They are suitable for any degree of toughness of the

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: . These are low cost, simple in design and suitable for rough handling and use. These drills suffer from the disadvantage of its low efficiency in terms of its input compressed air power. Also these drills are noisy and their exhausts generate mist and fog. They are suitable for any degree of toughness of the rock. 4.7.3 HYDRAULIC DRILLS Introduction of hydraulic drills in underground mines and tunnels is recent. Presently more than dozen-reputed manufacturers are in arena giving considerable different designs. Initially rotary hydraulic drills came up and later on the rotary percussive drills. The hydraulic drills operate by the intermittent application of high-pressure hydraulic oil to a double acting piston; the frequency oil application is controlled by the movement of piston or by the action of sliding or rotating valve or by a combination of both. Both may operate by the oil pressure, or by the piston or the valve. These drills have separate rotation motors giving adjustable rotation speeds maximum of 30 rpm. Some of the models are also fitted with reversible rotation mechanisms. One to four boom jumbos are available for their use in civil tunnels, and development and stoping operations in underground mines. The advantages claimed by their use are: ● ● ● ● ● 45% of the energy input in this case is converted into useful work as compared to 15% that in case of conventional drills. High penetration rates, longer bit’s and steel’s life. High drilling productivity comparing the pneumatic drills. Reduced noise level from 3–17 dB than the silenced pneumatic drills. Some of the models give adjustable piston stroke, thereby; larger strokes with less frequency can be used in tough rocks and where as short stroke with high frequency can be chosen for the brittle rocks. These drills require high capital investment, skilled operators and high degree of engineering and maintenance skills. Their use in small mines or tunnels with capital scarcity situations cannot be justified; but new mining ventures and tunnels aiming at high output could find them beneficial. As per the mechanical action upon the hole bottom they can be classified as percussion, rotary and rotary-percussion types of hydraulic drills. 4.8 DRILLING ACCESSORIES1,4,7 As shown in figure 4.2(d) to drill a hole apart from rock drill, some drilling accessories are required. These are integral drill steels, extension rods, shank adapters, sleeve couplings and bits. An integral drill steel as shown in figure 4.8 consists of a rod with a forged shank at one end and a forged bit with cemented carbide inserts at the other end. Thus, each steel is of a specific length and cannot be extended. Once first drill steel has drilled all the way into the rock; it is withdrawn and replaced by the longer one to drill further into the rock. The integral drill steels are available in increasing lengths with reduced diameters, as shown in table 4.2. Thus the smallest drill section has the largest diameter and its diameter is selected as per the size (diameter) of the explosive © 2005 by Taylor & Francis Group, LLC 72 SURFACE AND UNDERGROUND EXCAVATIONS Table 4.2 Integral drill steel series (size specifications).7 22 mm integral chisel bit hallow hexagonal drill steel Length, mm Bit diameter, mm Length, mm Bit diameter, mm 800 1600 2400 3200 4800 34 33 32 31 29 800 1600 2400 3200 36 35 34 33 Drilling accessories Reaming bits Integral drill steels Cross bit 35 – 51 mm Button bits 33 –127 mm Drilling accessories – integral drill steels, bits – cross, button, and x types; couplings, shank adopter, extension rods, etc. Figure 4.8 Common drilling accessories used during development drifting, raising and tunneling operations. (Courtesy: Atlas Copco; Sandvik – Tamrock)1,7 cartridge. The most common integral drill steel is chisel-type and other ty...
View Full Document

This document was uploaded on 02/12/2014.

Ask a homework question - tutors are online