INFLUENCE OF THE AIR-DISTRIBUTION SYSTEM ON THE VIBRATION AND ENERGY CHARACTERISTICS OF PNEUMATIC PERCUSSION MACHINES

V. I. Baburov, Zh. Kh. Seidaridi

(Presented by the Department of Mining Machines, Mine Transport, and Mining Mechanics)

In the national economy of our country, pneumatic percussion machines are widely used in the extraction of minerals, the driving of mine workings, the performance of breaking and riveting operations, the development of brick and stone masonry, the construction and repair of roads, etc. Their widespread use is explained by the simplicity of their design, the possibility of producing considerable impact power with low weight, and a relatively high efficiency compared, for example, with electric percussion machines.

However, this type of machine has two substantial shortcomings. Their use as hand-held mechanized tools leads to occupational diseases among workers. The latter develop vibration disease and hearing loss, caused by the significant vibrations and noise of the machine \[1, 2\].

The intensity of vibration and noise depends on a great many factors, some of which are related to the operating regime of the machine, while others are related to its design. The influence of the listed factors on the intensity of the mechanical and acoustic energy emitted by the machine is being studied by many researchers and by entire scientific teams \[2\].

In the present work we did not set ourselves the goal of studying the entire problem. We were interested in only one of its directions, namely whether the vibration characteristics of a pneumatic hand tool depend on the design of the air-distribution device and on the principle of its operation. We solved this problem analytically, using the theorem on the motion of a mass under the action of a force specified as a function of time \[8\].

The chosen path of investigation, namely the analytical one, greatly facilitates the solution of the problem posed, for the following reason. Comparison of machines by vibration indices is also possible experimentally, but this would require several types of machines with different air-distribution systems whose energy and weight parameters were close in value. This is practically impossible.

Existing pneumatic percussion machines have four air-distribution systems that differ substantially from one another: valve systems, spool-valve systems, distribution with micro-spools proposed by N. A. Klushin, and distribution without a special device \[2, 5, 9\].

In the latter case, the alternating supply of compressed air to the cavities above and below the piston is carried out through channels present in the pist—

not. This system did not become widely used and was not considered by us.

Air-distribution devices have been studied quite fully by G. I. Kusnitsyn, B. V. Sudnishnikov, M. D. Kolomiitsev [1, 4–7], and others. Their principal studies are devoted to calculating and establishing the influence of the operation of the device on the dynamics of the internal processes. V. I. Baburov showed that, for pneumatic machines with valve distribution, the presence of air counterpressure in the front cavity before the piston collides with the working tool is characteristic [1].

For machines with spool-valve distribution, at this moment the resultant pressure on the striker is either directed toward the tool or is equal to zero. These features naturally affect the dynamics of the working processes and the vibration of the machine [2].

The initial materials for our investigations were indicator diagrams of the KE-22 riveting hammer, recorded under the following conditions:

a) air pressure in the network: 5 atm;

b) pressing force: 26 kg (including the weight of the hammer);

c) hammer position—vertical;

d) workpiece material—a steel plate;

Fig. 1. Working cycles of pneumatic percussion machines with various air-distribution devices

e) length of the working tool—300 mm; diameter of its shank—16 mm.

In the calculations, the actual boundaries of the working cycle of the KE-22 hammer were shifted to the left or to the right. Shifting the boundaries to the left gives a working cycle characteristic of machines with spool-valve distribution (Fig. 1, a), and to the right—of machines with valve distribution (Fig. 1, c). The intermediate position of the cycle boundaries, i.e., when the value of the resultant force acting on the striker during the collision period is equal to zero, corresponds to the optimal course of the internal processes with spool-valve air distribution (Fig. 1, b). The three indicated working cycles were compared with one another and with the so-called cycle of B. V. Sudnishni-

... stroke. The last cycle was implemented by K. T. Klushin [3] in machines with micro-gold-beaters (Fig. 1, e).

All cycles have the same duration. This gives us the possibility of comparing the vibration characteristics of hammers with different air-distribution systems by the relative vibration displacement, which is the ratio of the amplitude of oscillations of the housing to the impact energy.

The essence of B. V. Sudnishnikov’s method for determining the amplitude of oscillations of the hammer housing from indicator diagrams is as follows [7].

Indicator diagrams are plotted on graph paper in “pressure—time” coordinates (Fig. 2). A line M–M is drawn (Fig. 2) corresponding to the value of the total reduced pressing force:

Fig. 2. For calculation of the vibration displacement of the hammer housing from pressure diagrams. \(P_3\) — diagram of air pressure above the piston; \(P_n\) — diagram of pressure below the piston

\[ q=\frac{P+Q\sin\alpha}{F_y},\ \text{kg/cm}^2, \tag{1} \]

where

\(q\) — total reduced pressing force;

\(P\) — pressing force on the handle, kg;

\(Q\) — weight of the hammer, kg;

\(\alpha\) — angle of inclination of the hammer to the horizontal \((\alpha=90^\circ)\);

\(F_y\) — cross-sectional area of the striker, cm\(^2\) \((F_y=7.07\ \text{cm}^2)\).

