List of equipment used in opencast coal mining are: 1. Bulldozer 2. Craper 3. Ripper 4. Tractor Shovel 5. Dipper Shovel 6. Stripper Shovel 7. Pull Shovel or Hoe 8. Dumpers or Tippers 9. Drag Line 10. Road Grader 11. Rock Drills.
Opencast mining is the oldest method of excavating minerals but the mining operations have been mechanised by the use of heavy earth moving machinery during the last 50 years resulting in excavations on a scale which was unthinkable half a century ago.
Equipment # 1. Bulldozer:
A bulldozer is often referred to simply as a dozer. It is a tractor with a pusher blades attached to the front portion. The tractor is the diesel-operated power unit equipped with either crawler chains or rubber tyred wheels for lifting. The pusher blade can be raised or lowered or tilted through small angles horizontally by rams operated through hydraulic pressure or by ropes.
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The dozer blade is used for pushing loose material or for digging in earth, sand and soft weathered rock. The machine is also engaged for leveling or spreading earth, for leveling of rock spoil in the dumping yard, grading and compacting temporary roads, pushing mineral into sub-ground level bunkers through grizzly, for towing dumpers, etc.
It also serves the purpose of pushing boulders, pulling down trees, and is an essential equipment to push scrapers. A dozer equipped with a fork like attachment is known as ripper and operates like a plough to loosen moderately hard rock. The loosened rock may be loaded by a scraper. A dozer can dig 1.2 m to 1.5 m below ground in earth or weathered rock.
Equipment # 2. Craper:
This machine is diesel-operated with pneumatic tyred wheels and has at the centre a bowl fitted with a cutting blade at bottom. The blade is reversible and can be replaced when blunt. Its working may be compared to that of a lawn power.
As a scraper is pushed forward by a dozer, its blade cuts a thin slice of earth usually between 75 mm and 225 mm thick over a distance of nearly 30 m. The earth is automatically collected in a central bowl whose capacity ranges from 3 m3 to 22 m3 and it takes nearly one minute for loading.
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When the scraper is fully loaded its bottom opening is closed by the operator through manipulation of a cable (rope) and the loaded scraper, with the bowl lifted, travels to the dumping yard on its own power. At the dumping yard, as the scraper moves, the bottom opening of the bowl is opened and the contents are unloaded in a layer 150 mm to 250 mm thick, over a distance of 30 to 70 m.
The bowl is always bottom discharging. Scrapers are unsuitable in soils with stumps, large boulders „ and hard rocks. When the ground is hard, it is necessary to rip the surface with the help of a ripper before loading by a scraper. Sandy soil is best for a scraper which has to be stopped during rains, if engaged, in aluminum.
Scrapers are used in coal mines for cutting and transporting weathered sandstone as well as coal. The coal excavated by it is however smaller in size. A Scraper may take 5 to 6 minutes for a complete cycle of loading and unloading if the total up-and-down distance of a trip is nearly 300 m. One-way traffic of loaded and empty scrapers is desirable for good results. One dozer is normally sufficient for every two scrapers used.
The scraper manufactured by BEML has the following main specifications:
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Flywheel H.P. of engine 332 at 2100 rpm;
Capacity- payload 23000 kg; struck 11.5 m3, heaped 16 m
Max., travel speed (forward) 44 km/hr
Overall dimensions mm length 12600; width 3470; height 3890.
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Net weight (no load) 26584 kg.
Equipment # 3. Ripper:
A ripper is a machine which cuts, as it travels, 0.6 to 1 m deep furrows in the ground, and it can be well compared with the farmer’s plough. The ripper is essentially a crawler mounted heavy duty diesel tractor with a ripper attachment.
Like a farmer’s plough, the ripper with the ripping tool thrust into the ground by hydraulic pressure, travels along close paths, 1.2 to 1.5 m apart and during the travel rips open the ground. The broken ground or rock can be dozed to form a stockpile for convenience of loading or can be loaded by a scraper.
If the overburden or mineral is suitable for ripping its breaking is possible with the help of a ripper and the process of drilling and blasting can be dispensed with. Soft rocks and medium hard rocks, below hardness 5 on Moh’s scale, which are laminated and stratified, provide suitable material for ripping.
