Some Results of a Study of the OM-506 Pneumatic Drilling Hammer

O. D. Alimov, V. F. Gorbunov

(Presented by Prof. P. M. Alabuzhev, Doctor of Technical Sciences)

The aim of the present investigation was to determine how the basic parameters characterizing the working processes of the percussion unit of the OM-506 pneumatic drilling hammer change as functions of changes in the feed force and air pressure.

The investigations were carried out according to a method developed by the Department of “Mining Machines and Mine Transport” of the Tomsk Polytechnic Institute [1].

The percussion units of pneumatic drilling hammers of the OM-506 type can operate both with a tool—during drilling—and without a tool, at idle. The devices that ensure operation of the pneumatic hammer at idle are intended for testing and running-in the hammer before drilling, and also for eliminating possible impacts of the piston-striker against the hammer body and other parts in the event of breakage or of the tool falling out of the hammer during drilling. The working process during idle operation of the hammer differs somewhat from the working process when the hammer operates with a tool. Therefore, in order to design new hammers and improve existing ones, it is necessary to know the course of the working process both under various drilling regimes and during idle operation of the hammer.

Operation of the Drilling Hammer at Idle

During idle operation of the hammer, the piston-striker performs reciprocating motion. The piston stroke is limited by air cushions, which are compressed by the piston in the front and rear cavities of the cylinder. Energy obtained by the piston from the compressed air entering the hammer during the remaining part of the cycle is expended on compressing the air cushions. Thus, during the forward stroke, after the front edge of the piston has passed the front edge of the exhaust port (position III, Fig. 1), the piston travels a distance \(l = 42\) mm, compressing the air cut off in the front cavity to a compression ratio

\[ \frac{V_1}{V_2} = 4.5 \]

(taking into account the volume of air in air-supply channel “B”). It may be assumed that the moment at which the opening of channel “B” is closed corresponds to the moment when the valve is shifted to the forward position. Having closed the opening of channel “B” (position I), the piston continues moving forward and compresses the air contained in the volume between the front wall of the piston and the intermediate bushing. The additional compression ratio of the air

\[ \frac{V_2}{V_3} \]

reaches fifteen.

Having expended all the kinetic energy acquired in the first part of the forward stroke on compressing the air cushion, the piston stops, and then, owing to the energy of the compressed air, begins to move backward with acceleration and opens the opening of the air-supply channel “Б.” Through channel “Б,” compressed air from the mains enters the front cavity of the hammer, and the piston continues to move with acceleration to the rear position, displacing air from the rear cavity through exhaust opening A. After the rear edge of the piston covers the exhaust opening (position IV), the air remaining in the rear cavity of the cylinder is partly displaced through opening “В,” which connects the cylinder cavity with the atmosphere. Having covered opening “В” (position V), the piston compresses the air remaining in the cavity. The compression of the air consumes the kinetic energy that the piston acquired at the beginning of the backward stroke. Before channel “В” is covered (position VI), from the beginning of the opening of channel “Б,” the piston travels a distance \(l_1 = 50\) mm. The retardation of the piston’s backward stroke begins, essentially, from the beginning of the covering of channel “В.” After the beginning of the covering of channel “В” and before stopping, the piston travels a path of 23–25 mm, not reaching the rear end of the cylinder (position \(O'\)) by 6–8 mm. The compression ratio of the air in the rear cushion after the covering of channel “В” is

Fig. 1. Model of the drilling hammer for processing piston-stroke diagrams.

\[ \frac{V_4}{V_5} = 3.7\text{–}2.75. \]

Because, during the backward stroke (return stroke), the piston compresses an air buffer of larger cross section and greater length than during the forward stroke, the stopping of the piston during the backward stroke occurs over a longer interval of time than at the end of the forward stroke. The deceleration time is:

a) during the forward stroke, 2–3 milliseconds, or \(1/15\) of the cycle time;
b) during the backward stroke, 5–7 milliseconds, or \(1/7\)–\(1/5\) of the cycle time.

