The Winding Engine - Continued
At the time of the disaster a Burns Cradle type brake was in use. It consisted of a pair of Ferodo lined brake shoes applied to the underside of the brake paths by the action of a compressed spring nest operating through a system of levers. The brake was released by using compressed air to counteract the force of the spring nest and move the brake shoes away from the drum brake path. During normal braking the winding engineman’s brake lever operated an Iversen type valve which controlled the flow of compressed air to a servo-cylinder. The force from the spring nest was transmitted to the main level of the brake system by a 2 inch diameter steel rod 8 feet 11 inches long which was located in the centre of the nest, constrained by a plate at the top and connected by a crosshead trunnion to the main lever at the bottom. The piston of the servo-cylinder was connected to the free end of the main lever and when compressed air was admitted to the cylinder this lever was forced down and the brake released.
If the emergency power supply failed or any of the safety devices operated, or the emergency button was pressed, the emergency brake solenoid was de-energised and the ‘un-grabbing’ gear immediately disengaged the engineman’s brake control level from the Iversen valve and caused the mechanical brake to be applied. The ‘ungrabbing’ gear could not be reset until the brake control level was returned to the ‘brakes on’ position and the safety circuits energised.
The automatic safety equipment was a Black’s controller driven from the drum shaft through a system of gears which also drove a vertical pillar type depth indicator which showed the position of the cages in the shaft. It was designed to cut off the power supply to the winder motor and cause the mechanical brake to be applied to prevent the cages, reaching an excessive speed in the acceleration, constant speed of retardation zones, being landed on the baulks at a speed exceeding five feet per second and travelling beyond a predetermined position above the highest landing. The controller had a centrifugal governor, dial mounted cams and overspeed and overwind switches which operated in conjunction with back-out switches and two electrical safety circuits.
One of the circuits, which included the brake solenoid and all the protective features provided by the automatic safety equipment was interlocked with the pilot safety circuit which had two contactors in parallel. These contactors could be tripped by various safety devices including the winding engineman’s emergency stop button, and overwind switch on the depth indicator, the Ward Leonard overcurrent relay and the slack rope and brake wear switches. The pilot safety connectors had contacts in series with two parallel connected main safety contactors which controlled the supply of electricity to the winder motor and the brake solenoid. Visual indicators in the engineman’s cabin showed correct or incorrect operation of the pilot and main safety contactors.
Before the winding engineman operated a lever which, through a linkage, altered the position of one of the cams on the automatic safety contrivance to enforce lower winding speeds. A switch on the lever gave a visual indication to the banksman, onsetter and the winding engine man that the lever was set for ‘man winding’ but did not positively prove the position of the cam on the automatic contrivance. A rope speed indicator was installed in the engineman’s cabin next to the ammeter about 13 years before but the associated tach-generator was not fitted and the indicator never operated.