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The alluvial surface deposits, weathered sandstones and shales underlying them in the coalfields can be easily ripped and the relative rippability of the rocks can be known with the help of an instrument known as Refraction Seismograph.
The Refraction Seismograph operates on the principle that “Sound waves travel subsurface material at different velocities, depending upon the degree of consolidation of the material”. It is believed that the same factors that affect consolidation also affect rippability. Thus poorly consolidated material with low seismic wave velocities could be ripped easily, while highly consolidated material with high velocities would be difficult to rip.
Equipment needed for seismic analysis includes a source of a sound or shock wave, a receiver, an electric counter, and a set of cables.
The main items are:
i. Refraction Seismograph – An electronic counter that determines the time interval between the strike of the hammer and the arrival of the seismic wave at the geophone.
ii. Geophone – Receiver of sound waves. A geophone is a velocity gauge suitable for detecting frequencies in the range of 1-100 Hz. The geophone converts the mechanical vibrations into its electrical analogue. The electrical signal is then amplified and transmitted to the monitoring station.
iii. Sledge Hammer and Impact Plate – Source of sound wave transmits sound through earth and also through an impact switch having direct connection with seismograph, through a connecting wire.
iv. A 30 m Tape – For measuring distances between the geophone and various impact points (wave sources).
The seismic wave is produced by a sledge hammer striking a steel plate at various distances from a geophone receiver. Immediately upon impact, a wave “front” composed of innumerable seismic waves travels in all directions away from the point of impact, or source.
Some of the waves are refracted into the layers of sub-surface materials and the angle of refraction is determined by Shell’s law which gives the following relationships:
Sine of angle of incidence/ Wave velocity in upper layer = Sine of angle of refraction /Wave velocity in lower layer
The geophone receiver is sensitive only to the first seismic wave that reaches it. Thus, either the wave which travels the shortest distance, or one which travels, a longer path but which includes a high velocity segment, arrives first at the geophone.
In addition to determining the degree of consolidation or rippability of each layer, it is also possible to determine the depth of each layer.
In iron ore areas of Goa the practical results obtained with seismograph were as follows- Seismic velocity in overburden (practically laterite) was 600 to 1,200 m per second. In iron ore it was 1,050 to 1,500 m per second, but in some cases velocities as high as 1,800 to 2,100 m per second were also recorded.
When selecting a tractor for ripping purposes, it is necessary to consider:
(a) The down pressure on the tooth to determine whether penetration can be accomplished,
(b) The tractor H.P. which should be capable of advancing the tooth through the rock and break it,
(c) Tractor weight which provides traction for full use of the H.P. in advancing the tooth.
The tractor speed is 0.8 to 2.5 km/hr during ripping. If the rock is soft it is advisable not to increase the speed but to add one or more ripping teeth. The distance between adjacent furrows during ripping may be 1 to 2 m and the harder the rock, the closer are the furrows. In some rock formations ripping is possible after sparse blasting of widely spaced charges.
Equipment # 4. Tractor Shovel:
It is essentially a diesel operated tractor with a bucket as the front attachment and is called a front-end loader or pay-loader. It may be on pneumatic tyres or crawler chains. The tractor shovel attachment consists of a push frame and a bucket that can be raised, lowered and dumped hydraulically or mechanically. The shovel usually has a pusher fan so that the dirt falling form the bucket will not be sucked back towards the operator and engine air intake.
For digging, the complete tractor shovel has to move forward toward the bench and for unloading the contents of the bucket the entire unit has to come back and position itself conveniently to empty the bucket on to a dumper. Its rate of digging and loading cannot, therefore, be as fast as with a revolving shovel.
Tractor shoveIs have been employed in some mines to load the stacked mineral at the siding into railway wagons or to push it into ground-level bunkers. Capacities of the buckets are from 0.57 m3 upwards. Heavy rock buckets for handling blasted rock carry teeth as standard equipment though the buckets used for coal handling need not be so equipped. Main specifications of two wheel-loaders (B.E.M.L.)
