Methodology for Testing Hammers Under Production Conditions

V. F. GORBUNOV, Yu. A. OPARIN, V. I. BABUROV

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

At present, manufacturers of pneumatic hammers monitor the weight of the hammer, the energy and frequency of blows, air consumption, and other indicators. However, neither when hammers leave the factory nor during their operation are their vibration characteristics monitored. This is due, first, to insufficiently strict sanitary-hygienic control over the working conditions of hand-held impact tools and, second, to the lack of simple measuring devices and test stands suitable for use under factory conditions.

In connection with the widespread use of pneumatic hammers and the recently increasing incidence of vibration disease among workers, the problem of monitoring and reducing hammer vibration has become especially urgent. Moreover, it is generally recognized that monitoring of the vibration characteristics of hammers must be carried out periodically during their operation, since, as a result of wear of the impact unit, the tool, and the air-distribution mechanism, the vibration characteristics of the hammer may change significantly.

In view of the above, it became necessary to develop and create a methodology and an experimental stand for testing hammers for vibration not in research laboratories (such installations exist), but under factory conditions. Such a methodology, in our opinion, must satisfy the following basic requirements:

1. Hammers must be tested under conditions close to production conditions.

2. The measuring devices, as well as the subjective abilities of the tester, must not affect the working process of the hammer.

3. The results of the inspection must be known immediately, without complicated calculations.

4. The design of the installation must allow hammers of one class (for example, hand-held hammers) of various designs and purposes to be tested.

The selection and justification of the individual units of the test installation, as well as the development of the general provisions of the methodology, are carried out on the basis of the studies we have conducted, taking into account recommendations available in the literature.

Let us consider the basic initial premises necessary for selecting and calculating the individual units of the experimental installation.

1. A pressing device simulating the action of the worker’s hand

When testing hammers for vibration, it is necessary to create a pressing force on the handle corresponding to actual production conditions. Numerous studies by engineers and hygienists have established that this force for hand-held machines varies from 7–10 kg to 25–30 kg. Such a force can be created manually on an experimental installation, but in that case we do not exclude the subjective influence of the operator on the test results. It is therefore necessary to introduce into the design of the stand a special pressing device that creates a quite definite pressing force on the hammer handle, both in magnitude and in character.

The technical literature contains descriptions of various experimental stands equipped with pressing devices.

A weight-type pressing device is the simplest in design and provides a constant pressing force, but it has substantial shortcomings. The total mass of the vibrating parts of the hammer is considerably increased, as a result of which the measured amplitude of oscillations of the hammer handle is considerably smaller than the actual one. Obviously, this is inadmissible, since a deliberately erroneous vibration index is entered into the machine characteristic. Attaching the weight to the hammer handle by means of a rope, as is done in some stand designs, does not eliminate the principal shortcoming of the weight-type pressing device.

In a spring pressing device, pressure on the hammer handle is effected by a pre-compressed spring with stiffness coefficient \(C\). In this device it is easy to ensure the required pressing force in magnitude. The change in force occurs within small limits as a result of deformation of the spring during oscillations of the handle

\[ \Delta Q = \Delta l \cdot C,\ \text{kg}, \]

where \(\Delta l\) is the range of oscillations of the handle, mm;
\(C\) is the stiffness coefficient of the spring, kg/mm.
If the spring is sufficiently compliant, it is obvious that oscillations of the pressing force will not have a substantial effect on the operating mode of the hammer. From this point of view, the suitability of such a pressing device raises no objections.

However, in this case another extreme is observed. The mass attached to the hammer handle is considerably smaller than that which is included in the oscillations when the handle is pressed by hand [3].

A pneumatic pressing device has become widely used in practice. It makes it possible to create the necessary pressing force and to regulate it easily depending on the type of hammer and the mode of its operation. The disadvantage of a pneumatic pressing device is that the air cushion has no stiffness, and therefore the nature of the feed differs somewhat from the force actually acting on the handle during manual pressing.

Studies of various pressing devices and analysis of mechanical analogues of the worker’s hand [1, 3] have shown that the greatest correspondence to real conditions is exhibited by a spring-mass system. Such a system consists of a combination of one or two masses and springs.

The simplest, yet meeting the requirements imposed on the pressing device, proved to be a mechanical model of the human hand consisting of two masses separated by a spring [1]. In such a device, a load weighing 2 kg is fixed immovably on the hammer handle, while a load weighing 18 kg rests freely on a spring with stiffness coefficient \(C = 1—1.1\ \text{kg/mm}\). Investigation of such a pressing device showed that the spectrum of vibration displacement coincides sufficiently closely with the spectrum of vibration displacement when the hammer is pressed by a human hand [1].