The exact position of the line K–K is established from the equation

\[ F_y(l_1-l_2-l_3)-(F_y-F_u)l_4, \tag{2} \]

where

\(F_u\) — cross-sectional area of the shank of the working tool, cm\(^2\);

\(l_1; l_2; l_3; l_4\) — areas of the figures determining the magnitudes of the impulses (shown in Fig. 2).

Having determined the positions of the centers of gravity of the indicated figures by hanging from three points, we calculate the value of the amplitude of oscillations of the machine housing according to the expression:

\[ A_k=\frac{1}{M}\,[l_1t_1-l_2t_2-l_3t_3] -\frac{(F_y-F_u)l_4t_4}{MF_y}, \tag{3} \]

where

\(M\) is the mass of the hammer housing without taking into account the mass of the striker;

\(t_1;\ t_2;\ t_3;\ t_4\) are the coordinates of the centers of gravity of the figures \(l_i\) relative to the line K–K.

From the known value of the vibration amplitude, the seating velocity of the housing on the limiting shoulder of the working tool was found:

\[ V_n=\sqrt{\frac{2gF_v\cdot A_k}{M}}. \tag{4} \]

It follows from formula (3) that, for a pneumatic machine, the amplitude will be the smaller, the smaller the value of the static moment of impulse \(l_1\).

This circumstance was precisely what B. V. Sudnitsnikov used in the theoretical development of a new working cycle for a machine with reduced recoil. Moreover, a reduction of this impulse in this case entails a decrease in the value of \(t_1\).

The energy parameters were also determined from pressure diagrams, which were then rearranged into a single resultant force diagram [2]. The resultant diagram is divided vertically by O–O into parts corresponding to the time of the working stroke \(t_p\) and the time of the return stroke \(t_o\) (Fig. 1, a). The division is performed by equating the static moments during the working and return strokes of the striker relative to the boundaries of the working cycle, according to the expression:

\[ F_1t_1=F_2t_2+F_4t_4-F_3t_3=m_yS_y \]

where

\(S_y\) is the striker travel, m;

\(m_y\) is the mass of the striker, kg·s\(^2\)/m.

The velocity of the striker is determined by the formula:

\[ V_y=\frac{F_1}{m_y}\cdot m_p\cdot m_t,\ \text{m/sec}, \tag{5} \]

where

\(F_1\) is the area of the force diagram characterizing the magnitude of the impact impulse (impact energy), cm\(^2\);

\(m_y\) is the mass of the striker, kg·s\(^2\)/m;

\(m_p\) is the scale coefficient of force, kg/cm;

\(m_t\) is the scale coefficient of time, sec/cm.

The rebound velocity is found in an analogous manner, i.e.,

\[ V_o=\frac{F_2+F_4-F_3}{m_y}\cdot m_p\cdot m_t. \tag{6} \]

The remaining parameters were determined from formulas known in the literature [1, 2, 7].

All values of the principal parameters of the calculated cycles are summarized in Table 1. Their analysis shows that, for one and the same cycle duration, machines with spool-type air distribution are characterized by a larger value of impact energy and a smaller value of vibration displacement. Their ratio is 0.68. For machines with a valve-type air-distribution device it is 1.06, i.e., 65% higher. In this case, the seating velocity of the housing on the tool is also greater. This indicates that valve distribution contributes to an increase in the vibration characteristics of the machine while simultaneously reducing the energy indices. Such an important indicator as the energy-transfer coefficient is lower in these machines.

Table 1

Values of the principal parameters of hammers with different air-distribution devices

Conclusions

1. The greatest effect among the cycles we compared is produced by the cycle of a machine with micro-spools. It is characterized by a high energy-transfer coefficient and a relative displacement that is several times smaller—namely, in comparison with spool distribution, by a factor of 3, and with valve distribution, by a factor of 4.6.

2. The foregoing and the data of Table 1 give grounds to assert that, when creating new pneumatic percussion machines, designers should orient themselves toward spool distribution and distribution by micro-spools. This will make it possible to reduce the vibration characteristics of the machines and the incidence of vibration disease among workers.

References

1. V. I. Baburov. Investigation of pneumatic chipping-riveting hammers. Candidate dissertation. Tomsk, 1964.

2. V. F. Gorbunov, V. I. Baburov, et al. Handheld pneumatic hammers. Mashgiz, 1967.

3. N. A. Klushin. New pneumatic chipping hammers. Mashinostroitel’, 7, 1967.

4. L. D. Kolomiitsev. Determination of the parameters of the plate valve of pneumatic hammers. Izvestiya VUZov. Gornyi zhurnal, 2, 1958.

5. G. I. Kusnitsyn. Investigation of air-distribution devices of pneumatic percussion machines. Candidate dissertation. Leningrad, 1959.

6. N. T. Nered. Analysis of the operation of pneumatic impactors with valved and valveless air distribution. In: Studies in Geology, Mining, and Ore Beneficiation of KMA. Rostekhizdat, 1962.

7. B. V. Sudnishnikov. On the theory of recoil of handheld percussion machines. In: Percussion Machines. Novosibirsk, 1953.

8. B. V. Sudnishnikov. On the motion of a mass under the action of a force specified as a function of time. In: Percussion Machines. Novosibirsk, 1953.

9. B. V. Sudnishnikov. Approximate theory of the distributing device of a pneumatic hammer. In: Percussion Machines. Novosibirsk, 1953.