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September, 1954

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In keeping with the I.C.I. policy of improving efficiency throughout every department of its many works and divisions, the anhydrite mine at Billingham, near Stockton-on-Tees, County of Durham, has implemented in recent years a large modernisation scheme affecting all aspects of its underground operations. Although the pillar-and-stall method of mining the 15-20 ft. Main anhydrite seam has been retained, the column-mounted pneumatic drifters, scraper-loaders and tub haulage are in the process of being completely replaced by trackless mining equipment; basically, this comprises diesel trucks of 14 tons payload capacity, Joy loaders and, at the present time, a variety of temporary drill carriages mounting percussive machines. Reference to these improvements was made, it will be remembered, in our issue of May 1952, p. 210, and a full description of the mine and the machines employed in both the old and new mining areas was recently given in our associated journal Mine and Quarry Engineering in a series of three articles.*

For many years, research has been proceeding with drilling techniques, and the advantages of electric rotary methods over percussive indicated at an early stage that the method should be finally adopted for production drilling. Apart from the more obvious economic advantages of adopting this type of drilling, two of the reasons affecting this decision were :—

Development

In 1949, a 3½-h.p. electric rotary machine, a scaled-up version of the Jeffrey A6 coal drill, provided the proof that anhydrite (hardness 3-4 on Moh's scale) could be drilled successfully by this method, although the machine itself was not found suitable for production use. The equipment required had to be capable of drilling 1 3/8 in. dia. shotholes 10 ft. long at any position in a face averaging 24 ft. wide and 18 ft. high at a rate in excess of 3,000 ft. per shift. At the 1949 Earls Court Mining Exhibition, the prototype 5-h.p. drifter rig exhibited by Hardypick Limited of Sheffield was seen and considered to he capable of development to meet the mine's requirements. Time and method studies were carried out on this rig and an exhaustive survey made of the various factors affecting drilling economics; this was published under the title "Economics of Electric Rotary Drilling" in three issues of this journal.**

A specification covering a crawler-track mounted twin-jib carriage was compiled and manufacture undertaken by Hardypick Limited who completed the prototype in January 1952. After trials in a quarry, the carriage went underground at the mine to await suitable drills.

The standard 5-h.p. Hardypick drill did not meet requirements but as a result of research with it, the specification of the TSD drill was supplied to this firm in November 1951. This was a compromise necessary to obtain a workable drill quickly to enable research to proceed. The TSD-l drill, delivered in August 1952, adopted the feed nut and screw principle, and the basic elements of the "feeler ram" preload system, found necessary for the successful rotary drilling of anhydrite, were also fitted.

Development of carriage and drill continued in 1952 and the requirements of the automatic control system determined. The modifications required to complete No. 2 drill were put in hand at the end of 1952 and the complete drill delivered in February 1954. Work had shown that two drills could be successfully operated when mounted on the tracked chassis and thrusts up to 3,000 lb. per drill could be applied without having to employ stabilising chassis jacks.

The next step, stimulated by a new requirement to operate on 1 in 4 gradients, was to investigate whether the drill mountings could he put on a pneumatic-tyred chassis. As the mine was standardising for its underground haulage fleet on the A.E.C. Mammoth Major chassis (with a special low and wide body), there would be many maintenance advantages if the same chassis could be used for a drilling carriage. Accordingly, one drill and its ancillary equipment was transferred in December, 1953, to this A.E.C. six-wheeled chassis.

We were recently invited to a demonstration of this prototype drilling carriage, which is now fitted with two Hardypick type TSD electric rotary drills; one of these has yet to be modified before the equipment is put on to production drilling later this year. We were able to see that all shothole locations on the face could easily be reached from a single positioning of the carriage. The first drill was moved on to the required drilling location by remote control, an operation which averaged over all holes, takes a minimum of 18 seconds. This operation of manoeuvring the drill into the required position is controlled from a portable switch box held by the operator. This allows him to observe and control the alignment of the drill from the most advantageous position. Fig. 3 shows the complete set of controls necessary for positioning the drills and drilling. The portable push-button box, seen on the left of the illustration is carried by the driller when positioning the drill; it is on a trailing cable which may be plugged in to control either drill mounting as required. Ten push-buttons are employed for controlling the five positioning motions, these being the turntable rotation, jib elevation, jib-rolling action, jib-extension and drill frame elevation. There is also a master on-off switch which isolates all buttons to prevent accidental operation.