The magnitude of the piston idle stroke, Fig. 1, increases as the air pressure increases. At an air pressure of 4–6 atm, the piston stroke length was 75–80 mm. Simultaneously with the increase in piston stroke as the air pressure increases, the time of one cycle decreases: from \(t_c = 37\) msec at 4 atm to \(t_c = 31\) msec at 6 atm; correspondingly, the number of piston strokes increases from 1620 to 1940 per minute. The ratio of the return-stroke time to the working-stroke time was

\[ \frac{t_x}{t_p} = 1.17\text{–}1.2. \]

The piston has its maximum velocity relative to the hammer body: a) during the forward stroke, at a distance of 40–42 mm from the beginning of the forward stroke, which precedes the opening of the exhaust port by the rear edge of the piston (position IV); b) during the backward stroke, at a distance of 50–57 mm from the beginning of the backward stroke, which precedes the opening of the exhaust port by the front edge of the piston (position III).

From the diagrams of the piston idle stroke one can calculate the maximum velocities of piston motion relative to the hammer body and the corresponding values of the kinetic energy of the piston, which, during idle operation of the hammer, are expended mainly in compressing the air buffers.

On the basis of an analysis of the diagrams of the piston idle stroke, the following conclusions may be drawn:

1. The magnitude of the piston idle stroke is 88–90% of the theoretically possible piston stroke.

2. The degree of compression of the air buffers during the forward stroke reaches 15, and during the backward stroke 3.7–2.75.

3. We assume that the piston cycle time can be shortened and, consequently, the number of cycles per minute increased by reducing the length of the rear buffer and thereby reducing the time of deceleration and acceleration of the piston in the rear extreme position.

Reduction of the rear buffer can be accomplished by reducing the length of the cylinder or by increasing the length of the piston.

Operation of a Pneumatic Drilling Hammer with a Tool

During operation of a pneumatic drilling hammer with a tool, the piston stroke is limited, during forward motion, by the end face of the drill shank, and during backward motion by the air buffer. A necessary condition for ensuring a normal drilling process is the application to the hammer body of an additional force in the direction of the face—the feed force. In the absence of an additional force, after the very first blows the hammer body moves away from the drill-collar stop to such an extent that the tool jumps out of the rotation bushing.

When the feed force is changed, a phenomenon occurs analogous to changing the length of the drill shank, i.e., the magnitude of the piston stroke and the parameters of the working process change.

Fig. 2 presents diagrams of the piston stroke when drilling sandstone at an air pressure at the hammer of 4 atm and at various feed forces.

At small feed forces, \(F = 24\) kg and less, the impact unit operates unevenly; some of the cycles with a smaller piston-stroke magnitude end זיך

Fig. 2. Diagrams of piston travel and recoil under various regimes of drilling sandstone
with an air pressure of 4 atm.

$F$ — feed force, kg; $V$ — drilling speed, mm/min;
$m_s$ — scale of piston travel, mm/mm;
$m_t$ — time scale, sec/mm; $A$ — piston-travel diagram,
$B$ — recoil diagram.

by striking the drill; the other part of the cycles, with a greater piston stroke, consists of idling cycles of the hammer at the given line pressure. Idle blows occur at the moment when the hammer body has moved considerably away (from the penetration line), as a result of which the drill shank leaves the internal cavity of the hammer. Owing to the different length of the drill shank inserted into the hammer cavity at the moment when the piston strikes it, the working cycles at a small feed force are not identical. With a further increase in the feed force the number of idle strokes decreases and the working process becomes more stable. Idle strokes cease when the maximum displacement of the hammer from the restraining collar of the drill (from the penetration line) does not exceed 10–12 mm.

The most stable operating process of the hammer is observed at feed forces that ensure a displacement of the hammer from the penetration line by 2–5 mm. In these cases the displacement of the hammer from the penetration line and the rebound of the drill from the rock are approximately constant; the piston blows on the drill shank occur at one and the same place, with the same velocity, and therefore the magnitude of the piston stroke varies only within small limits.

