Fig. 3.8 shows the general principle of a typical hydraulic feed mechanism (excluding the engine and frame) for a deep boring, the main features, starting from the bottom, being the chuck, the bevel-driving gears, the hydraulic cylinder and the ball-bearing suspension box. At the top is seen a lifting bail and water-swivel whose functions are self-evident.
There are three hollow rods, one within the other, namely, the inner bore rod, the middle drive rod and the outer piston rod. The hollow bore-rod is clamped to the drive-rod by the chuck. The drive rod is secured to and supported by the collar-plate in the suspension box, the latter remaining supported by being fixed to the hot tow piston rod which carries the piston in the hydraulic cylinder. This cylinder is firmly fixed to the engine frame work.
The rate of feed of the bore-rods is governed by the rate at which the piston descends, for this governs the descent of the suspension box and therefore, of the collor plate and drive-rod to which the uppermost bore-rod is clamped. It will be seen that the drive rod is free to rotate within the suspension box and within the hollow piston rod.
The drive-rod has on its outer surface ribs or splines which pass through slots or grooves in the box of the horizontal bevel drive-wheel. It can thus be rotated by the gear wheels (whose position is fixed) but it is free to descend through the horizontal wheel, carrying with it the bore-rods and boring tools at their lower end.
The position in the hydraulic cylinder may be moved either up or down by admitting water under pressure to the appropriate side of the piston through the inlet pipe and one of the controlling valves, V1 or V2) and by simultaneously releasing an equal amount of water from the other side of the piston through one of the valves V3 or V4 and the outlet. A single lever operates the four valves simultaneously to produce any desired pressure, either downward or upward, on the piston. Thus the weight of the rods may be partly taken off the boring tools by upward pressure; or the whole weight of the rods can rest on the boring tools and additional downward pressure can be applied. In this way complete control may be obtained over the pressure on the boring tool and over the rate of forward feed of the rods.
The cuttings are cleared from the drill hole by circulating water under pressure which is forced down the drill rods by a pump through a flexible hose pipe and water swivel connection. The return water from the hole goes to a settling tank and it is used over and over again.
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