P. A. SAMOILOV

ON THE QUESTION OF IMPROVING KTsM-4 SCREW FEEDERS

(Presented by the Department of Mining Machines and Mine Transport)

The rates of growth of the mining and mining-ore industry outlined by the XXII Congress of the CPSU urgently require further improvement and the creation of new high-performance drilling machines and their mounting-and-feeding devices.

As experience and specially conducted studies [1] show, the perfection of the mounting and feeding devices, which are the most important elements of drilling machines, has a significant effect on their productivity and on improving and easing the work of drillers.

In testing certain types of drilling hammers with piston and screw feeders it was established that the drilling speeds of these hammers under optimum conditions are greater when they operate with a piston feeder. On average it may be considered that hammers, for example, of the KTsM-4, RH-754-IW, and T-10 types, when operating with a piston feeder (in the case of drilling sandstone), give an increase in drilling speed, in comparison with operation on the KTsM-4 screw feeder, of 8, 16, and 38%, respectively.

However, the KTsM-4 screw feeder is much simpler to manufacture, lighter in weight, and more convenient in operation. It therefore appeared expedient to carry out investigations of precisely this feeder, with the aim of establishing ways to improve it.

In analyzing the reasons for the higher drilling speeds when drilling hammers operate with a piston feeder, it was observed that the KTsM-4 screw feeder under optimum conditions provides a more rigid character of feeding the hammer to the face (with smaller amplitudes of oscillation). This, of course, to some extent affects the reduction in drilling speed, but does not fully explain what is required.

Taking into account, at the same time, the substantial influence—established by a number of studies [2, 3, etc.]—of the feed force of the hammer on the face on the drilling speed, the answer must be sought in the difference in feed forces when the hammers operate under optimum conditions. In this connection it is necessary to know the forces with which the hammer, operating on a screw feeder, was fed during the drilling process to the face. It should be noted that during testing of hammers with a screw feeder, it was not the feed force that was measured (this was a difficult task), but the air pressure supplied to the feeder motor.

The feed forces provided by the screw feeder under the investigated conditions can be determined if the mechanical characte—

characteristics of the feeder motor, i.e., the dependence of the torque on the number of revolutions of the drive shaft of the motor or on the lead screw of the feeder. From these characteristics, knowing the parameters of the lead screw and specifying the value of the friction angle of the screw pair, it is easy to establish the required feed force. However, this method, like some other indirect methods for determining the required quantities, is insufficiently accurate.

Therefore we took a different path—the path of directly recording the traction characteristics of the screw feeder, i.e., the dependences of the feed forces on the feed speeds at various pressures of the air supplied to the feeder motor.

The traction characteristics of the KTsM-4 screw feeder were recorded on a stand specially designed and manufactured by us (Fig. 1). Let us explain the arrangement of the stand and the method of conducting the tests.

Fig. 1. Diagram of the stand for recording the traction characteristics of the KTsM-4 screw feeder.

On the sleeve 1 of the screw prop 2 the screw feeder under test with drilling hammer 3 is fastened. To the front end of the feeder frame 4 a pneumatic jack 5 is attached, serving to create counterpressure to the moving drilling hammer. The rod 6 of pneumatic jack 5 bears against the front part of the hammer through cup 7. This cup prevents rotation of the pneumatic-jack rod by the hammer’s rotary bushing during its operation.

Feed of the hammer is effected by feeder motor 8 through lead screw 9 with nut 10. The pressure of the air supplied to the feeder motor and to the hammer is regulated by valves 11, 12 and monitored by pressure gauges 13 and 14. Compressed air is supplied to the pneumatic jack through pipe 15, in which valve 16 and cock 17 are installed. This fitting serves to regulate the pressure of the air supplied to the pneumatic jack. The air pressure is then monitored by pressure gauge 18.

During the tests it was found that, with an air pressure supplied to it of more than 3 ati, at low feed speeds (less than 0.5 m/min) and with the hammer operating, the feeder motor develops considerably greater feed forces than the installed pneumatic jack can provide. Therefore the insufficient counterpressure was produced by weight 19, suspended from the hammer through pulley 20.

The feeder tests were carried out with the hammer operating, at air pressures supplied to the motor from 0.5 to 4 ati. The air pressure was changed up to 2 ati in steps of 0.5 ati, and above 2 ati in steps of 1 ati.

The resistance to the motion of the hammer, created by the pneumatic jack, was varied in steps of 0.25, 0.5, and 1 ati, i.e., by 8, 16, and 32 kg, which made it possible to obtain a sufficient number of points on each curve of the traction characteristics. To calculate the feed speeds, during the tests the distance traveled by the hammer and the time of its motion were measured. The distance was measured with a steel ruler to an accuracy of 1 mm, and the time was recorded with a stopwatch to an accuracy of 0.2 sec.

The traction characteristics of the KTsM-4 screw feeder obtained in the course of testing with the drilling hammer in operation are shown in Fig. 2.

