CAUSES OF VIBRATION IN HANDHELD PNEUMATIC HAMMERS AND METHODS FOR COMBATING IT

V. I. BABUROV, V. F. GORBUNOV, A. V. TRIKHANOV

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

The vibration of a handheld mechanized tool has a harmful vibrational effect on the human organism, leading to an occupational disease known as vibration disease [1]. At present, many scientific-research organizations are studying the vibration of handheld pneumatic hammers with the aim of creating machines that are safe for humans with respect to vibration. A study of the vibration of chipping and riveting hammers was carried out in 1963 at Tomsk Polytechnic Institute. The hammers were tested under conditions close to operating conditions. The hammers under study were installed obliquely to the workpiece being processed. Diagrams of the internal processes and vibrograms were recorded during operation of the hammer, which, while moving along the workpiece, cut a chip.

One of the main tasks of the investigations carried out was to establish the causes and sources of the oscillatory motions of the hammer body, so that, knowing them, data could be obtained for the further development of recommendations for reducing vibration.

A. Causes and Sources of Vibration

In the course of our investigations, we established the following main causes of oscillatory motions of chipping and riveting hammers that are harmful to humans, depending on the modes and conditions of their operation:

a) the action on the machine body of compressed-air pressure that varies in magnitude and time, i.e., the action of an internal disturbing force;

b) impacts of the striker on the body (bridge) of the hammer;

c) collision of the hammer body with the shank of the tool during rebounds, which usually occur during the return stroke of the striker;

d) rebound impacts of the tool on the hammer body;

e) shaking of the hammer body at the moment when the striker strikes the tool shank (rapid advance of the body following the penetrating tool under the action of the pressing force and friction forces in the guide bushing);

f) oscillations of the tool itself, transmitted to the worker’s left hand.

During the study of the operating conditions of hammers, postgraduate student V. I. Baburov of the TPI identified other additional sources of vibration in machines in service, namely:

a) hard, or direct, impacts of the striker against the rear wall of the barrel, which is usually observed in machines having a short rear buffer and considerable wear of the barrel and striker. During the return stroke of the striker, compressed air from the rear cavity passes through the gaps between the barrel and the striker into the front cavity and then into the atmosphere. For this reason no closed space (buffer) is formed, and the striker hits the handle (in hammers of the KE type) or the valve mechanism (in hammers of the MR type). Often, because of these impacts, the hammer handle is destroyed;

b) transverse oscillations of the hammer due to off-center impacts of the striker on the tool shank. Often, as a result of wear of the inner surface of the bushing, the tool shank becomes skewed; therefore, when the striker collides with the shank, a transverse component of the impact force is produced, which leads to transverse oscillations of the hammer body;

c) vibrations of the materials being worked are also a source of vibration; these vibrations are transmitted to the worker’s organism through the tool and the hammer body, or directly through the person’s limbs if he is in contact with the object being worked.

The causes of the occurrence of transverse oscillations were analyzed by the Polish engineer Lewandowski [2]. Among these causes he names: impacts of the striker on the lower part of the barrel, arising as a result of wear of the striker and barrel; impacts of the shank, during oscillation of the tool, against the side wall of the guide bushing (when there is wear of the bushing along its inner diameter). Lewandowski also attributes impacts of the spool of the air-distribution device against the body and handle of the hammer to sources of vibration of impact origin.

As is evident from the foregoing, the causes of vibration are varied and depend on the design of the hammers, operating conditions, service conditions, properties of the products being worked, the degree of wear of parts and assemblies, manufacturing technology, etc. The results of our studies of hammer vibration as a function of the modes and conditions of their operation may be used by designers and engineers in creating new machines that are safer with respect to vibration, as well as in calculating vibration-damping devices and elements.

