PROCEEDINGS
OF THE TOMSK POLYTECHNIC INSTITUTE
OF THE ORDER OF THE OCTOBER REVOLUTION AND THE ORDER
OF THE RED BANNER OF LABOR
NAMED AFTER S. M. KIROV

Volume 188                    1974

ON THE COEFFICIENT OF FRICTION OF PNEUMATIC HAMMERS

V. F. GORBUNOV, V. I. BABUROV, A. I. PASHNIN, G. A. KISELEV

In the reciprocating motion of the striker of pneumatic impact machines, part of the stored energy of the compressed air is expended on friction, being converted into heat. The coefficient of friction is one of the principal characteristics of the mechanical process and, to a certain extent, determines the efficiency of the machine. The values of the coefficient of friction available in the literature [1, 2, 3] cannot be used in determining energy losses due to friction in pneumatic hammers, since they were established without taking into account the specific features of the working process of these machines.

Fig. 1. Dynamometers for determining the friction force: a) \(D_1, D_2, D_3, D_4\) — PB-20-200A strain gauges; B — KBS-L-0.50 battery; \(P\) — connector; \(mA\) — microammeter; b) \(D_1, D_2\) — strain gauges; 1 — PET-3VM strain amplifier; 2 — MPO-2 magnetoelectric oscillograph; 3 — STV-300 voltage stabilizer

The coefficient of friction depends on the magnitude of the clearance between the striker and the barrel, the length of the striker, the type of lubricant, the velocity of the striker, and other factors. In analyzing these dependences, the following were considered: static friction, caused by the bonds of bodies when they are in contact, and kinetic friction, determined by the resistance during the relative displacement of two bodies.

The static friction force in pneumatic hammers was determined by electrical dynamometers on strain gauges, with direct reading of the indications on the scale of a microammeter (Fig. 1, a). To establish the kinetic friction force, the signal of the dynamometer was recorded on photographic film (Fig. 1, b); its characteristic was linear, and its sensitivity varied over a wide range. The experiments were carried out on pneumatic hammers with different degrees of wear, with a thin layer of lubricant, several microns thick; in this case the barrel of the hammer was installed horizontally.

The coefficient of static friction was determined at the moment when the striker began to move. The required number of tests was counted according to the coefficient of variation, whose value did not exceed 20%. Let us consider the dependence of the coefficient of static friction on the size of the clearance between the striker and the barrel of the pneumatic hammer, as well as on the length of the striker and the type of lubricant.

The clearance between the barrel and the striker, which depends on the type of fit and the degree of wear of the friction pair (barrel—striker), has a considerable effect on the magnitude of the coefficient of friction, since the clearance determines the real contact area of the rubbing pairs, which is calculated by Hertz’s formula (4)

\[ c = 1.6 \sqrt{p \frac{D_1 \cdot D_2}{D_1 - D_2} \left[ \frac{1-\mu_1^2}{E_1}+\frac{1-\mu_2^2}{E_2} \right]}, \]

where \(c\) is the radius of the circular contact area, mm;
\(D_1, D_2\) are the diameters of the rubbing parts, mm;
\(p\) is the load per unit length of the cylinder, kg/mm;
\(\mu_1, \mu_2\) are Poisson’s ratios of the materials of the parts;
\(E_1, E_2\) are the moduli of elasticity, kg/mm\(^2\).

Fig. 2. Dependence of the coefficient of friction on the magnitude of the clearance between the barrel and the striker: ×—×—×—magnitude of the clearance; ○—○—○—real contact area

The dependence of the real contact area on the clearance size for the KE-22 pneumatic hammer with strikers of different diameters is shown in Fig. 2. As can be seen from Fig. 2, the coefficient of friction and the real contact area are inversely proportional to the magnitude of the clearance between the striker and the barrel, i.e., as the clearance increases, the coefficient of friction decreases. However, increasing it beyond 0.09 mm is inadvisable, since the coefficient of friction decreases only slightly, while the energy losses due to leakage of compressed air through the clearance increase rapidly. Mass-produced pneumatic hammers have a clearance that is assigned mainly for technological reasons; thus, chipping and riveting hammers have a clearance of 0.01–0.075 mm. For the BS3-135 hammer, according to the drawing, the clearance is 0.008–0.045 mm. In operation the clearance changes constantly. Thus, for chipping hammers after six months of work the clearance increases, according to our measurements, to 0.15–0.19 mm, which significantly reduces their productivity.

