|
|
| The Beginning. | Description of Colliery. | Narrative of Overwind. | Rescue Operations. | Investigation. | Cause of Overwind. | Comments. | Recommendations. | Appendix. | List of Persons Injured. | Images. |
|
Brookhouse Colliery, Overwind. 4th March 1958.by A. E. Crook, C.B.E. H. M. Principal Inspector of Mechanical Engineering on the causes of, and circumstances attending, the Overwind which occurred at Brookhouse Colliery, Beighton, Yorkshire, 4th March, 1958. It is the only source of information used on this web page. REPORT(submitted to the Minister of Power. Lord Mills, K.B.E).INTRODUCTION2. During my investigation at the colliery I was accompanied by representatives of the National Union of Mineworkers, the British Association of Colliery Management, the National Coal Board and H.M. Inspectors of Mines. 3. My investigations showed that the overwind occurred as a result of the excessive speed of the winding engine which had not been sufficiently controlled by the application of both regenerative braking and mechanical braking. These were not as effective as they should have been when applied by the winding engineman and by the automatic contrivance to prevent overwinding, namely, a Lilly Controller which, however, was not properly equipped. Contents DESCRIPTION OF THE COLLIERY.General 4. Brookhouse Colliery, which is situated about seven miles south-east of Sheffield, has two circular shafts each 18 feet in diameter about three quarters of a mile apart. The Beighton shaft, the upcast, is 1,282 feet deep to the lowest entrance, which is for the Silkstone Seam, and it is used for carrying both materials and a relatively small number of persons between shifts. The Brookhouse shaft where the overwind occurred is the downcast and it is 1,285 feet deep to the lowest entrance, which is used for the Silkstone Seam. About 16,000 tons of coal per week and the majority of the 1,500 persons employed underground at the colliery are conveyed through this shaft. Management. 5. The colliery is in the National Coal Board's North Eastern Division No. 1 (Worksop) Area. The principal officials at the time of the overwind were:- Area General Manager Mr. F. W. Lane. Area Production Manager Mr. T. A. Carter. (Acting). Group Manager .Mr. H. Coates. Mine Manager Mr. R. Fairhurst. Area Chief Engineer .Mr. J. P. Todd. Area Mechanical Engineer . Mr. C. Hotchkiss. Area Electrical Engineer ..Mr. R. W. Barnes. Group Mechanical Engineer .. Mr. G. Hoole. Group Electrical Engineer . Mr. W. N. Littlewood. Mine Mechanical Engineer .Mr. I. G. Brothwell. Mine Electrical Engineer Mr. W. T. Corker. WINDING APPARATUS. Shaft and Headframe. 6. In the Brookhouse downcast shaft there are two double-deck cages, each deck holding two tubs of coal or 22 persons. Each cage is guided through the shaft by means of four wire rope guides that are suspended by clamps in the headframe and tensioned by deadweights in the shaft sump below the Silkstone Seam entrance. 7. At this entrance the cages are landed on four large wooden baulks which are supported by two steel compound girders 26 inches deep by 14 inches wide built into the shaft wall. The entrance is provided with two platforms so that both decks of a cage resting on the landing baulks may be loaded or unloaded simultaneously with mineral, material or persons. 8. The steel headframe above the shaft supports. and is connected to, some of the winding apparatus. including: two pulleys for the winding ropes attached to the cages and the winding engine, eight wire ropes for guiding the cages. two detaching plates that form part of the gear by which either cage is detached from its winding rope if it ascends too far when overwound, four sets of safety catches of the multiple type which are designed to support a cage that has been detached from the rope, and the equipment used for loading and unloading the cages. This equipment is located on two platforms, as at the Silkstone Seam entrance, so that both decks can be dealt with simultaneously. 9. The headframe is constructed so that a cage may ascend a distance of 21 feet 3 inches above its normal position at the loading and unloading platforms before it is detached from the winding rope. Winding Engine. General. 10. The winding engine involved in the overwind was supplied by The General Electric Company. Limited, Witton, and it was installed in the year 1929. Between that date and 1942 the engine was used infrequently to provide another means of egress from the mine that was being worked from the Beighton shafts. The engine was not used to raise coal until the mine and its equipment had been reorganised both on the surface and below ground so as to handle a projected output of about 15,000 tons of coal per week. In order to raise this output the nominal maximum speed of the winding drum was increased from 45.3 to 53.7 revolutions per minute as a result of modifications made to some of the electrical components. 11. The engine has a bi-cylindro-conical drum on to which the winding rope attached to one cage is coiled whilst the rope attached to the other cage is uncoiled. The drum is coupled directly to a D.C. winding motor that bears a name plate on which the following particulars are included: 1,200/2,040 h.p., 53.7 r.p.m., 425 volts, 2,260/4,000 amps. This motor is supplied with electrical energy by an induction-motor-driven Ward Leonard set with flywheel and slip regulator, which runs at a speed in the range of about 1,000 to 820 r.p.m. according to the magnitude and direction of the unbalanced load in the cages. The set includes a 900/1,700 kilowatt, 425 volt, D.C. generator, a flywheel weighing l1Ό tons and a 900/1,800 h.p., 3,300 volt, 3 phase, 50 cycles, slip-ring induction motor, which is controlled by a combined liquid starter and slip regulator. 12. The speed and direction of rotation of the winding motor are determined primarily by three field windings on the Ward Leonard generator. These windings consist of a self-excited field, a separately excited field regulated by the engineman's hand control lever and a differentially compounded field which is connected in series and is in opposition to the self-excited field during motoring conditions. The self-excited field is connected across the terminals of the generator and is subject in addition to two steps of resistance control introduced by the position of the engineman's hand lever. The three fields are so arranged that the armature current is limited to a safe value irrespective of the movement of the hand lever. The speed of the winding motor is also determined by the speed of the motor-driven Ward Leonard set and the magnitude and direction of the unbalanced load in the cages. The speed/load regulation of this control system between no load and full load is stated to be 11 per cent, but the actual regulation is about 19.75 per cent. owing to the large variation in the speed of the Ward Leonard set introduced by the slip regulator. The engineman's hand control lever moves inside a quadrant and it has three main positions, namely, a full speed "Forward" position at the end of the quadrant remote from the engineman, an "Off" position in the centre of the quadrant and a full speed "Reverse" position at 'the other end of the quadrant. 