Resistance to Drill Rotation in Percussive-Rotary Drilling

Ya. A. Serov

(Presented by Professor P. M. Alabuzhev)

General Provisions and Conditions of the Study

At present, little research has been done on percussive-rotary drilling of blastholes with independent rotation of the drill, which makes it possible to increase the drilling speed significantly. The works of N. N. Esin and O. D. Alimov \[1, 2\] initiated the study of resistance to drill rotation in percussive-rotary drilling.

The present work is one section of a general study of pneumatic drilling machines of percussive-rotary action and has as its aim the determination of the dependence of the torque on the drill on the feed force. This aim of the work determined the choice of the procedure and apparatus for carrying out experimental studies under laboratory conditions.

The experimental studies were conducted in the laboratory of percussive-action machines of the Tomsk Polytechnic Institute. In all experiments the blastholes were taken to be horizontal. As the drilling tool, drills with single-bit detachable crowns of diameter $d = 48\ \text{mm}$ were used, tipped with hard alloy of grade VK-15. Drill cuttings were removed from the blasthole by air flushing. The feed forces were taken from 13 to 71 kg, and the drill speeds were 100, 140, 180, and 220 rpm. Drilling was carried out in rock—skarn—of hardness $f = 12 \div 14$ according to Protodyakonov.

Experimental Setup and Method of Conducting the Studies

In accordance with the tasks set for the experimental study, in the laboratory of percussive-action machines a special drilling stand, a torque meter, and the necessary measuring instruments were designed and manufactured at the Tomsk Electromechanical Plant and in the laboratories of the Tomsk Polytechnic Institute. The design of the stand and the massive blocks of rock used made it possible to drill blastholes under conditions close to those of production.

The setup for conducting the studies consisted of a ZIF-ShVKS-5 compressor and a stand for mounting the drilling hammer with a torque meter and for securing the rock during drilling. The layout of the stand is shown in Fig. 1. The tested hammer 1 (Fig. 1) is fastened by clamping bars in carriage 2, which moves on wheels 3 in the grooves of

of the guide frame 4. The guide frame 4 is secured by means of a holder on the crosshead 5, fastened with clamps to the uprights of the welded frame of the stand 6. The design of the holder makes it possible to move the guide frame 4 along the crosshead 5, and also to rotate the guide frame relative to the vertical axis of the holder. The holder can rotate about the axis of the crosshead, which makes it possible to set the hammer at an angle to the axis of the frame. Fixing the guide frame in a definite position is carried out by tie bolts. The crosshead can be moved up and down along the frame uprights and, by means of clamps, fixed in the required position.

Fig. 1. Diagram of a stand for investigating percussive-rotary drilling.

The drilling hammer is installed on the stand without a drill holder. The rock specimen 7 intended for drilling is installed in the stand frame and fastened by means of jacks. The hammer is supplied with compressed air from an air receiver through a rubber hose 8 with an air-flow meter 9 and an automatic oiler 10 connected in series. The change in air pressure supplied to the hammer is effected by valve 11 and is monitored by pressure gauge 12.

To create the axial feed force on the hammer, a pneumatic pressing device was made, consisting of a cylinder 13 and a piston with rod 14. The cylinder 13 is connected by rubber hose 15 to the pipeline coming from the compressor. In cylinder 13 there is an opening connecting the cylinder with the atmosphere. By throttling the air from the feeder cylinder through this opening, a constant pressure is achieved and, consequently, a constant feed force during the drilling process. To eliminate the possibility of a change in air pressure in the feed cylinder due to its compression during oscillations of the hammer body, the piston feeder is connected to the network through hose 15 of increased diameter, \(d = 52\) mm, and length \(l = 4\) m. Oscillations of the hammer body at each blow cannot significantly change the pressure in the feeder, since, with a recoil amplitude of 2–3 mm, the degree of additional compression of the air buffer, taking into account the air-supply hose of larger diameter, will amount to less than 0.001 of the initial volume [3]. Iz...

—changing the feed force is accomplished by changing the air pressure in cylinder 13 by means of valve 16 and is monitored by pressure gauge 17.

To record diagrams of the oscillations of the hammer body during drilling, a rotating drum 18 is mounted on cylinder 13; it is driven by electric motor 19 through a gear reducer and belt drive. A rod 20 with pencil 21 is attached to the body of the hammer under test; this pencil records the oscillations of the hammer body over time on a sheet of paper fastened to the drum.

To measure the torque on the drill bit, after analyzing existing designs of torque meters, the torque-meter scheme developed by Professor A. M. Rozenberg and successfully used in the metal-cutting laboratory of the Tomsk Polytechnic Institute [4] was selected as the simplest and at the same time as providing the required measurement accuracy.

