Coal Cutting Machines This machine is now obsolete, however, it is studied for its historical importance. Coal cutting machines are used for providing an undercut, or overcut or shear cut in a coal seam. A coal cutting machine were used to make a groove (also called cut) 1m to 2,5 m in depth and 125 mm to 175 mm in height at a coal face. The cut was made across the whole width of a gallery in the case of bord and pillar headways and across the whole length of face in longwall working. A coal cutting machine was used in a colliery with the following objects: 1. To give a free face for blasting to result in an efficient blasting 2. To increase the coal output of a face. Advantages of a Coal Cutting Machines were:
Straight and systematic advance at the whole face Because of the systematic advance of the whole face, systematic roof supprots can be set to provide a uniform subsidence of the roof beds behind the moving face. This is important in longwall face. Dirt band or shale or other soft rock can be easily removed generally by giving a cut only in the dirt band.
Classification of Coal Cutting Machines The coal cutting machine were of four categories depending upon the manner by which the cutter sumps in and cuts the whole face:
Short wall CCM Long wall CCM Arc wall CCM Arc shearing CCM (for driving headings)
The CCM was classified depending on the position of the cut:
Under cutting machine that cuts at the floor level of the seam Middle cutting machine that cuts anywhere between floor of the seam and the roof of the seam Overcutting machine that cuts just below the roof of the seam
Construction The coal cutting machine had the following three major units assembled together by bolts: 1. Cutting Unit 2. Power Unit 3. Haulage Unit
Figure 1 Main units of CCM. 1: Drive for cutting units, 2: Motor Unit, 3: Haulage unit, 4: Jib, 5: Endless cutting chain with cutting picks Cutting Unit The cutting unit consisted of a Jib of 1.5 to 2.5 m long. Round the periphery of the jib was an endless chain with pick-boxes carrying coal cutting picks. The chain rested in the recess on the periphery of the jib and was driven by a sprocket wheel which receives its power from the motor through gear and clutches. The jib could be rigidly fixed in line with the body of the machine (in shortwall CCM) or may be able to move relative to the body of the machine nearly through 900 (in Longwall and Arc wall CCM). The chain had a series of pick boxes joined together by links and connection pins. Each pick box made a definite angle with the horizon and the cutter pick was fitted in the pick box by a set screw. In a chain all the picks making a definitie angle in the plane of the jib formed a line of picks and the number of ‘lines’ depended on the material to be cut. The harder the coal, the larger the number of line picks. The cutter picks were of 2 types:
The throw away type picks Tipped picks
The throw away type picks were of plain high carbon steel (0.6% Carbon). These had to be sharpened, hardened and tempered at the end of each shift in smithy shop for reuse. These were cheap and can be made in colliery workshop from the top portion of a rail or even from mild steel plates through the latter have less life and need frequent sharpening. A coal cutting machine driver carried with him one set of picks (usually 37 to 45) for each machine and one extra set, all sharpened at the beginning of the shift. He had to fix all the picks at the start of the work and take them out at the end of the shift. Extra picks were necessary to replace picks that get blunt. In course of time such picks become small and were discarded. Travel
Cutter Pick Pick Box
Pin
Link CUTTER CHAIN
Figure 2 Cutter chain of a CCM Picks with tungsten carbide tips were used in some collieries. They were costly but need not be taken out every shift for sharpening. They require sharpening after 50 to
100 cuts and as the tungsten carbide tip wears out the picks have to be discarded after 100 to 250 cuts depending on the hardness of the coal being mined. Reversible picks are also available. When such picks were worn out for one tip the pick was reversed to use the other tip. Cutter picks had to be fixed to a correct gauge in the pick box. During cutting the chain moved in only one direction with the cutter picks fixed. For reversing the direction of cutting, the picks had to be taken out and fitted in the opposite direction. Duplex picks can be used for cutting in either direction without changing their settings. During cutting the chain speed in the machine is constant and is about 120 to 170 m per minute. The cutting speed is the speed with which the jib with the cutter chain running and it is variable in 4 to 5 stages. Cutting speed in shortwall machine varies from 0.3 m/min to 1.5 m/min. Longwall and other machines have also cutting speed varied by a device known as cam-plate. The cutting chain should not be slack. A nut on a left and right threaded screw near the jib head can adjust the tension. The cuttings are brought outside the cut by a rotation of the cutter chain. To prevent their accumulation just outside the cut a member of the crew of the CCM removes them by shovelling outside the path of the machine. In some cases special