JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV ROCK FRAGMENTATION ENGINEERING Previous Question Paper (July/August - 2022 - R18)

A percussive drill machine operates on the principle of impact energy, where a piston strikes the shank end of a drill steel to transmit a shock wave through the rod to the bit, which then crushes the rock. The constructional features consist of a cylinder, a reciprocating piston, a valve mechanism, a rotation mechanism, and a flushing system.

Constructional Features:

1. Cylinder and Piston: The core component is a cylinder containing a heavy, reciprocating piston driven by compressed air.
2. Valve Mechanism: A valve (often a spool or plate type) controls the alternate admission of compressed air to the front and back of the piston, creating high-frequency reciprocating motion.
3. Rotation Mechanism: A rifle bar and nut assembly or a separate air motor rotates the drill steel during the return stroke of the piston to ensure the bit strikes a fresh rock surface with each blow.
4. Flushing System: A central tube or side port delivers water or compressed air through the hollow drill steel to the bit face to clear rock cuttings (sludge) from the hole.
5. Chuck: The front end of the machine holds the drill steel shank, allowing for both axial impact and rotational movement.

Operation:

Compressed air is supplied to the drill, where the valve mechanism directs air to force the piston forward at high velocity. The piston strikes the shank of the drill steel, generating a compressive stress wave that travels to the bit. Upon impact, the bit crushes the rock. During the piston's return stroke, the rotation mechanism turns the drill steel by a small angle. Simultaneously, the flushing medium is forced through the center of the rod to evacuate the crushed rock particles. This cycle repeats at high frequency (typically 1500–3000 blows per minute) to achieve rapid penetration.

Line Diagram Description: The diagram should illustrate the internal cylinder housing the piston, the rifle bar connected to the piston for rotation, the valve chest at the rear, the chuck assembly at the front, and the central flushing tube passing through the piston and shank to the drill bit.