An excavator, technically speaking, is any machine which excavates the rock or earth and swings or transports it, within narrow limits, to an adjacent place or dumps it on to a receptacle like a dumper or railway wagon.
In this sense, a tractor shovel which cuts or digs to some extent below the flow on which it stands, may well be considered an excavator – the name traxcavator for the tractor shovel manufactured by one company apply conveys the meaning- but, in earthmoving terminology the term excavator covers machines of the following type:
1. Power shovels like dipper shovels, stripper shovels and back-hose or pull shovels.
2. Draglines, and
3. Bucket wheel excavators.
A power shovel is a shovel using electric or diesel motive power for its operation, as distinct from a hand-operated shovel. The functions of power shovels are very simple. Basically, these machines lift fragmented rock, and swing it to a different location such as dumpers or spoil heaps.
The main components are:
1. Propelling arrangements consisting of either crawler chains or pneumatic tyres.
2. A deck or cab mounted on a turn table and housing the prime mover, all the controls for operation, cable (wire rope) drums and the operator’s seat. The deck or cab can swing through 360° independently of the propelling crawler chains or tyres.
3. Deck swinging mechanism. When the deck swings, all the equipment mounted on it, including the boom and the bucket also swings.
A shovels is made in three structural divisions. An automatically, the top or revolving unit is the head and torso, the mounting or travel unit is the legs, and the various attachments are the arms and hands.
A revolving and a travel unit together make up a basic shovel which may be fitted with any of the five front attachments. The machine may thus become any one of the following- a crane, a clam shell, a dipper shovel, a drag line, a pull shovel or a back-hoe.
Equipment # 5. Dipper Shovel:
This is a machine employed for excavating soft rock or loading fragmented rock from a bench and is very commonly used in mines. It is usually mounted on crawler chains. The cab carries the power unit which may be an electrical motor at 3300 V, supplied with power from an external source through a flexible electric cable, or a diesel engine.
The bucket (also called dipper) commonly used may be of 1 m3 to 4.5 m3 capacity. It is used for loading dumpers and for this purpose it has to stand on the floor of the bench. Watery conditions in the quarry are not suitable for efficient operation of this machine, as dumpers have to move inside the quarry.
During operation, the crawlers are stationary within 3 to 5m of the toe of the bench. To load the bucket, the operator crowds it into the fragmented rock with the dipper stick and hoists it. As it moves through an arc in the rock pile.
It is then retracted and the cab, along with all the machinery mounted on it, the boom and the bucket, is swung horizontally through nearly 90° to position the bucket over the dumper. The bucket is bottom discharging and its door is opened by the trip cable. Normally five buckets are required to load a dumper.
The teeth of the bucket wear out fast and when worn out, have to the built up to size by welding. The trip cable lasts for nearly 35 hours and the hoist cable, for nearly 100 hours. In one shift a shovel loads 450 to 500 buckets. Where the dumping yard is away from the quarry a dipper shovel loading into dumpers is advantageous.
Hydraulic shovels which eliminate use of wire ropes (cables) have become popular in recent years. The electric motor or diesel engine mounted on the shovel drives the hydraulic pump and the pressure developed is utilised for various operations of the shovel. Hydraulic motors are of low speed, high torque with hydrostatic braking. One example of such hydraulic excavator is Porcelain shovel of Larsen Toubro Ltd.
Figures in brackets show the working ranges in m of Tata dipper shovel, model 1055 B, at a boom angle of 50°. Boom length is 8.53 m.
A – Cutting height (10.83)B – Cutting radius, (11.96) C – Dumping radius (9.51) D – Dumping height (7.25) E – Cutting depth below crawler level (2.51)
Dipper shovels commonly employed in our mines are of 2 m3 – 4.6 m3 bucket capacities. Only a few mines employ shovels of 8.3 m3 or 10 m3 capacities, e.g. Malanjkhand Copper Project employs 10 m3 dipper shovels.
Under conditions existing in India the loading capacity of 2 m3 shovel in good condition and well fragmented rock is as follows:
Per hour- 80 passes of bucket.