The change in the velocity of the piston during the working stroke occurs smoothly; the velocity increases over the section from the beginning of the forward stroke to the opening, by the rear edge, of the exhaust port (position III) (Fig. 2). Then, up to position II, the motion proceeds almost at constant velocity. After the beginning of the closing of the opening of channel “B,” the velocity begins to decrease. The greatest decrease in velocity is observed after the opening of channel “B” is closed by the front edge. After collision with the drill the piston rebounds backward with a certain velocity. With this velocity the piston moves 2–5 mm; then the velocity of the piston motion slows somewhat, and only after the complete opening of channel “B” (position II) does the velocity increase. The increase in the piston velocity during the return stroke after striking the drill occurs more slowly than in the case of idle operation of the hammer, when the piston receives a considerable acceleration owing to the large degree of compression of the front cushion.

Over the section III–IV the piston moves at an almost constant velocity. Before the opening, by the front edge, of the exhaust port (position IV), the piston stroke begins to slow, and then the piston stops. The value of the piston stroke corresponding to the maximum drilling speed in sandstone and granite at pressures of 4, 5, and 6 atm is equal to \(52 \pm 1\) mm. In these cases, during the return stroke the piston stopped in position V, that is, when the rear edge of the piston covers the opening of channel “B.” Changes in the duration of one cycle and in the ratio between the duration of the working and return strokes in this case were insignificant. With a further increase in the feed force, a lower stability of the working process of the drilling hammer is observed. To start the hammer it is necessary first to pull the hammer body away from the face. At the moment when the hammer is pulled away from the face, the piston stroke naturally increases (section \(f—k\), test No. 25, Fig. 2), and then decreases. The cycle diagram also changes, especially the diagram of the second half of the piston return stroke. The front end position of the piston, as before, is limited by the blow of the piston against the drill at the moment when the lower edge of the piston has almost completely closed the air-supply channel “B” (position I). The rear end position of the piston in some cycles corresponds to the beginning of the opening of exhaust port “A” (position IV). The duration of individual cycles increases owing to an increase in the time of the return stroke. The slowing of the piston stroke occurs chiefly in the second half of the return stroke, at a distance of 5–12 mm before the opening of the exhaust port. It is characteristic that after a sharp slowing of the piston velocity, it then again moves with increasing velocity. Such a local slowing of the piston motion may be ex—

can be explained as follows: with accelerated backward motion of the piston, the difference between the air pressures in the front and rear cavities of the cylinder decreases; as a result, the acceleration of the piston and the torque that the piston can develop while rotating the drill decrease. With considerable resistance to rotation of the drill in the borehole, the pressure difference does not ensure backward displacement of the piston at the former velocity, and the piston velocity decreases (section \(c—d\), test No. 25), Fig. 2. But when the piston velocity decreases, the air pressure in the rear cavity decreases owing to the outflow of part of the cut-off air into the atmosphere through opening “B.” As a result, the pressure difference in the front and rear cavities increases, which increases the force acting on the piston and, consequently, the piston velocity as well (section \(d—e\), test No. 25). Thus, the presence of opening “B” makes it possible to increase the torque of the hammer at the end of the return stroke and thereby to ensure more stable operation of the hammer.

It should be noted that, at considerable feed forces, cycles are observed in which exhaust of air from the front cavity of the cylinder through the exhaust port does not occur, since the front edge of the piston in the rear extreme position does not uncover the exhaust port. In this case the compressed air from the front cavity of the cylinder flows into the atmosphere through the splines of the piston rod, which enter the front cavity of the hammer before the exhaust ports open. Since, when air is throttled through the splines, the pressure in the front cavity falls more slowly than when exhausting through special ports, the piston is delayed in the rear extreme position for a longer interval of time. This interval of time is sometimes equal to the duration of an entire cycle during stable operation of the hammer. The impact energy in such cycles is very small, owing to incomplete exhaust from the front cavity. The following cycle usually has a normal piston stroke.

Operation of the drilling hammer in regimes in which the rear extreme position of the piston corresponds to the beginning of opening of the exhaust port is unstable, and the hammer often stalls. The hammer usually stalls at the moment of the return stroke of the piston when the front edge of the latter does not reach the line of opening of the exhaust port by 6–10 mm. In this case the pressure differences are insufficient for further backward advance of the piston, while the excess air pressure in the front cavity does not allow the valve to be shifted into the position corresponding to the working stroke.

