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June, 1955

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The Bottom Busty seam, a section of which can be seen in Fig. 10, is normally 36 to 38 in. in thickness, and consists principally of vitrain containing thin dirt partings. The vitrain contains very pronounced cleavage planes and is friable. A durain band 6 to 8 in. thick present at the base of the seam shows very few cleavage planes and contains pyritic nodules at irregular intervals. A dirt band to 1 in. in thickness occurs immediately above the durain.

A good parting exists at the floor and the roof.

Test results

All the tests described were made with the 6-in. steel wedge.

The graphical results show the cutting force in lb. plotted against wedge penetration in inches. A number of repetitive results taken from all the tests are shown on each figure indicating that our conclusions are not based on a single experiment.

Bottom Busty seam, Morrison Busty Colliery

1. The first series of tests was made with the wedge carrying a 45 deg. planing blade, at floor level. Tests were made at 3 in., 6 in. and 9 in. from the free face, the results of which can be seen in Fig. 11.

The peak forces associated with the above cuts are approximately 3,000 lb., 5,500 lb., 6,200 lb.

On two occasions where nodules of pyrites were encountered the maximum cutting force of 19,000 lb. failed to take the cuts even when this thrust was maintained for some minutes.

2. The result of taking a 3-in, cut at 6 in. above floor level, before the cut at floor level, is to greatly reduce the force required to plane the durain. Fig. 12. The reduction from 3,000 lb. to less than 1,000 lb. is very pronounced. The force required to make the initial cut in the banded vitrain at 6 in. height, in which pyritic nodules do not occur, is also less than that required in the durain.

3. If the initial 3-in, cut is made at 12 in. above floor level, before the cut at floor level, there is a reduction in the cutting force required, but it is not so pronounced as in the previous test. Fig. 13.

The force required to take the initial cut at 12 in. above floor level corresponds with the force required at 6 in. height, both averaging about 2,500 lb.

4. The general effect of taking a cut at floor level was to bring down the coal to a height of 12 to 18 in. On many occasions the tops, although standing, were weak and for some distance came away easily.

The result of taking cuts at 12 and 18 in. above floor level after an undercut is, however, very interesting. Fig. 14. While the wedge is penetrating up to a distance of about 5 in., the coal planes with a force of about 1,000 lb. After penetrating beyond 5 in., the force immediately jumps to the value normally required without an undercut. This indicates that the influence of a prior undercut 10 in. in advance only extends for about 5 in.

5. A marked change in cutting force is associated with a change in the angle of the cutting tip, all other conditions remaining constant, compare the 3 in. cut of Fig. 11. with Fig. 15.

The ratios of the forces, from the major peaks for 30 deg., 45 deg., 60 deg. blades are 1.00, 1.93, 2.36.

The increase in cutting force associated with the change from 30 deg. to 45 deg. is particularly pronounced. It is probable that lack of maintenance of replaceable plane blades may well result in changes of this order, as the cutting edge is worn down over a period of time.

It was seen during the course of the tests, that the mechanism of coal failure under the action of the 60 deg. blade was very different from that under either the 30 deg. or 45 deg. blades. With the 60 deg. blade strain energy was apparently stored in the coal and the coal yielded violently, often flying a distance of 3 or 4 ft.

When the face, on which these tests were made, went into production, the O.M.S. of the coal producers (pneumatic pick) was recorded. The average was 8.99 tons per manshift, a figure indicating normality for the seam.

Brockwell seam, Morrison Busty Colliery

The Brockwell seam a section of which is seen in Fig. 16. is usually 25 to 27 in. in thickness and consists mainly of vitrain containing thin dirt partings. The cleavage planes although well developed in the vitrain are not as pronounced as in the Busty seam, and again are practically nonexistent in the durain band, 5 to 7 in. thick, at the base of the seam.

A good parting exists at the floor and the roof. This seam has, and is, being successfully worked with a Haarmann scraper.

The test site was located in the south side development area in solid, newly exposed coal. The test stalls were made at intervals of 10 yards as the winning advanced. Fig. 17.

