JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B. Tech III Year MINE MECHANIZATION - II Previous Question Paper (March - 2021 - R18)

Pit top and pit bottom layouts are critical infrastructure components in underground mining that facilitate the safe and efficient transfer of personnel, materials, and minerals between the surface and the underground workings. These layouts must be designed to minimize congestion, ensure ventilation efficiency, and maintain high throughput.

Pit Top Layout: The pit top serves as the interface between the shaft and the surface transport system. Its primary functions include receiving mineral-filled tubs or skips, discharging them, and returning empty units to the shaft. A typical layout includes:

• Shaft Headgear and Landing: The structural frame supporting the winding pulleys and the landing platform where cages are positioned.
• Tippler/Unloading Station: Located near the shaft to receive loaded tubs. Rotary or side-discharge tipplers are commonly used to transfer coal onto conveyor belts.
• Circulation Tracks: A system of tracks designed for the continuous movement of tubs, often utilizing gravity or creepers to move empties back to the shaft.
• Safety Devices: Includes keps (to support the cage), arresting gear, and signaling systems to prevent over-winding or accidental descent.

Pit Bottom Layout: The pit bottom is the underground hub where the shaft meets the main haulage roads. It must handle high volumes of traffic while maintaining structural stability. Key features include:

• Shaft Inset: The widened area around the shaft where loading and unloading occur.
• Loading/Unloading Sidings: Parallel tracks or bypasses that allow loaded tubs to be marshaled for hoisting while empties are distributed back to the working faces.
• Pump Room and Sump: A dedicated area for managing mine water, usually located at the lowest point of the shaft bottom.
• Ventilation Air-locks: Essential doors or regulators to prevent short-circuiting of the main ventilation current at the shaft bottom.
• Storage Areas: Buffer zones for materials, equipment, and explosives, kept separate from the main traffic flow for safety.

In a standard layout, the Pit Bottom is often designed as a 'double-track' or 'loop' system to ensure that the winding cycle is not interrupted by traffic bottlenecks. The efficiency of these layouts is measured by the time taken for cage decking and the capacity of the shaft to handle the required daily output.

A safety detaching hook is a critical safety device installed between the winding rope and the cage or skip to prevent accidents during an overwind. It consists of two main outer plates and two inner plates, which are held together by a central copper shearing pin and a main shackle pin.

The constructional details include:

• Outer Plates: Two steel plates shaped with a central slot and a specific profile that allows them to engage with the detaching bell.
• Inner Plates: Two inner plates that pivot on a central pin and contain a slot for the winding rope shackle.
• Shearing Pin: A copper pin that keeps the inner and outer plates in their normal operating position, designed to shear under a predetermined force.
• Detaching Bell: A heavy steel cylinder fixed at the top of the headframe, featuring a tapered internal bore that acts as the trigger for the hook.

The functioning during an overwind is as follows:

1. If the cage is hoisted beyond the normal tipping point, the detaching hook enters the detaching bell at the headframe.
2. The tapered profile of the detaching bell forces the outer plates of the hook to move inward.
3. This movement causes the copper shearing pin to break, allowing the inner plates to rotate.
4. As the inner plates rotate, the winding rope shackle is released from the hook, effectively detaching the rope from the cage.
5. Simultaneously, the outer plates expand into a locking position above the bell, securely suspending the cage in the headframe and preventing it from falling back down the shaft.