As a result of comparing the considered designs of pressing devices, we come to the conclusion that for factory vibration tests of hammers it is most expedient to adopt a stand with a spring-mass pressing device according to the scheme given in [1]. This is all the more so because this stand was developed with the participation of sanitary-hygienic specialists and, consequently, meets the requirements from the physiological-hygienic point of view.

2. Position of the Hammer Being Tested

Pneumatic chipping and riveting hammers are used to perform various types of work: cleaning castings, riveting rivets, removing scale from rolled products, and dressing cracks. When all these types of work are performed, the position of the hammer relative to the horizon varies from vertically downward to vertically upward. At the same time, the pressing force required for the hammer’s operation changes. As can be seen from the formula

\[ F = F_1 \mp G \cdot \sin \alpha,\ \text{kg}, \]

the total pressing force \(F\) on the hammer consists of the force \(F_1\) created by the worker and the component due to the weight of the hammer, \(G \cdot \sin \alpha\). Thus, when the hammer operates vertically downward, the pressing force required for operation, for example in the limiting basic mode, will be less by the amount of the hammer’s weight. When operating vertically upward, however, the worker, for operation of the hammer in this same mode, must create a pressing force that will be greater by the amount of the hammer’s weight. Consequently, the position of the hammer affects the magnitude of the pressing force that must be created for its operation in a given mode.

Factories produce hundreds of thousands of hand pneumatic hammers, which operate in different positions. It is impossible to predict in advance that one or another hammer will be used for a strictly defined type of work. Therefore it is necessary to agree on the position in which the hammer will be tested.

In known designs of stands [1, 2], which were created for studying pneumatic hammers in the laboratory, the hammer is installed vertically downward or horizontally. In the horizontal position it is somewhat more complicated to install the hammer, owing to the need to have special guide devices on the stand, which, moreover, must create little resistance to the movement of the hammer. However, because in the horizontal position the weight of the hammer is excluded from the calculation of the pressing force and does not affect the working process of the hammer itself and of the tool, for detailed studies of a number of hammer sizes it is expedient to adopt their horizontal position.

For the rapid determination of the vibration characteristics of each hammer from a large batch, the speed with which the hammer is installed on and removed from the stand is an essential factor. Control of the weight and of the pressing force

is carried out in this case only once for the entire batch of hammers. In this connection, the vertical position of the hammer is more acceptable than the horizontal one.

Therefore we shall assume that, in factory tests, the hammer will be installed in a vertical position, with the inserted tool downward.

3. Operating mode of the hammer

Studies have established that, during hammer operation, three modes are possible, determined by the oscillations of the body: floating, limiting basic, and under the action of reverse blows of the tool [6, 7].

Each mode can be characterized by the pressing force that must be applied to the hammer handle, and by the amplitude of oscillation.

In the floating mode the velocity of the body changes smoothly, and the displacement amplitude is the smallest within the working range of the pressing force. Such a mode is favorable from the hygienic point of view. The pressing force corresponding to this mode is determined by the formula [6]:

\[ R_n = \frac{n}{60} \left[ \sqrt{2A_y \cdot m}\,(K_0 + 1) + f_n \int_{0}^{t_n} P_p \cdot dt \right], \ \text{kg}, \tag{1} \]

where \(A_y\) is the impact energy, kg·m;
\(m\) is the mass of the striker, kg·s\(^2\)/m;
\(K_0\) is the rebound coefficient of the striker;
\(f_n\) is the cross-sectional area of the shank of the working tool, cm\(^2\);
\(P_p\) is the variable air pressure in the front cavity, kg/cm\(^2\);
\(t_n\) is the duration of action of the pressure \(P_p\), s.

Formula (1) includes the following quantities: the rebound coefficient of the striker \(K_0\) and the variable pressure \(P_p\).

The value of \(K_0\) depends on the condition of the surface being worked, its material, the shape and material of the impacting surfaces, etc. [18]. The magnitude of the rebound of the striker from the tool determines the character of the change in pressure in the front cavity.

Thus, in order for the hammer to operate in the floating mode, it is necessary to maintain the pressing force characteristic of it; and this force depends on many variable factors and is practically difficult to achieve.