13. In order to lower a cage, for example, the under-rope cage, from the landing places in the headframe to the Silkstone Seam entrance, the engine-man after receiving the necessary signals moves the control lever towards the "Reverse" position and releases the mechanical brakes on the drum. Consequently the winding engine accelerates and the motor generator set decelerates to allow the flywheel to give up some of its kinetic energy and thereby reduce the peak demand for electrical energy to drive the induction motor. This continues until the winding motor and the motor generator set attain speeds determined by the position of the control lever, the slip regulator and the magnitude and direction of the unbalanced load in the cages. When the cage is travelling at the appropriate speed at a certain place towards the end of the wind as shown on the depth indicator, the engineman normally moves the control lever towards the "Off" position and allows some of the kinetic energy in the winding engine to be converted into electrical energy which accelerates the flywheel and the motor generator set and thereby decelerates the engine. In other words lie obtains regenerative braking. At the appropriate place the engineman applies the mechanical brakes and eventually brings the cage to rest where required. Abnormally if the engine is travelling at a greater speed at this place, the engineman may move his control lever through the "Off" position towards the "Forward" position to try to obtain a greater rate of deceleration or regenerative braking, a matter which is considered later in this Report. 14. At the commencement of a wind the rope attached to the cage at the landing places in the headframe is on the large diameter of the drum, namely, 17 feet 6 inches, whereas the rope attached to the cage at the Silkstone Seam entrance is on the small diameter of the drum, namely, 11 feet 0 inches. Consequently the effort required to start or accelerate the winding engine is much less than it would be if both ropes coiled on to and off the same diameter of the drum. The speeds of the two cages at any instant are directly proportional to the varying diameters of the drum on to which the ropes are coiling and thus the cages travel at the same speed for a long distance in the shaft, but their speeds are in the ratio of 1 to 1.59 when they are in close proximity to the Silkstone Seam entrance and the landing places in the head- frame, respectively. For example, a landing speed of 5 feet per second at the Silkstone entrance corresponds to a speed of nearly 8 feet per second at the above landing places. Brakes. 15. The winding drum has two brake paths each 12 feet 6 inches in diameter and 8 inches wide, to which brakes of the bottom-anchored four post type, with rigid shoes having Ferodo C R friction linings, are applied by a dead weight that is controlled by a brake engine actuated by compressed air at 60 p.s.i. The brakes are applied or released, respectively, when compressed air is exhausted from or admitted to the underside of the piston in the air cylinder of the brake engine. The movement of the air is controlled by two valves, namely, a valve of the Iversen type and an air escape valve. 16. The Iversen Valve is designed to produce a braking effort directly proportional to the displacement of the winding engineman's hand brake lever. But this object may not be attained without proper adjustment and maintenance of the complete valve, which incorporates a small piston valve that moves inside a sleeve that also moves inside the valve box. The Iversen Valve is actuated for normal braking by the hand brake lever and for emergency braking by an emergency solenoid that is controlled by the safety equipment of the winding engine. This includes the Lilly Controller or automatic contrivance that is fitted to the winding engine in order to prevent overwinding of the cages as required by Regulation 7 of the Coal and Other Mines (Shafts, Outlets and Roads) Regulations, 1956. 17. The air escape valve is actuated by the combined action of the emergency solenoid and the apparatus that controls the movement of the throttling valve in the cataract cylinder of the brake engine in such a way that the air escape valve may function only during "Fast Braking" as described in the next paragraph. 18. There is a cataract or damping cylinder incorporated in the brake engine to provide for "Fast Braking" or for "Slow Braking". This is achieved by the movement of a throttling valve in the pipe that connects the two ends of the cataract cylinder, which is kept full of fluid and contains a piston. The throttling valve is connected to cams on a dial driven by the winding engine so that "Fast Braking" is available for about two revolutions of the winding drum from either end of a wind and "Slow Braking" is enforced for the remainder of a wind. Safety Equipment. 19. The Lilly Controller referred to in paragraph 16 is designed - (a) to prevent the speed of winding in mid-shaft from being excessive; (b) to enforce a progressive reduction in speed as the cages approach the end of the wind; (c) to prevent the cages from being overwound at a dangerous speed; (d) to stop the winding engine as quickly as practicable whenever a cage passes the extreme position that it attains during normal winding operations; (e) to prevent the engine from being re-started in the wrong direction after a cage has been overwound ; and (f) to permit 'the speed of winding during he accelerating period to be greater than the speed at the corresponding place as the cages approach the end of the wind. 20. The elements provided on the Controller for the purpose of attaining the aforesaid objects include centrifugal governors and dial-mounted cams, overspeed and overwind switches that work in conjunction with backing out switches, accelerating solenoids, electrical safety circuits that are designed to "fail to safety" and a man-safety element which makes one of the governors of the Controller more sensitive when persons are being carried in the cages. The latter element is controlled by a man-safety lever located in the engine- man's cabin. When this lever is moved into the man-winding position an indicator in the engine house and indicators at the landing places in the headframe are illuminated automatically, indicating to the banksman that the Controller or automatic contrivance to prevent overwinding is fully engaged before he allows any person to enter the cage. 21. The safety features of the winding engine are divided into two sections, namely, a "Slow Braking Group" and a "Fast Braking Group ". The "Slow Braking Group" contains four features by which the emergency brake contactor coil circuit may be tripped, namely, overload of the exciter, overload of the winding motor, the winding engineman's emergency stop button and the overspeed switch of the Lilly Controller. When any of these features cause a trip in the "Slow Braking Zone" described in paragraph 18. regenerative braking is produced and maintained automatically by means of two pressure switches which retain the excitation contactor closed and thereby keep the motor field energised until the mechanical brakes become effective after a lapse of some seconds. One switch is connected to the lower side of the air cylinder and so arranged that its contacts, which are normally closed, are opened automatically as soon as the pressure of the air in the cylinder is reduced when the Iversen Valve is moved into the exhaust position as a result of the "trip ". The other switch is connected to the lower side of the cataract cylinder and so arranged that its contacts, which are normally open, are closed automatically as soon as the falling piston in the cylinder establishes pressure and that they remain closed until the pressure is reduced to a low value as the mechanical brakes are applied effectively. At that moment the switch opens to drop out the excitation contactor and thereby remove the winding motor field. When any of the above features causes a trip in the "Fast Braking Zone" the mechanical brakes are applied relatively quickly without regenerative braking because both pressure switches open quickly and trip the excitation contactor. 