Why do mines end up with trucks
which are not able to carry their nominated payload? What is the problem
with truck capacity? SAE Standard J-1363 is still used by most suppliers
of truck bodies to define the capacity. However, with the advent of
larger and larger trucks (and loaders) more sophistication is demanded of the
truck tray capacity. Many mines simply don’t (and most can’t) achieve the
truck’s nominal capacity on average without the addition of a door on the rear
and/or hungry boards. A calculation of the geometry shows that the field
volume can be 5-15% below the SAE rated volume. The main error in SAE
Standard J-1363 is that the capacity requires a 2:1 heap from all sides and 1:1
slope off the rear to the point where it intersects the top of the body
sides. The problems with this are;
1. There are virtually no
materials which will stack at 1:1.
2. To put the 2:1 heap on top of
the 1:1 at the rear is wrong. Some manufacturers will take the spoil off the
back at 2:1.
3. Spoil when dumped will form a
cone. Therefore the angular top of the truck body cannot be filled
completely.
These three points
are demonstrated in the accompanying figure 1 which is from Hagenbuch (2000).
Figure 1
4. The angle of repose is almost
never 2:1 (26.6o). The problem is magnified the larger the
angle is. Interestingly enough dragline engineers are taught that the
angle of repose is 37o. In reality it is rarely that
high. Most angles of repose are between 30 and 35o.
5. The angle towards the front is
almost always shallower than the angle at the rear and the angles on the
sides. The difference between front and rear is up to 7o. The
difference on the sides is not consistent and has been measured from -7o
to +6o compared with the rear angle, (Hagenbuch 2000).
The final difficulty then is the
determination of density of material in the truck. This is again broken
into three confounding variables;
- Different material has
different density.
- Different materials will have
different swells upon loading, which will often be different to that in the
dipper or bucket, and
- The operators loading technique
may alter the density in the truck.
As a further confounding issue,
the operators’ placement of spoil in the truck may reduce the effective
capacity due to loading on the axles. This is not covered in this blog
but is very important in the optimisation process.
When the five issues are
considered the actual volume can be 5-15% below the SAE J-1363 Standard.
Now I do need to say that there are a number of truck tray manufacturers in the
market who are doing this much smarter than others and are providing a more
accurate calculation of nominal capacity. However, if truck supplier X
says they will carry 291 tonnes in their tray and truck tray manufacturer YY
says that theirs will carry 285, guess which one most choose? The problem
is that the standard tray which comes with most 291 tonne (320 ton) trucks may
only carry 270 tonnes. Maybe the truck tray manufacturer YY can carry 285
tonnes but most of the time they won’t? Some do consistently carry what
they say they will, however, most truck suppliers know that their trays won’t
carry the nominal payload. The problem here is that unless you model it
you simply don’t know.
In the second figure I have put
a sample of truck makes and models and the payload they carry in “best
practice” operations. The trendline of average is also provided.
This clearly shows the reducing payload as a fraction of nominal load as
capacity increases. What this means is that you can’t expect to achieve the
nominal payload for any truck over a Cat785 size. You might get it but
more than likely you won’t. For a truck in the 327 tonne (360 ton) size,
you might get 20-30 (or more) tonnes lower payload than you expect for the
20,000+ times the truck is filled per annum. For a fleet of 8 trucks (and
you might need more as I will discuss in the next few weeks), this is 4M tonnes
of payload lost per annum. How is your mine plan looking? Scary
thought.
Figure 2
Reference
Hagenbuch, L.G. 2000, Adapting
the Off-Highway Truck Body Volumetric Process to Real World Conditions, SAE
Technical Paper Series No. 2000-01-2652, International Off-Highway &
Powerplant Congress & Exposition Milwaukee, Wisconsin September
11-13, 2000
No comments:
Post a Comment