Oscillations of the hammer in the limiting basic mode are characterized by a sharp seating of the body on the collar of the tool. The displacement amplitude in this mode is the greatest. The pressing force is determined by the formula

\[ R_n = \frac{n}{60} \left[ \sqrt{2A_y \cdot m}\,(K_0 + 1) + f_n \int_{0}^{t_n} P_p \cdot dt + m_k \cdot V_p\,(K_k + 1) \right], \ \text{kg}, \tag{2} \]

where \(m_k\) is the mass of the body, kg·s\(^2\)/m;
\(V_p\) is the velocity of seating of the body on the collar, m/s;
\(K_k\) is the rebound coefficient of the body.

This expression is formula (1) to which the term \(\dfrac{n}{60} m_k V_p (K_k + 1)\) has been added; in this term the seating velocity \(V_p\) of the bod-

the release and the rebound coefficient of the body \(K_{\mathrm{k}}\)—are variable quantities. But they vary less, since the condition of their impacting surfaces changes less; moreover, the mass of the body is several times greater than the mass of the tool. The expression \(\dfrac{n}{60} m_{\mathrm{k}} \cdot V_{\mathrm{p}} (K_{\mathrm{k}} + 1)\) is numerically greater than the other two; therefore, despite a considerable fluctuation of their numerical values, the overall result changes little.

In the third mode, oscillations occur under the action of reverse blows of the tool. The amplitude of displacement in this mode is small, while the magnitude of the pressing force reaches 30–35 kg. The magnitude of the pressing force is determined by the formula

\[ R_{\mathrm{n}} = \frac{n}{60} \left[ \sqrt{2 A_{\mathrm{y}} \cdot m}(K_{0}+1) + f_{\mathrm{i}} \int_{0}^{t_{\mathrm{p}}} P_{\mathrm{n}} \cdot dt + \int_{0}^{t_{\mathrm{o}}} F_{\mathrm{o.y}} \cdot dt \right] + R_{\mathrm{k}}, \text{ kg}, \tag{3} \]

where \(F_{\mathrm{o.y}}\) is the force of the reverse blows, kg;
\(R_{\mathrm{k}}\) is the contact force between the body and the shank of the tool, kg.

The greatest influence on the overall result of the formula is exerted by the forces \(F_{\mathrm{o.y}}\) and \(R_{\mathrm{k}}\), which are larger in magnitude than the others. The magnitude of these forces fluctuates little; therefore the overall result also changes little.

From the physiological-hygienic point of view, the floating mode of oscillations and the mode under the action of reverse blows are the most favorable, since the amplitude of oscillations in them is the smallest. In terms of the magnitude of the pressing force, the mode of oscillations under the action of reverse blows is less favorable, since the corresponding pressing force is difficult for the worker to withstand for a long time.

The extremely basic mode differs from the others by the greatest amplitude of displacement and by the magnitude of the pressing force most frequently encountered in production; therefore we accept that hammers should be evaluated on the stand in this mode.

4. Working Medium and Tool

Studies have established that both the medium and the tool exert a substantial influence on the working process of the hammer [9, 10, 11, 15], since different materials possess different moduli of elasticity \(E\). When working with one and the same tool on different materials, the magnitude of the striker’s rebound from the tool is the greater, the greater the value of the modulus of elasticity. With a change in the striker rebound, the character of the working process changes, and the vibration changes.

It has been noted that with an increase in the striker rebound, the frequency of blows and the vibration of the hammer increase [8].

A direct effect on the vibration of the hammer is exerted by the weight and shape of the working part of the chisel.

In practice, chisels with different shapes of the working part and different lengths are used [5]. The energy transmitted by the chisel depends on the shape of the chisel sharpening and on the condition of the surface being worked. It has been established that the efficiency of energy transmission is greatest when one surface is spherical and the other is flat [11, 15]. Such a situation is observed in riveting and embossing.

With the maximum efficiency of energy transmission from the chisel to the material, the oscillations of the hammer are intensified under the action of the elastic properties of the material. Therefore we obtain the maximum possible amplitude of vibration displacement.

But cases of working with a chisel having a spherical working part are rare; therefore, when testing a hammer under such conditions, we obtain results several times higher than the actual ones. Testing hammers in this way is inadvisable, especially since, in the process of metal chipping, the amplitude of vibration displacement tends to decrease.

