On the Calculation and Design of Drill Rotation Mechanisms in Pneumatic Drilling Hammers

O. D. Alimov

Studies of the regimes of percussion-rotary drilling \[1; 2; 3; 4; 5; 6; 7\] show that the drilling rate changes significantly depending on the relationship between the number of impacts and the number of revolutions of the drilling tool, that is, on the angle of rotation of the drill between two impacts. Thus, for example, when drilling blast-holes 40–45 mm in diameter \[5\], the drilling rate varied by a factor of 1.5–2 depending on the angle of rotation of the drill between impacts, all other drilling conditions being the same.

In this connection, the question becomes urgent: how should devices for rotating the drill in machines for percussion-rotary drilling be most correctly calculated and designed? There is as yet no satisfactory answer to this question in the literature, even for such widely used machines in the mining, construction, and coal industries as pneumatic drilling hammers.

The most expedient value of the angle of rotation of the drill between impacts, from the standpoint of the minimum energy consumption of the fracture process, depends on the strength of the rock being broken, the impact energy, the diameter of the blast-hole or borehole, and the shape of the tool \[5\].

To determine the optimal angle of rotation as a function of these factors, various formulas have been proposed by a number of researchers \[1–3\]. However, it does not appear possible for either a design engineer or an operating engineer to use them, owing either to the indefiniteness of the values of the terms entering into these formulas \[1\], or to the incorrectness of the assumptions made in deriving the formulas. Therefore it is recommended that, where the design of the machine permits, the optimal angle of rotation be determined experimentally.

On the basis of the study of the mechanism of rock destruction in percussion-rotary drilling, carried out recently at the Tomsk Polytechnic Institute \[5; 6\], it has been possible to specify the values of the most expedient angles of drill rotation between impacts for the case of drilling blast-holes 40–45 mm in diameter with single-chisel drill bits in rocks of high strength. In this case it is recommended \[6\] to adopt angles
\(\alpha = 19^\circ;\ 21.2^\circ;\ 24^\circ;\ 27.8^\circ;\ 32.7^\circ;\ 40^\circ;\ 51.5^\circ\). Drilling at such angles of drill rotation between impacts proceeds with a lower energy consumption in the process of rock destruction and, consequently, with the highest rate. The definiteness of the values of the optimal angles of drill rotation makes it possible to analyze the rotation devices of modern drilling hammers and to outline ways of improving them.

In all modern pneumatic drilling hammers, rotation of the drill is carried out depending on the operation of the impact unit. The mechanism for rotating the drill consists of a rod 1 with a screw thread (the so-called helical rod), Fig. 1, and a helical nut inserted ...

... the striker piston 2. The piston rod is connected by means of splines to the rotary bushing 3, and the latter to the bushing 4, which has a hexagonal opening for the drill shank. The head of the helical rod with ratchet pawls 5 is placed in the ratchet bushing 6.

When the piston moves from right to left (working stroke), the pawls of the ratchet device do not hinder the rotation of the helical rod in the ratchet mechanism, and at this time the piston can move forward without rotating. When the piston moves from left to right (return stroke), the pawls bear against the teeth of the ratchet bushing and do not allow the helical rod to rotate. This causes the piston to turn; its rod, by means of the splined connection, turns the rotary bushing 3, the bushing 4, and the drill. Thus, the drill-rotation mechanism is engaged only during the return stroke of the piston, and therefore it has been called intermittent. The simplicity and compactness of such mechanisms have led to their wide use in modern pneumatic drilling hammers.

Fig. 1

The number of revolutions of the drill per unit time, and consequently the angle of rotation of the drill between two blows during operation of pneumatic drilling hammers, is recommended to be determined [9] as follows.

Fig. 2

Figure 2 shows the developed view of a part of the helical rod corresponding to the length of the piston stroke—$S$. The length of the arc $x$, corresponding to the angle of rotation of the rod in one stroke, is equal to

\[ x = S \cdot \operatorname{tg} \gamma, \tag{1} \]

where $\gamma$ is the angle of inclination of the thread of the helical rod.

The angle of rotation of the rod $\alpha$, corresponding to this arc, or, with the rod immobile, the angle of rotation of the piston and drill, is

\[ \alpha = \frac{180 \cdot x}{\pi \cdot R}. \tag{2} \]

or, after substituting the value of \(x\) from equality (1) into (2),

\[ \alpha=\frac{180\cdot S\cdot \tg \alpha}{\pi R}, \tag{3} \]

where \(R\) is the mean radius of the thread of the helicoidal rod.