B. Methods and Means of Combating Vibration

At present there are many ways and methods of combating vibration. They have been developed by Soviet and foreign scientists and engineers. We shall briefly analyze the main directions in reducing the vibration of handheld pneumatic hammers.

a) Changing the shapes of pressure diagrams

One of the principal sources of hammer vibration is the action of the internal disturbing force on the walls of the body. To reduce the amplitude of oscillations of the machine caused by the action of the aforementioned force, B. V. Sudnishnikov proposed changing its form [3]. In 1955 this proposal was implemented by N. A. Klushin [4]. The results of the tests he carried out showed that pneumatic hammers operating with the use of the so-called “Sudnishnikov cycle” have a vibration amplitude much smaller than machines with the usual cycle. In na-

At the present time, a number of fundamentally new and structurally simple pneumatic hammers operating according to the Sudnishnikov cycle have been created at the Siberian Branch of the Academy of Sciences of the USSR.

At the Tomsk Electromechanical Plant, several chipping hammers with reduced vibration (by a factor of 2–4) have been designed and manufactured. The reduction in vibration amplitude here was also achieved by changing the shape of the diagram of the resultant force.

A fundamental design of a pneumatic percussion machine, in which vibration reduction was likewise achieved by changing the shape of the diagram, was developed by A. M. Petreev [5].

The vibration amplitude of this machine, compared with the OMPS-5 breaker hammer when operating under the same conditions, is 1.2 times lower. Changing the shape of the pressure diagrams in the hammer cavities makes it possible not only to reduce the amplitude of oscillations, but also to reduce the pressing force, which is especially important for hand-held machines.

Studies by G. A. Sovetov [6] established that the shape of the pressure diagrams in the machine cavities can be changed not only by using special air-distribution devices (N. A. Klushin) and by changing the cross section of the rear cavity (Petreev), but also by changing the cross section of the channels supplying air to the machine cavity.

Thus, changing the shape of the pressure diagrams in the cavities of pneumatic machines is one of the most important and practically feasible measures for reducing machine vibration.

The basic idea of this method is as follows. By the most acceptable (simple) methods, it is necessary to ensure that the force produced by the pressure of compressed air on the rear wall of the housing changes as smoothly as possible over time. To preserve the machine parameters in this case, it is recommended to increase the air-pressure impulse in the front cavity. However, in our opinion, the method considered for reducing vibration amplitude is not universal, since it does not reduce hammer vibration caused by rebound impacts of the tool and by other causes.

b) Use of elastic handles

Reducing the vibrational effect of a hand tool on the worker by means of elastic handles is now a generally recognized and more universal method compared with those considered above. The handle body is separated from the hammer barrel by springs of low stiffness (3–5 kg/cm). According to the law of oscillations of a mass on an elastic foundation [7], a sliding handle is characterized by a smaller amplitude of oscillatory motion than the oscillating barrel.

Some studies in this direction were carried out by V. I. Kopytov, P. M. Alabuzhev, and Yu. P. Tsivinsky [8, 9, 10]. A real machine with an elastic handle was replaced by a two-mass system with two degrees of freedom. With certain assumptions, this hammer model was described mathematically and investigated.

These studies are of great theoretical and practical importance. At present they are used in calculating pneumatic hammers with an elastic handle, namely in selecting the spring stiffness coefficient as a function of the masses of the handle and housing, as well as of the frequency of the exciting force. The studies showed that the effectiveness of vibration damping by means of elastic handles depends on the ratio of the frequency of forced oscillations, corresponding to the number of blows per second, to the frequency of the system’s natural free oscillations.

Similar studies of a two-mass system as applied to a chipping hammer with an elastic handle were carried out in 1962–63 on an electronic model by V. F. Gorbunov and A. V. Trikhanov [11, 12], who obtained results showing the influence of the hammer’s operating conditions on the effectiveness of vibration isolation.

At present, in many research, design-engineering, and production organizations, hammers with vibration isolation by means of elastic handles have been created. A vibration-damping handle for pneumatic hammers has been developed at the Skochinsky Institute of Mining, Academy of Sciences of the USSR, at the Institute of Mining of the Siberian Branch of the Academy of Sciences of the USSR (by A. N. Klyushin), and also at the Tomsk Polytechnic Institute [13]. Vibration reduction by means of a handle separated from the body by three originally arranged springs has been achieved in a hammer created by S. I. Dobroborsky (the “Pnevmatika” plant, Leningrad). The design of a KE-type hammer with vibration damping was also developed by Leont’ev, a fitter at the Kharkov Turbine Plant. In Leont’ev’s hammer, reduction of handle vibration is achieved by means of a cylindrical spring and a pneumatic buffer formed between the walls of the handle and the body.