Per shift- (8-hours) 500 passes
Per day (2-shifts)- 950 passes or 190 dumper loads or 1070 m3 solid.
Per week- 5,800 m3
Per month, dry- 23,000 m3
Per year- 2,53,000 m3
Loading capacities or performance of other shovels may be considered as follows:
3.5 m3 capacity- 400,000 m3 per year.
4.5 m3– 550,000 m3 per year.
Equipment # 6. Stripper Shovel:
A stripper shovel is only a modification of dipper shovel with a long boom and is used for casting fragmented rock or earth into a dump of overburden. It is mostly deployed for overburden.
Equipment # 7. Pull Shovel or Hoe:
A pull shovel, is also known as a hoe, back hoe, drag shovel. It is used for loading dumpers and its best application is for digging below the level on which it stands. The shovel and the dumpers can stand at a higher level free from water and mud of the quarry floor. As the attachments to the bucket are by dipper stick and not by cables, the bucket is under positive control of the operator and therefore suitable for hard digging.
The shovel is used for stripping top soil, and making shallow cuts and trenches upto a depth of 3.5 to 6 m. Compared to dipper shovel, the hoe is slower in digging and less efficient for loading trunks.
In deep digging, the face should be kept fairly straight and the shovel should be as far back from the edge as possible, otherwise there may be danger of caving of the edge.
Equipment # 8. Dumpers or Tippers:
These are heavy duty trucks with a container-body of steel open at the top for receiving material loaded mechanically by tractor shovel, dipper shovel, dragline, etc. All dumpers/ tippers are provided with arrangements to lift the loaded body by utilizing hydraulic pressure to force a ram out.
The body swings from its horizontal position round a fulcrum through nearly 70° to dump its load and the hydraulic system also functions to pull the body back on its seat i.e., the chassis. A typical hydraulic system layout for the tipping gear of a dumper. From an oil tank oil flows by gravity to hydraulic pump.
When the driver engages the power take off (P.T.O.) control lever, power from the engine is transmitted from the transmission countershaft to the power take off which drives the pump. The oil under high pressure from the pump goes to the control valve whose lever can be manipulated for 4 different positions.
(a) Raise Position:
High pressure oil goes through the hose pipes to the bottom of the hoist cylinder and the ram is then forced out. Oil at the top of the hoist travel back to the control valve through the hose connected to the piston rod.
(b) Hold Position:
Both the passages between the control valve and hoist are closed so that oil at the bottom and top of the hoist is at a standstill and the latter is unable to move in either direction.
(c) Float Position:
Both hose passages between the control valve and hoist are open so that oil at either end of the hoist can flow either way. The hoist can then travel in either direction depending upon the direction in which the force is applied.
(d) Lower Position:
High pressure oil goes to the top of the hoist which then telescopes itself by the oil pressure and the oil at the bottom of the hoist travels back to the tank via the control valve. The body is thereby lowered on to the chassis.
Steering on all the heavy duty dumpers is mechanical but assisted by hydraulic power, generated by the engine. The dumper operator’s exertion in thereby considerably reduced. Mechanical transmission from the engine to the rear wheels is the standard practice now-a- days, though for some years the rear wheels were driven by individual electric motors controlled from operator’s cabin.
Medium sized mechanised quarries employ dumpers of 25-50 te carrying capacity. 50-60 te coal haulers are on the manufacturing line of B.E.M.L. and Hindustan Motors. Future planning of large projects is for employment of 100-150 te dumpers which will be fed by shovel of 8-10 m3 capacity. Bottom discharging coal haulers of 55 te payload, 43 m3 struck capacity (model GB 60C) are manufactured by BEML.
Brakes on dumpers are operated by compressed air. Some dumpers are equipped with hydraulic retarder (hydrotarder). This is a device used on some trucks and dumpers to prevent the speed from exceeding certain limits when travelling a steep down- slope and also to produce a breaking action on the vehicle.