When drilling with an air pressure of 5 and 6 atm, the pattern of change of the piston-stroke cycles with change in feed force is the same as when operating at 4 atm. In contrast to operation at 4 atm, when drilling at 5 and 6 atm the region of unstable operation of the hammer at small feed forces expands owing to the large rebound of the hammer body from the limiting collar of the drill. Stable operation of the OM-506 hammer at an air pressure of 5 and 6 atm is ensured at a feed force of 47–60 kg.

When drilling granite, the piston-stroke diagrams at various feed forces and air pressures retain, in the main, all their characteristic features (Fig. 3). However, in view of the greater hardness of granite, the region of stable operation of the hammer shifts toward greater feed forces than when drilling sandstone. Figs. 4 and 5 compare the piston-stroke cycles under various regimes of drilling sandstone and granite.

Relationship between the piston stroke and the rebound of the drilling hammer

During idling operation of the hammer, the piston-stroke diagram and the rebound diagram are curves resembling sinusoids. During drilling, the piston-stroke diagram changes its configuration because of the impact of the piston on the drill shank, as described above. The rebound curve of the hammer—the motion

of the hammer body over time also undergoes a corresponding change. At small feed forces, the hammer moves away from the face not only during one cycle, but oscillations of the body are observed over several cycles. In this case, the character and magnitude of the oscillations of the body change with the change in feed force. Over the entire extent of the rebound diagram

Fig. 3. Diagrams of piston travel and rebound under different regimes of drilling granite with an air pressure of 6 atm.

\(F\) — feed force, kg; \(V\) — drilling speed, mm/min;
\(m_s\) — scale of piston travel, mm/mm; \(m_t\) — time scale, sec/m;
\(A\) — diagram of piston travel; \(Б\) — rebound diagram.

(Fig. 2), test No. 20, periodic increases and decreases in the displacement of the hammer are observed; moreover, the increase in displacement occurs smoothly over the course of 8–13 blows (cycles), while the decrease in displacement—the approach of the hammer to the face—

—occurs more rapidly, over the course of 3–4 cycles. The recoil of the hammer decreases to 5–7 mm, and then the recoil again increases, i.e., the next period begins. Under optimal feed forces, the recoil of the hammer from the face is approximately the same throughout the entire operation (experiment No. 23).

As a result of a change in the magnitude of the recoil of the hammer body from the face when drilling with a small feed force, the piston stroke, at one and the same feed force and air pressure, is not constant (Fig. 2), experiment No. 20. The piston stroke increases with an increase in the recoil of the hammer body from the face. But the increase in piston stroke is not proportional to the increase in hammer recoil from the face (from the line of penetration); the increase in piston stroke is, as a rule, less than the increase in recoil of the hammer body from the face. This can be explained by the fact that when the hammer body recoils from the face, after impact on the rock the drill rebounds backward and strikes the drill shank against the hammer body. As a result, the next impact of the piston on the drill occurs at the moment when the drill shank is either near the hammer body or at a slight distance from it. After impact on the drill, the piston stops or rebounds somewhat backward, while the drill moves toward the face, strikes the rock with the bit, and rebounds again. The average magnitude of the drill rebound from the rock during drilling under optimal conditions may, without great error, be taken as equal to the average magnitude of the recoil of the hammer body from the line of penetration. The maximum drilling speeds correspond to an average recoil magnitude of \(h_{\mathrm{av}} = 2\text{–}6\) mm.

With an increase in feed force above the optimal value, cycles with a sharp change in the velocity of piston motion during the return stroke correspond to cycles of the recoil diagram with additional oscillatory motions of the hammer body (experiment No. 23, Fig. 2). Cycles with additional oscillatory motion are usually more extended in time; the average amplitude of body oscillation in this case is smaller.

As is evident from the foregoing, the presence of a certain constant recoil of the hammer from the line of penetration ensures constant rebound of the drill from the rock and determines stable operation of the hammer.

