On the Performance of Core Drills

L. T. Dvornikov

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

To drill a single blast hole of depth \(l\) at drilling speed \(V\), it is necessary to spend a time

\[ t_p=\frac{l}{V}, \ \text{min}. \]

Both \(l\) and \(V\) are variable quantities, and therefore \(t_p\) may vary over a wide range.

The average depth of blast holes used in driving preparatory workings at present is \(1.8\text{–}2.0\ \text{m}\). However, in some cases blast holes with depths of both \(1\ \text{m}\) and \(3.5\ \text{m}\) are used. Drilling speeds have a considerably wider range—from \(50\text{–}100\ \text{mm/min}\) to \(4000\ \text{mm/min}\) and more.

In Fig. 1 are presented the dependences of the change in the time spent on drilling proper on the drilling speed for blast holes of different depths [1]. Analysis of these dependences shows that:

Fig. 1. Influence of drilling speed and blast-hole depth on drilling time

1. At a constant blast-hole depth, an increase in drilling speed leads to a nonlinear, inversely proportional decrease in drilling time. Thus, for \(l=2\ \text{m}\), increasing the drilling speed from \(0.4\) to \(0.5\ \text{m/min}\), i.e., by \(0.1\ \text{m/min}\), gives a reduction in blast-hole drilling time of 1 min, while increasing the drilling speed from \(2\ \text{m/min}\) to

4 m/min, i.e., at 2 m/min gives a reduction in drilling time of only 0.5 min.

1. Increasing the drilling rate above 2 meters per minute practically does not lead to any significant reduction in drilling time. Since the time of auxiliary operations when working with core electric drills is a rather large quantity (4–8 min), increasing the drilling rate above 1.5–2.0 m/min cannot give a noticeable increase in productivity.

2. The depth of the blastholes has a substantial effect on drilling time at low drilling rates; at high drilling rates this effect is considerably reduced.

The productivity of the drilling machine \(Q\), in blastholes drilled per unit time, is determined as

\[ Q=\frac{1}{t_p+t_x}; \tag{1} \]

where \(t_x\) is the time of auxiliary operations.

According to the data of O. D. Alimov, the time of auxiliary operations [2] includes several components:

\[ t_x=t_1+t_2+t_3+t_4, \]

where \(t_1\) is the time for setting up the drilling machine to drill each blasthole and beginning the collaring;

\(t_2\) is the time spent extracting the drilling tool from the blasthole;

\(t_3\) is the time for changing the drilling tool;

\(t_4\) is other auxiliary operations associated with switching the machine on and off, lubricating it, supplying water for flushing the blasthole, etc.

On the basis of numerous timing observations of the operation of the EBK-2M and ESGP-4 core drills, carried out by KuzNII [3], as well as by the Tomsk Polytechnic Institute, it has been established that all auxiliary operations associated with producing one blasthole with a core electric drill take from 4 to 12 min, depending on the method of installing the drill in the heading. When the drill is installed on the boom of a loading-machine manipulator, the time of auxiliary operations is from 4 to 8 min.

Figure 2 presents the dependence of the productivity of a drilling machine on the time spent on drilling proper, for various values of the time spent on auxiliary operations.

From Fig. 2 it is clear what a great influence the time of auxiliary operations has on the productivity of a drilling machine. Thus, at \(t_x=8\) min, the productivity of the drilling machine, no matter how much we increase the drilling rate, regardless of the blasthole depth, cannot exceed 0.125 holes/min. At \(t_x=2\) min the same productivity can be achieved at a drilling rate of 0.25 m/min (for \(l=1.6\) m) or 0.35 m/min (for \(l=2\) m).

Depending on the time of auxiliary operations, the theoretical maximum possible productivity of drilling machines is expressed by the curve

\[ Q_{\max}=\frac{1}{t_x} \]

(Fig. 3, curve 1).

In practice, at a drilling rate of 2 m/min for a blasthole depth of 2 m, the productivity of the drilling machine is expressed by curve 2 (Fig. 3).

Figure 2. Influence of the actual drilling time and the time of auxiliary operations on the productivity of a drilling machine

Figure 3. Productivity of a drilling machine:
1 — at a very high drilling speed and 2 — at a drilling speed of 2 m/min

It follows from this that the productivity of a drilling machine cannot exceed 1 hole/min, even when \(t_x = 0\). For \(t_x\) greater than 2 min, the difference between the theoretically possible productivity and the productivity at \(V = 2\) m/min and \(l = 2\) m is insignificant. For small values \(t_x < 2\) min, this difference increases substantially.

Thus, when the time for auxiliary operations is reduced to 1 min or less per hole, a significant reserve for increasing the productivity of drilling machines may be an increase in the drilling speed to more than 2 m/min.

For each face, a definite drilling-and-blasting work schedule is characteristic. The number of holes is determined by the cross-sectional area of the face, the strength of the rocks being drilled, and the intensity of the applied blasting agent.

Fig. 4. Influence of drilling-machine productivity and the number of holes on the time required for drilling a face

In the practice of drilling-and-blasting operations, the number of holes in one face \(n\), depending on the indicated factors, may vary from 10 to 100.

The time for drilling a face will therefore be determined as

\[ t_z = \frac{n}{Q}, \ \text{min}; \tag{2} \]

where \(Q\) is the productivity of the drilling machine, holes/min.

