Analysis of the Operation of Drilling Hammers on Pneumatic Supports
Abstract Generated abstract
This paper analyzes the use of pneumatic supports with hand-held pneumatic drilling hammers, focusing on the feed forces needed to use hammer power efficiently without excessive physical effort by the driller. It develops calculation formulas and a nomogram relating optimal feed force, hammer and support weight, support angle and length, hole height, air pressure, and support cylinder diameter. The analysis indicates that many existing supports provide rational operating conditions only at lower air pressures, while at 5 to 6 ati they may require additional forces that drillers cannot sustain. The paper recommends design criteria for pneumatic supports, concludes that they are suitable for hammers with optimal feed forces up to about 130 kg, and argues that more powerful hammers require other feeding devices.
Full Text
PROCEEDINGS
OF THE TOMSK ORDER OF THE RED BANNER OF LABOR
POLYTECHNIC INSTITUTE named after S. M. KIROV
Vol. 108 STUDY OF DRILLING MACHINES 1959
ANALYSIS OF THE OPERATION OF DRILLING HAMMERS
ON PNEUMATIC SUPPORTS
I. G. Basov
The studies we have carried out have shown that, for the most efficient use of hammer power, drilling must be performed with definite feed forces. For drilling hammers which, by weight category, are classified as hand-held, the optimal feed forces at \(p = 4—6\) ati lie within the range of 48—140 kg [4].
When working without any supporting-and-feeding devices, i.e., when bearing the weight of the hammer and feeding it to the face by hand, the driller can, over a sufficiently long period of time, produce a feed force of no more than 25—30 kg [3]. From a comparison of these values with the optimal feed-force values for the hammers investigated, it is clear that, for manual work, drilling hammers with power no greater than that developed by the OM-506 and PM-508 machines at \(p = 4\) ati are suitable. More powerful hammers, including high-frequency ones, should not be used for manual work.
As the mine tests we have conducted have shown, drilling blast-holes with such hammers as the RH-754 and T-10 by hand does not lead to any substantial increase in labor productivity in comparison with drilling by OM-506 hammers, and it causes rapid fatigue of the drillers. The question of the expedient use of high-frequency drilling hammers can be resolved only on the basis of an analysis of the mounting-and-feeding devices required for them.
At the present time, in the practice of operating drilling hammers, pneumatic supports are becoming increasingly widespread. Their extensive use in the mining industry is explained by the simplicity of their design and operation, as well as by the insignificant expenditure of time required for installation at the face.
It should be noted that at first pneumatic supports were created mainly to support the hammer at a certain hei-
in height, and only recently, after works that drew the attention of designers to the influence of the feed force on the drilling rate [1—2], have requirements begun to be imposed on pneumatic supports to ensure the necessary feed forces [5].
Until recently, industry produced pneumatic supports that were insufficiently perfected and in small quantities. In most cases pneumatic supports were made directly at mines on the proposals of innovators. Pneumatic supports made in this way for the same hammers and the same operating conditions had very different parameters. There are still no sufficiently substantiated recommendations in the literature for choosing the parameters of pneumatic supports. Therefore operating personnel, as well as designers, have no clear opinion not only regarding the necessary parameters of pneumatic supports, but also regarding the advisability of their use in general. Whereas in the ore-mining industry drilling with hand-held hammers has been completely transferred to drilling only with pneumatic supports, in the coal industry and in mine construction a considerable proportion of production personnel underestimate the possibilities of these devices. In a number of cases, instead of using a pneumatic support, they prefer to have an additional worker to help the driller support the hammer and carry out its feed during the drilling process.
In view of the foregoing, we shall dwell on an analysis of the operation of hand drilling hammers on pneumatic supports, with the aim of substantiating the most expedient field of application of pneumatic supports and the choice of their basic parameters for hammers of various power.