To the right of Fig. 3 may be seen the permanently mounted box with duplicate controls, one to each drill. Having positioned the drill, the operator selects the desired hole length to be drilled and presses the feed button. No further action is normally required as the drill automatically preloads the jib, drills the selected length of hole, retracts on completion of drilling and switches off in readiness for being repositioned for the next hole. Should the driller wish to retract the drill whilst drilling, he uses the "non-auto retract" button.

The red light near the top of the box appears when the drill bit requires changing. The arrowed switch controls all electrical supply to the drill control circuits and acts as the drill motor starter.

Drill carriage

The design specification for the DC. 1 drill carriage required a machine that would conform to the following requirements :—

After this specification was drawn up, in 1949, it was found necessary to operate the carriage on 1 in 4 gradients, and one of the two turntables is being modified to enable this to be done. As the original specification and mountings were for the tracked chassis, the overall height has been increased by 18 in. since changing over to the tyred chassis; future carriages will, therefore, be 18 in. lower than the one demonstrated.

The chassis is a 3 axle A.E.C. Mammoth Major type 3671K, 8 ft. wide model with a wheelbase of 11 ft. 8½ in. It has a fully articulated rear bogie with drive to both axles, and the rear springs have been replaced by solid balance beams. A 5-speed gearbox in conjunction with a 10 1/3: 1 differential and 11.00 x 20 x 14-ply tyres gives a maximum road speed of 21 m.p.h. at a gross loading of 24 tons; in use, speeds are limited to a maximum of 10 m.p.h. An A.E.C. 11.3 litre D.I. diesel engine is fitted that has been down rated from 150 b.h.p. to 100 b.h.p. at 1,800 r.p.m. in order to ensure that the neat diesel exhaust does not contain more than 1,000 parts per million of oxides of nitrogen or 2,000 parts per million of carbon monoxide under full load conditions. With the exception of the solid balance beams, the chassis is identical with the A.E.C. trucks used for stone haulage from the working faces to the underground crusher. Power-assisted steering will be fitted later in the year. The turning circle, as measured at the outer front wheel, is approximately 50 ft. dia. and provides adequate manoeuvrability. Tests have verified that the fully loaded chassis has ample power and adhesion for negotiating normally surfaced 1 in 4 gradients.

The drilling equipment proper is mounted on a 10 ft. 10 in. x 5 ft. 4 in. x ¾ in. soleplate suitably stiffened and bolted to the sub-chassis, which consists of braced 9 in. x 3 in. channels bolted to the AEC. chassis. Two turntables are mounted on the soleplate.

Each turntable, comprising a ¾ in., 5 ft. 4 in. dia. top plate on 10 roller assemblies, is powered by two single-acting hydraulic rams of 4¾ in. dia. and these, in conjunction with a chain and sprockets, impart 220° of rotation to the turntable. As the rams are hydraulically locked when not in use they also act as a brake preventing rotation of the turntable under drilling loads. The main turntable brake is an internal expanding type in which two (moving) Ferodo-lined brake shoes are pressed against a stationary brake drum attached to the inner diameter of the turntable track ring. This brake is automatically released by a hydraulic ram whenever the turntable operation movement is selected. Also mounted on the turntable are the complete electro-hydraulic system, drill motor contactor and jib roll motor contactor necessary for its set of drilling equipment.

The jib movement is controlled by twin double-acting 4¾ in. dia. rams which raise the jib through approximately 90°. A 2 h.p. jib-rolling motor mounted on the trunnion casting drives through epicyclic gearing, chain reduction gear and an irreversible worm reduction gear to the worm wheel mounted on the jib centre tube. The final rotational speed imparted to the jib is 1/3 r.p.m.

The jib is telescopic and largely fabricated from aluminium sections by rivetting. Each jib is in halves to enable the drill to nest inside the jib when drilling holes on the jib centre-line. Two 2 in. dia. double-acting rams extend and contract the jib.