In a way, it acts as a governor. It uses the hydraulic friction to produce the breaking action. Unline the regular brakes, the hydrotarder will not completely stop the vehicle but will slow it down preparatory to stoppings with the familiar friction brakes, operated by compressed air or hydraulic pressure. The retarder essentially consists of a vane type rotor turned by the driven shaft, a fixed casing or stator fitted with vanes and an oil circulation system.
The machines deployed in the opencast mines, at the crushing and ore preparation plant have to be of matching capacities. At Kudremukh Iron Ore Project, one of the largest opencast mines in India, the capacities of some of the machines are : shovels 10.7 m3, production trucks 108 te, front end loaders 10 m3 , electric drills for 310 mm dia. blast holes, gyratory crushers 4000 te/hr.
Equipment # 9. Drag Line:
A drag line is a machine used for excavating earth, sand or soft rock and consists essentially of a revolving deck, a long light boom, crawler chains, and a special type of bucket held in position and controlled by cables. The bucket, when it has to be loaded, is lowered in the earth or loose rock by manipulation of the cables and is dragged by them.
As it is dragged it gets loaded. Hence the name dragline. A dragline is operated by diesel engine or a motor which is supplied power at high voltage from external source through a trailing cable. The depth to which a dragline digs is limited by the capacity of the drums to hold the hoist cable. When digging, the bucket, after it is loaded, is hoisted up, the boom given a swing through 90° and the contents then unloaded by manipulation of the cable.
A dragline is suitable for digging alluvium, sandy soil, unconsolidated rock or blasted coal/rock. It digs below the level, at which it stands and from position can dig over a wide working place and cast the earth over a wide area within the reach of the boom.
It is generally not employed to load dumpers as the accurate positioning of the dragline bucket over a limited area of the dumper delays the cycle of operation and the common application is for dumping overburden. It is suitable for working a quarry with watery conditions as the dragline works from a higher and, therefore, dry position.
Loading Capacity of a Dragline:
A dragline is capable of dealing with the following quantities of rock/earth (solid) in a year (12 to 14 hours work daily).
A drag line may be crawler mounted or of waling type. Crawler mounted machines have travelling speeds of 0.25 to 5 km/hr. A walking dragline has a travelling speed of 0.18 to 0.6 km/hr.
Equipment # 10. Road Grader:
This is a machine for leveling the road surface by smoothening out the ups and downs and for casting aside the boulders on the road. It is always pneumatic tyre mounted with only rear wheel drive and the front wheels are small.
The grading blade is attached to a circle that is hung from the overhead frame and pulled by a drawbar fastened to the front of the frame. The blade is usually 3.5 to 4 long having replaceable edges on the sides and bottom. Steering is direct-connecting mechanical by a hand wheel though a hydraulic booster is fitted on some models.
The motor grader (Mode GD 605 R-2) of B.E.M.L. has the following main specifications- Engine flywheel HP 145 at 1800 RPM; operating weight 12,650 kg; Max. Drawbar pull 7,280 kg; Max. speed forward – 43.6 kmph; steering – full hydraulic; overall length-8415 mm; width 2375 mm; height-3200 mm, minimum turning radius 10.4 m.
Equipment # 11. Rock Drills:
Rock drill is the term applied to all machines using compressed air for drilling holes into rock by combined percussive and rotary action. The hole diameter is normally upto 100 mm.
The rock drills are classified mainly as follows:
i. Jack hammers (also called Sinkers)
ii. Wagon Drill.
A jack hammer, so familiar to mine workers, is a hand-held and unmounted drill used for vertically downward drilling. It weighs from 15 to 25 kgf and is used for drilling upto a depth of 2 m (rarely 3 m); hole dia. is generally 30 to 37 mm and rarely 50 mm. In a few cases a jack hammer may be mounted on an air leg. Though ordinary used for dry drilling, it can be adapted for wet drilling as well.
A drifter is a mounted drill, generally designed for horizontal drilling. It is heavier than the Jack hammer and is used in quarries and for tunnel driving. The widely used mounting is the column and arm and the drill may be used for wet as well as dry drilling. Its working is like a jack hammer.
A stopper is a drill for drilling upward and derives its name from its widespread use in mine stopes. It is used normally for wet drilling.