As was already noted above, a pneumatic support is a device intended for simultaneously supporting the drilling hammer and creating a feed force in the direction of drilling the blastholes. Structurally, a pneumatic support is a telescopic device consisting of a cylinder and a piston with a rod (Fig. 1). The supporting part of a pneumatic support may be either the rod or the cylinder; in some designs provision is made for the possibility of using either one or the other as the support, as required. Recently, factories have begun to manufacture mainly pneumatic supports with a supporting cylinder, since such a design is more convenient when manually repositioning its lower supporting part.
The working procedure of a driller with a hammer mounted on a pneumatic support is as follows. After the support has been connected to the hammer and the air and water hoses have been connected, the hammer is raised to the height at which the blasthole is to be collared (horizontal). In this process the pneumatic support is set at a certain angle to the horizontal; for this purpose, if necessary, it is first extended somewhat by compressed air supplied to the cylinder. The magnitude of the extending ...
the force of the pneumatic support is established from a calculation that ensures a certain support of the hammer and the creation of a small feed force necessary for collaring. Collaring is performed with a short drill steel. Further drilling is carried out either with one drill steel for the entire depth of the blasthole, or with a set of drill steels. In the process of drilling the blasthole, the driller regulates the extending force, supports the entire installation against lateral displacement, and, as necessary, repositions the supporting part of the support.
In order for the drilling hammer to operate on the pneumatic support without the driller applying an additional force in the direction of drilling the blasthole, it must have a device that ensures the production of the necessary extending force \(R\) (Fig. 1), allowing, above all, the entire installation to be supported at the level of the height at which the blasthole is started. If it is assumed that the weight of the pneumatic support is uniformly distributed along its length, and that the weight of the drill steel has no substantial influence on the magnitude of the extending force, then it must be selected from the condition of ensuring the vertical component
Fig. 1. Diagram of the installation of a drilling hammer on a pneumatic support.
\[ G = G_{\mathrm{m}} + \frac{G_{\mathrm{p}}}{2}, \tag{1} \]
where \(G_{\mathrm{m}}\) is the weight of the hammer, kg;
\(\phantom{\text{where }}G_{\mathrm{p}}\) is the weight of the pneumatic support, kg.
In addition, the magnitude \(R\) must ensure operation of the drilling hammer under the optimum regime, i.e., in the direction of drilling the blasthole there must act a force equal to the optimum feed force \(F_{\mathrm{op}}\). When drilling horizontal blastholes, it will be equal to
\[ F_{\mathrm{op}} = G \operatorname{ctg} \alpha = \left(G_{\mathrm{m}} + \frac{G_{\mathrm{p}}}{2}\right)\operatorname{ctg}\alpha . \tag{2} \]
From this expression it is evident that the feed force which the pneumatic support must develop depends on its weight and on the weight of the hammer—
as well as on the angle of inclination of the pneumatic support to the horizon, \(\alpha\). For certain weight characteristics of the hammer and pneumatic support, the optimum feed force can be ensured at angles of inclination of the support to the horizon that can be found from the expression
\[ \alpha = \operatorname{arctg} \frac{2G_{\mathrm{m}} + G_{\mathrm{p}}}{2F_{\mathrm{op}}}. \tag{3} \]
Given a certain angle \(\alpha\) and the maximum height of blasthole placement \(H_{\mathrm{sh}}\), one can determine the length of support necessary to obtain the optimum feed force,
\[ l_{\mathrm{p}} = H_{\mathrm{sh}} \operatorname{cosec} \alpha. \tag{4} \]
As the blasthole is drilled, the hammer moves toward the face, the pneumatic support extends, and its angle of inclination changes. After the pneumatic support has extended by the accepted amount of piston travel, its bearing part is moved closer to the face. If the amount by which the pneumatic support extends between repositionings of its bearing part is denoted by \(l_x\), then the structural length of the pneumatic support in the extended state will be equal to
