Cutting Processes
It has been stated that a longwall advances by cutting
slices off the block. This is relatively simple for hand worked faces and with
a coal plough, but with mechanized longwalls using shearers the means of doing
this is not as straightforward as it would at first appear because of the
complexity of the equipment. To start with, the cutting machine has to cut into
the face after each slice is taken to line itself up to cut the next web. It is
incapable of cutting at right angles to the face, so has to be eased in at an
angle. This is achieved by "snaking" the AFC on which the shearer travels, so that the
cutter drum can cut a wedge shaped section of coal until the full depth of the
web is attained.
It is possible to cut a full web in one pass and to do
this in either direction, a process known as "Bi-directional or Bi-di cutting". An alternative is to cut in one direction only,
known as "Uni-directional or Uni-di cutting", often actually cutting only part of the web height
in one direction and the remainder in the reverse direction. There are other more complicated
processes involving only taking half the web width in one pass and the
remainder on the return.
At first glance Bi-di cutting would appear to be the
quickest way to advance and frequently this is the case. However there are
advantages with the other processes, relating to simplicity of operation,
"steering" the face equipment, effect on the coal haulage system, power
requirements on the shearer, location of operators in relation to dust sources,
etc which can result in better productivity overall. The best system will often
be different for different mines, particularly in different seam thicknesses
and possibly even with different personnel.
If the full web is cut in one pass, more power for
cutting is required and the shearer will move slower than is possible if only part
of the seam is cut. This, and the simpler overall process, may allow Uni-di or
other processes to attain similar production levels to Bi-di.
If a shearer cuts coal at a given rate, the effect on
the haulage system will be different depending on the direction of cut –
when cutting towards the tailgate the coal is carried away from the shearer
while the shearer is moving away from the cut location; when cutting to the
maingate the shearer is travelling in the same direction as the cut coal and so
tends to load fresh coal on top of that already on the AFC. The total coal load
for each web cut is the same, but there are higher peak loads on part of the
AFC when cutting to the maingate. This variation can be evened-out with methods
which do not cut the full web in one pass.
There are two primary sources of dust on a longwall face,
the cutting machine and during support advance. With Uni-di operations it is
possible to keep operators on the intake side of these sources most of the
time, particularly with remote or automatic chock operation. With Bi-di cutting
this is not possible at all times.
Another aspect of the cutting process which requires
strict attention is the straightness of the face and its angle to the gate
roads. To begin with, the face equipment is a fixed overall length apart from a
small amount of play between items. If the face contains excessive curvature,
particularly in the plane of the seam but also to some extent perpendicular to
it, it could happen that the face end(s) will be within the longwall block and
short of the gate roads. If curvature of the face is too great it is also possible
for the pans to become locked and unable to be advanced.
Face alignment is maintained by checks with a string
line across the face. If out of alignment, a "straightening cut" is done
whereby the AFC pans are only partially advanced by different amounts up to the
string line and only a part web is cut across most of the face.
A face will seldom move in exactly the required
direction when advanced. If there is a dip across the face, the chocks and pans
will tend to move down dip all the time. If the snake is always in the one
direction the face will tend to move towards the face end where the snake
begins. If chock side shields only touch one adjacent chock, that chock will
tend to be pushed away from the one it is touching. If the face line is not
perpendicular to the gate roads, the face will tend to move towards the gate
which is lagging. All these factors can act together to tend to move the face
towards one gate or the other, this movement being known as "face creep". If it is allowed to go too far the maingate
equipment can run into one of the ribs and the tailgate end may either run into
the rib away from the face or will not reach the edge of the block.
The usual method of "steering" the face is to
intentionally cut it at an angle other than a right angle to the gate roads so
one end leads the other and the movement so caused counteracts the
unintentional movement which is occurring. The required angle is created by
cutting a "fly cut",
whereby the AFC is set up to a string line set to the required angle. A wedge
shaped web is thus cut. It may be necessary to take more than one fly cut to
achieve the desired angle.
Note that if too great an angle is present this can
have the same effect as curvature on the face where the equipment may not reach
the gate roads.
A further aspect of cutting relates to grading. The
need for this may arise from:
-
Presence of seam
discontinuities (faults)
-
Gate roads floors cut
below normal face cutting height for whatever reason
- Need to cut extra face
height (possibly for a number of reasons, but frequently when approaching the
end of a block)
Shearer drums have the ability to cut some distance
below the bottom of the AFC as well as above the normal roof height, so any
profile can be cut within these limits. Whenever grading is required however it
is necessary to keep in mind that changes must be made gradually. Longwalls
cannot handle sudden changes well in any direction. Grades along the face must
be within the limits of vertical movement between pans. In the direction of
face advance, large steps are to be avoided (the AFC may jam up against a large
step or chock bases may bridge across steps and have poor floor contact or the AFC
toe may dig into the floor, especially if soft, and be difficult to level off
again).
The position of the drum cutting the roof level can
usually be observed visually, especially if there is a good "marker bed" in the
strata (which may be the seam roof) or can be judged relative to the chock
canopy position. The floor drum may not be so easy as it cannot be readily
seen. The use of a measuring stick is a simple but effective means of checking.
Attempts have been made to control cutting height
automatically by sensing some strata level (eg seam roof) and maintaining a
constant height relative to this, but these attempts have not been entirely
successful
Another method of control involves carrying out a
manual shear during which a control computer "learns" the cut profile and will
then repeat the profile automatically. The "learning" has to be repeated
whenever a changed profile is needed for whatever reason.
When cutting with a shearer, an important aspect to be
kept in mind at all times is that the angle of the cutting drums is fixed
relative to the shearer body and the latter is governed by the alignment of the
AFC. If the AFC is tilted towards or away from the face, then so will be the
drums and to a position exaggerated by the distance they extend in front of the
AFC. If not controlled carefully it can become very difficult to guide a face
back to its desired position within a seam, especially if a face begins to dive
into a soft floor.