The system used for controlling the various motions required in positioning the drill may best be described by considering a single motion as a double acting hydraulic ram. Each port of the ram is connected via a restrictor valve to a pressure port on a 3. position hydraulic valve. The restrictor valves serve only to restrict oil leaving the appropriate ram port and do not interfere with flow into the ram Their purpose is to limit the maximum speed of the ram operation under varying load conditions. They apportion the total output of the hydraulic pump so that other rams may be used simultaneously; this is necessary so that when the drill is moving from hole to hole all necessary movements are operating simultaneously and concluding at approximately the same time. The three-position hydraulic valve is operated by 2 x 25 V. d.c. solenoids with a spring centre position which hydraulically locks the ram. In order to prevent accidental selection of jib movement, all 25 V. control circuits are supplied only through operation of the switch controlling the hydraulic pump motor, this switch being mounted on the operator's push-button control panel.

Though the "feeler" rams are mounted on the drill, they are essentially part of the jib. Their function is to prevent movement of the drill trunnion mounting whilst drilling, to prevent binding of the drill rod in the hole and to hold the front drill rod guide immovable for easy hole-starting. This is done by attaching two rams to the end of the jib. The pistons are pumped out against the face before drilling commences and thus the jib is preloaded to approximately 1 1/3 tons, a figure in excess of the maximum permissible bit pressure. This preload takes up all bearing clearances which together with deflection of the cantilever jib results in a jib end movement of several inches in extreme cases. In effect, the rams are then hydraulically locked and drilling commences. Since the drill thrust is transmitted directly back to the trunnion mounting on the front jib, it tends to further deflect the jib. This, however, cannot take place since each 1,000 lb. of drill thrust merely reduces the pressure in the "feeler rams" by the equivalent of the 1,000 lb. thrust and the jib remains in its original preloaded position under the original 1 1/3 tons preload. In practice, due to slight leakages in the jib hydraulic system and the indentation of the face under the rams, the "feeler ram" pressure reduces more rapidly than the increase in drilling pressure and it is found necessary to ensure that the feeler ram pressure does not fall below the equivalent of a 4 ton thrust by maintaining this pressure by pumping oil into them.

Drilling may also be carried out by both preloading at 4 ton and maintaining this pressure in the system. However, the rod-wear effects resulting from the different "feeler-ram" systems have yet to be studied to determine the most suitable one.

Drill

The Hardypick electric rotary anhydrite drill type TSD.2 is powered by a totally-enclosed fan-cooled electric motor using a Higgs type 190.G rotor-stator unit. The motor is continuously rated to give 5 b.h.p. at 2,400 r.p.m. on a 3-phase, 400 V, 40 cycles supply. It is foot-mounted to the underside of the drill frame. The rotor shaft is extended rearwards to drive a Plessey type 2H19 hydraulic pump through a flexible coupling, the front extension engaging a two-stage epicyclic reduction gearbox giving a ratio of 26 : 1. Drive to the lower feed screw is transmitted by Renold duplex chain and sprocket drive while the upper feed screw is driven at the same speed, 140 r.p.m., by means of a gear train from the lower feed screw. The feed screws are supported in the rear light alloy casing by Salter roller bearings, while they are manufactured from 1 3/8 in. o.d. thick walled tubing and are cut with a 1 in. pitch single start left-handed acme thread of 12 ft. 8 5/8 in. length and two diametrically opposite keyways 12 ft. 2 5/16 in. long. The feed screws pass through the travelling head gearbox, while the sleeve gears are keyed to their respective feed screws so that they rotate at the same speed as the feed screws but are free to slide along them. Both gears mesh with a single drive shaft gear which extends forward through the gearbox casing to receive the splines on the drill chuck. The drive shaft gear extends to the rear to terminate in a dog clutch cut on the end face and is counterbored to carry the clutch shaft. A dog clutch is keyed to the clutch shaft and has a gear formed on it which meshes with the feed nuts that engage the screw threads on the feed screws. When the clutch shaft is held forward, the drive shaft gear drives the dog clutch which in turn drives the feed nuts. With the feed screw rotating at 140 r.p.m., the gear train gives a rotational speed of 153.5 r.p.m. to the feed nuts. Thus there is a relative motion between the feed screws and the nuts of 13½ r.p.m. and this coupled with the 1 in. pitch left-handed screw gives a forward motion of 134 in. per min. to the travelling head gearbox whilst the drill is penetrating.

A number of Londex type QO switches are attached to the drill frame in such a manner that the travelling drill head closes them in passing and in so doing a selected one operates the clutch motor causing the drill to retract after drilling a pre-selected length of hole Hole lengths varying in 6 in. from 8 ft. to 10 ft. may be pre-selected. In the event of a switch failure, the drill is retracted after 10 ft. of drilling by a fixed stop.

