Investigation of the impact unit of an electropneumatic drilling hammer
Abstract Generated abstract
This paper reports experimental investigations of the impact unit of a TEMZ-EM electropneumatic drilling hammer intended to reduce reliance on costly compressed air in hard-rock drilling. Using a laboratory installation with controlled impact, rotation, feed, and flushing systems, the authors drilled sandstone and granite specimens while varying the position of the air-leakage compensation opening and the length of the air cushion, then evaluated drilling speed, power consumption, stability, and volumetric work of destruction. The tests indicate that the most favorable regime for the studied hammer occurs with leakage compensation through the sixth opening and an air-cushion length of 46 mm, giving maximum drilling speed and minimum volumetric work of destruction. The authors further suggest that reduced drill rotation speed and increased feed force could substantially improve drilling performance.
Full Text
INVESTIGATION OF THE IMPACT UNIT OF AN ELECTROPNEUMATIC DRILLING HAMMER
A. S. SIMONOV, Ya. A. SEROV, V. F. GORBUNOV, N. N. RODYUKOV
(Presented by Prof., Doctor of Technical Sciences P. M. Alabuzhev.)
Introduction
At the present time, blast-hole drilling in hard rocks is carried out mainly by pneumatic drilling hammers. A significant shortcoming of this method of drilling is the high cost of compressed-air energy. Hence follows the need to create electric drilling machines capable of drilling hard rocks.
Recently, scientific-research institutes and plants have designed and manufactured several new designs of electric and electropneumatic drilling hammers [1].
The operating principle of the impact units of electropneumatic hammers has been set forth in sufficient detail by P. M. Alabuzhev in his doctoral dissertation [2]. The staff of the Department of “Mining Machines and Mine Transport” of the Tomsk Polytechnic Institute carried out additional investigations of the performance of the impact unit of an electropneumatic hammer when drilling rocks of various hardness. The purpose of these investigations was to determine the most advantageous location of the compensation opening and the rational magnitude of the air cushion, ensuring the greatest drilling speed with the minimum specific expenditure of power of the designed TEMZ-EM powerful electropneumatic drilling hammer.
Description of the experimental installation and processing of experimental data
The experimental installation consists of the following main parts: an impact and rotation unit, a drilling tool, devices for blowing the blast-hole, for feeding the drill to the face, a support unit, and control-and-measuring apparatus.
The impact unit (Fig. 1) consists of cylinder 1, piston 2, and striker 3. During operation of the impact unit, the striker and piston perform reciprocating motion in cylinder 1. During forward motion, the striker delivers impacts on the end part of the adapter sleeve, in which the drilling tool is fastened. The reciprocating motion of the piston is effected by means of crank-and-connecting-rod mechanism 4. The crank-and-connecting-rod mechanism is driven by a three-phase electric motor 5 (power \(N = 2.7\) kW, rotational speed \(n = 1460\) rpm) through V-belt transmission 6 with transmission ratio \(i = 1.12\). Between the piston and the striker there is an air cushion, the magnitude of which is varied by changing the length of the shank of the adapter sleeve with the aid of washers 7 within the range from
0 to 65 mm. To compensate for air leakage from the air cushion along the cylinder barrel, there is a row of compensation openings 8, which can be closed with screw plugs.
Fig. 1. General view of the experimental setup.
The drill-rotation unit consists of a direct-current motor 9 and a reduction gear 10. The reduction gear consists of two pairs of spur cylindrical gears with a total gear ratio \(i = 5.2\). The gear housing is attached to the front wall of the cylinder and to the flange part of the electric motor. The drilling tool receives rotary motion from the transmission sleeve 11, on which the driven gear of the reducer is mounted. The transmission sleeve has an elongated keyway and, owing to this, can not only rotate together with the driven gear of the reducer, but also have longitudinal displacement relative to the gear 12 mounted on it. As the drilling tool, a drill with a single-bit crown \(d = 42\) mm, tipped with hard alloy VK-15, was used. The drill is made of drill steel, \(d = 25\) mm, \(l = 1\) m. With its conical shank the drill enters the end conical recess of the transmission sleeve 11, which transmits to it both rotary and translational motion. To remove drilling dust formed in the hole, a side method was used for supplying compressed air into the hole through a special coupling 13 mounted on the transmission sleeve. Air compressed to a pressure of 2–3 atm is supplied to the coupling through a flexible hose 14 from a ShVKS-5 compressor. From the coupling the air enters the transverse groove of the transmission sleeve and then, through the axial opening of the transmission sleeve and the drill, into the hole. To remove drilling dust from the laboratory room, a “P khodka 500” fan was used.