Fig. 2. Traction characteristics of the KTsM-4 screw feeder.

It should be noted that the traction characteristics of the feeder with the drilling hammer switched on were obtained when the hammer was operating at an air pressure of 2.5 ati. A higher air pressure at the hammer, as our additional tests showed, does not have a significant effect on the traction capacity of the feeder. In addition, at an air pressure of 2.5 ati the drilling hammer could operate idle for a longer time without danger of putting it out of action.

As can be seen from Fig. 2, the traction characteristics of the KTsM-4 screw feeder constitute a family of hyperbolic curves. Mathematical processing of the experimental results showed that these curves can be described by the formula

\[ P_{\mathrm{п}} = \frac{(7.6p^{5} - 73.5p^{4} + 281.7p^{3} - 491.2p^{2} + 398.3p - 144.9)\,v_{\mathrm{п}}} {(0.2p^{2} - 0.5p + 1.3)\,v_{\mathrm{п}} + 1} + \frac{90.5p + 112.8} {(0.2p^{2} - 0.5p + 1.3)\,v_{\mathrm{п}} + 1}, \tag{1} \]

where \(P_{\mathrm{п}}\) is the feed force, kg;
\(v_{\mathrm{п}}\) is the feed speed, m/min;
\(p\) is the pressure of the air supplied to the feeder motor, ati.

The values of the feed forces provided by the KTsM-4 screw feeder, established experimentally and also calculated by formula (1), indicate that drilling hammers with this feeder...

operate at optimal regimes with greater (Table 1), than with a piston feeder, feed forces. At large feed forces of the hammer at the face, smaller in magnitude departures of the hammer body from the limiting collar of the drill are obtained, which, as is known [4], lead to deterioration of the internal working process of the hammer and to a reduction in the efficiency of impact-energy transmission in the “piston—drill—rock” system. Hence the drilling speed is also lower than with a piston feeder.

Table 1

Note: The optimal feed forces for the piston feeder, the optimal air pressures supplied to the motor of the KTsM-4 screw feeder, and the drilling speeds at optimal regimes were obtained in tests of drilling hammers with these feeders on sandstones with a strength coefficient \(f = 6 \div 8\). The feed forces provided by the KTsM-4 screw feeder at optimal air pressures (\(p\)) supplied to its motor were established from the traction characteristics (Fig. 2) for \(p = 1\) and \(2\) atm, and for \(p = 0.75\), \(1.25\), and \(1.75\) atm were calculated by formula (1).

The feed force of screw pairs is determined by the following formula, known from applied mechanics [5]:

\[ P_{\text{p}} = \frac{2M_{\text{kr}}}{d_{\text{sr}}\operatorname{tg}(\alpha+\rho)}, \tag{2} \]

where \(M_{\text{kr}}\) is the torque applied to the screw;
\(d_{\text{sr}}\) is the mean diameter of the screw;
\(\alpha\) is the helix angle;
\(\rho\) is the friction angle of the screw thread.

From formula (2) it is evident that, all other conditions being equal, the magnitude of the feed force is inversely proportional to the helix angle of the screw. Thus, naturally

one may suppose that lower feed forces at the optimal operating regimes of drilling hammers and, accordingly, an increase in the drilling speed with a hammer mounted on a screw feeder can be obtained by using a feeder lead screw with a larger helix angle.

Such a screw feeder (of the PVD-1 type) was created with the participation of the author at the Department of Mining Machines and Mine Transport of the Tomsk Polytechnic Institute. Its lead screw has a helix angle of \(22^\circ 20'\), instead of \(17^\circ 40'\) in the KTsM-4 screw feeder.

Comparative tests of the PVD-1 and KTsM-4 screw feeders showed that the PVD-1 feeder provides an increase in drilling speed by an average of 35%. Consequently, the identified path for improving the screw feeder deserves attention. Further investigations are required to establish the optimal helix angle of the feeder lead screw, which, it is to be hoped, should lead to the creation of a high-productivity screw feeder.

References

1. O. D. Alimov, P. A. Samoilov. Some results of testing experimental specimens of long-stroke feeders. Izvestiya TPI, vol. 108. Investigation of drilling machines. Metallurgizdat, 1959.

2. O. D. Alimov. The influence of feed force on drilling speed with pneumatic drilling hammers. Izvestiya TPI, vol. 75, Tomsk, 1954.

3. I. G. Basov. Investigation of the influence of feed force on the drilling speed and operating regime of pneumatic drilling hammers. Candidate dissertation, TPI, Tomsk, 1958.

4. V. F. Gorbunov. Experimental investigation of the working process of pneumatic drilling hammers. Candidate dissertation, TPI, Tomsk, 1958.

5. V. S. Polyakov, V. N. Kudryavtsev, M. P. Zubanov, A. S. Anosov, I. D. Barbash, V. D. Myagkov. Machine Parts. Mashgiz, 1953.