Magnitude of the Piston Stroke of the OM-506 Drilling Hammer during Drilling

Comparison of piston-stroke diagrams at one and the same feed force (Figs. 4, 5 and Tables 1 and 2) shows that when the air pressure is increased by 1 atm, the piston stroke increases by 4–7 mm; moreover, in the zone of small feed forces this increase amounts to 7 mm/1 atm, and in the region of large forces to 4–5 mm/1 atm. In the region of stable hammer operation, when the feed force is increased by each 10 kg, the piston stroke decreases by 4–5 mm, Figs. 6, 7. When the piston stroke is reduced to 50 mm, the hammer operates stably. A further decrease in piston stroke due to an increase in feed force leads to unstable hammer operation. With a piston stroke of less than 44 mm, the exhaust port at the end of the piston return stroke does not open (with drill shank length \(l_x = 107\) mm), and the exhaust of compressed air occurs partly through the clearances between the splines of the piston rod and the rotary bushing. When the piston stroke is reduced to 42 mm, the hammer stalls. This corresponds to the feed forces given in Table 3.

Figs. 8a and 8b present a comparison of piston-stroke cycles corresponding to optimal feed forces (ensuring the maximum drilling speed) at different air pressures (Fig. 8a) and when drilling different rocks (Fig. 8b).

Table 1

Table 2

Table 3

Table 4

Fig. 4. Comparison of piston-stroke cycles under various sandstone drilling regimes:
a — feed force 24 kg, b — 36 kg, c — 47 kg, d — 59 kg, e — 71 kg, f — 83 kg; — air pressure 4 ati, — — 5 ati, — · — 6 ati.

From Figs. 8a, 8b and Table 4 it is evident that the piston stroke of the OM-506 hammer under operating conditions that ensure the maximum drilling speed at different air pressures and when drilling different rocks is a relatively constant quantity, equal to \(S = 52 \pm 1\) mm.

Fig. 5. Comparison of piston-stroke cycles under different granite-drilling conditions:
\(a\)—feed force 24 kg, \(b\)—36 kg, \(v\)—47 kg, \(g\)—59 kg,
\(d\)—71 kg, \(e\)—95 kg; ——— air pressure 4 atm,
– – – 5 atm, – · – 6 atm.

Compression of the front and rear air cushions

The degree of compression of the air cushions is evaluated as a percentage of the total cushion length [1]. With an increase in the feed force, the degree of compression of the air cushions decreases both when drilling pestanik and when drilling granite. At the same feed force, the deformation of the air cushions is greater the higher the air pressure (Fig. 9). The maximum compression of the rear air cushion corresponds to the idle stroke of the piston and is \(\sim 90\%\). With an increase in feed force by 10 kg, compression of the rear cushion decreases by 7–3%; with an increase in air pressure by 1 atm at the same

Fig. 6. Dependence of the piston-stroke magnitude on air pressure and feed force when drilling sandstone:
○ — 4 atm, △ — 5 atm, □ — 6 atm.

Fig. 7. Dependence of the piston-stroke magnitude on air pressure and feed force when drilling granite:
○ — 4 atm, △ — 5 atm, □ — 6 atm.

the same feed force, the compression increases by 5–12%. The minimum compression of the rear buffer is, when drilling granite, 45%, and when drilling sandstone, 43%, which corresponds to a piston stroke of \(s = 43\) and \(42\ \text{mm}\), i.e., to an unstable operating regime of the hammer. In this case, on the return stroke the piston does not completely cover channel “B”.

Fig. 8a. Comparison of piston-stroke cycles during drilling at different air pressures and with optimum feed forces.
\(a\)—operation of the hammer at idle; \(b\)—drilling sandstone; \(c\)—drilling granite; — \(4\) atm; --- \(5\) atm; -·- \(6\) atm.

Fig. 8b. Comparison of piston-stroke cycles when drilling different rocks with optimum feed forces.
\(a\)—at a pressure of \(6\) atm; \(b\)—at a pressure of \(4\) atm; \(c\)—at a pressure of \(5\) atm; — sandstone; --- granite.