If we take into account that \(\tg \gamma=\dfrac{2\pi R}{S_{cm}}\), where \(S_{cm}\) is the thread pitch of the helicoidal rod, then the angle of rotation of the drill between two blows may be determined by the simpler formula

\[ \alpha=360\frac{S}{S_{cm}}. \tag{4} \]

If the number of blows of the hammer per minute is taken to be \(n_{y\partial}\), and the angle of rotation of the drill between two blows is \(\alpha\), then the number of revolutions of the drill per minute will be

\[ n=\frac{\alpha\cdot n_{y\partial}}{360}. \tag{5} \]

As can be seen from the calculation given above, its principal premises are the assumption that rotation of the drill occurs only during the return stroke of the piston, whereas during the working stroke of the piston the drill does not rotate. From formula (4) it follows that the angle of rotation of the drill between two blows depends only on the ratio between the piston stroke \(S\) and the pitch of the helicoidal rod, \(S_{cm}\). Since the pitch of the helicoidal rod is a constant quantity, and the magnitude of the piston stroke during stable operation of the hammer varies within insignificant limits, on the basis of formula (4) the conclusion is drawn that the value of the angle of rotation of the drill of a pneumatic hammer is a constant quantity and characteristic for the given hammer. The impossibility of changing the angle of rotation of the drill between two blows when drilling rocks of different hardness with one and the same hammer is considered the most substantial shortcoming of modern pneumatic drilling hammers with intermittent-action rotation devices. In this connection, conclusions are drawn as to the advisability of equipping pneumatic drilling hammers with several helicoidal rods and nuts having different thread pitches. A number of designs with an independent drive for drill rotation are also proposed.

Meanwhile, in our opinion, the premises that served as the basis for the calculation presented above are inaccurate, and, as a consequence, the conclusions obtained are incorrect. The assumption that rotation of the drill in pneumatic drilling hammers occurs only during the return stroke of the piston is entirely unsubstantiated. Indeed, the kinematic chain of the rotation device can close and, consequently, transmit kinetic energy from the piston to rotate the drill only during the return stroke of the piston. But this does not exclude the fact that the transfer of energy may be effected by a short impulse over a period of time shorter than the duration of the return stroke, and also that the drill may rotate by inertia during the working stroke of the piston, when the ratchet device does not close the kinematic chain of the rotation mechanism. This is confirmed by Table 1, in which the angles of rotation of the drill between two blows, calculated by formula (4) and obtained directly during drilling, are compared.

If a decrease in the angle of rotation during drilling to below the angle \(\alpha\) can still be explained by a decrease in the piston stroke, then an increase of its value above \(\alpha\) can be explained only by the fact that rotation of the drill also occurs at the moment of the working stroke of the piston.

Table 2

The investigations we carried out [8; 10] showed that, in reality, the rotation of the drill during drilling with pneumatic drilling hammers occurs both on the return stroke and on the working stroke, with some slowing of rotation at the moment when the tool penetrates the rock. The magnitude of the rotation angle is substantially influenced by the moment of resistance to rotation of the drill in the blast hole. The latter depends to a considerable degree on the feed force of the drilling hammer on the face. Therefore, with a change in the feed force (Figs. 3, 4), the angle of rotation of the drill between two blows also changes, which is one of the reasons for the change in drilling rate (Fig. 5). The magnitude of the angle and the nature of the drill rotation also have a considerable influence on the stability of operation of the hammer.

Fig. 3.
Vertical axis: Drill rotation angle, deg.
Horizontal axis: Feed force, kg.
In the plot: PM-508, No. 5472; curve labels “4 holes,” “5 holes,” “6 holes,” “7 holes”; dashed reference line \(\alpha' = 30^\circ 12'\).

In order to reveal some of these dependences, let us examine in more detail the kinematics and the geometrical relationships of the drill-rotation mechanism of pneumatic drilling hammers.

Let us assume that on the return stroke of the piston the drill turns through an angle \(\alpha\), determined by formula (4); then, as is seen from Table 2, the value of this angle is different for different hammers. In turn, on the working

Fig. 4. Feed force, kg, as a function of drill rotation angle, degrees.

• Horizontal axis: drill rotation angle, deg.
• Vertical axis: feed force, kg.
• Hammer: PR-30K, No. 9.
• Curve labels: 40 mm, 50 mm, 70 mm, 80 mm.

