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May, 1955

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The first coal plough of German manufacture and design was first introduced into British coal mining in 1947, when a standard plough was installed in Co. Durham at the Morrison Busty Pit in the Bottom Busty Seam. Several papers on this installation have been read, and ploughs and their modifications have been the subject of many other contributions to Kings College Mining Society and other institutions. The standard plough has been a success at Morrison Busty Pit and has encouraged the installation of plough equipment in other seams in Co. Durham and in other coalfields.

The winning of coal by ploughing is recognised as a definite contribution towards the achievement of continuous mining in longwall working. Ploughing possesses many advantages over other systems, in that shotfiring at the coal face can be eliminated or considerably reduced, the use of machines employing high-speed rotating pick-chains is obviated, with the consequent reduction in the dust produced at the coal face.

In the Northern Coalfield, the coal plough has other advantages which make it attractive to the mining engineer. Thin seams are predominant in this coalfield and the plough or coal plane can be constructed with shearing blades of any height to suit seam conditions. This is particularly important in Durham where large reserves of the highest-quality coking coal exist in seams less than 18 in. to 20 in. in thickness. No really satisfactory system for mining such seams economically has been found, and since coal planing appears to offer a solution, it is not surprising that many pioneering coal planing projects have been initiated in the Durham Division.

Some of these installations have not produced the results anticipated and it is apparent that a lack of basic information exists on the seam characteristics upon which successful planing depends. It was in the hope that some of these problems could be solved by a scientific approach, that our investigations were initiated.

Asbeck has referred to attempts to measure such basic information in German mines, but this was without a great deal of success. The National Coal Board at their Central Research Establishment have also been engaged on work in this field. The measurement of "ploughability" is also mentioned in the Ruhr Report, the first publication referring in detail to the design and development of the original German coal ploughs. Ploughability is meant to imply an assessment of the practicability of planing a seam based upon an accepted standard, and in our own coalfields this standard has already been laid down by the successful Morrison Busty installation.

In order to obtain information about the plough able prospects of a coal seam, an apparatus has been constructed for testing the coal in situ and in the laboratory. This equipment enables a record to be made of the force required to plane strips of coal of varying thickness at any horizon in the seam.

The basis of this comparative index was the force required to plane a strip of fixed thickness in the Bottom Busty seam at Morrison Busty Colliery.

It will be appreciated that for the test results to be of scientific value they must prove to be repetitive. The initial tests in the Bottom Busty seam have been used for such an analysis and at the same time have established the basic ploughability index.

The testing equipment

The apparatus consists of three main parts: the pumping unit, the hydraulic ram, and the portable recorder. Fig. 1.

The hydraulic ram is mounted in the laboratory test frame seen in Fig. 2, used in the development of the equipment. In the actual underground tests the ram is mounted, through a sliding sleeve, on an adjustable screw-jack prop.

The pumping unit. Fig. 5. Oil from the unit can be delivered through a selector valve to either side of the double-acting ram, at pressures up to 2,000 lb. per sq. in. by means of a small band-operated pump. In order to minimise pulsations in the oil delivery a Greer-Mercier hydro-pneumatic accumulator of approximately 10 c.c. capacity has been incorporated in the delivery line. This small accumulator is mounted behind the oil tank.

The hydraulic ram. The double acting ram having an internal cross-sectional area of 9.63 sq. in. is capable of exerting thrusts up to 8.6 tons. The piston can be fitted with steel wedges having vertical heights of 6 in., 4 in. or 2 in., and each steel wedge can take planing blades having 30 deg. 45 deg. or 60 deg. cutting edges. The blades seen in Fig. 6 are made of case-hardened mild steel and are held in position on the wedge face by three 3/8 in. diameter countersunk bolts.

The wedge is prevented from rotating as it is forced into the coal, by a steel guide rod, running through a brass lined steel sleeve mounted on the cylinder body and screwed into the rear of the wedge head.

The portable recorder. This instrument automatically records both the pressure fluctuations and depth of blade penetration during a test. It incorporates a special pressure helix sensitive to pressure from 0 to 2,000 lb. per sq. in,. which can withstand instantaneous falls in pressure from 2,000 lb. per sq. in. to 0, without damage. This is essential as the pressure drops immediately the coal yields.

The instrument chart is rotated at a speed of one revolution in 15 minutes by a clockwork drive. It was found necessary to have a relatively high speed of rotation in order to prevent overlapping of the rapidly fluctuating pressure pen recordings.

The pen recording penetration is operated by means of a Bowden cable attached to the guide rod of the hydraulic ram, through a spindle projecting from the back of the recorder, against the pull of a powerful spring. Operating against a loaded spring ensures that the pen returns to its zero position when the piston is retracted. A fine adjusting screw mounted on the hydraulic cylinder is used to tension the Bowden cable and to bring the pen recording the penetration to its zero position for each test.

Record analyser. Fig. 7. The pressure and corresponding penetration values may be read directly from the circular graphs and booked ready for calibration corrections and plotting, using a circular analysing device. The analyser has a time scale graduated in seconds around its outer edge. At the centre of the graduated circle a spindle takes the graph to be analysed. A perspex cursor having two index lines from which can he read the pressure in lb. per sq. in. and penetration in inches at the corresponding time, rotates on the central spindle. The time is read from the outer scale using the edge of the cursor.

