ON ASSESSING THE QUALITY OF MACHINES FOR DRILLING BLAST HOLES

O. D. Alimov

The quality of a given drilling machine can be assessed with sufficient completeness only by taking into account such characteristics as the productivity of the drilling machine itself; the labor intensity and labor productivity of the workers engaged in servicing the machine; the durability and economy of the machine; and also the safety of the workers’ labor. Let us consider possible assessments of some of these characteristics.

The productivity of a machine is assessed by the number of products it makes in a definite interval of time, or by the reciprocal quantity—the duration of making one unit of product. Machines for drilling blast holes are most often used in carrying out preparatory workings. The productivity of such machines, in our opinion, is most expediently assessed by the number of blast holes drilled by the machine in 1 hour of operation, or by the time spent on drilling a set of blast holes or a single blast hole. The basis for this is that drilling blast holes in the driving of workings is one of several operations that make up the complex of heading work. The duration of the cycle of these operations is the decisive factor in determining the degree of perfection of the equipment used for this purpose and of the organization of the work. Consequently, the separate component parts of this cycle—drilling, blasting, ventilation, loading, etc.—should expediently be assessed by the duration of the work performed over time. In addition, operations connected with drilling blast holes at the face of a preparatory working at present, as a rule, take less time than one working shift, and with the use of more advanced machines will take only 1.0–2.5 hours. This time also includes the time spent preparing the drilling machine for work, setting it up at the face before drilling, dismantling it, and transporting it to a safe place after drilling. Having finished drilling the face, the heading worker performs other

work—loading, support installation, etc. Proceeding from these conditions, to determine the productivity of the machine and the worker by the number of linear meters of blast holes drilled during a shift, as proposed by a number of researchers, is in our opinion inexpedient.

A face of one and the same cross section may be equipped with different numbers of drilling machines. Thus, for example, in driving workings with a cross section of 9–14 \(m^2\), where 25–40 blast holes are drilled, from one to 12–14 drilling machines are used. As a result, one machine accounts for from 40 to 2 blast holes. Under these conditions, the expediency of using one machine or another can be assessed by the duration of drilling the set of blast holes assigned to one machine, or by the duration of drilling one blast hole. In this case, by a set of blast holes we mean the number of blast holes in the development cycle assigned for drilling by one machine. When determining the productivity of machines, the time expenditures for the entire work cycle must be taken into account, including the drilling process proper, preparatory and concluding operations, and machine downtime associated with carrying out the necessary auxiliary operations.

Proceeding from the foregoing, the productivity of a machine for drilling blast holes in preparatory workings, expressed by the duration of drilling a set of blast holes, may be determined by the formula

\[ T = n_{\text{ш}} t_{\text{ш}} + T_{\text{п.з}}, \tag{1} \]

where \(t_{\text{ш}}\)—time spent drilling one blast hole;
\(n_{\text{ш}}\)—number of blast holes in the set;
\(T_{\text{п.з}}\)—time of preparatory and concluding operations (delivery of the machine and its installation in the face, dismantling of the installation and transportation of the machine away from the face after it has been drilled).

If the productivity of a machine is evaluated by the average duration of drilling one blast hole, then in this case it is advisable to use the formula

\[ t'_{\text{ш}} = \frac{T}{n_{\text{ш}}} = t_{\text{ш}} + \frac{T_{\text{п.з}}}{n_{\text{ш}}}. \tag{2} \]

In this case, the average duration of drilling each blast hole is determined with allowance for the time spent on preparatory and concluding operations.

The productivity of a drilling machine, expressed by the number of linear meters drilled during one hour, may be determined as

\[ L = \frac{60 l_{\text{ш}}}{t'_{\text{ш}}}\ \text{lin. m/hour}, \tag{3} \]

where \(l_{\text{ш}}\)—average depth of a blast hole, \(m\).

Let us consider the component parts of equations (1) and (2). The number of drill holes in a set depends on the total number of holes in the face and on the number of drilling machines. The number of holes in the face depends on the strength of the rock and on the quality of the explosive. In part, it also depends on the quality of the drilling machines, since, when the latter have high productivity, it is sometimes expedient to drill more holes and thereby obtain better breakage, fragmentation of the rock, and greater advance of the face in one cycle.

The duration of the preparatory-and-final operations depends on the weight of the drilling machines, the type of mounting and transport devices, the kind of energy used to drive the drilling machines, and, naturally, on the organization of labor. For modern designs of drilling machines and the organization of work during their operation, the duration of preparatory-and-final operations when drilling the faces of mine workings is from 20 to 50 min.