attachment called ‘Gummer’ is fitted to the gear head for automatic cleaning of the gumming. The gummer is a short spiral worm fitted to the gear head and driven by the gear. The gummer receives the cuttings thrown out by the chain and conveys them clear off the machine. A gummer is required on undercutting machines only and on longwall undercutting machine it is a standard fitting. The over cutter or middle cutter machine do not need a gummer. During cutting, for suppression of dust, wet cutting is practised under the mining rules water spread by jets from pipes is arranged on the cutter picks as they enter the cut and also as they leave the cut. If the water jets are suitably disposed they can quench the frictional sparks during cutting. Power Unit It is a motor driven by compressed air, a.c. or d.c. Where a.c. is used it is a 550 volt squirrel cage reversible motor of 40-75 hp. The machine is switched direct into the line. At the start of cutting the machine takes 5 to 6 times of its normal working current and light may dim slightly. With underground substation of inadequate capacity, othr machines lke pump etc. may stop. To reduce heavy starting current motors are now a days with double cage rotors. Such rotor has two sets rotor bars, one of higher resistance than the other. At the starting the high resistance bars are in action. This gives a high starting torque with reduced starting current, which is reduced to nearly 4 times. At full speed, the low resistance bars are in action and the machine then runs like an ordinary squirrel cage motor, which has a good running efficiency. Motor is of one-hour rating. Motion from the rotor is transmitted to various moving parts, drums etc. through gears and clutches. The CCM needs to move from face to face. It is therefore supplied power through a flexible trailing cable, each core of which is 14.75 mm2 in cross section and 100 m in length. Normally the cable is of 4 core (3 power, 1 earting), however for remote controlled machines 5 core cables are used. A gate end box , placed 100 m outbye the face is supplied power at 550 V through armoured cable from the underground substation and the trailing cable receives its power from the gate-end-box. Gate end
boxes are placed at suitable locations in a district where the same machine needs to move over a wide area. The gate-end-box contains a 3 pole air break main contactor (an electromagnetically operated switch) with magnetic blow out coils and are shields on each pole, a single phase voltage transformer giving the pilot voltage of 25-30 V and other protection like core balance leakage protection, earth continuity protection, pilot core protection. It also provides interlock to prevent: 1. The opening of the box whilst the switch is on. 2. The accidental closing of the switch while the box is open. Remote Control Operation CCMs are operated by remote control system in gassy mines. In this system the actual make and break of the heavy current required for the motor takes place, not on the CCM but in the gate-end-box. The CCM is fitted with a pilot and reversing switch which is manipulated by the nmachineopwerator's controls. This controls a low voltage pilot circuit at 25 V. This circuit magnetically operates a contactor switch in the gate end box, which operates the main power circuit of the coal cutting motor. The main advantage is that make and break of the heavy current of coal cutting machine does not take place at the face which may be gassy, but in the gate-end-box which is at outbye and therefore at a better ventilated place, free from gas. Haulage and Control Unit The coal cutter has hauling and control unit at the rear of the machine (away from face). All the controlling levers and handles are mounted on the control unit. The coal cutter can be Skid mounted, Crawler mounted or wheel mounted on rail. Skid Mounted: The coal cutter is fitted with the help of anchor props, guide pulleyes (deflection pulleyes) on the coal cutter and haulage ropes of the drum mounted on the coal cutter. The drums are driven by the motor of the machine through gearand clutch arrangements. Each drum can be operated independently as desired. For mild gradients (in 5) only single haulage drum is often used, as the operation with two drums requires an additional worker to hold anchor prp of the other drum and an extra rope for that drum. For steep gradients two haulage drums are essential. Coal cutting machines can work on 1 in 5 gradients without any special attachments. One haulage drum carries about 25 m of 18 mm diameter flexible wire rope with steel core. The machine can be used only where roof and floor are hard and height of gallery is up to 2.4 m. Making holes in the roof for the anchor pipes with miner’s picks is difficult in a gallery of 2.2 m or more height. The flitting speed of the machine is fixed and in short wall coal cutter it is 8 to 12 m/min. Interlocking arrangement is provided to prevent the haulage drums operations at flitting speed when the machine is cutting. The arrangement of haulage rope, guide pulleys and anchor props when flitting and giving a cut with a shortwall skid mounted undercutter is shown in Figure 3 for mild gradients where only one haulage drum is used.