22. The "Fast Braking Group" comprises four features by which the excitation contactor coil circuit is tripped, namely the overspeed switch of the motor-driven Ward Leonard set, the failure of the winding motor field, the failure of air pressure for the mechanical brake and the overwind switches of the Lilly Controller. When any of these features causes a " trip" in the "Fast Braking Zone" described in paragraph 18, regenerative braking is not produced because the emergency brake contactor is opened to apply the mechanical brakes relatively quickly and consequently the pressure switches do not prevent the excitation contactor from opening and removing the winding motor field. 23. When a trip is caused by any of the above features the winding engine is brought to rest automatically and it cannot be moved until any defect in the safety equipment has been rectified and the winding engineman has reset that equipment. For this purpose the control lever is put into the "Off" position and the brake lever into the "Reset" position and the "Reset" push button is pressed. In the event of an overwind, however, the engineman must first leave his cabin so as to close the appropriate backing out switch on the Ward Leonard controller which is operated by the control lever. This switch then short circuits the overwind switch on the Lilly Controller and enables the engineman to reset the equipment in the manner described. He may then apply power to the winder motor, but only in the opposite direction to that in which the overwind occurred because any attempt to apply power in the other direction would cause the backing out switch to open and cut off the power and apply the brakes. Contents 24. The overwind occurred about 6.30 a.m. on Tuesday, 4th March, 1958, during a wind that was made after 260 persons had been raised out of the mine and 507 persons lowered into it by the man-winding operations that had commenced about one hour previously. In this wind the ascending overlap-rope cage was empty and the descending underlap-rope cage was carrying the maximum number of persons permitted by the manager, namely, 44 persons of whom there were 22 in each of the two decks of the cage. 25. The descending cage was landed on the wooden baulks at the Silkstone Seam entrance at such an excessive 'speed that 28 of the persons in the cage sustained fractures of the lower limbs and the remainder suffered from shock and bruises as a result of the impact. The names of the persons who were injured and the nature of 'their injuries are given in Appendix II. The impact also partially fractured one of the wooden baulks. The descending winding rope continued in motion until the winding engine stopped by which time a further 45 feet of this rope had been uncoiled from the drum. Part of the rope and the cage chains were lying on top of the cage. 26. The ascending cage travelled past the landing places in the headframe at a speed that was l.59 times that at which the descending cage had struck the baulks at the Silkstone Seam entrance. The cage continued to ascend and it was detached from the winding rope by the combined actions of the detaching hook in the cage suspension gear and of the detaching plate that was located below the overlap-rope pulley in the headframe. After detachment the cage continued to ascend until it had lost its momentum and then it dropped back owing to the effect of gravity until it was arrested by the safety catches. Contents RESCUE OPERATIONS.27. The onsetter at the Silkstone Seam entrance saw the descending cage for a fleeting moment before it crashed on to the wooden baulks at that place and caused the men in the cage to crumple and drop. He and the other workmen and officials at the entrance assisted by the few men in the cage who had sustained only shock and bruises, immediately took steps to extricate the injured men from the cage. 28. The manager of the mine did not know that there had been an overwind involving a large number of men until 7.00 a.m. when he was told at his home by a messenger who had been sent by the under-manager of the mine. The delay in notifying the manager was caused by the inability of the telephone exchange operator at the mine to contact him because he had not been informed that the telephone numbers of the manager and some of his officials had been changed two days before when the Woodhouse Exchange was put on to the automatic system. The manager arrived at the mine at 7.10 a.m. assessed the situation and made arrangements for stretchers, ambulances and first aid equipment, stretcher bearers both below and above ground and casualty stations on the surface so that the men could receive medical attention before they were sent to the Sheffield Royal Infirmary. He informed the Infirmary of the probable number of injured men and asked for their reception and treatment. H.M. Senior District Inspector of Mines, the Area General Manager and other officials were notified of the accident. 29. The injured men were put on to stretchers and carried for about three quarters of a mile below ground from the Brookhouse downcast haft to the Beighton upcast shaft where they were raised to the surface. They were then carried a short distance to the ambulances which conveyed them to the casualty stations near to the top of the Brookhouse shaft. The first of the injured men arrived at these stations at 8.30 a.m. and the last at 10.30 a.m. Here they received medical attention under the supervision of Dr. R. McL. Archibald, National Coal Board No. 1 (Worksop) Area Medical Officer, before they were sent to the Infirmary. Contents INVESTIGATION OF THE OVERWIND.Investigation before Normal Winding was resumed. 30. The investigation of the overwind by officials of the National Coal Board and H.M. Inspectors of Mines in the North Eastern Division commenced about two hours after the overwind and it continued until the morning of 6th March, 1958, by which time the automatic contrivance had been modified and tested with satisfactory results. 31. The overwound ascending cage was held in the headframe by a safety catch under one end of the middle deck and by another catch under the opposite end of the bottom deck. The catches were not in proper working order because they had not been properly maintained. Consequently nine of the catches in one set and 11 in the other set had not returned to their operative positions after they had been pushed back by the ascending cage. In addition, four catches of the first set and one catch of the other set had only partially returned to those positions. Ineffective operation of many of the safety catches had caused the falling cage to tilt before it was finally arrested by the aforesaid two catches, which were not directly opposite. As a result of the tilting, one of the frames containing a set of catches was bent outwards about 4+(5/16) inches. When the cage was examined it was found to have sustained damage to the hoop of its middle deck, which was repaired before the cage was used again. 