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

A headgear, or headframe, is a structural tower erected over a mine shaft to support the winding pulleys (sheaves) and guide the winding ropes during the hoisting of men, materials, and minerals. It serves as the critical interface between the underground shaft and the surface winding machinery.

The constructional details of a typical headgear include the following main components:

• Main Legs (Backstays): These are the primary structural members that support the vertical load of the sheaves and the tension of the winding ropes. The backstays are inclined to counteract the resultant force exerted by the winding ropes, ensuring the stability of the structure against overturning.
• Front Legs (Vertical or Inclined): These support the weight of the headgear structure itself and the vertical component of the load from the winding sheaves.
• Sheave Platform (Headframe Deck): Located at the top, this platform supports the winding sheaves. It is designed to withstand high dynamic loads and is equipped with maintenance access.
• Winding Sheaves: Large diameter wheels over which the winding ropes pass. They are designed to minimize rope fatigue by maintaining an appropriate D/d ratio (sheave diameter to rope diameter).
• Detaching Hook Plate (Bell): A safety device located at the top of the headgear that catches the cage or skip in the event of an overwind, preventing it from crashing into the sheaves.
• Guide Ropes and Tensioning Weights: The headgear provides the anchorage for the shaft guides (rigid or rope guides) that keep the conveyance stable during travel.
• Landing Levels: Platforms at the surface level where the conveyance stops for loading and unloading.

The structure is typically constructed using lattice steel girders or reinforced concrete to provide high strength-to-weight ratios and durability. The design must account for the static weight of the conveyance, the dynamic loads during acceleration and deceleration, and the tension forces from the winding ropes directed toward the winder house.

Diagram Description: Imagine a triangular frame where the vertical or slightly inclined front legs support the sheaves, and the robust backstays are anchored into deep concrete foundations to resist the pull of the winding ropes coming from the winder drum. The sheaves are mounted at the apex, with the cage/skip suspended vertically below them within the shaft collar.

Balancing the torque on a winding drum is essential to reduce the peak power demand on the motor and ensure smooth operation. The primary methods used to achieve this balance are:

• Tail rope (Balance rope): A rope of weight equal to the winding rope is attached to the bottom of the cages or skips to compensate for the varying weight of the suspended ropes.
• Counterweight system: A separate weight is moved in a dedicated compartment to balance the dead weight of the cage and part of the payload.
• Conical or Bi-cylindro-conical drums: The varying radius of the drum changes the effective leverage (torque arm) as the cage moves, compensating for the changing rope weight.
• Ward-Leonard control system: An electrical method using a motor-generator set to control the speed and torque characteristics of the hoist motor.

The bi-cylindro-conical drum is designed to provide mechanical torque balancing by utilizing two cylindrical sections of different diameters connected by a conical transition section. The torque-time diagram for this system typically exhibits the following phases:

1. Acceleration period: The torque rises sharply to overcome the inertia of the system and the static load. As the rope begins to wind onto the smaller diameter, the mechanical advantage is high, helping to limit the peak starting torque.
2. Transition period: As the rope moves from the small cylinder onto the conical section, the effective radius increases. This increase in radius compensates for the reduction in the weight of the ascending rope, keeping the torque requirement relatively stable.
3. Constant speed period: Once the rope reaches the larger cylindrical diameter, the torque remains nearly constant. The larger radius at the ascending side balances the weight of the descending rope and cage, effectively neutralizing the gravitational torque imbalance.
4. Deceleration period: The torque drops significantly as the motor acts as a generator (regenerative braking) to slow down the inertia of the moving masses. The torque curve shows a negative value during this phase as energy is returned to the system.

By matching the drum profile to the specific depth and load requirements, the bi-cylindro-conical drum minimizes the difference between the maximum and minimum torque, leading to a more uniform power demand and reduced energy consumption.