However, most often, for drilling a face, not one drill is used, but two, or even four (which, however, occurs very rarely because of the absence of the necessary mounting-and-feed devices).

If the number of drilling machines is denoted by \(M\), then formula (2) takes the form:

\[ t_z = \frac{n}{QM}. \tag{3} \]

In Fig. 4 the dependence of the time for drilling the face on the productivity of drilling machines is presented for different numbers of blastholes in the face. In all cases we took \(M\) to be equal to 2.

From Fig. 4 it may be noted that, when drilling a face of any area with drilling machines having a productivity of 0.6 holes/min or more, the drilling time does not exceed 1 hour.

On the basis of numerous time-study observations carried out by TsNIIpodzemshakhtostroem [4], KuzNIIUI [3], the Tomsk Polytechnic Institute [5], and others, ratios have been established for the time expenditures among the individual operations of the heading cycle (Table 1). On average it may be assumed that the drilling operation accounts for 30% of the total cycle time.

On this basis, in Fig. 5 dependencies are constructed for the change in the rate of advance of the face on the time spent drilling the face when using blastholes of different depths.

Fig. 5. Influence of the time for drilling the face and of the depth of the blastholes on the rate of driving mine workings

Labels in the figure: vertical axis—productivity of drilling machines, holes/min; horizontal axis—time for drilling the face, min; \(t_{\text{face}}\), h. Curves: \(n=100\) holes, \(n=60\) holes, \(n=40\) holes, \(n=20\) holes, \(n=10\) holes.

These dependencies are expressed by the formula

\[ W=\frac{60\cdot 30\cdot l\cdot k_{\text{ish}}}{100\cdot t_{z}}\cdot 25\cdot 24 =\frac{1080\cdot l\cdot k_{\text{ish}}}{t_{z}}, \ \text{m/month}, \]

where \(k_{\text{ish}}\) is the coefficient of utilization of the blasthole,

25, 24 are the number of days in a month and the number of hours in a day (we assume a four-shift working day).

The coefficient of utilization of the blasthole is taken by us to be 0.8 [6].

When the time spent drilling the face is 4 hours or more, the rate of driving mine workings cannot exceed 100 m/month. To drive workings at rates of 200 meters per month or more, the time for drilling the face must not exceed 2 hours.

From the graphs shown in Figs. 1, 2, 4, and 5, one can compile a diagram of the change in the driving rate of preparatory workings as a function of the productivity of the drilling machines used in the face. Analyzing this diagram (Fig. 6), one can arrive at a number of conclusions that are very important for finding ways to increase the rates of driving workings.

1. For the successful solution of the tasks set before the coal and mining industry by the Seven-Year Plan for the development of the national economy of the USSR and by the Program of the CPSU, the average rates of drivage of workings must be increased to 300–500 m/month [7]. Such rates of drivage of workings can be provided by drilling machines having a productivity of 0.3 shpur/min and higher.

Fig. 6. Diagram relating the driving rate of workings to the productivity of drilling machines

1. For faces of large cross section, the productivity of drilling machines has a more substantial influence on the driving rate of workings than for faces of smaller cross section.

2. At one and the same productivity of the drilling machine and for faces identical in cross section, the driving rate of workings depends to a considerable degree on the borehole depth used. Thus, for a working having 40 sh. at a drilling-machine productivity of

machine productivity of 0.35 holes/min; increasing the blasthole depth from 2 to 2.4 m gives an increase in the rate of advance of the working by 50 m/month. The higher the productivity of the drilling machine, the more significant the influence of blasthole depth on the rate of drivage.

Table 1

Thus, with an increase in the productivity of drilling machines, the use of deeper blastholes is highly expedient.

1. For faces having more than 40 blastholes, rates of drivage greater than 200 m/month can be obtained only when the productivity of drilling machines is more than 0.4 holes/min. It follows from this that the time for auxiliary operations in servicing the drill must not exceed 2 min.

REFERENCES

1. O. D. Alimov, I. G. Basov, V. F. Gorbunov, D. N. Malikov. Drilling Machines. Gosgortekhizdat, 1960.

2. O. D. Alimov. On evaluating the quality of machines for drilling blastholes. Izvestiya TPI, vol. 108, 1959.

3. A. N. Volkov. Research in the field of creating high-productivity electric rock drills. In: Studies on Problems of Mechanization of Mining Operations and Automation, KuzNIUI, No. 6, Gosgortekhizdat, 1959.

4. Kh. I. Abramson, V. A. Rumyantsev. Field of application and economic efficiency of rotary drilling of blastholes. TsNIIpodzemshakhtostroi, Institute Proceedings, issue 1, Gosgortekhizdat, 1962.

5. M. K. Tsekhin. Study of drilling-and-blasting operations and increasing their efficiency in the driving of horizontal workings in rock at the mines of the Prokopyevsk ore district. Candidate dissertation, Tomsk, 1955.

6. E. O. Mindeli. Drilling-and-Blasting Operations in Driving Mine Workings. Gosgortekhizdat, 1960.

7. N. D. Skoba, V. G. Polozhenko. High-Speed Driving of Horizontal Workings in Strong Rocks. Gosgortekhizdat, 1962.

8. Kh. I. Abramson. Reserves for increasing the labor productivity of drivage workers and the rate of driving horizontal workings by the drilling-and-blasting method. Underground Construction, Gosgortekhizdat, 1961.