A Jib Bracket Pulley G
i
ii
iii
A
Figure 3 Flitting and cutting with one haulage rope: A-Anchor Prop, G- Guide Prop. i. Sumping in, ii. Cutting across, iii. Withdrawal It should be noted that with one drum serving, the machine can cut at all the faces only from left to right (or from right to left). It is a good practice to use one drum for some period (say three months), and the other drum for a similar period thereafter. The wears on drum and other gears are then even. The crew consists of 3 men including two helpers and one operator at the control (at the haulage end). As the rope coils on the drum, the machine moves on towards the anchor prop. The wire rope may last for 10 to 40 shifts or more in moderatly inclined seams. A skid mounted undercutting machine can be made suitable for cutting nearly 0.3 m above the floor level by placing two girders between the machine and the skid plate. This arrangement can be done in colliery workshop and may be desirable when cutting dip watery places. Crawler Mounted The crawler mounted CCM had a crawler chain as in a tractor for flitting. Flitting speed is faster than that of skid mounted machine and is nearly 26 m/min and limiting gradient is 1 in 5. Wheel Mounted on Rails Such machine was mounted on a trolley moving on wheels with 0.6 m gauge of the mine track. The trolley was propelled by motor of the machine through gears and clutches. With a gate-end-box placed in the middle of the road, the machine could travel nearly 200 m with 100 m long cable. Where the distance was long, the trolley was pulled by locos, if they are used, for quick flitting. Limiting gradient for flitting was 1 in 12. But in practice it rarely exceeded 1 in 20. A trolley mounted machine could cut a face with the trolley standing on the track. Rail pieces of 1.5 m, 3m and 4.5 m were kept ready for extension of track after each cut of 1.5 m. Pneumatic tyre mounted CCM are seldom used.
Differences of Short Wall and Long Wall CCM Feature Jib
SW CCM
LW CCM
Fixed rigid in line with the body. In most of the case the driving sprocket wheel for cutterchain is at the gear head, though in a few models it is at the haulage end and the jib then extends up to the
Here the jib can be moved relative to the body in a horizontal plane and can be locked so as to be in line with the body or at 900 to the body on either side. During cutting on a longwall face jib is locked in a position at 900 to the body. The jib is alwas up to the gear
haulage end.
head only.
SW CCM can not be used as LW CCM can be used as SW CCM. LW CCM. Dimension
Dimension are smaller as it works in narrow clear space of longwall face.
Haulage drum
Haulage drum is protruding a. Haulage drum is flush with the side beyond the sides as LW machine has to travel practically brushing the coal face.
Base plate
Base plate is wider
Gummer
The base plate is narrow, it coincides with the breadth and width of the machine. This machine usually carry a gummer.
Model KMP-3 (MAMC) Specifications Elelctric motor
3 phase, 50 Hz, AC Squirrel Cage 500/550 V, 1460 rpm, 1 hr rating (52 kW), continuous rating (23 kW)
Cutting chain speed
2.3 m/sec.
Haulage speed
a. Cutting: 0.3-1.4 m.min b. Flitting: 8.6 m/min
Kerf thickness
140 mm
Depth of cut
2m
Chain
Endless, 76 mm , picks provides 9 lines of cut.
Cutting position
Undercutting floor level
Rope pull
a. cutting: max-10 te b. Flitting: 5 te
Weight of machine
3.4 te (without gearbox)
Control
Start and stop by push button
Cable
5 core flexible trailing cable of dia 49.65mm
Maintenance 1. Picks: Number of picks must be complete in the chain and in complete lines designed. A few gape of the picks or the use of irrelevant pick box not matching with the designed lines does harm to the machine. Gauge of the picks, lubrication for the movement of the chain in the jib and chain tension are important factors. 2. Jib: Timely replacement of jib considering i.
wear of the tensioning device
ii.
wear on the guard plates forming chain recess all around the jib
3. Electrical: Proper maintenance of the trailing cable, its support and coiling while working and preventive maintenance eof the Gate-end –box 4. Timely shifting of the gate-end-box so a s to reach the machine to all the working faces 5. Enough numer of props availabel in the vicinity of work 6. Long shovel for the cleaning of the machine cut. Any compromise on this factor shall deteriorate the power factor very heavily. (Coal cutting machines are now a days becoming obsolete. Indian Coal Mining industry deployed a large number of coal cutting machines in its early days of mechnisation of bord and pillar mining. In 1988, the status of this machine in Coal India Limited was as shown in Table 1) Table 1 Population of Coal Cutting Machines in CIL and their Status in January 1988 Item
Total in CIL
ECL BCCL
CCL
WCL SECL
No. of CCM on Roll
736
338
57
52
104
185
No. of CCM waiting repair
233
102
45
32
21
62
No. of CCM working
374
236
12
20
20
123