The time expended on drilling one hole must be determined with allowance for the duration of all operations directly associated with the process of drilling each hole:

\[ t_{\mathrm{h}} = t_1 + t_2, \tag{4} \]

where \(t_1\)—the time expended on the actual drilling of the hole, which we shall hereafter call the machine drilling time;

\(t_2\)—the time expended on auxiliary operations in drilling the hole.

The machine drilling time is determined by the formula

\[ t_1 = \frac{l_{\mathrm{h}}}{v_b}, \tag{5} \]

where \(v_b\)—the average machine drilling speed, mm/min.

The length of holes used in the practice of driving workings varies within a comparatively small range, from 1.5 to 3 m. The drilling speed varies over a much greater range. Depending on their power, operating conditions, tool geometry, and the strength of the rock, modern drilling machines can provide hole-drilling speeds from 40–50 to 3000–4000 mm/min, i.e., this speed may vary by 60–100 times! In this connection, drilling speed is one of the principal indicators of the quality of a drilling machine.

The auxiliary operations in drilling each hole include moving the machine at the face, setting it up for drilling and collaring, extracting the tool from the drilled hole, replacing the drilling tool, and a number of additional operations connected with observing the operation of the machine and servicing it. The time expended on these operations is

\[ t_2 = t_3 + t_4 + t_5 + t_6, \tag{6} \]

where \(t_3\) is the time for setting up the drilling machine for drilling each blasthole and for beginning collaring; it depends on the perfection of the setting devices of the manipulators and attachments that facilitate collaring;

\(t_4\) is the time spent on extracting the drilling tool from the blasthole,

\[ t_4=\frac{l_{\mathrm{sh}}}{v_{\mathrm{sh}}}, \tag{7} \]

where \(v_{\mathrm{sh}}\) is the maneuvering feed speed of the drilling tool of the drilling machine; in modern designs of drilling machines \(v_{\mathrm{sh}}\) ranges from 2000 to 10,000 mm/min; hence, taking into account the possible length of the blasthole, \(t_4\) may vary within the limits from 0.15 to 1.5 min.;

\(t_5\) is the time for changing the drilling tool; it depends on the number of rods required for drilling one blasthole and on the wear resistance of the drilling tool; when EBK-2 type drills are used, a blasthole 2.0 m long is drilled with a set of three rods; changing rods during the drilling of each blasthole takes from 0.5 to 5 min.;

\(t_6\) comprises other auxiliary operations connected with switching the machine on and off, lubricating it, supplying water for flushing the blasthole, etc.

Thus, the quality of a drilling machine from the standpoint of its productivity can be assessed only with allowance for a number of factors, whose magnitudes depend both on the parameters of the drilling machine itself and on auxiliary equipment, mining-engineering conditions, and the organization of work at the face.

It is difficult, in carrying out each investigation or test of a machine, to determine and refine all these factors in detail. Therefore, for the most characteristic operations, the results of investigations should be evaluated by aggregated indicators. Thus, in comparative tests of several drilling machines having identical setting attachments and intended for work under the same mining-engineering conditions, sufficiently objective conclusions about the advantage in productivity of one machine or another can be drawn from the time expenditures for drilling one blasthole, found by formula (4).

In the investigation of machines that provide the same machine drilling speed and differ only in feeding devices (feeders) and setting attachments (manipulators), the best machine can be identified by the minimum time expenditures for auxiliary operations, calculated by formula (6).

In investigating drilling modes, principal attention should be paid to the greatest possible reduction of the machine drilling time (5). In this connection it is important to assess correctly the magnitude of the drilling speed.

Many researchers take the drilling rate as the basic criterion of the quality of a drilling machine and of its operating regimes. However, the drilling rate depends not only on the parameters and operating regimes of the drilling machine, but also on such factors as the physico-mechanical properties of the rock, the method of its destruction, the shape and area of the hole bottom, the initial geometry of the drilling tool and its change during drilling, the method of removing drill cuttings from the hole, etc. Therefore, in order to determine the productivity of a given drilling machine under particular mining-engineering conditions, it is necessary to know the value of the drilling rate with these factors taken into account as well. To this end, drilling machines are tested under laboratory or mine conditions, and the obtained values of drilling rates are entered in the corresponding reference books and standard-setting manuals. The change in drilling rate under the greatest deviations from the conditions that are possible in practice is evaluated by coefficients.

In investigating drilling regimes and testing drilling machines, experiments in most cases are carried out with a sharpened drilling tool. This fact is always emphasized by researchers as confirmation of the objectivity and comparability of the data obtained. Indeed, the drilling rate of any machine depends to a considerable extent on the geometry of the drilling tool, which changes during drilling as a result of wear. Therefore it is correct that comparable data can be obtained only while maintaining, as far as possible, a constant geometry of the drilling tool. This can most simply be achieved when drilling with sharpened bits.