32. The capping of the overlap winding rope was lying on the ground just in front of the winding engine house. It had made a hole in the concrete ramp to the basement and demolished some railings. The rope had been damaged in the vicinity of the capping, and consequently 25 feet of the rope had to be cut off before it was recapped and used again. It was found that the winding drum had rotated about l.33 revolutions after it should have stopped and that about an additional 73 feet of the overlap rope had been coiled on to the drum. 33. The winding engine was examined by the investigators who found that the supply of electric power thereto was cut off, the brake solenoid was in the "Brakes On" position, and the overwind switch was open on the over-rope side of the automatic contrivance, a type C Lilly Controller. These circumstances indicated that the automatic contrivance had operated so as to cut off the supply of power and to apply the brake. 34. When the overwind occurred the winding engine was being operated by Mr. J. Mozley, aged 63 years, who came to work just before 5.30 a.m. and relieved Mr. H. Leadbeater who had operated the engine during the night shift. Mr. Mozley had not been to bed that night on account of his wife's illness and he was very upset by the accident. He said that he felt there was a large unbalanced load on the engine at the start of the wind, which proceeded normally, however, until the drum was about nine revolutions from the end of the wind, when he moved his control lever towards the "Off" or reverse position, so as to apply regenerative braking. This action apparently did not reduce the speed of the engine, so he moved the lever progressively into the full reverse position. but even this had no noticeable effect on the speed of the drum. By this time, the drum was only about five revolutions from the end of the wind, so he moved the brake lever into the "On" position, and kept the power control lever in the full reverse position. The two relief winding engine. men, who were in the engine house at that time, heard the screech of the brakes which eventually brought the drum to rest about one and one third revolutions past the position at which it was normally stopped. 35. Mr. Mozley said that the man-safety lever was in the correct position to set the Lilly Controller for carrying persons, which was confirmed by several witnesses, and that the overspeed switch and the overwind switch on the Controller did not trip the safety circuit as he expected. He also said that some 15 minutes after the overwind he pressed the emergency push button which tripped the safety circuit, because he heard a thud on the brake handle that was caused by the falling brake solenoid. 36. Mr. Mozley's statement about the loss of regenerative braking and the failure of the Lilly Controller to operate caused such grave concern that it was decided to make exhaustive examinations and static tests of the winding engine and the Controller with its ancillary apparatus both mechanical and electrical. No fault was found in the electrical apparatus which could prevent the Controller from functioning correctly. Likewise no fault was found in the mechanical apparatus which could result in the loss of braking effort or which could prevent the Controller from functioning correctly. For example, when the overspeed or overwind switches were opened by the action of the speed governor or the overwind limit cam respectively. These tests also showed at an early stage of the investigation that the backing out switches were in the open position. Lubricating oil from the mechanism of the Controller was found to have run down the vertical conduit containing the electrical wiring connected to the Controller. Although tests of the wiring in situ did not reveal any defect the wiring was renewed. The old wiring was specially examined after it had been removed and it was found to be free from any defect that could prevent the Controller from functioning correctly. 37. In view of the satisfactory results of the above static tests, the winding apparatus was made ready for a series of landing speed tests, in which the Lilly Controller or automatic contrivance was fully engaged for carrying persons. These tests established beyond any doubt that: (a) The winding engine drum attained a speed of about 68 r.p.m. when the ascending cage was empty and the descending cage contained a load equivalent to the maximum number of men allowed in it at any one time. This speed is equivalent to a rope speed of 63 feet per second when the rope is coiling on the large diameter of the drum. (b) The descending cage eventually passed the artificial landing, used for the purpose of the test, at a speed of about 19 feet per second despite the operation of the Lilly Controller, which functioned correctly. 38. The tests showed quite clearly that the winding apparatus should not be used again for carrying persons until the automatic contrivance on the winding engine had been modified to satisfy the requirements of Regulation 7 of the Coal and Other Mines (Shafts, Outlets and Roads) Regulations, 1956. 39. The automatic contrivance was fitted, therefore, with a Man-Safety Tappet, so constructed and adjusted as to prevent the cage from travelling at a speed greater than 40 feet per second when it was carrying persons. Subsequent tests of the automatic contrivance indicated that it prevented the cage from passing the artificial landing at a speed exceeding 5 feet per second and that it was able to satisfy the requirements of the Regulation. All the representatives agreed, therefore, that the winding apparatus could be used again for normal winding which started at 2 p.m. on 6th March, 1958. Investigation after Normal Winding was resumed. Brakes. 40. An examination of the arrangement, construction and operation of the Lilly Controller, ancillary apparatus and the brake engine indicated to the Area Chief Engineer that in certain circumstances there might be undue delay in the application of the mechanical brakes. Consequently on 16th. March a series of tests was made to determine the performance of the brake engine when the brakes were applied by the winding engineman's brake lever, the Lilly Controller or the combined action of the lever and the Controller. 41. On 17th and 18th March I examined the brake engine and the detailed drawings thereof, considered the results of the tests and came to the conclusion that the Iversen Valve was neither set correctly nor working properly. The management readily accepted my suggestion that they should alter the Valve setting and arrange for representatives of the Fraser and Chalmers Engineering Company, a subsidiary of the General Electric Company, Limited, to make a thorough examination of the Valve as soon as possible. The examination was made on the night of 20th March, when it was found that the sleeve inside the valve box was so tight that it could only be moved by force whereas it should have worked quite freely. After the sleeve had been thoroughly cleaned, eased and put back to work the Iversen Valve functioned properly. The performance of the brake engine was then determined by another series of tests. 42. The information shows that, both in the "Slow Braking" and the "Fast Braking" Zones, the defective Iversen Valve caused an alarming delay when the brake was applied by the engineman's brake lever or by the combined action of that lever and the Lilly Controller. It also shows that prior application of the brake by the brake lever prevented the Controller from applying the brake anything like so quickly as it would have done otherwise. This was caused, firstly, by the defective Iversen Valve which did not allow the compressed air to exhaust as rapidly as it should have done and, secondly, by the linkage used for operating the brake engine by means of the brake lever, the fast and slow braking device and the emergency solenoid which was controlled by the Controller and associated safety devices. This linkage incorporated a slotted link by which the solenoid was reset after the Controller had operated. As a result of this arrangement, prior application of the brake by the engineman's brake lever prevented the solenoid from failing more than about half an inch instead of the two inches that it would have fallen otherwise. Consequently the solenoid could not open the air escape valve on the brake engine cylinder in the fast braking zone and thus the time of application of the brake in that zone was dependent on the speed at which the air was exhausted through the defective Iversen Valve. which caused a perilously long delay. Lilly Controller. 