The front air buffer, during the idle stroke, is deformed by up to 95%. In this case, between the front end of the piston and the intermediate bushing there remains at times a buffer with a thickness on the order of \(0.5\)–\(1.0\ \text{mm}\). In the range of small feed forces (up to \(25\ \text{kgf}\)), the compression of the front buffer is 50–80%. With a further increase in the feed force, the degree of compression of the air buffer decreases rapidly. At feed forces ensuring pressing of the housing

of the hammer to the limiting shoulder of the drill bit, compression of the front buffer, after the closure of intake channel “Б,” generally does not occur. As noted above, under these conditions the rebound of the piston backward after impact is smaller than at low feed forces, when closure of channel “Б” and some compression of the front air buffer precede the moment of impact. Consequently, compression of the front buffer has a greater influence on the magnitude of rebound than the pressing of the drill bit against the rock before it is struck by the piston.

Fig. 9. Relationship between piston stroke and degree of compression of the air buffers under different granite-drilling conditions:
○ — 4 atm; △ — 5 atm; □ — 6 atm;

A — exhaust port, Б — front intake port, В — tangential channel, а — change in the degree of compression of the front buffer, б — position of the front edge of the piston at the extreme rear position (degree of opening of the exhaust port), в — change in the degree of compression of the rear buffer, \(l_n\) — height of the piston head. 0–VI — characteristic positions of the piston in the hammer cylinder (see Fig. 1).

The operating regimes of the hammer that ensure high drilling speeds and stable operation correspond to the following compressions of the air buffers: rear—60–65%, front—17–20%.

The character of the change in piston stroke and in the deformation of the air buffers is the same both when drilling granite and when drilling sandstone. The only difference is that when drilling granite, the decrease in compression of the front and rear buffers occurs over a somewhat larger range of feed forces than when drilling sandstone.

Impact energy

The general character of the change in impact energy (the kinetic energy of the piston before collision with the drill bit) as a function of feed force at various air pressures (Fig. 10) is analogous to the character of the change in drilling speed. At first, at low feed forces, the piston energy before collision with the drill bit increases and, at feed forces of 45–55 kg, reaches a maximum. With a further increase in feed force, the kinetic energy of the piston begins to decrease.

The feed forces corresponding to the maximum kinetic energies of the piston before collision with the drill bit are close to the feed forces that ensure the maximum drilling speed. The maximum piston energy before collision is attained at a somewhat smaller feed force than the maximum drilling speed.

In Table 5 are given the values of the kinetic energy of the striker piston as a function of the feed force \(F\) and the air pressure \(p\) for the drilling

Fig. 10. Dependence of impact energy on air pressure and feed force when drilling granite:
□ — 4 ati, △ — 5 ati, ○ — 6 ati.

Table 5

hammer OM-506 when drilling granite, and in Fig. 11 is presented the dependence of the change in impact energy on the magnitude of the piston stroke.

In constructing Fig. 11, two types of characteristic curves are distinguished:

1) dependences of impact energy on piston stroke at constant air pressure and various feed forces \(A_y = f_1(S, F)\) are shown by solid lines;

2) dependences of impact energy on piston stroke at constant feed force, but variable air pressure \(A_y = f_2(S, p)\) are shown by a dashed line.

The first dependences are expressed by convex curves having maxima

\[ \text{at } p = 6 \text{ ati}, \quad S = 60 \text{ mm}, \quad F = 60 \text{ kgf}, \]

\[ \text{at } p = 5 \text{ ati}, \quad S = 57 \text{ mm}, \quad F = 50 \text{ kgf}, \]

\[ \text{at } p = 4 \text{ ati}, \quad S = 50 \text{ mm}, \quad F = 45 \text{ kgf}. \]

When the feed force is decreased, the piston stroke increases, while the impact energy decreases because the kinetic energy of the piston is expended in compressing the air cushion in the front chamber of the hammer cylinder. When the feed force is increased above the optimum, the impact energy decreases rapidly because of a reduction in the active part of the piston stroke (from the rear extreme position to the opening of the exhaust port) and because of deterioration of the exhaust of air from the front chamber of the cylinder, as a result of which the remaining air offers considerable back pressure during the working stroke. In the diagram in Fig. 11, the shaded part denotes the region of rational operation of the rock drill; it corresponds to higher drilling speeds.

Fig. 11. Dependence of impact energy on piston stroke:
□—4 atm, △—5 atm, ○—6 atm.