The impact unit with the tool is fed by means of a load fastened to a cable. The cable is passed over a pulley and its other end is fastened to the moving part of the stand. The moving part of the stand is a plate on which the impact and rotary units are mounted. The plate, by means of a carriage, has free longitudinal movement along the guide grooves of the screw feeder.
The support unit consists of a horizontal crossbeam and a cage moving along it. The crossbeam is fastened by clamps to vertical posts welded to the installation frame. The installation frame is welded from two channels and two transverse angles. In the cage of the crossbeam there is mounted a screw feeder with a carriage and the hammer located on it. The design of the cage
allows the drilling direction to be changed in both the horizontal and vertical planes.
The blast holes were drilled in granite with a strength coefficient \(f = 13\text{--}15\) and in sandstone with a strength coefficient \(f = 6\text{--}8\).
The rock specimens were installed in the frame of a special stand and secured with screw jacks.
Parameters of the impact unit
| Name | Indicators |
|---|---|
| Number of blows, blows/min | 1300 |
| Cylinder diameter, mm | 68 |
| Piston stroke, mm | 87 |
| Crank radius, mm | 43.5 |
| Striker weight, kg | 2 |
| Diameter of the compensation hole, mm | 5 |
| Distance between compensation holes, mm | 10--15 |
On the basis of the investigations, the following quantities were determined:
a) drilling rate
\[ V = \frac{l}{t}\cdot 60\ \text{mm/min}, \]
where \(l\) is the depth of the hole drilled in the test, mm,
\(t\) is the drilling duration, sec;
b) power consumed by the impact unit
\[ N_{ud} = 3 \cdot W \cdot 30\ \text{watts}, \]
where \(3\) is the number of phases,
\(W\) is the wattmeter reading in divisions,
\(30\) is the instrument constant for the given current and voltage measurement ranges [3];
c) power consumed by the rotation unit
\[ N_{vr} = U \cdot I\ \text{watts}, \]
where \(U\) is the DC voltage, volts,
\(I\) is the current consumed, amperes;
d) number of hammer blows
\[ n_m = \frac{n_{ud}}{i_{ud}}\ \text{blows/min}, \]
where \(n_{ud}\) is the number of revolutions of the electric motor of the impact unit,
\(i_{ud}\) is the transmission ratio of the V-belt drive;
e) number of drill revolutions
\[ n_b = \frac{n_{vr}}{i_{vr}}\ \text{rev/min}, \]
where \(n_{vr}\) is the number of revolutions of the electric motor of the rotation unit,
\(i_{vr}\) is the transmission ratio of the reducer.
To construct graphs from the tables of experimental results, the following were determined.
- Arithmetic mean values:
a) drilling speed, mm/min,
b) power expended on impact, watts,
c) power expended on rotation, watts.
By the formula:
\[ A_{\text{avg. ar.}}=\frac{\Sigma A}{n_{\text{op}}}, \]
where \(\Sigma A\) is the sum of the values of the quantities for each series of tests (for the given compensation opening or the given value of the air cushion),
\(n_{\text{op}}\) is the number of tests in the series.
- The volumetric work of destruction by the formula:
\[ \Delta A_v=\kappa \frac{N_{\text{avg. ar.}}}{V_{\text{avg. ar.}}}, \ \text{kgm}/\text{cm}^3, \]
where \(N_{\text{avg. ar.}}\) is the arithmetic mean value of the power for the given series of tests,
\(V_{\text{avg. ar.}}\) is the arithmetic mean value of the drilling speed for the same series of tests,
\(\kappa\) is the coefficient of the drill-steel diameter, which may be determined as follows:
\[ \frac{\text{watt}}{\text{mm}/\text{min}} = \frac{0.001\ \text{kW}\cdot 60\ \text{sec}}{0.1\ \text{cm}} = \frac{61.2\ \text{kgm}}{\text{cm}}, \]
since 1 cm of drill steel corresponds to a volume
\[ \frac{\pi}{4}\cdot d^2\cdot 1=0.785\,d^2\text{ cm}^3, \]
we obtain:
\[ \frac{\text{watt}}{\text{mm}/\text{min}} = \frac{61.2}{0.785\,d^2}\ \text{kgm}/\text{cm}^3, \]
and since \(d\) of the drill steel \(\approx d\) of the bit, then
\[ \frac{\text{watt}}{\text{mm}/\text{min}} = \frac{61.2}{0.785\cdot 4.2^2} = 4.37\ \text{kgm}/\text{cm}^3, \]
therefore,
\[ \kappa=4.37\ \text{kgm}/\text{cm}^3. \]
- The value of the root-mean-square error for each arithmetic mean value by the formula:
\[ \sigma=\sqrt{\frac{\Sigma \Delta^2}{n_{\text{op}}-1}}, \]
where \(\Sigma \Delta\) is the sum of deviations of the experimental quantities from their arithmetic mean value,
\(n_{\text{op}}\) is the number of tests. The coefficient of variation (percentage index of the root-mean-square error):
\[ v=\frac{\sigma\cdot 100}{A_{\text{avg. ar.}}}\%. \]
The magnitude of the root-mean-square error and the coefficient of variation makes it possible not only to assess the accuracy of the experiment, but also to judge the stability of the operation of the impact unit under the given regime.