43. The Duplex head on the Controller, i.e. a head with two governors, was fitted by the Metropolitan-Vickers Electrical Company. Limited, in July, 1953. Examination of the sheet of data which the former Area Chief Engineer of the National Coal Board forwarded to that Company when the Duplex head was ordered showed that the normal winding speed was declared as 49.2 feet per second and that the Company was not asked to provide for a reduced maximum speed for man winding. In other words the Company was not asked to provide a Man-Safety Tappet. 44. The Controller is designed to prevent the winding engine from travelling at an excessive speed and to enforce a progressive reduction in speed as the cages approach the ends of the wind. The reduction of speed is effected by two deceleration or retardation cams. One cam controls the last four and one quarter revolutions of the winding drum when the overlap- rope cage is descending, which is equivalent to the last 149 feet of travel of the cage. The other cam controls the last four revolutions of the winding drum when the underlap-rope cage is descending, which is equivalent to the last, 140 feet of travel of that cage. This means that the winding engine may be accelerated until it attains a speed just less than that permitted by the Controller and that it may run at that speed until the descending underlaprope cage is nearly 140 feet from the end of the wind. 45. The accelerating solenoids are fitted to the Controller so that during the early part of a wind the winding engine may be accelerated at a greater rate than the minimum rate that it must be decelerated towards the end of a wind. This is achieved by interfering with the normal effect of the appropriate deceleration cam on the Controller during the accelerating period only. Unfortunately, however, some overwinds have occurred on a very small number of winding engines as a result of defects in the accelerating solenoids which caused them to function incorrectly during the decelerating period. In view of these dangerous occurrences, the accelerating solenoids on the Controller of this winding engine were specially examined and found to be in proper working order. 46. Between July 1953 and October 1955, periodic tests of the operation of the Lilly Controller were made under the direction of the Area Chief Engineer's Department. These tests involved the application of power to the winding engine when, as required by the Regulations, attempts were made to land the descending cage at an excessive speed. The results of the tests indicated to the management that the cage could not be landed at the pit bottom at a speed exceeding 5 feet per second and that the maximum speed of winding either men or minerals was 57 feet per second; this speed was 16 per cent, higher than the speed declared by the management when they ordered the Duplex head for the Controller. But Mr. Leadbeater, one of the winding enginemen, said that he had lowered materials at a speed of 60 feet per second without being stopped by the Controller. Therefore the maximum speed of winding was probably higher than 57 feet per second, a condition which had not been revealed by the method of testing. 47. Between October 1955, and the time of the overwind, periodic tests were made without the application of power and thus they were not nearly so thorough as formerly. This method of testing without power was adopted because the Area Chief Engineer was concerned about a crack in one of the cast iron scrolls of the winding drum and he thought that it would not be wise to continue to use the previous method of testing which might have an adverse effect on the crack. He was convinced, however, that the results of the tests indicated that the performance of the automatic contrivance was such as to satisfy the statutory requirements. 48. In addition to the above tests, the operation of the Lilly Controller was tested every night by the winding engineman on duty. Such a test was made on the night immediately prior to the overwind when Mr. Leadbeater reported in the winding engineman's daily log book that he had tested the overwind switches, the emergency push button, the man-riding signals and the emergency stop signals and that he found them in order. Brake Contactor. 49. It came to light that Mr. E. Williams, assistant mine electrical engineer, went into the engine house about ten minutes after the overwind and asked the two relief enginemen, the engineman who was going off duty and Mr. Mozley what had happened and why the latter had not used the emergency push button. When Mr. Mozley said he had not thought about using the push button, Mr. Williams opened the right hand side door of the contactor cubicle where he saw that the brake contactor was closed. He re-closed the door without noticing whether any other contactor was closed and returned to the electricians' shop without telling anyone of his discovery. A few minutes later he sent an electrician to instruct the winding enginemen that they should not touch anything. Shortly afterwards one of these men came to the shop and said that Mr. Mozley wanted to see him. He returned to the engine house where Mr. Mozley said that before the above instruction was received he had operated the emergency push button and had heard the contactor drop out. Mr. Williams then said to him "I could not understand why that contactor was closed ". He again opened the door of the contactor cubicle and found that the brake contactor was now open. 50. Although the evidence on the reporting of the above situation is somewhat conflicting and disturbing, it would appear that some of the senior engineers and H.M. inspectors of Mines were not told that Mr. Williams had found the brake contactor closed until after the winding engine had been put back to work. My investigations and the replies to my questions showed that the brake contactor might have been found closed by Mr. Williams after the overwind as a result of any of the following circumstances: - (I) Failure of the Lilly Controller owing to electrical or mechanical defects or incorrect adjustment. (2) Failure of the brake contactor. (3) Resetting of the safety equipment after the overwind in the manner described in paragraph 23. 51. Unfortunately no investigation was made as it should have been immediately after Mr. Williams had looked inside the contactor cubicle, the four winding enginemen were not instructed immediately that they must leave everything undisturbed and Mr. Mozley's action in pressing the emergency push button seriously interfered with the subsequent investigations. Nevertheless the exhaustive examinations and tests that were made did not reveal any defect or incorrect adjustment that could cause either the Controller or the contactor to fail. Moreover all the conditions and the evidence, except that of Mr. Mozley and Mr. Williams, indicated that the Controller and the contactor did not fail and that they had never failed. Regenerative Braking. 