JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III MINE MECHANIZATION - II Previous Question Paper (February - 2022 - R18)

A pit bottom layout with shunt back arrangement is a track layout designed to facilitate the efficient movement of mine cars between the shaft bottom and the main haulage roads. It is primarily used to manage the flow of full cars arriving from the mine and empty cars returning from the shaft, ensuring that the hoisting cycle remains uninterrupted.

The layout operates on the principle of gravity or mechanical shunting to reverse the direction of travel without requiring complex track switches or excessive manual handling. In this arrangement, the track is laid out such that the incoming full cars are directed onto a specific siding. Once the cars reach the end of this siding, they are shunted back onto a parallel track that leads directly to the shaft landing. This 'shunt back' movement allows the locomotive or haulage rope to remain on the same side of the train, effectively reversing the direction of the cars.

Key components and operational steps include:

• Approach Tracks: Tracks leading from the main haulage roads to the pit bottom, often graded slightly to assist car movement.
• Shunt Back Siding: A dead-end track section where the train of cars is brought to a stop. The length of this siding must be sufficient to accommodate the entire train.
• Reversing Mechanism: Once the train enters the shunt back, the locomotive detaches or the haulage rope is released, and the train is moved onto the return track leading to the shaft cage.
• Shaft Landing: The area where cars are loaded into the cage. The layout ensures a continuous supply of full cars and a clear path for empty cars to be returned to the haulage system.

A diagram of this layout would show the main haulage road branching into two parallel tracks at the pit bottom. One track serves as the entry for full cars, which then enters the shunt back siding. From the siding, the cars are moved onto the second track, which acts as the loading station for the shaft. This configuration minimizes congestion at the shaft bottom and allows for high-capacity hoisting by ensuring that the shaft is never waiting for cars.

The primary advantage of the shunt back arrangement is its simplicity and reliability, as it avoids the need for complex turntables or traversers. It is particularly effective in mines where the haulage system is based on track-bound mine cars and where space at the pit bottom is constrained.

The King detaching safety hook is a critical safety device installed between the winding rope and the cage in vertical shaft mining. Its primary purpose is to prevent accidents caused by overwinding by automatically detaching the cage from the rope and suspending it in the headframe if the cage travels beyond the designated landing level.

The construction consists of two main steel plates (side plates) held together by a central pin and two outer pins. Between these plates are two inner plates (shackle plates) that can pivot. The assembly is held in the 'normal' operating position by a copper shear pin. The top of the hook is connected to the winding rope shackle, while the bottom is connected to the cage chains. The outer edges of the plates are shaped with a specific profile, including a 'neck' and a 'shoulder' section.

The working principle is as follows:

1. During normal operation, the copper shear pin keeps the plates aligned, allowing the load to be transmitted directly through the central pin.
2. If an overwind occurs, the hook is pulled into a detaching bell, which is a heavy steel cylinder fixed in the headframe.
3. As the hook enters the bell, the tapered outer shoulders of the plates strike the internal rim of the bell.
4. The force of the impact shears the copper pin. This causes the inner plates to pivot, forcing the top shackle (connected to the rope) to be pushed out of the hook mechanism.
5. Simultaneously, the 'shoulders' of the hook lock onto the top of the detaching bell, effectively suspending the cage securely in the headframe and preventing it from falling back down the shaft.

In summary, the device acts as a mechanical fuse that ensures the cage remains stationary in the headframe in the event of a hoist control failure, protecting personnel and equipment from the consequences of a rope detachment or fall.