Most chipping and riveting hammers produced by factories are used for chipping work, and only a very small part for riveting work. Therefore, all hammers should expediently be tested as chipping hammers, with the corresponding tool. In the practice of chipping work, chisels with a wedge-shaped working part, a sharpening angle of \(40^\circ\), and a length of 250–300 mm have become most widespread. We therefore adopt such a tool for testing hammers.

Since the materials most frequently encountered in chipping—steel and cast iron—have similar moduli of elasticity \(E\), steel is adopted as the material on which the hammers will be tested. If it is necessary to test the vibration of hammers operating on nonferrous metals, a bronze support is installed on the stand.

5. Requirements for apparatus for measuring displacement amplitude

According to existing sanitary standards [4], the vibration hazard of impact-action machines is assessed by the magnitude of the displacement amplitude of the fundamental vibration frequency. The permissible amplitude is reduced by a factor of 2–3 when harmonics with a frequency of 120–250 Hz are present in the spectrum. Below we consider apparatus intended for measuring the displacement amplitude of the fundamental frequency.

In laboratory and production practice there are a number of instruments that make it possible to measure displacement amplitude [12, 13, 14, 17, 19, 20]. Some of them have a pointer indicator, allowing the readings to be monitored during the measurement process. Most instruments, however, operate together with an oscillograph, on which the measurement results can be recorded on motion-picture film or on the screen of a cathode-ray tube. In instruments of the latter type, processing the results takes a great deal of time; moreover, the instruments themselves are complex and bulky.

An instrument that meets the following requirements best satisfies production needs:

1) the instrument is of a portable type,

2) the results of measuring the displacement amplitude are known during the measurement process or immediately after it,

3) the instrument readings are automatically set to zero after the measurement is completed,

4) the instrument does not require adjustment during the measurement process,

5) the instrument’s sensor is of a remote type and weighs 100–150 g.

Fulfillment of the listed requirements depends on the type of sensor used with the instrument.

In practice, potentiometric, inductive, capacitive, resonance sensors and acceleration sensors are used for measuring displacement amplitude.

Potentiometric sensors are simple in design [12], are statically calibrated, and, depending on the diameter of the wire used for the measuring element, provide the required accuracy of measurement. The sensors can be used both with an oscillograph and with a pointer instrument (when the supply voltage of the coil is increased). A disadvantage of such sensors is their short service life, since the measuring coil is subject to mechanical wear as the contacts move over it.

Inductive and capacitive sensors have the same advantages. They are also simple in design and possess high sensitivity, which is determined by the size of the gap between the moving and fixed parts. The sensitive element is not subject to mechanical wear. A high-frequency current is needed to power the sensors. Sensors of any length can be made, and they can be sufficiently compact. Inductive and capacitive sensors are used with a pointer indicator and with an oscillograph. Their calibration is static.

Resonant sensors are, in essence, also instruments, since they directly indicate the measured quantity. Some types of such sensors have a number of sensitive elements, one of which oscillates in resonance with the object being measured. In others, however, there is only one sensitive element and it must be tuned to resonance with the object being measured; therefore, in this case the result of the measurement is influenced by the mass of the operator’s hand. Resonant-type sensors must be calibrated on special calibration stands.

In acceleration (seismic) sensors the sensitive element is suspended on springs; therefore there is no need—as in potentiometric, inductive, and capacitive sensors—for a fixed support. Such sensors are calibrated on calibration stands. Since the sensors measure acceleration, in order to obtain displacement a transformation by double integration is necessary. Seismic sensors have the smallest weight in comparison with the others and do not require the operator’s intervention in the measurement process.

The simplest means of measuring displacement amplitude is the measuring wedge [21].

Of the sensors considered, potentiometric sensors do not meet production requirements, since they are not durable; resonant sensors, on the other hand, have a large weight (on the order of 0.5–0.7 kg).

The most suitable for production conditions are seismic sensors and the measuring wedge, since they have low weight and their readings are set to zero after the measurements are completed. We therefore adopt them.

Capacitive and inductive sensors may be used, but for them it is necessary to solve the problem of setting the indicator to zero after the measurements.

The blow frequency of the hammer, which determines the vibration frequency, does not have sharp deviations from the tabulated value. Therefore it is sufficient to check in the laboratory the blow frequency of one or two hammers from the batch.

Thus, for measuring vibration on the stand we adopt a measuring wedge or an instrument with a pointer indicator having an external sensor of the seismic type.