52. In view of Mr. Mozley's statement that his attempt to apply regenerative braking immediately prior to the overwind did not result in the speed of the drum being reduced, the Area Chief Engineer conducted some tests on regenerative braking which were witnessed by representatives of the General Electric Company, Limited. These tests showed that the regeneration obtained for various movements of the power lever from the full speed forward position towards the full speed reverse position were substantially the same as those measured when the winding engine was commissioned some 16 years ago. Nevertheless the regeneration obtained when the power lever was moved from the full speed forward position to the full speed reverse position was only 70 per cent, of the full load and it was very much less than that obtained when the lever was moved to only the 60 per cent., full speed reverse position. In other words much less regeneration was obtained when the lever was in the full reverse position, into which it was put by the winding engineman immediately prior to the overwind. 53. Three of the winding enginemen commented on regenerative braking: Mr. Anderson said that it didn't always come the same and that on odd occasions it seemed to take longer to act: Mr. Leadbeater said he had been troubled occasionally by losing power in reverse; and Mr. Johnson said ; that sometimes when a heavy load was going down and the engine was going fast regenerative braking does not occur. The last statement is very significant, because when the overwind occurred there was a heavy load going down, the engine was going fast and Mr. Mozley said that he seemed to lose reverse power. Mr. S. Luxmore, H.M. Electrical Inspector of Mines, observed that at speeds in excess of 50 feet per second when the engineman's control lever was moved into the full reverse position, regenerative braking far from being increased was in fact reduced. Contents CAUSE OF THE OVERWIND.54. The tests which were made before the Controller was fitted with a Man-Safety Tappet after the overwind revealed that, on one test when the engine was lowering an unbalanced load equivalent to the 44 men who were descending the shaft when the overwind occurred, the ascending cage attained a speed of about 63 feet per second before the Controller tripped in the "Slow Braking Zone." Although the Controller caused the application of regenerative braking and mechanical braking to the winding engine, the descending cage passed the artificial landing used for the purpose of the test at a speed of about 19 feet per second. At the same time the ascending cage passed the corresponding artificial landing at the surface at a speed of almost 30 feet per second and subsequently it travelled about 60 feet past that landing before it was stopped. 55. If an artificial landing had not been used for the purpose of the above tests, the descending cage would have crashed into the baulks at the Silkstone Seam entrance at a speed of about 19 feet per second, which is about three and three quarter times the maximum landing speed of 5 feet per second prescribed by the statutory Regulation. Likewise the ascending cage would have passed the landing places in the headframe at a speed of about 30 feet per second and it would have been detached from the winding rope. In other words, there would have been an overwind very similar to that which occurred on 4th March, 1958. 56. During the above tests the Lilly Controller functioned correctly, but the defective Iversen Valve delayed the application of the mechanical brakes in the "Slow Braking Zone". The air escape valve, however, was opened automatically in the 'Fast Braking Zone". 57. During the overwind on 4th March, the defective Iversen Valve delayed the application of the mechanical brakes by the winding engineman in the "Slow Braking Zone" and, moreover, the air escape valve was not opened in the "Fast Braking Zone". It could not be opened because the engineman had put his brake lever into the "On" position and the linkage prevented the valve from being opened by the action of the Controller, as described in paragraph 42. The winding engine was decelerated, therefore, at a lower rate than it was during the above tests until the descending cage crashed on to the wooden baulks at the Silkstone Seam entrance. When that happened, however, the mechanical brakes became more effective automatically and the rate of deceleration increased considerably. In consequence of these circumstances the average rate of deceleration both during the overwind and the above tests was probably about the same. 58. As a result of the investigations I have come to the conclusion that the overwind on 4th March, 1958, occurred because, firstly, the winding engineman allowed the engine to reach a speed which was higher than the normal speed when the cages were approaching the end of the wind; secondly, the Lilly Controller was not equipped to prevent the engine from attaining that speed ; and thirdly, the actions of the engineman and the Controller did not cause the speed to be reduced as quickly as the engineman expected and at the rate essential to prevent the overwind because: (I) the regenerative braking was not sufficient to deal with the descending unbalanced load and the high speed of the engine, owing to the combined effect of the relatively high speed of the Ward Leonard set, the control system and the way in which it was operated, especially when the engineman moved the control lever into the full reverse position (2) the defective Iversen Valve on the brake engine prevented the mechanical brakes from being applied effectively until many seconds had elapsed after the engineman moved the brake lever into the "On" position, in which period it is reasonable to assume that the Lilly Controller would trip ; and (3) the movement of the brake lever into the "On" position, however, automatically prevented the brake solenoid that was controlled by the Lilly Controller from opening the air escape valve on the brake engine in the "Fast Braking Zone" and consequently prevented the mechanical brakes from being applied effectively and quickly as they should have been. In other words the brake solenoid was prevented from doing its job. 59. It follows from my conclusions that, from the point of view of preventing the overwind, it was immaterial whether the Lilly Controller tripped during the overwind because a trip could not have increased the rate at which the speed of the winding engine was already being reduced. Contents COMMENTS.60. Tests of the Lilly Controller before it was altered after the overwind showed that it was not able to prevent the winding engine from moving the cages at a maximum speed of about 63 feet per second. This was a far higher speed than the management expected from the results obtained by their method of testing. The speed was dangerously high when persons were being carried because the stopping distance that was provided for by the Controller was much less than the distance in which a cage moving at that speed could be stopped by the application of the regenerative and mechanical braking. 61. After the Lilly Controller was altered it was able to prevent the winding engine from moving the cages at a speed exceeding 54 feet per second when winding mineral and 40 feet per second when winding persons. This reduction in the maximum speed at which persons could be carried has effected a great improvement in the standard of safety because the stopping distance for a cage is directly proportional to the square of the speed at which it is moving when the brakes are applied by the action of the Controller. Consequently the stopping distance required for a speed of 40 feet per second is only two fifths of that required for a speed of 63 feet per second. 