When the dog clutch is withdrawn to the fully rearward position, it engages the stationary clutch retraction dogs which are keyed into the gearbox casing. When thus withdrawn the dog clutch and its meshing feed nuts are held stationary. Relative motion between feed nuts and feed screws is now 140 r.p.m. giving a drill retraction speed of 140 in. per min.

It should be noted that by virtue of duplicating feed nuts and feed screws, the drilling thrust on the travelling drill head is entirely resisted by the feed screws; hence this head may be supported in guide rails which are designed to carry only its weight. This duplication was adopted to eliminate possible ill effects which could result from a single feed nut drill of this type in which the drill thrusts of up to 3,000 lb. have to be resisted as a bending moment imposed on the guide rails.

The drill rod is supported at the front end by a closefitting sleeve carried in bearings in a cast-steel casing, termed the front rod guide. When the drill is fully retracted, the drill bit is withdrawn just inside this guide. When drilling started the guide is in close proximity to the face and by virtue of the immovable mounting provided by the drill carriage, the drill bit is forced to start to drill no matter how unfavourable the face presented. Any contact between drill rod and sleeve causes the latter to rotate at drill rod speed, largely eliminating wear on the drill rod. It should be emphasised that the development of this front guide feature in 1949 was the single fundamental development which had to be made before it was possible to proceed with design and manufacture of the highly productive DC.1 twin jib drill carriage and associated drills.

In view of the extreme flexibility of 1 3/8 in. dia. drill steels it is necessary to provide an intermediate rod guide to prevent bowing of the long steel. This guide is bolted to a carrier, supported on the guide rails. The feed screws are also supported in this carrier. In the fully retracted position a spring clip holds the rear guide carrier against the travelling head and maintains this position until 5 ft. of drilling has been completed, when the spring clip releases the carrier and the intermediate and rear guides remain stationary during the second 5 ft. of drilling. By virtue of the rod guide features, the drill rod and feed screws can be totally guarded in the form of the drill frame and thus prevent the type of accident associated with the conventional drill and its exposed rotating parts.

The drill frame is based on 2 in. x 2 in. x 3/16 in. angle members tied together at approximately 20 in. intervals by 2 in. aluminium alloy bulkheads to give the frame the necessary torsional stiffness. Near the centre of the frame a ¼ in. plate box section reinforcement is used which carries the drill trunnions. This section provides the base attachment for the "feeler rams," which are supported in straps at their forward ends.

From the description given above, it will be noted that the drill is designed to operate at a constant penetration rate, as distinct from a constant drilling pressure. This decision resulted from the original research carried out in connection with the economics of rotary drilling, and further experience may ascertain that the drill operation should be based on a constant drilling pressure irrespective of penetration rate. At present bits are removed for regrinding when a drilling thrust of one ton is required. The character of the anhydrite being drilled has been found to differ greatly from hole to hole, figures of 10 ft. and 150 ft. per bit regrind having been encountered in areas only 3 ft. apart. The penetration rate used is determined largely by considerations of obtaining the lowest overall cost per foot and is nowhere near the fastest possible penetration rate. A limiting factor in determining the economic penetration rate is the frequency with which the driller has to change bits on the two drills he is operating. Further research is at present being directed into the question of bits which are of the tungsten-carbide type M supplied by Victor Products (Wallsend) Limited.

When electric rotary drill carriages of this type are put into production use in the near future and the pneumatic percussive machines now employed in both the old and new mining panels withdrawn from service, great improvements in the overall drilling efficiency at Billingham will result.

Acknowledgments

We would like to express our appreciation to Imperial Chemical Industries Limited for arranging the visit to the Billingham mine and the demonstration of this drill carriage. We would like particularly to thank Mr. G. Eland Stewart, the mine agent and manager, and Mr. K. N. Emmott, who was largely responsible for its design and development.

* "The Billingham Mine", Mine and Quarry Engineering, p. 432, December, 1953, p. 2, January, 1954, and p. 66, February, 1954.

** "Economics of Electric Rotary Drilling", K.N. Emmott, B.Sc. Colliery Engineering, p. 531, December 1952, p. 22, January 1953, and p. 61, February, 1953.

Drawings and Photographs accompanying the article