A wagon drill is essentially a drifter type drill capable of movement up and down a vertical guide and mounted on a portable frame fitted with wheels. The hole dia. is from 50 to 100 mm and the depth drilled ranges from 3 to 15 m.
Compressed air was the motive power for wagon drills till recently but now-a-days some wagon drills are operated by hydraulic power, as hydraulic power is more efficient than compressed air power.
It is a compressed air operated drill to which air is supplied from external compressors through hose pipes at a pressure of about 6 kgf/cm2. The drill weighs 15 to 25 kgf and drills holes of dia. 30 mm to 38 mm (rarely upto 50 mm) upto 3 m depth. The drill rod is hexagonal in cross-section, suitably shaped at one end to form the shank and the other end is so shaped as to form a non-detachable single chisel bit with a tungsten carbide insert.
Drill rods may also be equipped with detachable X type tungsten carbide drill bits. In a shift of 8 hrs, two workers who hold the drill can drill 60 holes, each 1.2 to 1.5 m deep in sand stone, laterite, etc. When hand-held, the machine drills vertically downward holes only but if mounted on air legs, it may be used for drilling inclined holes.
An oil bottle placed between the drill and., the air receiver, and connected by hose pipes to both, provides lubrication to the drill when working. For dust suppression a jack hammer can be adapted to wet drilling by some modifications so that the drill cuttings mixed with water come out of the hole in the form of a sludge. The air consumption is generally 2-2.5 m3 of free air/min.
ii. Wagon Drill:
A compressed air operated drifter mounted on a mobile frame and capable of travel up and down a mast is known as wagon drill. The frame is usually tyred wheel mounted though crawler chain mounting is provided in a few models. Tyred wheel mounted wagon drills can be pulled by the operator and his helper to the hole sites on a level ground.
A wagon drill, is used to drill holes of dia., varying from 50 mm to 100 mm for depth of 3 m to 15 m. The mast for the drifter is usually 3 m long providing for nearly 3 m vertical travel of the latter. This travel is possible with the help of a compressed, air driven feed motor through chain (known as chain feed).
The drifer provides the rotary motion as well as the percussive action to the drill rods, and in turn, to the drill bit. The drill bit is detachable X type with tungsten carbide insert. Compressed air fed through the hollow drill rods blows away the cuttings to the surface.
Total meter age drilled in an 8-hours shift is 60-70 m in rocks like sandstone, coal, etc. including the time spent on shifting the drill from hole to hole. The mast is capable of swiveling from vertical to a horizontal position and it can be kept fixed at any angle between the horizontal and the vertical, thereby facilitating vertical, horizontal or inclined drilling upto 40°. The drill is not self-propelling, and receives air from external compressor.
The maximum air consumption is 8 to 19 m3/s min. of free air at 6 kgf/cm2 including air blowing for drill bit of 60-70 mm dia.
Though a detachable X-bit is the drill bit on most of the wagon drills, some wagon drills used for 100 m dia. holes used down the hole hammers. Such down-the-hole hammers are used for larger dia.
In a large size wagon drill using a drifter a considerable portion of the drifter’s energy is utilised in overcoming the inertia of the drill string making up the column of the drill rods and in rotating them. Such loss of the drill energy increases with depth. This waste of energy is considerably reduced by the use of the down-the-hole hammer.
The drill bit used may be a carset bit (a X-bit with little modification) or a button bit which is fitted in the hammer. The compressed air going down the hollow drill rods forces the piston which directly hammers the drill bit without any drill rod in-between. The number of blows is from 500 to 2400 per min.
When using down-the hole hammer the drifter is replaced by a rotary head placed at the top of the drill string and driven by a built-in piston type air motor. The rotational speed of the drill rods is nearly 15-25 r.p.m. The rotary head is also used to tighten and loosen threaded joints on rods.
The up and down travel of the drill rods is by a chain feed. The down-the-hole hammer, type 100 ASS used on HALCO drills for holes of 100 mm to 125 mm dia. Its specifications: Outside dia. 89 mm, length without bit 94 cms; weight without bit 31 kg.