The Haarmann had not worked in this area at the time the tests were made, all experience with it had been gained on the east side of the pit.

Test results

1. A series of tests taking cuts at 3 in., 6 in. and 9 in. from the free face was made at floor level with the wedge carrying the 45 deg. planing blade. Fig. 18.

The peak forces for this series are approximately 4,000 lb., 9,000 lb., 13,000 lb., a practically linear relationship. The value of the forces is very much higher than for the Bottom Busty seam and so is their ratio.

2. A change in the angle of the cutting tip again produced a marked change in the value of the cutting force required, the ratio for 45 deg.: 60 deg. blades being approximately 1 : 2. Compare the 3 in. cut of Fig. 18. with Fig. 19.

Determination of horse-power requirements

The opportunity of checking the practical value of this research against a working installation occurred in the Brockwell seam. A Haarmann scraper was working in this seam and its isolating switch carried an ammeter. The power consumption could, therefore, be measured directly.

In order to calculate the total theoretical horsepower required by a plough installation it is necessary to know the value of the following individual force components.

For a given rate of travel :—

(a) The force required to plane a slice of known thickness in the seam.

(b) The frictional resistance of the plane and attachments over the seam floor.

(c) The frictional resistance of the ploughed coal moving over the seam floor.

(d) The weight component to be overcome on gradients.

From a time-study analysis it was found that the scraper boxes took a mean time of 17.79 seconds to travel 66 ft.

A total of 155 tests gave a mean value for the coefficient of friction of steel over the clean seam floor of 0.3952.

A total of 103 tests gave a mean value for the coefficient of friction of steel over the seam floor, covered with small coal, of 0.3792.

A total of 151 tests gave a mean value for the coefficient of friction of coal over the seam floor of 0. 6739.

There are eight scraper boxes on the face, four of which are 10 ft. 6 in. in length, the other four being 8 ft. 10 in. in length. The volume of steel in each box was measured and its weight calculated.

Weight of large type box	=	1,208 lb.
Weight of small type box	=	1,097 lb.
Total weight of eight boxes	=	9,220 lb.

The average weight of coal carried by a scraper box per trip from 443 observations was 0.304 tons.

The maximum weight delivered in the same period was 0.655 tons. A check measurement of the internal volume of a large box showed that it is theoretically capable of carrying 0.608 tons, but as coal piles up on top of the box it is probable that it carries more.

The gradient along the face calculated from the colliery surveyor’s levels in the main gate of this and the two adjoining faces, was 1 in 52.

The angle of the cutting blades on the scraper boxes was measured as 58 deg. with a 4 deg. back rake. In view of this the cutting force determined with the 60 deg. planing blade, has been employed in assessing the probable cutting force required per box.

The sequence of operations on the face ensures that a maximum of two boxes are cutting at any instant, and it is probable that only one box at a time is cutting on the majority of occasions.

It was very difficult to assess the exact depth of cut taken by the scraper boxes, but from careful observation of both the face and the pneumatic rams on which the boxes “float,” as they pass, it was estimated that a maximum cut of 2 in. was possible.

The average value of the major peaks from the results obtained with the 60 deg. blade taking a 3-in, cut at floor level is 750 lb. per sq. in. It was shown that an approximately linear relationship existed between cutting force and depth of cut; therefore, for a 2-in, cut a value of 500 lb. per sq. in. has been employed.

The power supply to the Haarmann hauler is 3-phase, 50 cycle at 500 volts. Assuming a power factor of 0.8 the number of amperes equivalent to 1 h.p.

746	= 1.08amp.
√3 x 500 x 0.8

Any horsepower calculations can, therefore, be converted to equivalent ampere values and compared directly with the haulage motor ammeter readings.

The ammeter reading was observed when the haulage motor was running, turning the gears, and driving the main clutch plates but with the clutch out and the ropes idle.

The reading was 57 amp. This checked against the reading of the transformer ammeter of 58 amp.

Drawings and Photographs accompanying the article