At a constant feed force and with a change in air pressure, the piston stroke and impact energy increase as the air pressure increases. In the range of small feed forces, an increase in pressure by 1 atm corresponds to an increase in impact work by 1.8 kg·m and an increase in piston stroke by 6–7 mm. In the range of high feed forces, a change in pressure by 1 atm corresponds to a change in impact energy by 0.5–1.0 kg·m and in piston stroke by 4–5 mm.

A theoretical substantiation of the indicated relationships can be made with sufficient accuracy only after an additional study of the change in air pressure in the different chambers of the rock drill as a function of the feed force.

Duration of One Piston-Stroke Cycle

The duration of one piston-stroke cycle varies depending on the air pressure and the feed force (Fig. 12).

As the air pressure increases, the duration of one cycle decreases and, consequently, the number of blows increases. With an increase in press—

[[unclear: beginning of line]] of air pressure by 1 atm, the duration of the cycle decreases on average by 0.0025–0.003 sec.

Vertical axis: cycle time, ms
Horizontal axis: feed force, kg
Curve labels: 4 atm; 5 atm; 6 atm

Fig. 12. Dependence of the cycle time of the piston stroke on the feed force and air pressure when drilling sandstone.

When the feed force is varied in the case of drilling sandstone, the duration of the piston-stroke cycle changes as follows:

1) when the feed force is increased from 0 to 20 kg (for a pressure of 4 atm), Fig. 12, and to 35 kg (for 6 atm), the cycle duration increases;

2) when the feed force is increased from 20–35 kg to a feed force 5–10 kg greater than the optimum, the cycle duration gradually decreases. This interval is the larger, the greater the air pressure;

3) when the feed is increased beyond 50–60 kg, the cycle duration increases.

Such a change in the duration of the cycle can be explained by the influence of the following factors: in the first interval \((F = 0 \div 20–35\ \text{kg})\), the duration of the cycles increases owing to a decrease in the degree of compression of the front air cushion, as a result of which the acceleration time of the piston at the beginning of the return stroke increases. In these cases rebound of the striker from the drill is not observed. In the second interval, the reduction in the cycle time occurs, first, owing to a decrease in the length of the piston stroke and, second, owing to the acceleration of the piston at the beginning of the return stroke as a result of the rebound of the piston from the drill. In the third interval, the cycle duration increases owing to an increase in the time of the return stroke, due to the high resistance to rotation of the drill, and owing to an increase in the time of the working stroke, due to incomplete preliminary exhaust of air from the front cavity of the cylinder.

In contrast to sandstone, when drilling granite the reduction in the duration of the piston-stroke cycle in the second interval occurs up to larger absolute values of the feed force. In addition, the maximum durati-

the cycle duration when drilling granite is less than when drilling sandstone. This can be explained by the influence of the rebound of the striker, which is greater when drilling granite than when drilling sandstone.

Relationship between the duration of the return (idle) stroke and the duration of the working stroke

With a change in the feed force, the duration of the return stroke \(t_x\), the working stroke \(t_p\), and their ratio \(t_x : t_p\) change (Fig. 13). During idling the ratio \(t_x : t_p\) is close to unity. In a number of cases, at a pressure \(p = 6\) atm, values of \(t_x : t_p\) less than unity (0.9–0.8) were obtained, that is,

Fig. 13. Influence of feed force on the value of the ratio of the piston return-stroke time to the direct-stroke time when drilling granite:
\(\square\) — 4 atm, \(\triangle\) — 5 atm, \(\bigcirc\) — 6 atm.

the idle-stroke time in these cases was less than the working-stroke time. With an increase in feed force from 0 to 30–35 kg, the ratio \(t_x : t_p\) increases from 1.0–0.8 to 1.3–1.6. At optimal feed forces the ratio is \(t_x : t_p = 1.4 \div 1.9\). This range is the same when drilling both granite and sandstone with different air pressures (from 4 to 6 atm). When the feed force is increased above the optimum, most cycles have \(t_x : t_p = 1.7 \div 1.8\), but cycles with \(t_x : t_p = 2 \div 2.5\) are encountered.