Air consumption at 7 kgf/cm2 is 5.5 m3/min.
Some of the heavy duty wagon drills are powered by hydraulic pressure system. It is equipped with a rock drill model COP 1308 HB manufactured by the same company. In the drill, compressed air is replaced by hydraulic pressure and the prime mover for the hydraulic power pack is an air cooled diesel engine.
The absence of exhaust air results in a much lower noise level when compared with air-powered rock drill. It can drill holes of dia. 65 mm to 127 mm and can therefore be used as a well hole drill for 127 mm dia. holes for depth upto about 12 m. The hole is flushed with compressed air at 10 kgf/cm2.
The rate of penetration in hard rock is generally 1 m/min using 90 mm dia bit. The rock drill 1038 HB is equipped with a hydraulic system incorporates indicators rock condition. The hydraulic system incorporates indicators which point out any fault or malfunction in the system. The boom system is operated by hydraulic pressure.
Automatic disc brakes contribute towards increased safety for the operation when travelling along steep inclines.
This is usually a crawler mounted drill operated by a diesel engine or by an electric motor which is supplied power from an external source through a trailing cable. It drills holes of 125 mm to 300 mm diameter, depth varying from 6 m to 18 m. It has a long mast, 3 m to 6 m, to accommodate the length of the drill rod.
The mast is collapsible and the drill should not be moved over an appreciable distance with the mast raised. The drills are of percussive as well as rotary type but the latter is common in coalmining areas. The drilling tool of rotary drill is a tricone bit on most of the drills but on the machines which are known as “down-the-hole percussive drills” (sometimes called “down-the-hole hammer drill”), the drilling tool is a cross bit (carset bit), or a button bit. In down-the-hole hammer drill the assembly of the drill and its short length pipe is called down-the-hole hammer.
In the rotary drill the string is rotated by the prime mover through suitable gearing. The tricone bit attached at the end of the drill string is thus rotated and it is kept pressed against the rock by hydraulic or pneumatic pressure.
In down-the-hole percussive drill the rotation of the hollow drill rods is provided by a rotary placed at the top of the drill string and driven by a built-in air motor. The air motor is also used to tighten and loosen threaded joints on rods and bits. The up or down travel of the drill rods is by a chain, operated by a reversible piston type air motor (Chain Feed).
A compressor mounted on a well hole drill helps to clean the hole as it is drilled. During drilling, the machine is leveled with the help of 3 hydraulic jacks. Normally twenty holes, each 9 m deep, can be drilled in one shift in sand stone, shale and coal.
Only vertically downward drilling is possible on most models though holes 20° off vertical can be drilled by a few machines. On some machines the drill-rods and the tools are at one end and on others, in the middle of the machine.
The latter arrangement is permissible where the burden of blast hole is large and the ground at the quarry edge strong enough to support the weight of the machine; but where this is not practicable drills rigs with the drill rods and tool at one end have to be used.
A well hole drill appears like a wagon drill suitable for large diam. A rotary well hole drill can drill in a shift of 8 hours nearly 20 holes, 200 mm dia. each 9 m deep, in sandstone, coal, shale and similar rocks.
Where the overburden consists of soft rock which can be conveniently removed by ripper and scraper-dozer combination an alternative to ripper and scraper-dozer combination an alternative to ripper is the method of drilling nearly horizontal blast holes and blasting them. Vertical (or nearly-vertical) blast holes have to be drilled where the overburden consists of hard rock like sandstone, laterite, etc.
30° off vertical may be considered to be the limit for inclined drilling of nearly-vertical holes on a bench. Larger angle increases the length of the hole, difficulties in charging it with explosives of fixed shaped cartridges, proportion of stemmed section of the hole and gives face inclination unsuitable for travel of the shovel bucket.
The toe of a bench can be removed by extra drilling of short length horizontal holes only in the toe and blasting them, or by resort to inclined drilling of the main (nearly vertical) blast holes. In vertical as well as inclined blast holes for the face, it is always essential to extend the hole slightly beyond the level of bench floor to secure proper fragmentation of toe if the hole is terminating in hard rock.