But in the course of operation a real drilling machine works with a sharp tool only at the initial moment after it has been installed. As the tool wears, the drilling rate also changes.

For modern mechanical methods of drilling holes—rotary, percussion-rotary, and rotary-percussion—the change in drilling rate with the depth of the holes can be expressed, with sufficient accuracy for calculating machine productivity, by the following relation:

\[ v = v_{\max} - lk, \tag{8} \]

where \(v_{\max}\) is the maximum drilling rate provided under the given regime by a sharpened bit;
\(l\) is the number of running meters of holes drilled;
\(k\) is a coefficient characterizing the intensity of the drop in drilling rate as a result of blunting of the tool.

The intensity of the drop in drilling rate varies. It depends on the physico-mechanical properties of the rocks, the initial geometry and quality of the material of the drilling tool, and the method and regimes of drilling.

In connection with this, to determine the productivity of a drilling machine, formula (5) should be supplied not with the maximum possible drilling speed with a sharpened bit, obtained under one or another operating mode of the machine, but with the average speed

\[ v_{\mathrm{av}}=\frac{v_{\max}+v_{\min}}{2}, \tag{9} \]

where \(v_{\min}\) is the drilling speed under the given mode at the maximum permissible wear of the drilling tool.

Moreover, a larger value of the average drilling speed may not correspond to the mode under which the drilling speed with a sharpened bit is greatest. Thus, comparing dependences I and II (see figure), one may conclude that when drilling a blasthole to a depth \(l_1\), in case II a higher average drilling speed will be achieved, and consequently a higher productivity of the machine, than in case I, where the speed with a sharpened bit is greater.

Comparing the quality of machines on the basis of drilling speeds with sharpened bits is possible only in the case where they operate under such modes in which the intensities of change in drilling speed with the number of drilled running meters of blasthole are approximately the same.

Change in blasthole drilling speed as a result of dulling of the drilling tool.

Thus, when conducting investigations of the drilling tool, drilling modes, and drilling machines, in addition to obtaining data on the drilling speed with sharpened bits, it is necessary to determine the dependences of the change in drilling speed on the number of drilled running meters of blasthole. Naturally, this requires carrying out a large volume of drilling work. Therefore, it is expedient to conduct the majority of experimental investigations as laboratory-and-production investigations, i.e., under laboratory conditions to make a preliminary evaluation of the drilling tool, modes, and drilling machines as a whole according to the maximum drilling speed in the most characteristic samples of rocks, and under production conditions (in open pits and mines) to determine the change in drilling speed with the number of blasthole-meters and, on this basis, to determine the average drilling speeds for the most characteristic mining-engineering conditions.

In laboratory studies, in addition to the maximum drilling rates with sharply sharpened bits, one can also obtain preliminary data on average drilling rates. For this purpose, when studying the processes of rock destruction, drilling conditions, and drilling machines, the drilling rate should be determined both with sharply sharpened and with extremely blunted bits.

At present there is no definite opinion in the technical literature as to at what wear a drill bit is considered extremely blunted and must be replaced by a sharply sharpened one. Evidently, this concept, like tool life in mechanical engineering, must be substantiated on the basis of economic considerations. The solution of this question can be considerably facilitated if sufficient data are available on changes in drilling rates as a function of the number of meters drilled. Until such relationships have been established, the limiting permissible wear of the tool may be taken as that at which the average width of the end wear land of each blade of the drilling tool is 2.5–3 mm. This is the criterion currently used in practice by drillers when deciding whether to replace the tool during drilling.

The drilling rate can also be expressed in a more general form, namely:

\[ v_b=\frac{102\cdot 60N\eta}{sA_{\mathrm{ob}}}\ \text{cm/min}, \tag{10} \]

where \(N\) is the power developed by the drive of the drilling machine during drilling, kW;

\(s\) is the area of the bottom of the blast hole destroyed by the drilling tool, \(\text{cm}^2\);

\(A_{\mathrm{ob}}\) is the volumetric work of rock destruction during drilling, \(\text{kgm}/\text{cm}^3\);

\(\eta\) is the efficiency factor characterizing power losses in transmitting it from the drilling machine to the destruction of the rock.

For the most common case—drilling blast holes with a solid bottom—formula (10) can be transformed into

\[ v_b=\frac{102\cdot 60N\eta}{\frac{\pi d^2}{4}A_{\mathrm{ob}}} =\frac{7800N\eta}{d^2A_{\mathrm{ob}}}, \tag{11} \]

where \(d\) is the blast-hole diameter, cm.