Figure 1 gives a schematic of the torque meter. The arrangement and operating principle of the torque meter are as follows. The bit of the drilling hammer under test is driven in rotation by a separate electric motor through a two-stage gear reducer. The electric motor for rotating the bit is a direct-current motor, so that it is possible to vary the bit speed within the required limits. The rotary mechanism of the drilling hammer was disabled by removing the pawls from the helical rod.

Gear 22 of the torque-meter reducer rotates the bit of the hammer under test by means of a detachable hexagonal bushing; at the same time the bit is able to move freely longitudinally in the hexagonal bushing. Gear 22 is meshed with parasitic gear 23, fixed on shaft 24, which rotates in ball bearings mounted in two yokes 25. The yokes are seated on ball bearings on shaft 26, which carries gear 27, and are rigidly connected to one another. When rotation is transmitted from the electric motor to the bit through the reducer and, consequently, through gears 29, 28, 27, 23, and 22, circumferential forces arise on parasitic gear 23; these forces tend to turn it together with yokes 25 counterclockwise (when the bit also rotates counterclockwise, if one looks at the bit from the side of the rock being drilled) about the axes of gears 27 and 28. The yokes transmit, through cylindrical rod 30 to a hydraulic capsule, circumferential forces proportional to the torque developed on the bit. The capsule is rigidly fastened to a bracket of the reducer housing.

Since the liquid in the capsule is practically incompressible, yokes 25 almost do not move, but they create a pressure on the piston of the capsule that is the greater, the greater the torque on the hammer bit. This torque can thus be recorded on a moving paper tape (tracing paper) by a self-recording pressure gauge connected to the capsule by a tube with an internal diameter of 2.5 mm. To eliminate oscillations of the liquid in the capsule–tube–pressure-gauge system, a steel wire was inserted into the tube, reducing its cross section. To eliminate clearances in the measuring system, a preliminary preload is created; for this purpose the yokes, through special spring 31, are pressed by a screw against the capsule. The magnitude of the preliminary preload was approximately 20% of the limiting load of the torque meter.

Calibration of the torque meter was performed with the aid of a lever and suspended weights. The lever was mounted on the front end of the bit, with the reducer gears wedged. The calibration scheme of the torque meter and the calibration curve are shown in Fig. 2. Calibration was carried out

10 times both in the direction of loading and unloading; in this case the scatter of the points was approximately 1.5%, which should be considered a quite good result. In the course of the investigations the torque meter was systematically subjected to control calibration, and the results coincided with the original ones. Such a calibration method is simple and quite accurate [4].

Fig. 2. Calibration scheme and calibration curve of the torque meter.

The design of the stand makes it possible to drill blastholes at various angles to the horizontal; to record during the experiment the length of the blasthole drilled during the test; to determine the number of blows and the number of revolutions of the drill per test; and to determine the drilling speed of various rocks at different compressed-air pressures and different feed forces. With the aid of the torque meter installed on the stand, it is possible during the experiments to record a diagram of the torque on the drill.

As indicated above, the torque diagram was recorded by a self-recording pressure gauge. Its arrangement is seen from the diagram in Fig. 3. From drum 1 the paper tape (tracing paper) is unwound, passes around drum 2 and roller 3. After passing around roller 3, the paper tape goes to recording table 4. The tape then moves between rubber-covered rollers 5 and 6, one of which is the drive roller and the other the pressure roller. Drive roller 5 receives rotation from electric motor 7 through gear reducer 8 and belt drive 9. Pressure roller 6, by means of steel springs 10, presses the paper tape against the dri-

to the drive roller 5, thereby preventing the belt from slipping on the drive roller. The instrument drive consists of an electric motor 7 with a power of 200 watts, at 1500 rpm, a reducer 8, and a belt transmission 9.

Diagram of motion of the paper belt

Torque-meter strain gauge

Fig. 3. Diagram of a self-recording torque meter.

At the front, in the upper part of the mechanism for drawing the belt, a pressure gauge is mounted for recording the torque, and on the side, on the wall of the mechanism, a pen for recording the zero line. Fig. 3 shows the pen for recording

Torque curve

Zero line

Fig. 4. Sample tape of the self-recording torque meter.

the zero line 11 and pen 12 for recording the torque diagram. The pens used were pen nibs. A sample tape with a recording of the torque diagram at a small feed force (20 kg) is shown in Fig. 4, and at a large feed force (60 kg)—in Fig. 5.

Torque curve

Zero line

Fig. 5. Sample tape of the self-recording torque meter.