\[ l_{\mathrm{k}} = H_{\mathrm{sh}} \operatorname{cosec} \alpha + l_x. \tag{5} \]
It should be noted that, when determining the structural length of the pneumatic support, the height of possible blasthole placement should not be taken as more than \(1.5\ \mathrm{m}\), since otherwise, while maintaining the required installation angles of the pneumatic support, its length, and consequently its weight, increase considerably. For drilling blastholes located higher than \(1.5\ \mathrm{m}\), it is advisable to arrange platforms or to break down the upper part of the face from loosened, broken rock. Taking into account also the extension of the pneumatic support during drilling, its value should expediently be taken as \(l_x \leq 0.5\ \mathrm{m}\), proceeding from the fact that such an elongation of the support increases the feed force to such an extent that it does not leave the range of rational feed forces, where the hammer provides not less than 90% of the maximum possible drilling rate under stable operation [1] (Fig. 2). An extension greater than \(0.5\ \mathrm{m}\) leads to a considerable increase in \(R\), and hence to an excess over the optimum feed force. If, however, \(l_x\) is taken as more than \(0.5\ \mathrm{m}\), but during drilling of the blasthole the value of \(R\) is reduced by lowering the air pressure in the support cylinder, then in that case it is necessary to hold the drilling hammer, i.e., active intervention by the driller, which is undesirable in the operation of high-frequency hammers from the standpoint of occupational hygiene [3].
However, pneumatic supports must have a telescopic device that allows their length to be changed by more than \(0.5\ \mathrm{m}\).
This is necessary so that, when drilling blastholes with a setup height of up to \(1.5\,m\), it is possible to vary the length of the support considerably in order to maintain the constancy of the angle \(\alpha\), which ensures optimal operating conditions of the hammer without the driller applying additional effort.
The spreading force of the pneumatic support depends on the diameter of its cylinder \(D_{\mathrm{p}}\) and on the pressure of the air supplied to it, \(p_{\mathrm{p}}\), and may be found from the expression
\[ R=\frac{G}{\sin\alpha}=\frac{\pi D_{\mathrm{p}}^{2}}{4}\,p_{\mathrm{p}}. \tag{6} \]
For a given type of hammer and an adopted weight of the pneumatic support, the diameter of its working cylinder may be found from the expression
\[ D_{\mathrm{p}}=2\sqrt{\frac{G}{\pi p_{\mathrm{p}}\sin\alpha}}, \tag{7} \]
whence, noting that \(G=F_{\mathrm{op}}\tan\alpha\), we obtain
\[ D_{\mathrm{p}}=2\sqrt{\frac{F_{\mathrm{op}}}{\pi p_{\mathrm{p}}\cos\alpha}}. \tag{8} \]
Thus, from equation (8) it is seen that the diameter of the pneumatic-support cylinder depends on the optimal feed force of the drilling hammer, the air pressure \(p_{\mathrm{p}}\), and the angle of inclination of the support. In calculating \(D_{\mathrm{p}}\), the angle \(\alpha\) must be selected from the condition of ensuring the optimal feed force of the hammer. The value \(p_{\mathrm{p}}\) should be taken as \(1\) atm below the line air pressure \(p\), for which the optimal feed force of the hammer is calculated. The diameter of the pneumatic support selected on the basis of these conditions will provide rational operating conditions of the hammer and the required increase in the spreading force \(R\) when the support is lengthened during drilling by \(l_x<0.5\,m\). In addition, by reducing the design \(p_{\mathrm{p}}\) by \(1\) atm, losses of air pressure along its path into the support cylinder are also taken into account.
The relationships:
\[ F_{\mathrm{op}}=f(a,G_{\mathrm{m}},G_{\mathrm{p}});\quad a=f(H_{\mathrm{sh}},l_{\mathrm{p}}); \]
\[ R=f(H_{\mathrm{sh}},G_{\mathrm{m}},G_{\mathrm{p}})\ \text{and}\ D_{\mathrm{p}}=f(R,p_{\mathrm{p}}), \]
calculated by formulas (2--6), are given in the form of a nomogram in Fig. 2. In the lower left quadrant of this nomogram we have introduced the dependence, found by us, of the optimal feed force of a drilling hammer on the area of its piston and the air pressure, and calculated by the formula
\[ F_{\mathrm{op}}=\frac{pQ}{3.56}+Cp, \tag{9} \]
where \(Q\) is the area of the piston on which the compressed air acts during its forward stroke, \(\text{cm}^2\);
\(C = \kappa Q = 4\) is a constant coefficient found experimentally.
