On smaller machines, driven either by hand-power or mechanically, the feed is by a screw feed arrangement comprising a series of differential gears.
In this arrangement the drill rods pass through a hollow screw shaft, threaded on the outside, and provided with a long keyway. A bevel pinion, rotated by the bevel gear of the main driving shaft, has feathers engaging the keyway on the screw shaft, to which it imparts rotation. It also drives gear wheel A, engaging with gear wheel B on a. countershaft, Usually three different combinations of gear are provided here, any of which can be utilised to vary the rate of advance to suit the type of rock. Gear B, through the counter-shaft, drives C, which engages a fourth wheel D, threaded internally to fit the threads of the screw shaft. If, for example, the number of teeth on" gears A, B, C and D are as shown in the diagram, viz. 38, 36, 24 and 25, one revolution of A= 38/36 revolution of B and C, = 38/36 x 24/ 25 revolutions of D. Therefore 75 revolutions of A will cause D to rotate 75 x 38/36 x 24/25 = 76 times. Consequently for every 75 revolutions of A, D revolves 76 times, and the screw shaft moves forward by a distance equal to the pitch of its thread. If this is 6 mm, then the rods advance 24 mm for every 300 revolutions. The movement of the screw shaft is imparted to the rods by the chuck.
A pressure gauge attached to the roller friction collar records the varying pressure on the bit.
The setting of the feed decided upon is obtained by sliding a lever to the desired position for locking the appropriate loose gear, giving the speed required for the ground being bored.
The feed ratio may be changed while the drill is operating; but if any necessary change is neglected, so causing the drill rods to advance too rapidly (e.g. when a softer stratum is suddenly encountered) the rate of advance is automatically checked by the slipping of a spring-loaded friction cone-clutch on the bottom of the countershaft
Fig. 3.7. A screw feed head on a diamond core drill Engine
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