Power developed by the impact unit. Volumetric work of destruction

The power developed by the hammer impact unit varies depending on the air pressure and the feed force (Fig. 14). When the feed force is increased from 0 to 50, 60, 65 kg (for 4, 5, and 6 atm), the impact power increases. This is accompanied by a rapid increase in drilling speed. With a further increase in feed force, the impact power decreases. A decrease in impact power leads to a decrease in drilling speed.

The effectiveness of the impact power developed by the hammer for breaking rock under particular operating conditions of the hammer can be most fully evaluated from the value of the volumetric breaking work (Fig. 15).

Fig. 14. Dependence of the power of the impact unit on feed force and air pressure when drilling granite:
□ — 4 atm, △ — 5 atm, ○ — 6 atm.

Fig. 15. Dependence of the volumetric work of rock breaking on feed force and air pressure.

By the volumetric work of destruction in this case is meant the work expended by the impact mechanism on destroying \(1\ \text{cm}^3\) of rock. The power expended in doing so on rotation of the drill steel was not taken into account.

It is evident from Fig. 15 that, both in drilling granite and in drilling sandstone, the magnitude of the volumetric work of destruction decreases as the feed force is increased from zero to \(45\text{--}80\ \text{kg}\). The minimum volumetric work corresponds to feed forces equal to, or somewhat greater than, the feed forces that ensure the maximum drilling rates. With a further increase in feed force, the volumetric work of destruction increases. Consequently, the most complete transfer of impact energy from the striker piston through the drill steel to the destruction of the rock occurs when the mean rebound of the hammer body is reduced to zero.

Conclusions

1. The pressure of the air supplied to the hammer and the feed force during drilling have a substantial effect on the working process of pneumatic drilling hammers of the OM-506 type.

2. When designing a drilling hammer with dependent rotation of the drill steel, the design and dimensions of the impact unit should be selected with consideration of:

a) ensuring stable operation of the hammer at no load;

b) ensuring stable operation of the hammer during drilling when the feed force varies considerably.

1. The increase in drilling rate when the feed force is increased from 0 to \(40\text{--}60\ \text{kg}\) is achieved through a change in the working process of the hammer, as a result of which:

a) the number of strokes of the striker piston ending with an impact of the piston on the drill steel increases;

b) the energy of the piston before collision increases (impact energy);

c) the number of impacts increases;

d) the efficiency of energy transfer from the piston to the drill steel and from the drill steel to the destruction of the rock increases.

1. The stability of the working process of the impact unit when operating at substantial feed forces can be ensured if the drilling hammer develops on the drill steel the torque required to rotate the drill steel.

2. The torque developed on the drill steel of drilling hammers with an air-distribution device of the OM-506 type (RP-17, PR-30, PR-35, etc.) can be increased by means of a special channel connecting the rear cavity of the hammer with the atmosphere. This opening should expediently be located in the hammer body in such a way that it is closed by the rear edge of the piston at the moment when the front edge of the piston begins to open the exhaust port.

3. On the basis of experimental data it may be assumed that the number of impacts of the OM-506 drilling hammer can be increased by reducing the length of the rear buffer by \(4\text{--}5\ \text{mm}\) (reducing the cylinder length by shortening the distance between the rear edge of the exhaust port and the valve box). The impact energy can be increased by increasing the effective portion of the piston working stroke. For this purpose it is advisable to reduce the length of the exhaust port by \(6\text{--}8\ \text{mm}\), shifting the rear edge of the exhaust port by this amount toward the front part of the hammer cylinder.

On the basis of the studies carried out on the kinematics of the OM-506 drilling hammer, we know the velocities of motion of the striker piston at every point of its path; from these it is possible to determine the accelerations and the forces moving the piston. To verify and refine the forces obtained in this way, it is advisable simultaneously to record indicator diagrams of the drilling hammer.

REFERENCES

1. Alimov O. D., Basov I. G., Gorbunov V. F., Serov Ya. A. Methodology for conducting experiments in the study of the kinematics of impact units of pneumatic drilling hammers and processing of experimental data. Izvestiya TPI, vol. 88, 1956.