62. This overwind and the investigation thereof show quite clearly that every automatic contrivance to prevent overwinding in accordance with the statutory Regulation should restrict the speed of winding persons so as to ensure that when the brakes are applied by the actions of the contrivance they will reduce the speed of the cage to a safe speed within the stopping distance provided for by the contrivance. This is but one of the fundamental precautions against overwinding which have been considered in several publications, especially in two papers*that were presented in 1946 and 1951. There is no doubt that dangerous overwinding of persons could not occur if the winding apparatus at every mine was constructed, operated, maintained and tested in accordance with those fundamentals. 63. The brake engine and ancillary apparatus included some features and aspects that were either unsatisfactory or not as good as they might have been. The lversen Valve had not been properly lubricated, examined, tested and maintained. The air escape valve could not be opened in the "Fast Braking Zone" by the emergency solenoid in the circumstances which have been described earlier. The throttling valve in the cataract system which provided slow braking was not constructed to ensure that it had a minimum aperture through which the fluid could pass. Thus there was a danger that incorrect adjustment of the linkage that operated the valve or the failure thereof might greatly delay or even prevent the application of the brake. This dead weight type braking system was arranged so that slow braking was enforced during the major part of the wind where normally the cages move quickly. This was done to reduce the undesirable stresses on the winding apparatus which are inherent disadvantages produced by the momentum acquired when the dead weight falls to apply the brakes rapidly. For this and other good reasons modern braking systems do not use dead weights except where they are required as a safeguard against the failure of any fluid medium that is normally used to apply the brakes. 64. The method of testing the automatic contrivance that was used until October, 1955, was not as satisfactory as it might have been because it failed to reveal, amongst other things, the maximum speed at which the cages could be moved in the shaft. The method of testing used between that date and the overwind, however, was unsatisfactory as it did not show that the Iversen Valve was defective and that the automatic contrivance could not prevent overwinding in some circumstances. The method of testing should be designed so as to try and defeat the automatic contrivance, because it is only by this means that the testing engineer may prove whether the automatic contrivance can prevent overwinding. The reports of the results of the tests that were made were not circulated to all the officials who should have received them. 65. The winding engine was not fitted with the accelerating and decelerating cams that form part of the normal cam gear to control a Ward Leonard winder that was manufactured some years ago. Although the control system of the engine was such that accelerating cams were not necessary, the decelerating cams would automatically have provided some regenerative braking and returned the power control lever at the same position of the cages in the shaft and thus warned the engineman if he had not taken the appropriate action to control the engine properly. Thus the primary safeguard of a Ward Leonard winder had not been fitted in this case. 66. The emergency push button, by which the winding engineman could operate the safety devices and bring the engine to rest was beyond his reach and was therefore not readily available in an emergency as it should have been. 67. A winding engineman said that on one occasion some five years ago grease or rope oil had got on to the brake paths and caused the drum to slip through the brakes. He thought the grease might have dropped from two plates that were used to prevent sunlight from dazzling the engineman. There was no evidence, however, to show that this had occurred recently. 68. The safety catches in the headframe were provided with linkages by which they could be manually actuated for testing purposes from the top of the headframe. This was a most inconvenient and unsuitable position which obviously resulted in a complete lack of regular testing and contributed to the rusting and inadequate maintenance of the catches. Consequently the catches did not operate properly during the overwind, the cage was tilted and it bent one set of catches that was not sufficiently supported. 69. The breakdown in communications which caused the delay in notifying the colliery manager of the occurrence might have had more serious consequences in other circumstances such as a fire or explosion. 70. In recent years the number of serious casualties caused by overwinding accidents has been very small in relation to the one half million or so persons carried through the shafts every working day. Notwithstanding this apparently satisfactory position, there is no doubt that a violent overwind may result in a far greater disaster than any recent underground fire or explosion. The potential hazards of overwinding are increasing in these days of reorganisation, concentration, deeper shafts and high unit outputs. Formerly the maximum number of persons at risk at one time in a shaft at some of the larger mines was 128, but before long the number may be 240 and in the future it may even be 480, where there are four large cages in a shaft. Although a winding engineman rarely makes a mistake, he is human and therefore liable to do so or to lose control of the engine at any time. It follows, therefore, that some other means of bringing the engine to rest safely must be provided. An additional engineman is not adequate for this purpose and consequently it is imperative to have an automatic contrivance and ancillary apparatus that are designed, constructed, tested and maintained so that they will perform this function. The overwinding and other hazards of raising and lowering persons in shafts are so great that they demand the use of winding apparatus which conforms to the highest standards of mechanical, electrical and civil engineering practice. Contents RECOMMENDATIONS.71. As a result of my investigation of the overwind, I make the following recommendations which should be attended to urgently: - 1. The performance of every automatic contrivance to prevent over- winding as required by Regulation 7 of the Coal and Other Mines (Shafts, Outlets and Roads) Regulations, 1956, should be critically examined forthwith having regard to the results of the investigation of the overwind which is the subject of this Report. 2. Effective steps should be taken forthwith to eliminate the deficiencies that are found in any automatic contrivance to prevent overwinding. 3. The Code of Testing the Safety Equipment of Winding Engines which the National Coal Board have nearly completed should be reviewed in consultation with the manufacturers of winding engines and safety equipment. The Code should be modified where necessary in the light of this Report and then applied without delay to the winding engines at the Board's mines. Copies of the Code should be made available to the owners of other mines where winding engines with automatic contrivances are used. 4. The safety equipment of every winding engine should be constructed, tested and maintained so that nothing can prevent any part of the equipment from taking that action by which it fulfils its purpose. 