The real contact area of the rubbing pair depends on their length; consequently, the coefficient of friction is also determined by the length of the striker (Fig. 3, a). To reduce the coefficient of friction, it is advisable to manufacture the striker with annular grooves, which reduces the length

the friction pair and improves the lubrication conditions by its accumulation in the grooves. When reducing the length of the striker in order to reduce friction, one must not forget the energy parameters. It is known that the dependence of the impact power of a hammer on the striker length is not monotonic, but has a maximum [5]. This is clearly seen from Fig. 3, b.

Fig. 3. Dependence of the coefficient of friction on the striker length:
△—△—△ — striker length; —○—○— power developed by the striker

The question of selecting a lubricant for pneumatic hammers is of considerable interest. We carried out investigations in this direction as well. Lubricants with different kinematic viscosities, from 5 to 52 centistokes, were introduced into the barrel of the KE-19 hammer. Experiment established that any lubricant increases the coefficient of friction in comparison with dry friction, since lubrication promotes an increase in the molecular adhesive forces between the barrel and the striker. With increasing lubricant viscosity the coefficient of friction increases from 0.26 to 0.35 and then again decreases to 0.2 (striker weight 273 g). The values of the coefficient of static friction for the standard sizes of pneumatic hammers with different degrees of wear, according to experimental data, are 0.2–0.3.

Fig. 4. Dependence of the coefficient of friction on the velocity of motion of the striker for the BS3-135 hammer:
— — — — calculated; ×— —×— experimental

Friction depends substantially on the relative velocity of motion of the rubbing parts. The final maximum pre-impact velocity of motion of the striker in most types of hammers reaches 9–11 m/s at an air-line pressure of 5 atm. During the working stroke of the striker it varies from 0 to 9–11 m/s, and during the idle stroke—from 0 to 6–7 m/s. The nature of the dependence of the coefficient of sliding friction on velocity may be highly varied. It is determined by the magnitude of the specific load, which in the case under study is equal to 12 kg/cm². Owing to the viscosity of contact, the force required to break down mutually interpenetrating elements depends on the rate at which the load is applied and on the sliding velocity. At the same time, however, conditions are created for the leveling of the surfaces of the rubbing pair. Therefore, as the sliding velocity increases, the coefficient of friction passes through a maximum (Fig. 4). Within the velocity range from 0.2 to 9 m/s, the coefficient of friction varies according to a linear law. In this case the slope of the linear section of the curve depends substantially on the weight of the striker. As a result of processing the experimental data for the linear section of the curve (Fig. 4), the following dependence was obtained:

\[ K_{\text{fr}}=\frac{0.28-0.025V}{\sqrt[4]{Q}}, \]

where \(V\) is the velocity of motion of the striker in m/s,
\(Q\) is the weight of the striker (within the range 0.2–0.8 kg).

Using this formula, one can calculate the value of \(K_{\text{fr}}\) as a function of the velocity of motion, the weight, or the dimensions of the striker, and consequently the energy losses due to friction.

Knowing the magnitude of the friction forces, it is easy to calculate from the coefficient of friction the mechanical efficiency of the impact unit (barrel–striker), i.e.,

\[ \eta=\frac{A_{3}-A_{\text{fr}}}{A_{3}}, \]

where \(A_{3}\) is the stored energy of the compressed air in the impact unit;
\(A_{\text{fr}}\) is the work of the friction forces.

Conclusions

1. As the clearance increases, the coefficient of friction decreases; however, increasing the clearance beyond 0.09 mm is inadvisable, since the energy losses of the compressed air increase due to its flow into another cavity.

2. To reduce the coefficient of friction, it is advisable to make grooves on the striker.

3. The dependence presented in this paper of the coefficient of friction on the velocity of motion of the striker and its weight makes it possible to calculate the work expended on overcoming friction forces in dynamics.

References

1. V. F. Gorbunov. Investigation of the Working Process and Vibration of Pneumatic Hammers. Doctoral dissertation. Tomsk, 1965.

2. I. A. Begagon. Investigation of Wear Resistance and Methods for Increasing the Service Life of Drilling Machines. Abstract of doctoral dissertation. Dnepropetrovsk, 1966.

1. I. V. Kragel’skii, I. L. Vinogradova. Coefficients of Friction. Mashgiz, 1962.

2. Mechanical Engineering. Encyclopedic Handbook, vol. 1, part II. Mashgiz, 1947.

3. G. I. Sidorenkov. Chipping pneumatic hammers. Vestnik mashinostroeniya, No. 11, 1956.