This blog continues to
investigate the issue of why many trucks are being perfectly loaded in 2.5 or
3.5 passes. In this discussion I am looking at rope shovel capacity and
why we need so much steel to carry what is often a very poor payload.
How is it possible that best practice in
dipper performance provides a payload of 2.16 times capacity but the dominant
manufacturers provide dippers which only achieve around 1.70 times
capacity? This is more than 20% less payload for the same capacity and
around the same weight of steel. This rhetorical question actually has a
real answer. It is because the mines don’t care. So
long as it keeps going and is supported when it breaks then that is OK.
Many mines don’t even complain when the loader truck match is 2.5 or 3.5.
To someone who has worked in equipment productivity for over 20 years this is
really depressing.
Looking at some issues which
impact shovel payload. Firstly, dipper issues which the mine can have
some impact on. The tooth attack angle is really important. Payload
increases by around 0.5% per degree as the tooth attack angle is
increased. However, it is not possible to simply keep steepening the
tooth attack angle of the dipper due to the interaction between the heel and
the bank. Relative heel wear rises exponentially after about 65 degrees
tooth attack angle. By 70 degrees the heel wear is probably unacceptably
high. Many buckets are in the range 50-55o and are losing a
lot of payload.
The concept of Bail vs Bail-less is a function of where the hoist
connection is made to the dipper. The
connection of hoist ropes at the rear of the dipper increases payload.
Where the connection is 25% along the dipper the difference is -10% which is
significant.
The width : height : depth ratios
as well as teeth arrangements have an impact on payload but there is little
impact site people can have on these issues once you have the dipper so I won’t
expand on these issues here.
The other side of the payload
issue is operational issues. Many of these can be controlled by the
mine. What is being dug causes variation in average payload by up to 20%
in the same dipper. Herein lies a significant issue relating to truck/shovel
matches. It is possible that the same dipper, even on the same minesite,
can get differences in payload of 20% simply due to the spoil being dug. The key to higher
payload is the degree of fragmentation. The highest payloads are achieved
in spoil where there is a range of particle sizes; not all large and not all
small. The implication is that payload is significantly enhanced by good
blasting practices.
The power made available to the
operator has a major impact on payload. In harder digging, ie. blocky,
poorly shot, etc., increased power provides increased payload up to 120% of the
standard power level. In softer spoils the shovel dipper achieves higher
payloads at lower power levels. In summary, it is beneficial (in terms of
payload) to increase power to the maximum.
Bench height plays a major role
in determining payload. At any bench height greater than 30% of boom
point height a full payload can be achieved consistently. Similarly, the
distance from the face has a major impact on payload. The variation from
cycle to cycle is quite large but a consistent trend is seen for each digging
position. The first few digs have the loading unit very close to the
face. During these cycles the payloads are reduced possibly due to the
inefficient application of power to the trajectory of the dipper / bucket.
The payload increases as the face “moves” away from the shovel. Once the
dipper starts having trouble reaching the face the payload reduces quite
quickly. The decision about when to move the loader is not an easy
one to get right. Generally the operator will decide to move the loader
when they encounter difficulty in loading the truck in the designated number of
cycles. To optimise the productivity a range of factors need to be
considered, including, payload, fill time, another truck waiting, what the face
is like. As a general observation, if the loader is under-trucked, it
would appear prudent to move the loading unit frequently. If the shovel
is over-trucked it becomes a multi-dimensional equation as to when the most
efficient time to move is.
It became evident from a very
early stage in the work on shovels that on some loading equipment the
efficiency of the bucket / dipper was severely compromised by large voids
inside the dipper / bucket (Figure 1). These voids ranged from 5% inside
a backhoe bucket up to 25% inside rope shovel buckets. The impact of
these voids is included in the previously described impacts on payload.
Finally I would direct your
attention to Figure 2. This shows the variation in dipper payload for
P&H and Cat (previously Bucyrus), (both unidentified) and VR Mining
Dippers. I have spent my career helping mines be more productive and the
VR Mining dipper is the most efficient dipper design I am aware of. I am
aware there are maintenance, support and financial issues to purchasing a
dipper but speak to dipper manufacturers, not just the OEM, the next time you
want a dipper.
Just so you know: I worked for
VR Mining in 1997 and 1998; before they designed this dipper. GBI has had
a number of small consulting jobs from VR Mining over the last 10 years.
I had no input into the VR design. Neither I nor GBI receive anything
from anyone for the comments made here. They are simply my honest opinion
– the VR dipper is the best and the mines are costing themselves a bundle by
not looking at it. Even if the mines used this fact to put pressure on
P&H and Caterpillar to do better, the industry would benefit.
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