The terms entering formulas (10) and (11) define sufficiently well the dependence of the drilling rate on the principal parameters and operating conditions of the machine, the size of the tool, and the strength of the rock.

Indeed, the potential capability of a drilling machine to attain the maximum drilling rate is determined by the power of its drive. The power of the drive of drilling machines is limited by their weight and by the strength of the drilling tool. At present, the drive power of machines for drilling blast holes ranges from 1.0 to 6 kW.

The resistance of rock to one or another type of destruction, as experiments by many researchers have shown, can be assessed quite fully by the magnitude of the volumetric work of destruction. The data necessary for calculating the magnitude of the volumetric work of destruction may be determined either on the basis of processing experiments on the study of drilling regimes, or by special investigations. To determine the average drilling rate, the average value of the volumetric work of destruction should be substituted into formula (10) or (11):

\[ A_{\text{vol. avg}}=\frac{A_{\text{vol. min}}+A_{\text{vol. max}}}{2}, \tag{12} \]

where \(A_{\text{vol. min}}\) is the volumetric work of destruction in breaking the rock under the given regime with a sharpened bit;

\(A_{\text{vol. max}}\) is the volumetric work of destruction when drilling with a completely blunted bit.

In the future, to determine the average volumetric work of destruction for various rocks and various drilling methods, express methods should be developed.

The efficiency of energy transmission from the drive to the destruction of the rock must take into account power losses in the various links of the machine and the drilling tool. As numerous studies show, the magnitude of these losses depends on the design of the drilling machine, the regimes of its operation, and the drilling tool.

Investigations of drilling regimes are often carried out with the aid of industrial models of drilling machines. In this case it is difficult to determine how much energy is expended directly on the destruction of the rock and how much is lost in the drilling machine and tool. In such cases, drilling regimes are evaluated by the volumetric work of destruction, which is defined as the quotient of the energy transmitted by the drilling machine divided by the volume of rock destruction. This characteristic takes into account both the work of destruction and the coefficient of energy transmission. The magnitude of the volumetric work of destruction obtained in this way makes it possible to assess quite fully the quality of the operating regimes when testing one or several drilling machines of the same type.

The labor productivity of workers engaged in servicing drilling machines may be assessed by the number of linear

meters of holes drilled during one hour and referred to one worker.

\[ L_{\mathrm{p}}=\frac{\sum n_{\mathrm{sh}} l_{\mathrm{sh}}}{B T_{\max}}, \tag{13} \]

where \(\sum n_{\mathrm{sh}}\) is the number of holes in the face;

\(B\) is the number of workers engaged in servicing drilling machines in the face;

\(T_{\max}\) is the greatest duration of drilling of a set of holes attributable to one drilling machine, found from expression (1).

Formula (13) indirectly takes into account the productivity of each machine, the number of drilling machines, and the workers required for servicing them.

The required number of workers for servicing the drilling machines installed in the face may be determined from the formula

\[ B_{\min}=\frac{c}{m}, \tag{14} \]

where \(c\) is the number of machines in the face;

\(m\) is the maximum number of machines that can be serviced by one worker without overlap of auxiliary operations.

The maximum possible hourly labor productivity of a driller when servicing \(m\) machines is

\[ L_{\mathrm{p}.\max}=60\,\frac{l_{\mathrm{sh}}}{t_{\mathrm{sh}}}\,m \ \text{linear m/hour}. \tag{15} \]

Substituting the values of \(t_{\mathrm{sh}}\) and \(m\) into formula (15), we obtain

\[ L_{\mathrm{p}.\max}=60\,\frac{l_{\mathrm{sh}}}{t_{2}} . \]

When testing machines for which the operating time expenditures on preparatory and concluding operations are the same, \(L_{\mathrm{p}.\max}\) may serve as the principal characteristic for determining the best drilling installation—i.e., the drilling machine together with the setting-and-feed devices.

Such characteristics of drilling machines as durability, economy, and safety in operation can be assessed sufficiently fully only on the basis of studying the designs of the machines and generalizing experience with their operation under various mining-engineering conditions.

The overall assessment of the perfection of a given machine must be made with allowance for the aggregate of the basic characteristics indicated above. However, it should be noted that, owing to the very diverse

With diverse purposes and conditions for conducting both drilling operations and mining operations as a whole, assessing the quality of a given drilling machine solely on the basis of summing positive and negative factors is not always possible. In a number of cases, the final conclusion regarding the advisability of using a drilling machine under certain mining-engineering conditions may also be made on the basis of a single factor, but the one most important for the given conditions.

The relationships indicated above for determining productivity can be used both to assess the quality of already existing drilling machines and to substantiate the basic parameters of newly designed machines.