The Winding Engine
The winding engine was a 440 h.p. Ward Leonard supplied by the British Houston Company Limited of Rugby. The mechanical parts were designed and manufactured by Markham and Company Limited of Chesterfield. The engine was installed at the Do Well Colliery which was owned by the Staveley Coal and Iron Company Limited in 1921 and transferred to the Markham No. 2 shaft about 1930. In 1945 a cylindrical drum was fitted with a centre ring and in 1949 a replacement automatic contrivance of the torque controller type was installed and commissioned by Tudor Auto Services Limited which at the time of the accident was called Blacks Equipment Limited.
In 1952 the original deadweight brake was replaced by a servo-spring brake unit which was in use at the time of the accident. At the same time a Lockheed hydraulic impulse brake tripping system was installed but this was replaced in 1960 by a Blacks high pressure oil tripping system. In the same year the 30 hertz alternating current apparatus was converted to operate from a 50 hertz power supply.
In 1961 the number of men permitted to ride in the cage was increased from 24 to 32. Thicker barrel plates and a new centre ring were fitted to the drum in 1963 and the cast iron brake shoes and fulcrum brackets were replace by mild steel parts the following year.
The engine was used to wind only men, materials and limited quantities of stone and as this did not require a constant winding cycle, it was left to the enginemen to regulate the speed and rates of acceleration. The ropes were attached to the drum by means of white metal swords capels which were bolted to the drum sides next to the brake paths and were double layered when they were fully wound on. A new rope had about 11 ‘dead’ coils to allow for recapping. The drum was connected by a slid coupling to a direct current winder motor which had separately excited field windings and an open loop control system. The armature was supplied by the generator of a Ward Leonard motor generator set which was driven by an alternating current slip-ring induction motor housed with the associated control equipment in a separate building adjacent to the winding engine house.
The speed and the direction of rotation of the winder motor was controlled by the engineman’s control lever within a quadrant which had a middle ‘off’ position. Forward or backward movement of this level operated electrical contacts which determined the polarity of the generator field and thus the direction of rotation of the winder motor. The lever also operated a rheostat which varied the generator field strength and hence the output voltage of the generator. Except at low speeds the winder motor speed was proportional to the voltage for a particular load but for each change in the load these proportionally altered.
One of the winding enginemen described a normal wind with equal numbers of men in each cage, he said that after receiving the necessary signals, he moved the control level in the appropriate direction from the ‘off’ position, gradually applying power to the winder motor and slowly releasing the mechanical brake. The voltage applied to the motor was then increased to accelerate the winding drum until the voltage reached 300 to 320 volts which gave a drum speed of about 20 feet per second (13.6 miles per hour). This speed was maintained until the position of the cages in the shaft corresponded to about 10 drum revolutions (283 feet) from the end of the wind when the generator voltage was gradually reduced by moving the control level towards the ‘off’ position to increase braking. This regenerative braking was available to reduce speed during a wind and if there was a descending out-of-balance load, to maintain a selected speed. During this regenerative braking, the winder motor, which was then being driven by the load, became a generator with an output voltage higher that that of the Ward Leonard generator. This caused the generator to drive the alternating current motor which in turn became a induction generator feeding power back into the colliery supply system which thus produce a braking effect on the winding engine. The engineman continued this braking as the wind continued by progressively reducing the voltage to about 200 at 6A drum revolutions (184) feet from the end of the wind, when a warning bell rang. He continued to decrease the voltage and the speed of the cages by gradually bringing the level to the ‘off’ position. At about four revolutions (113 feet) from the end of the wind he progressively applied the mechanical brake until the cages come to rest. With a heavy out-of-balance load descending, the engine was controlled in a similar manner but the mechanical brake was applied earlier.
The engineman estimated the loads in the cages from readings of the winder motor current on a centre zero ammeter which was connected in the main direct current loop.
When loads in the two cages were similar the starting current was about 800 amps and this reduced to zero as the cages approached mid-point of the shaft. As the weight of the descending ropes became predominant, the motor current automatically reversed to produce the regenerative braking. The value of the current was indicated on the opposite side of the ammeter scale. With a minimum out-of-balance load descending the starting current was lower, regenerative braking developed earlier and the magnitude of the braking current was greater than that obtained with equal loads in the cages. Although the regenerative braking provided and efficient means of reducing the speed of the Ward Leonard winding engine, it was available in the No. 3 winding engine only when the electrical systems were operating normally. There was no regenerative braking if the power supply failed, if any of the safety devices operated or if the emergency stop button was pressed.