The calibration of the recorder. The pressure element of the recorder was calibrated against pressures imposed in an oil cylinder by known weights operating through a lever. The gauge was taken up to 2,000 lb. per sq. in. in 50 lb. per sq. in. increments and down to zero in decrements of 250 lb. per sq. in. This was repeated three times and on each occasion the maximum deviation on the pressure chart for any individual reading was 10 lb. per sq. in. over the whole range.

After a continuous period of service, of six months, underground, and in the condition in which it returned from the colliery, the pressure element was recalibrated. Repetitive tests showed that no individual reading differed from the true value by more than ± 10 lb. per sq. in.

The penetration pen was calibrated against readings of the pressure chart for increments and decrements of 0.5 in. up to full piston extension of 10.4 in. The piston was advanced and retracted three times and on each occasion the Bowden cable, transmitting the piston movement to the recorder pen, was moved to a different position of rest. The pen was adjusted to its zero reading each time by means of the fine adjusting screw attached to the hydraulic cylinder.

No variation greater than the line thickness scribed by the recorder pen was observed on any occasion.

The "no-load" oil pressure required to extend the piston was determined and allowance is made for it in the calculation of the results.

The underground test

Selection of site for coal-planing tests. It was decided to select a test site which satisfied, as nearly as possible, the following conditions:-

The last point was considered to be essential as it was thought that a variation, such as exists at the normal working face, would have some unknown effect on the value of the test results obtained at different times.

It was considered that an area of solid, newly exposed coal, remote from a working face would best satisfy these three conditions. A strong argument in favour of such a place is that pre-installation tests will probably be made in such conditions. If the tests were conducted on a conventional working face, the roof loading would probably differ from that existing in the basic tests and this influence would be unknown.

Consultation with the senior officials at Morrison Busty Colliery resulted in the selection of the site. Fig. 8.

This was a newly won double-unit longwall face, in fairly solid ground, in the area in which the coal plough had successfully operated. It was not required for immediate production and was a considerable distance from any working face. The site was accepted as being nearly ideal.

The test procedure adopted

It was known, from tests conducted in the laboratory, that the screw-jack prop employed as the support for the hydraulic cylinder would only take a side thrust, at its mid-point, of 3,000 lb. before starting to bend. In view of this fact, it was evident that the thrust of the hydraulic piston would have to be carried by some other body. The only body offering such support was the solid coal. Test places, or stalls, were, therefore, prepared in the solid coal face by means of a pneumatic pick. The stalls were made one yard wide, to suit the overall length of the hydraulic cylinder mounted on the screw jack prop with coal wedge fitted. They were made approximately 4 ft deep in order that they would be well back into the solid coal, and placed at intervals of ten yards along the coal face, five stalls being made on each side of the Main Gate.

The stalls were placed at intervals of ten yards along the face, a comparatively wide spacing, in order that they would have very little, if any, weakening effect on one another, and also, in order to cover a fairly long length of face to give a representative result.

The individual tests were conducted in the following manner: Fig. 9.

1. The first 12 to 15 in. of lift No. 1 (a lift is one full piston advance, i.e., 10.4 in.) marked A is removed by hand. This coal is usually noticeably "blown" up to about 10 in. from the free face, but the remainder of the coal to the back of the stall is generally very sound.

2. A clean vertical test face, approximately 15 in. long, was now prepared and any loose coal very gently trimmed off the front by hand pick. All irregularities were next trimmed off the floor around the face.

3. The width of coal slice to be ploughed off (3, 6 or 9 in.) was measured off from the free face by means of a rigid steel rule, and marked by a chalk line on the coal.

4. The hydraulic cylinder with wedge and prop attached was next lifted into position with the edge of the cutting blade lying against the chalk line on the coal. The prop position was then adjusted until the cylinder was lying parallel to the prepared test face, the screw jack prop was then tightened to the roof.

The cylinder was aligned with the test face by placing a wooden straight edge along the test face, the straight edge projecting across the mouth of the stall. The cylinder was then adjusted until it was lying parallel with, and the width of the coal slice from, the straight edge.

5. Any gap. existing between the screw jack prop and the solid coal was packed with hard-wood pads.

6. The piston was next advanced approximately half an inch until a fixed mark on the guide rod coincided with a fixed mark on the cylinder. This served to tension the Bowden cable against the powerful return spring of the recorder and remove any sag.

7. The slight pressure required to advance the piston was now released by reversing the handle of the selector valve on the pump unit, and the trace of the pressure pen examined for deviation from its zero. Any zero adjustment was then made, by means of an adjusting screw on the link mechanism coupling the helical pressure element to the pressure pen arm.

8. The test was then made by operating the pump with the selector valve in the forward position. At the end of the cut the width of coal slice ploughed off was checked by measurement, to ascertain whether or not any deviation of the wedge had taken place.

9. Detailed notes were kept of each test, containing such information as width of cut, position of cut in the vertical plane, size of wedge employed, blade angle used and any general information regarding the visible presence of weakness planes in the coal. The full seam height at each stall was recorded and any variation in the number, thickness and position of seggar bands noted.

(To be continued)

Drawings and Photographs accompanying the article