Fig. 5. Feed force, kg, as a function of drilling speed, mm/min.

• Horizontal axis: drilling speed, mm/min.
• Vertical axis: feed force, kg.
• Hammer: PM-508, No. 5472.
• Curve labels: 40 mm, 50 mm, 60 mm, 70 mm.

Table 2

during the piston stroke the helical rod would have to turn through the same angle, and, consequently, through the corresponding integer number of teeth in the ratchet bushing. However, as can be seen from Table 2, this cannot be ensured in any of the modern drilling hammers, since the angle $\alpha$ does not correspond to an integer number of teeth of the ratchet bushing. This is also understandable, since rotation of the drill, and together with it of the piston, occurs not only during the return stroke, but also at the moment of the working stroke. Therefore

Fig. 6

in design practice the number of teeth of the ratchet bushing is not linked with the angle $\alpha$, but is selected only from the condition of ensuring the required magnitude of the so-called “idle stroke.” For example, the PM-508 hammer has a ratchet bushing with the number of teeth $Z = 23$ and two ratchet pawls. Then, dividing the number of teeth by the number of ratchet pawls $a = 2$,

\[ \frac{Z}{a} = \frac{23}{2} = 11.5, \]

we see that only one pawl can be in engagement with the ratchet bushing at the same time, while the other ratchet pawl is offset from the corresponding tooth of the ratchet bushing by $0.5$ tooth (Fig. 6). Since, at the beginning of the return stroke, in most cases neither of the pawls is in engagement (Fig. 7), the rotation of the drill in fact begins not from the very beginning of the return stroke, but only after this clearance in the rotation mechanism has been taken up. As a result, even in the most unfavorable case (jamming of the drill during penetration) the ratchet device will not lock the piston in its extreme position, but, having turned through the angle $\alpha_3 = 0.5 \alpha_2$, will ensure the return stroke of the piston (“idle stroke”)

\[ S_1 = \frac{S_{\mathrm{st}} \cdot \alpha_3}{360}, \]

where $\alpha_2$ is the angle of rotation of the helical rod by one tooth of the ratchet bushing.

The presence of “lost motion” ensures the necessary stability of operation of the drilling hammer, and also eliminates the possibility of large dynamic loads on the turning mechanism during penetration of the drill. In the practice of design bureaus there is no established opinion as to the most expedient dimensions of the “lost motion.” As can be seen from Table 2, different plants, even for the same types of drilling hammers (OM-506 l and OM-506 m; KTsM-4 and KTsM-4 k), use different ratios of the numbers of teeth of the ratchets and pawls \(a\), and, consequently, different values of \(S_1\). Most often, two pawls are used in the ratchet mechanisms of drilling hammers, providing rotation of the helicoidal rod during the “lost motion” by 0.5 tooth (RPM-17, OM-506 m, PM-508), or four pawls, providing rotation by 0.25 tooth (PA-23; PT-30; PR-35; KTsM-3 fl) and by 0.5 tooth (TP-4 and KTsM-4 k).

Taking account of the “lost motion,” the angle of rotation of the drill during the return stroke of the piston is

\[ \alpha_1 = \alpha - x_3 . \]

Comparing the value of \(\alpha_1\) (Table 2) with the experimental data of Figs. 3 and 4, it may be established that, under feed conditions ensuring operation of the drilling hammer with an angle of rotation of the drill between blows of \(2^\circ\!-\!4^\circ\), the drilling hammer begins to operate unstably, the drilling speed decreases, and with further decreases in the angle of rotation of the drill it may stop altogether. This may be explained by the fact that, with considerable resistance to rotation of the drill and a decrease in the angle of rotation between blows to less than \(\alpha_1\), movement of the piston through the full stroke \(S\) can no longer be ensured, and the piston, not reaching the upper extreme position, is unable to overcome the resistance force to rotation of the drill; it becomes, as it were, jammed by the thread of the helicoidal nut in the thread of the fixed helicoidal rod.

Fig. 7

Thus, the minimum angle of rotation of the drill per blow \(\alpha_1\) during stable operation of the hammer is determined by the design ratios of the hammer: the piston stroke \(S\), the pitch of the helicoidal rod \(S_{\mathrm{cm}}\), and the magnitude of the “lost motion” \(S_1\).