In the upper left quadrant the dependence is shown of the feed force provided by the pneumatic support on its angle of inclination, and also on the weight of the hammer and the support. In the upper right quadrant are given the dependence of the support inclination angle, for different
Fig. 2. Nomogram for calculating the principal parameters of pneumatic supports.
lengths, on the possible height of drilling-hole placement and the dependence of the thrust force of the support on its angle of inclination for different reduced weights of the installation \(G\). In the lower right quadrant the dependence of the support piston diameter on its thrust force is given.
The procedure for using the nomogram is as follows. On the basis of the known piston area of the hammer and the air pressure supplied to the hammer, the optimum feed force is determined. From the magnitude of this force, proceeding from the weight of the hammer and the support, the maximum possible angle \(\alpha\) is found at which the hammer can operate at the optimum feed force without pri-
the driller’s application of additional force. From the value of the angle $\alpha$ and the height at which the hole is set, the length of the support is determined; or, for a known support length, proceeding from its maximum possible angle of inclination, the maximum possible height at which the hole may be set is found. Knowing the optimum value of the angle $\alpha$ and the reduced weight of the unit, one can also determine the extension force of the support, from which, for a given air pressure, the diameter of the piston of the pneumatic support is then found.
On the basis of the foregoing and of the results of investigations of the operating conditions of high-frequency pneumatic drilling hammers, we analyzed the basic parameters of pneumatic supports P18L, P22L, “Atlas-Diesel,” and “Tampella,” used respectively with hammers PM-508, PR-25, PR-20, RH-754, and T-10. The analysis established that the existing lengths of the above-mentioned pneumatic supports, only at an air pressure at the hammers of 4 ati, ensure their rational operating conditions; with an increase in pressure to 5–6 ati, this requires the application of additional forces of 30 to 70 kg. The driller cannot provide such forces for long periods. Consequently, the lengths of the supports must be increased accordingly, or else the drilling of holes must be carried out at a height of less than 1.0 m. Otherwise, the power of the drilling hammer will be used irrationally.
According to the nomogram (Fig. 2), drilling hammers whose optimum feed forces are 130 kg should not be operated with pneumatic supports, since the latter would have to be made of very great length and weight. This would deprive pneumatic supports of their main advantage over other feeding devices—small expenditures of time on auxiliary drilling operations. If pneumatic supports of smaller dimensions are used, the power of the hammers will not be fully utilized. Consequently, for powerful hammers other, more powerful feeding devices are required.
Thus, on the basis of the analysis performed of the operation of drilling hammers on pneumatic supports, the following conclusions can be drawn.
The magnitude of the feed force that can be provided by a pneumatic support depends on its weight, length, and angle of inclination, on the height at which the hole is set, and also on the weight of the hammer.
The majority of pneumatic supports, in terms of their principal characteristic—length—ensure optimum operating conditions for drilling hammers only at air pressures below 5 ati. Considering that in most mines and ore mines the air pressure at the faces has been brought up to 5–6 ati, pneumatic supports must also be designed on the basis of this condition.
The investigation carried out showed that, at air pressures of 4–6 ati, it is advisable to operate with pneumatic-support-
...drilling hammers whose optimal feed forces do not exceed 130 kg, i.e., hammers having a piston area on which compressed air acts during its forward stroke of up to 55 cm². For calculating the principal parameters of pneumatic supports for such a hammer, the method set forth above may be recommended.
To ensure the optimal feed force during drilling with a hammer mounted on a pneumatic support, the latter must have a device that makes it possible to regulate smoothly the air pressure in its cylinder.
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