5. The advantages and disadvantages of using dynamic, regenerative or other electrical braking in conjunction with mechanical braking that is applied slowly when the automatic contrivance of an electric winding engine trips during that portion of a wind where normally the cages move quickly, should be reviewed at an early date in the light of this Report and of modern practice both in this and other countries. 6. Every emergency push button should be situated so that the winding engineman can easily and readily operate it from the place where he normally controls the winding engine. 7. Appropriate and effective steps should be taken where still necessary to prevent grease or oil from being deposited on the brake paths of every winding engine. 8. The safety catches in any headframe should be adequately supported and arranged so that they may be readily examined, tested and maintained in proper working order. 9. Effective arrangements should be made at every mine to ensure that any change in the telephone number or address of any official or person who must be notified of any emergency is circulated and posted where necessary at the appropriate time. ACKNOWLEDGMENTS.72. I wish to record by appreciation of the co-operation received from representatives of the National Union of Mineworkers' and the British Association of Colliery Management, officials of the National Coal Board and workmen at Brookhouse Colliery and H.M. Inspectors of Mines in the North Eastern Division. I am particularly indebted to Mr. J. H. Longstaffe, H.M. Inspector of Mechanical Engineering and Mr. S. Luxmore, H.M. Electrical Inspector of Mines and Quarries, and also to Mr. F. W. Lane and other officials of No. 1 (Worksop) Area, for supplying technical information facilitating my investigation. I have the honour to be, My Lord, Your Lordship's obedient Servant, A. E. CROOK. * Precautions Against Overwinding ", by A. F. Crook, M.I.Min.E., excerpt from the Trans. lnstn. Mm. Engrs., Vol. 105, Pt. II and Vol. 106, Pts. 4, 5 and 6. Safety Aspects of Winding Apparatus in Mines" by A. F. Crook, M.I.Min.E., presented at the Thirty-ninth National Safety Congress and Exposition of the National Safety Council, Chicago, III., Oct. 8-12, 1951. Contents APPENDIX I.System of Winding Two cages are wound by a direct coupled D.C. electric winding engine with a bi-cylindro-conical drum arranged for Ward Leonard control. The 2-deck cages land on baulks at the shaft bottom and on keps at the landing place in the headframe and they are decked simultaneously. Shaft Diameter- . .18feet. Depth (to lowest entrance)- 1,285 feet. Upcast or downcast.. .Downcast. Guides. Type of guides .............. Rope Number of guide ropes per cage 4. Disposition of ropes ....Near corners of cages. Method of suspension and tension ...Clamps in headframe and tensioned by .......deadweights in sump. Diameters of guide ropes-inches ...1Ό, 1(3/8), and 1½. Weight suspended from each rope-tons . Varies from 4. 6 to 5.95. Headframe. Construction ... Steel joist. Number of pulleys .2. Diameter of pulleys-feet ..16. Method of loading and unloading cages ...Pneumatic rams for each deck. Safety catches for cages .2 sets of multiple safety catches. Maximum overtravel distance before detaching hooks operate . .21 feet 3 inches. Cages. Number of decks per cage ..2. Number of tubs per deck .....2. Weight of cage-tons .5.78. Weight of suspension gear-tons ....1.15. Tare of 4 tubs-tons ...1.65. Nett load of coal (4 tubs)-tons .3.79. Total load suspended from rope capping when winding coal-tons ..........12.37. Winding Ropes. (a) Underlap Rope Type Locked coil. Diameter-inches ...1.55. Weight-lb./per foot .5.95 Actual breaking load (when new)--tons. .133.45. (b)) Overlap Rope. Type Locked coil. Diameter-inches ....1.50. Weight-lb./per foot ....5.65. Actual breaking load (when new)--tons. .123. Drum. Type Bi-cylindro-conical. Diameters-feet ..11.0 to 17.5. Rope distribution dead turns on small diameter ..2Ύ. live turns on small diameter ... ..3Ό. turns on scroll .5. turns on large diameter .17Ό. Mechanical Brakes. Number of brake paths ..2. Diameter of brake paths ..12feet 6 inches. Width of brake paths .8 inches. Type of brake .....Bottom anchored post. Type of friction lining ..Ferodo CR. Fabric. Method of application .Deadweights. Method of control ...Compressed air brake engine. Type of control valves ..........Iversen and air escape. Other controls .......Cataract cylinder with provision for fast and slow braking. Electrical Braking. System Regenerative. Winding Duty. Maximum number of winds per hour 75. Nett load of coal-tons. 3.79. Maximum number of men per cage .44. Winding Cycle. Duration of wind--seconds. .40. Decking time-seconds ..8. Total winding time--seconds ..48. Speeds. Nominal maximum winding speed-feet per second ..49.2. Nominal maximum drum speed--r.p.m 53.7. Winder Motor. Method of coupling .Direct. Rated horsepower (nameplate details) .. .1,200/2,040. Rated speed-r.p.m . .53.7. D.C. voltage ±425. Current-amperes ....2,260/4,000. Motor Generator Set. Main Motor. Type .3-phase, 50 cycles slip- ring induction. Rated horsepower .. .. 900/1,800. Voltage 3,300. D.C. Generator. Rating-kilowatts ....900/1,700. Voltage ±425 Speed-r.p.m .1,000/820. Flywheel. Weight-tons .11Ό . Automatic Contrivance. Type .. Type 'C" Lilly Controller with Duplex Governors. Contents Appendix II.List of Persons Injured.(36).Brian Jones, 24 years of age, occupation,Collier. Eric Hargate, 32 years of age, occupation,Haulage. Albert Baugh, 65 years of age, occupation,Fitter. George Dolman, 21years of age, occupation,Collier. Maurice Parkin, 25 years of age, occupation,Fitter. Harry Turner, 36, years of age, occupation, Belt Mechanic. Arnold Clarke, 30 years of age, occupation, Belt Mechanic. George Wall, 32 years of age, occupation, Bricklayer. Kenneth Crouch, 20 years of age, occupation, Filler. Arnold Keye, 35 years of age, occupation, Belt Mechanic. Frank Jones, 27 years of age, occupation, Collier. Stanley Cooper, 35 years of age, occupation, Signalman. George Brightman, 26 years of age, occupation, Collier. Ronald Beaumont, 22 years of age, occupation, Collier. Douglas Totty, 34 years of age, occupation, Belt Mechanic. Harold Doncaster, 35 years of age, occupation, Repair Filler. John Glossop, 30 years of age, occupation, Collier. Harold Shaw, 21 years of age, occupation, Collier. Ronald Bowler, 19 years of age, occupation, Electrician. Keith Collins, 20 years of age, occupation, Filler. Harry Gregory, 38 years of age, occupation, Haulage. Patrick Jones, 23 years of age, occupation, collier. Ronald Bradder, 23 years of age, occupation, Collier. William Powell, 32 years of age, occupation, Electrician. Thomas Brocklehurst, 24 years of age, occupation, Collier. Arthur Davison, 33 years of age, occupation, Belt Mechanic. James Brashaw, 28 years of age, occupation, Electrician. Barry Warriner, 21years of age, occupation, Collier. Terry Bolton, 24 years of age, occupation, Mechanic. John Garratt, 38 years of age, occupation, Safety Staff. Patrick Freeman, 20 years of age, occupation, Haulage. Chris. Pilkington, 20 years of age, occupation, Haulage. Frank Gilson, 24 years of age, occupation, Electrician. Dennis Smithson, 16 years of age, occupation, Haulage. John Briggs, 16 years of age, occupation, Fitter. Brian Blogg, 23 years of age, occupation, Haulage. NOTE: Of these reportably injured men, 17 sustained a fractured femur, five sustained a fractured tibia and six sustained fractures of other bones. Contents Photo. Photo. Photo. Plan. |