Under operating conditions of the hammer corresponding to the maximum drilling speed, the bit turns between impacts through an angle \(>\alpha_1\). If it is taken into account that, under steady operation of the hammer, the helical rod between two impacts turns in the ratchet bushing by one and the same number of teeth, then the values of the bit rotation angles, depending on the resistance to rotation, may be determined by the formula

\[ \alpha=\alpha_2 z_2 k, \]

where
\(z_2\) is the number of teeth through which the helical rod turns,
\(k\) is the corresponding value of the “lost motion” \((k=1.0;\ 0.5;\ 0.25)\).

The numerical values of the possible bit rotation angles under steady operating conditions of hammers are given in Table 2, from which it is seen that the values of the bit rotation angles vary, as in the experimental data, stepwise, and their value differs for different drilling hammers. The most gradual variation of the rotation angles occurs in the drilling hammers PA-23, PT-30, PR-35, and KTsM, where the value of the “lost motion” corresponds to rotation by \(1/4\) of a tooth. The most abrupt changes in the rotation angles occur in the OM-506 drilling hammer (of the Leningrad “Pnevmatika” plant), which has a “lost motion” equal to the rotation of the helical rod by 1 tooth.

From a comparison of the possible bit rotation angles in modern drilling hammers with the optimum rotation angles obtained by us in previous studies [5; 6], where \(\alpha_{opt}=24^\circ;\ 27.8^\circ;\ 32.7^\circ;\ 40^\circ\), it is seen that reducing the ratio

\[ \frac{S_{ct}}{S} \]

below 12 is not advisable, since this increases the value of the minimum angle \(\alpha_1\), which determines the region of stable operation of the hammer. Therefore, it is more expedient to manufacture the helical rods in PM-508 hammers, for example, with a thread pitch \(S_{ct}=650\ \text{mm}\), rather than with \(S_{ct}=550\ \text{mm}\). The closest agreement of the bit rotation angles between impacts with the optimum values is achieved in the drilling hammers RPM-17, OM-506 m, PM-508 \((S_{ct}=650\ \text{mm})\), and KTsM-4. In this case, the most probable agreement of the bit rotation angles between impacts with the optimum values is possible in the hammers RPM-17, OM-506 m, and PM-508. In our opinion, the ratchet mechanism with 33 teeth and two ratchet pawls also deserves attention. With this ratio, operation of the drilling hammer can be ensured with bit rotation between impacts through angles whose values are close to the optimum ones.

Thus, intermittent-action bit rotation devices can ensure operation of the drilling hammer with different bit rotation angles between impacts. The possible bit rotation angles between two impacts are determined by the number of teeth of the ratchet bushing, the number of ratchet pawls, the piston stroke, and the pitch of the helical rod.

Changing the bit rotation angle during drilling can be accomplished by changing the feed force. Therefore, requirements must be imposed on the design of the automatic feeder not only from the calculation of ensuring automatic feed, but also from the standpoint of the possibility of changing the bit rotation angle between impacts when drilling rocks of different hardness. In view of the fact that the moment of resistance to bit rotation in the blasthole varies considerably depending on the feed force [6], automation of drilling with drilling hammers must be based on changing the feed force as a function of the torque developed by the device for rotating the bit.

References

1. Uspensky N. S. A Course in Deep Drilling by the Percussive Method. Izd. Neft. Prom., Moscow–Leningrad, 1924.

2. Epstein E. F. Theory of Drilling and Cutting of Rock with Hard Alloys. GONTI, 1939.

3. Medvedko A. I. On the mechanism of rock destruction during drilling. Mining Journal, No. 12, 1947.

4. Schreiner L. A. Physical Foundations of the Mechanics of Rocks. Gostoptekhizdat, 1950.

5. Alimov O. D. Investigation of the Mechanism of Rock Destruction in Percussive-Rotary Drilling and the Initial Parameters of Drilling Hammers. Tomsk, Poligrafizdat, 1953.

6. Alimov O. D. On the mechanism of rock destruction in percussive-rotary drilling. Izv. TPI, vol. 75, 1954.

7. Pokrovsky I. S. Theory of percussive drilling. Mining Journal, No. 12, 1949.

8. Alabuzhev P. M., Alimov O. D. The influence of feed force and bit rotation angle between blows on the speed of percussive-rotary drilling of blast holes. Izv. TPI, vol. 75, 1954.

9. Voroshilin I. R. Mechanization of Mining Operations. Metallurgizdat, 1952.

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

Noted Misprints