000 hydraulic shovel loading Cat793
trucks. These have not been chosen for any particular reason except it
should be a comfortable three pass match. The average PC8000 loader will
require 7.5 average Cat 793 trucks. Four crews plus spares plus trainees
(you should always have a pool of people training) probably means around 40
truck drivers. If a mine then goes and purchases Cat797 trucks the
typical method of determining number of trucks is to simply work out the
proportional capacity. New trucks = old trucks * 793 capacity / 797
capacity. Using this formula five new Cat797 trucks would be purchased
with the expectation that around 13 people would be saved along with reduced
running and maintenance costs. Unfortunately, this scenario is
fictitious. In the real world the PC8000 on average needs 5.8 * 797
trucks and only saves 9 people. Bigger trucks cost more to buy and more
to run, so how far ahead are you?
OK so returning to the real
point of this column; technology is progressing fast. We now know that
trucks are not carrying the nominal payloads. This has not gone unnoticed
by companies which make their way in the world by making equipment work
better. For the OEM the real money seems to be in the chassis and
tyres. Improvements in payload are coming from specialist tray
suppliers. Truck trays are no different to most other mining
equipment. What the equipment carries is made up of steel and payload and
the aim is to maximise the payload and minimise the steel while achieving
acceptable life. In the past with trucks this was a nothing equation
because OEM’s told the mine what payload the truck would carry. We now
know this was almost always wrong. Truck trays seem to be following where
the industry has been with draglines. Now Bucyrus and P&H build
draglines and shovels but CQMS currently build the most efficient dragline
buckets while VR Mining have the most efficient shovel dippers. In trucks
you have specialised truck tray manufacturers like DT HiLoad, Duratray, Esco,
Philippi-Hagenbach, Westech, etc. who seem to get it; the chassis is built to
carry a certain load and if you can reduce tonnes of steel and increase tonnes
of payload then the mine must be ahead.
It is my proposal that we must
here and now dispose of SAE Standard J-1363 for calculating truck capacity the
same way suppliers have disposed of the CIMA formula for dragline bucket
capacity. We must also stop rating trucks based on a nominal
payload. We should establish a rated capacity for the truck trays which
is struck capacity (contained capacity with no heaping according to computer
models) multiplied by a factor. With dragline buckets the factor is 0.9
which I have always disagreed with but everyone knows it and accepts it.
I believe the rated capacity of a truck tray should be equal to the struck
capacity, (factor = 1). In the same way that we have a Bucket Efficiency
Ratio for draglines and a Dipper Efficiency Ratio for shovels, which is payload
/ rated capacity, we need a Tray Efficiency Ratio (payload / rated capacity)
for trucks - TER. There is also a steel weight ratio (Tray Unit Weight
(TUW)), which is the weight of the tray divided by the rated capacity. The
formula for the optimum truck tray rated capacity is then;
OTC
= GVM – Chassis Wt
TER + TUW
Only then can we get the best
tray design with the right capacity to meet the gross vehicle mass. At
least then we will be covering Step 1 in the optimisation process; mines will
be selecting the right piece of gear.
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