Experimental studies of hammers with elastic handles show that, with proper selection of the spring parameters, a considerable reduction in vibration amplitude can be achieved (by a factor of 4–14).

The use of an elastic connection between the handle and the body reduces not only the vibration amplitude, but also lowers the maximum value of the recoil force and gives it a smooth character of variation.

The main and very substantial advantage of this method of vibration isolation is that it makes it possible to reduce vibration produced by many forces acting on the hammer body.

The disadvantages of this method of vibration isolation include:

a) the need for a separate calculation (selection) of elastic elements for each hammer size;

b) at certain values of the pressing force, shocks of the handle may occur as a result of impacts of the body against the handle through the holders (handle-position retainers);

c) the difficulty of transmitting compressed air into the pre-spool chamber of the distribution device;

d) lengthening of the hammer because of the need to place elastic elements between the handle and the body;

e) an increase in the weight of the hand tool;

f) complication of the design of the machine as a whole.

c) Vibration damping by means of pneumatic shock absorbers

The use of pneumatic shock absorbers in hammers is very promising. In this case the oscillating barrel of the hammer compresses not a spring, but the air located in a closed chamber between the handle and the barrel. The greatest vibration-isolating effect with pneumatic shock absorbers is achieved when the pressure in the chamber is automatically regulated with changes in the pressing force on the hammer handle. A scheme of such a shock absorber was proposed by Yu. P. Maksimovich and A. M. Petreev [14].

In the work of A. Levandovsky [2] it is indicated that, at the Hungarian Institute of Labor Protection, an air cushion was used as a shock absorber, in which the average air pressure is regulated depending on the properties of the materials being worked and the magnitude of the pressing force.

However, achieving vibration reduction with pneumatic shock absorbers is complicated by the presence of considerable friction in the seals between the walls of the handle and the outer surface of the hammer barrel. In our opinion, friction can be reduced to a minimum if, instead of a closed chamber, a shock absorber is used that consists of a rubber envelope filled with air. Regulation of the pressure in the envelope will take place automatically as a result of the change in the volume of the envelope when it is compressed.

г) Use of elastic grips and special gloves

To reduce the vibrational effect of the tool on the human organism, in 1935 the Soviet engineer A. I. Pavlov proposed covering the entire hammer, including the handle grip, with a rubber casing (Patent No. 44493, February 1, 1935). This method is simple and makes it possible to reduce the effect of high frequencies on the worker; however, the effect of vibration with large displacements essentially remains the same.

At present, some enterprises use special gloves with pads sewn into them made of porous rubber or other elastic materials. Tests carried out by A. G. Mogilnichenko to determine the effectiveness of these methods showed that they significantly reduce high-frequency longitudinal vibration [15].

A reduction in the vibrational effect through the use of elastic grips and gloves has been noted by hygienists in many works. They recommend using for this purpose new plastic materials, cellular rubber, caoutchouc, etc. [16]. Despite a certain vibration-isolating effect obtained with the aid of the listed elastic materials, they still do not completely protect a person from vibrational exposure.

д) Damping vibration from rebound impacts

For a long time, researchers did not pay attention to the vibration of hammers caused by rebound impacts of the tool. Therefore, methods for combating this type of vibration have still not been sufficiently developed or studied. Recently, however, a number of authors have proposed design solutions for damping rebound impacts. A. Levandovsky [2] recommends using a special guide bushing to damp collisions of the body with the tool. Levandovsky’s vibration-damping bushing consists of two metal sleeves, between which a rubber ring is placed. The proposed device is simple in design and, apparently, its use will improve working conditions. However, in our opinion, the device has substantial shortcomings. When working with the hammer, the worker, holding the tool with the left hand, presses it downward. As a result, the shank becomes skewed in the bushing, which may give rise to transverse oscillations of the front part of the hammer due to off-center blows of the striker on the shank.