The obtained values of the quantities were entered into tables, on the basis of which the graphs were also constructed (Figs. 2, 3, 4, 5, 6, 7).
Analysis of the Obtained Relationships
Influence of the Position of the Compensation Hole on the Operation of the Percussion Unit
From the graph, Fig. 2, we see that the drilling speed of the hammer varies along a certain broken curve and reaches its maximum value when operating on 5, 6, and 7 compensation holes (Fig. 1).
When the percussion unit operates on the extreme compensation holes, the drilling speed decreases sharply and has its lowest value at 8 and 10 compensation holes. It should be noted that at low drilling speeds the most stable operating regime of the percussion unit is observed both when drilling in sandstone and in granite. When drilling in sandstone, the percussion unit operates most steadily on 4 and 10 compensation holes; when drilling in granite—on the third hole. At high drilling speeds (when operating on 5, 6, and 7 compensation holes) the percussion unit has less stable regimes.
Fig. 2. Dependence of drilling speed on the position of the compensation hole.
Comparing the drilling speeds of the hammer when the percussion unit operates on various compensation holes and proceeding from the stability of the regime, compensation hole 6 should be singled out. Hole 6 is located at a distance of 82 mm from the front wall of the piston (which corresponds to 17.8% of the length of the air cushion), when the piston is in the rear extreme position, and at a distance of 5 mm from the edge of the piston in its front position (which corresponds to 10.8% of the length of the air cushion).
When the percussion unit operates on the 6th compensation hole, the hammer has the maximum drilling speed and a relatively stable operating regime. Since the experiments were carried out at a constant number of blows per minute and a constant number of drill rotations, it may be assumed that the drilling speed changes in proportion to the change in impact power. The power curve when drilling in sandstone (Fig. 3) somewhat resembles the drilling-speed curve; that is, an increase in drilling speed corresponds also to an increase in the power consumed by the percussion unit. The power reaches its greatest value when operating on 6 and 7 compensation holes (which corresponds to the maximum drilling speed), and then falls and again incre—
increases when leaks are compensated at the 9th and 10th holes, and reaches its maximum value when operating at the 11th compensation hole. When drilling granite, the power curve reaches its maximum value when operating at the 7th and 8th compensation holes. The power curve for drilling granite lies somewhat higher than for drilling sandstone. The power consumption per blow when drilling granite is 30–50% greater than when drilling sandstone (with the same impact-unit operating conditions). As can be seen from the curves (Figs. 2, 3), the power consumed by the impact unit does not correspond to the impact energy at all compensation holes. At the 10th compensation hole the impact power has a considerable value; however, the impact energy is small and the drilling speed is low.
Fig. 3. Dependence of impact power and rotational power on the position of the compensation hole.
The power consumed at the eleventh compensation hole is greater than at the sixth and seventh, while the drilling speed is lower.
Consequently, as the compensation hole approaches the rear extreme position of the piston, the impact unit operates less steadily and consumes greater power at lower drilling speeds.
From comparison of the graphs (Figs. 2, 3) it follows that the power required for rotating the drill is inversely dependent on the drilling speed and, consequently, on the impact energy; that is, the maximum impact energy (at the 6th compensation hole) corresponds to the minimum rotational power, while the minimum impact energy at the 10th compensation hole corresponds to the maximum rotational power.
When drilling granite, the power consumed by the rotation unit is less than when drilling sandstone (by 30–50%).
From Fig. 4 we see that the volumetric work of destruction varies along a curve having a minimum at the 6th compensation hole (both when drilling sandstone and when drilling granite). As the number of the compensation hole increases (as it approaches the front wall of the piston), the volumetric work of destruction increases by a factor of 2–3, and when drilling granite it is approximately twice as great as when drilling sandstone.