The resistance to drill rotation during drilling was investigated as follows. The required compressed-air pressure was set in the hammer and in the feeder. The drilling hammer and the electric motor for rotating the drill were started simultaneously. The electric motor of the drive mechanism for the strip of the recording manometer was started somewhat earlier than the hammer, so that, when recording the torque diagram, the speed of the strip would be constant. It is difficult to use ordinary tachometers to determine the rotational speed of the drill, since, in addition to rotation, the drill performs reciprocating motion at high speed. In view of the absence of special instruments, the rotational speed of the drill during the experiments was measured by means of a cord. The essence of this method is that, before drilling begins, the end of a thin cord is fastened to the drill. During operation of the hammer the cord is wound onto the drill; the number of turns wound onto the drill corresponds to the number of revolutions of the drill during the experiment. By timing the operation of the hammer with a stopwatch and measuring the number of turns of cord on the drill after drilling, the necessary data for calculating the number of revolutions of the drill per minute are obtained. The numerical values of the torque were determined from the recorded diagram and the calibration curve, and the feed force as the product of the compressed-air pressure in the feeder and the area of its piston. Each experiment was repeated 4–5 times; mean values were calculated, on the basis of which the graphs shown in Figs. 6, 7, 8, and 9 were plotted.

Fig. 6.

Results of the Experimental Investigations

The investigation data are presented in the form of graphs in Figs. 6, 7, 8, and 9. Examination of the graphs shows that the resistance to drill rotation increases with increasing feed force. When the drilling hammer operates at compressed-air pressures of 4, 5, and 6 ati, the resistance to drill rotation remains approximately constant as the feed force increases; however, with an increase in the air pressure in the hammer and in the torque on the drill, it decreases somewhat. The latter can be explained by the fact that, with an increase in impact energy (as the compressed-air pressure in the hammer is raised), the rebound of the drill from the bottom hole increases and, consequently, the drill [[unclear: text continues on next page]]

freely between blows. In Figs. 7 and 8 this conclusion is not confirmed. This can be explained by the fact that the resistance to rotation of the bit decreases with increasing impact energy, apparently only slightly, and since the rocks drilled were heterogeneous, the scatter of the points exceeded the numerical value of this increase.

The torque on the bit increases with an increase in the number of revolutions of the bit (with an increase in the angle of rotation of the bit between blows). If, at a bit speed of \(n=100\) rpm, with a feed force of 20 kg (Fig. 6), the resistance to rotation of the bit when drilling with the hammer at 4 atm is equal to

Fig. 7.

39 kg·cm, then under the same conditions, but at \(n=220\) rpm (Fig. 9), the resistance to rotation of the bit is already 64 kg·cm; with a feed force of 60 kg, it is respectively 78 and 143 kg·cm. This is explained by the fact that after the blow of the bit against the bottom of the borehole, the latter rebounds and is then pressed by the feed force against the bottom of the borehole. During this time the moment of resistance to rotation of the bit increases; a new blow follows, rebound occurs, and the resistance to rotation of the bit drops. Consequently, the greater the angle of rotation of the bit between blows, the greater the average moment of resistance will be, and vice versa. Therefore, when designing drilling hammers, one should strive to ensure that the rotation device provides the minimum possible number of revolutions of the bit, proceeding from the condition of creating the optimum angle of rotation of the bit between blows; otherwise the rotation device will have to overcome unnecessary resistance to rotation of the bit.

As already indicated above, in carrying out the investigations, diagrams of the torques on the bit were recorded directly during drilling. In Fig. 4 a diagram is given of the torque on the bit at a small feed force (20 kg), and in Fig. 5 at a large feed force, 60 kg. From the diagrams it is evident that the resistance to rotation of the bit at a small feed force is uniform, while at a large feed force it has a step-like character. Therefore the torque values given for large feed forces are averages.

Fig. 8.

Graph labels:

• Resistance to drill rotation, kg·cm: 0, 20, 40, 60, 80, 100, 120, 140, 160
• Feed force, kg: 0, 20, 40, 60, 80

Legend:

• \(n = 180\) rpm
• ○ — 4 atm
• △ — 5 atm
• □ — 6 atm

Optimal feed forces for series-produced pneumatic hand-held drill hammers lie in the range from 30 to 70 kg; the resistance to rotation of the drill within these limits is respectively

[Figure: graph of drill-rotation resistance versus feed force. Vertical axis: “Resistance to drill rotation, kg·cm”; horizontal axis: “Feed force, kg.” Legend: ○ — 4 atm; △ — 5 atm; □ — 6 atm. Note: \(n = 220\) rpm.]

Fig. 9.

from 25 to 150 kg·cm. From this it may be concluded that for drill hammers having an increased value of the optimum feed force, a turning device is required that is capable of developing a considerably greater torque.

LITERATURE

1. Esin N. N. Investigation of light drill hammers, Novosibirsk (candidate dissertation), 1952.

2. Alimov O. D. Investigation of the mechanism of rock destruction in percussive-rotary drilling and of the initial parameters of drill hammers, Tomsk (candidate dissertation), 1953.

3. Calculations, design, and testing of mining machines (second collection). Edited by engineer A. V. Topchiev. Moscow, Ugletekhizdat, 1955.

4. Rosenberg A. M. Dynamics of milling. Moscow, Sovetskaya Nauka, 1946.