Normally core barrels of 0.5 m to 3 m length are employed. For removal of core during the conventional method of drilling, all the drill rods have to be withdrawn to the surface, after drilling a length equal to length of core barrel. The withdrawal of the rods and their re-introduction into the borehole with the additional drill rod, after removal of core, takes a considerable time, nearly 75 to 90% of the total time spent on drilling. A wire line drilling technique is an improvement, to reduce this time. The rods are not taken out to remove the core which is collected in the core barrel tube during drilling. The tube is pulled to the surface through the drill rods with the help of a catcher which is lowered through the rods by a 5 mm dia. wire rope. The catcher grips the tube containing the core. Core size less than BX is not possible with wire line coring equipment which is therefore used for drilling holes of NX and BX size only. The speed of drilling with this equipment is nearly 18 m per shift (8 hours) in the types of rocks met with in the coalfields. All the drill rods need to be withdrawn to the surface only when the bit has to be changed.
Wire line drilling is possible upto a depth of 1000 m. As stated earlier, the rods used for wire line drilling have specifications as laid down in "Q" series decided by DCDMA. An ordinary drilling equipment can be adopted to wire-line drilling and hoisting equipment with suitable modifications.
Some of the recent drills in the market are equipped with hydrostatic drive. In such drive an electric motor or a diesel engine drives a water pump. Its pressure is used for rotation of drill rods through a hydraulic motor and also for hydraulic feed. It’s main advantage is that speed can be varied from zero to a certain limit without any fixed ratios that are possible in a geared drive. Fig. 3.8. Hydraulic feed mechanism
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