It follows from the foregoing that the maximum drilling speeds correspond to the minimum volumetric work of destruction. Thus, both when drilling sandstone and when drilling granite, it is most expedient to operate at compensation hole 6 (Fig. 1).
Influence of the Length of the Air Cushion on the Operation of the Impact Unit
In studying the influence of the length of the air cushion on the operation of the impact unit, compensation for air leakage from the air cushion was carried out through hole 6. When the length of the air cushion is changed, the drilling speed (Fig. 5, drilling in sandstone) changes along a curve that has a maximum at an air-cushion length equal to 46 mm. With a decrease or an increase in the length of the air cushion, the drilling speed decreases by 30–40%. Experiments conducted during drilling in granite confirm the maximum value of the speed at an air-cushion length of 46 mm. However, when drilling in granite, the drilling speed is 2–3 times lower than when drilling in sandstone.
1 — granite; 2 — sandstone
Vertical axis: volumetric work of destruction, kgm/cm³
Horizontal axis: number of the compensation hole
Fig. 4. Dependence of the volumetric work of destruction on the position of the compensation hole.
The power consumed by the impact unit (Fig. 6) decreases somewhat as the length of the air cushion increases. A decrease in impact power also leads to a decrease in drilling speed. When drilling in granite, the impact unit consumes more power than when drilling in sandstone. The power consumed by the rotation unit (Fig. 6) changes little with changes in the length of the air cushion, but increases somewhat as the length of the air cushion increases, which corresponds to the decrease in impact energy and drilling speed. The curve of rotation power during drilling in granite lies lower than during drilling in sandstone.
The volumetric work of destruction (Fig. 7—drilling in sandstone) varies along a curve that has a minimum when the length of the air cushion is \(46\) mm. With an increase or decrease in the length of the air cushion, the volumetric work of destruction increases. The minimum volumetric work of destruction
Fig. 5. Dependence of the drilling speed on the magnitude of the air cushion.
Fig. 6. Dependence of the impact power and rotational speed on the magnitude of the air cushion.
at an air-cushion length of 46 mm is confirmed by experiments in drilling granite. But in drilling granite the volumetric work of destruction is much higher than in drilling sandstone.
In the graph: 1—granite; 2—sandstone. Vertical axis: Volumetric work of destruction, kgm/cm³. Horizontal axis: Air-cushion length, mm.
Fig. 7. Dependence of the volumetric work of destruction
on the length of the air cushion.
Thus, in the course of the investigation it was established that the most advantageous operating regime for the given hammer should be considered operation:
1) with an air-cushion length equal to 46 mm and with the ratio
\[ \frac{R}{l_0}=\frac{43.5\ \text{mm}}{46\ \text{mm}}=9.45, \]
where \(R\) is the crank radius, mm,
\(l_0\) is the length of the air cushion, mm;
2) with compensation of leakage through opening 6.
When the piston is in the rear extreme position, the ratio of the distance of compensation opening 6 from the front wall of the piston to the length of the air cushion is 1.78; the ratio 0.1 corresponds to the forward extreme position of the piston;
3) when operating at the sixth compensation opening, the hammer has the maximum drilling speed and a stable operating regime. Operation in the above-mentioned regime provides the indicators given in Table 1.
With the recommended parameters of the impact unit, the drilling speed can be increased by 1.5–2 times by:
a) reducing the rotational speed of the drill from 360–420 to 150 revolutions per minute [4];
b) increasing the feed force from 20–25 to 50 kg [5].
Table 1
| Rock | Maximum drilling speed, mm/min | Maximum specific power consumption, W·min/mm | Minimum volumetric work of destruction, kgm/cm³ | Minimum electric energy consumption per 1 linear m of blast-hole |
|---|---|---|---|---|
| Sandstone | 525 | 11.78 | 52 | 0.197 |
| Granite | 210 | 25.48 | 132 | 0.425 |
REFERENCES
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Alabuzhev P. M., Alimov O. D. On the question of designing electropneumatic machines of impact action. Izvestiya TPI, vol. 78, 1955.
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Alabuzhev P. M. Investigation of the working process of electropneumatic machines of impact action. Dissertation, TPI, 1954.
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Instructions for the set of measuring instruments KIP, 1954.
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Alimov O. D. On the mechanism of destruction of rocks in impact-rotary drilling. Izvestiya TPI, vol. 75, 1954.
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Alimov O. D. On the influence of feed force on the drilling speed with pneumatic drilling hammers. Izvestiya TPI, vol. 75, 1954.