Many researchers, in order to soften rebound impacts of the tool and reduce the speed with which the body seats on the collar, have proposed installing elastic elements made of rubber, cylindrical disk springs, vulkolan, etc., between the body and the collar. Undoubtedly, all these methods significantly reduce hammer vibration. In production, however, they are not used, which is often explained by the absence of the indicated elastic elements and also by the fact that, in fettling castings, working tools without limiting collars are used (with conical and combined shanks).

e) Preventing blows of the striker against the body (bridge)

At present, the only way to combat blows of the striker against the hammer barrel is to create an air cushion in the front cavity. This method is justified when the appropriate manufacturing technology is used for hammers and working tools. Otherwise, i.e., with rapid wear of the mating surfaces of the piston and barrel, the bushing and the shank, no closed space is formed, and the piston strikes the bridge, which often destroys the barrel (longitudinal cracks appear in it). At present, the barrels of TEZ chipping and riveting hammers are not hardened and therefore wear out quickly. In addition, in demolition work, tapered shanks are used which, during hammer operation, allow air to leak from the front cavity. In our opinion, to protect the bridge from impacts it is necessary to abandon the use of tapered bushings, improve the manufacturing technology of hammers, or develop a series of vibration-damping devices whose use would make it possible to reduce vibration with any design of the tool shank.

ж) Protecting the worker’s left hand from the action of tool vibration

During work, the demolition worker holds the tool with the left hand and guides it. The riveter usually holds the hammer by the front part of the barrel with the left hand. In both cases, the left hand must be protected from vibration. For this purpose it is proposed to use special elastic sleeves fitted onto the tool. A sleeve design with vibration damping was developed by Candidate of Technical Sciences S. I. Dobroborsky. Tests of it showed good results. To reduce the effect of chisel vibration on the left hand, demolition workers put a rubber sleeve on the chisel or work with gloves having rubberized elastic pads. To protect the left hand of a riveter, V. F. Gorbunov, V. I. Kopytov, and I. F. Vysotsky recommend using hammers with an elastic handle, which have an additional so-called false barrel [13]. Elastic sleeves reduce the longitudinal and transverse displacements transmitted by the oscillating tool to the demolition worker’s hand.

з) Other methods of reducing the effect of vibration

In addition to the methods considered for reducing the vibration of pneumatic hammers, preventive measures also affect the intensity of development of vibration disease [1].

Studies by hygienists have shown the beneficial effect of heating the air, washing the hands under a dispersed jet of heated water (38–40°), reducing noise, ensuring monitoring of workers’ health, nutrition, the workday schedule, organization of the workplace, etc. All these measures play an important role and help to reduce vibration sensitivity. No less important is monitoring the condition of the hammer itself, i.e., the wear of its parts. In the authors’ opinion, hammers with worn barrels should not be allowed into service, since a new source of longitudinal and transverse vibration appears as a result of blows of the striker against the rear wall of the hammer and off-center collisions with the tool. The internal surface of the barrel must not have scoring; it must be lubricated, especially when working in dusty and dirty rooms.

Conclusion

The brief analysis carried out of the methods and means for reducing the vibrational effect of hammers on the human organism shows that each of them has its own positive and negative aspects, and that only their combined application will make it possible to solve the problem of vibration isolation. This must be preceded by fundamental studies of the various elements and systems of isolation and vibration damping. In our opinion, the greatest effect in reducing the action of vibration can be achieved through the combined use of the following principal methods: elastic handles, modified forms of pressure diagrams, damping of rebound impacts and impacts of the striker on the barrel, as well as the use of elastic couplings and holders on chisels. At present, the institute has developed a number of basic schemes for chipping-and-riveting hammers with vibration-damping devices; working drawings will be prepared for these in the near future, and experimental machines will then be manufactured.

References

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13. V. F. Gorbunov, V. I. Kopytov, I. F. Vysotskii. Some results of testing a prototype paving breaker with an elastic handle. IVUZ. Machine Building, 10, 1963.

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