Conclusions

We consider it possible to take the following provisions as the basis of the methodology for testing pneumatic riveting-chipping hammers for vibration:

1. During testing, pressure on the hammer handle is applied by a spring–mass system.

2. The hammer is tested in a vertical position.

3. Measurements of displacement amplitude are carried out in the extremely basic operating mode of the hammer.

4. Hammers intended for work on steel and cast iron are tested on a steel plate, and those intended for work on nonferrous metals—on a bronze support.

1. The tool used is a symmetrically sharpened chisel with a sharpening angle of 40°.

2. The displacement amplitude is measured with a measuring wedge or with a portable-type instrument with a dial indicator. The instrument sensor is of the remote seismic type.

REFERENCES

1. G. S. Vilner. A stand and apparatus for vibration testing of pneumatic chipping hammers. Scientific Works of the Institutes of Labor Protection of the All-Union Central Council of Trade Unions. Issue 1 (27). Proftekhizdat, 1954.

2. G. V. Rodionov. A universal stand for investigating pneumatic machines of impact action. In: Machines of Impact Action. West Siberian Branch of the USSR Academy of Sciences, Novosibirsk, 1953.

3. Dickman D. Ein schwingungsmaschines Model für das schwingungs erregte Hand-Armsystem des Menschen. V. D. I. Zeitschrift, No. 1, 1959.

4. Collection of Important Official Materials on Questions of Occupational Hygiene and Industrial Sanitation. Issue 2, Medgiz, 1962.

5. D. I. Sudakevich, G. I. Bernadskii. Handbook on Mechanized Hand Tools. Mashgiz, Moscow, 1961.

6. V. I. Baburov. Investigation of pneumatic chipping and riveting hammers. Candidate dissertation. Tomsk, 1964.

7. B. V. Sudnishnikov. On the theory of recoil of hand-held machines of impact action. In: Machines of Impact Action. West Siberian Branch of the USSR Academy of Sciences, Novosibirsk, 1953.

8. V. F. Gorbunov, V. I. Baburov, L. S. Redutinskii, A. V. Trikhanov, G. G. Pestov. Investigation of the vibration of a hand hammer when working on various materials. Mining Machines and Automation, No. 56, 1964.

9. N. N. Esin. On the initial conditions of operation of pneumatic chipping hammers. In: Machines of Impact Action. West Siberian Branch of the USSR Academy of Sciences, Novosibirsk, 1953.

10. N. I. Nikishin. Rebound and its effect on the operation of scaling hammers and concrete breakers. Investigation and Calculation of Impact Mechanisms. Proceedings of VNIIstroydormash, Issue XXX, Moscow, 1961.

11. Schlobach E. Bestimmung der Leistung von Drucklufthämmern mit dem Einheitsprägerät. Glückauf, Nos. 2, 3, 1937.

12. V. F. Gorbunov, V. M. Gusarov, L. S. Berezin. An electrical method of recording vibrograms of pneumatic hammers. Proceedings of Higher Educational Institutions, Mining Journal, No. 12, 1962.

13. Leningrad Mechanical Plant. Vibrometer types VK-2, VK-3. Operating manual. Leningrad, 1957.

14. N. P. Ryashentsev, E. M. Timoshenko. An induction sensor for investigating the working process of machines of impact action. Proceedings of Higher Educational Institutions, Mining Journal, No. 7, 1961.

15. E. V. Aleksandrov. Coefficient of restitution, or coefficient of loss of relative velocity. In: Improvement of the Development of Coal Deposits. Ugletekhizdat, Moscow, 1959.

16. A. T. Belevtsev. Potentiometers. Oborongiz, 1962.

17. V. V. Yunin. Determination of the parameters of the internal process of mine pneumatic machines of impact action by the method of electrical measurements. Author’s abstract of Candidate dissertation. Dnepropetrovsk, 1960.

18. M. N. Yashnikov. Periodic motions of a vibro-impact system at various coefficients of restitution. In: Theory of Mechanisms and Machines. Issue 105—106. Nauka, Moscow, 1965.

19. M. S. Antsiferov. New vibrometers of the sound range. In: Vibration Technology, No. 3 (7). Moscow, 1962.

20. L. A. Osipovich. Precision small-sized piezoquartz sensors of vibration accelerations, type PKD. Instrument Engineering, No. 5, 1964.

21. E. Ts. Andreeva-Galanina. Investigation of Vibration. Occupational Safety in Production. Reference Manual. Investigations and Tests. Metallurgizdat, Moscow, 1964.