Design of a Manual Pneumatic Hammer with Vibration-Damping Devices

Yu. A. Oparin, V. F. Gorbunov, V. I. Baburov

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

The vibration of hand-held mechanized tools causes a dangerous occupational disease known as vibration disease. At present, many research and production organizations are engaged in the development and study of hand-held hammers with reduced vibration. Such organizations include: the State Plant of Pneumatic Machines “Pnevmatika” (Leningrad), the A. A. Skochinsky Institute of Mining, the Institute of Mining of the Siberian Branch of the Academy of Sciences of the USSR, the Tomsk Electromechanical Plant, the Central Design Bureau for Mechanized Tools (Moscow), the Novosibirsk Electrotechnical Institute, and many others. For a number of years the Tomsk Polytechnic Institute has also been working in this field [1, 2].

The present paper describes a new design of a hand-held pneumatic hammer with vibration damping, created by the authors in 1964, and also considers certain questions in the design of such hammers.

Figure 1 shows the design of the pneumatic chipping hammer that was developed. The handle 1 (Fig. 1), by means of an elastic member—a conical spring 2, which rests on the intermediate member 3 serving to fasten the air-distribution device 4—is separated from the barrel 5 of the hammer.

Along the barrel 5, together with the handle 1, a false barrel 6 moves in the axial direction.

Under emergency operating conditions of the hammer, the striker 7 strikes the vibration-damping bushing 8, on which rubber sleeves (vibration dampers) 9 and 10 are fitted. These impacts are transmitted through vibration damper 10 to the front nut 11 and then to the barrel 5. Vibration damper 9 serves to soften collisions between the barrel and the working tool 12. The collisions are caused by reverse blows of the chisel, pick, or caulking tool, as well as by reactive forces when the barrel seats against the limiting collar of the tool [1].

The distinguishing features of this schematic design of hammers are:

a) the use of conical springs installed as elastic elements between the handle and the barrel;

b) the use of a special vibration-damping bushing, which reduces the vibration of the hammer caused by the action of a number of forces, namely, the blows of the striker on the bridge, the rebound impacts of the working tool on the body, and reactive forces when the tool seats against the collar;

c) the supply of compressed air from the handle to the air-distribution device is carried out through a flexible hose;

d) the presence of a casing, or false barrel.

Fig. 1. Diagram of a manual pneumatic hammer with vibration-damping devices.
1—handle; 2—conical spring; 3—intermediate link; 4—air-distribution device; 5—barrel; 6—false barrel; 7—striker; 8—bushing; 9 and 10—rubber sleeves; 11—front nut; 12—working tool; 13—stop; 14—intermediate-link retainer; 15—flexible hose; 16—rubber ring.

Let us explain these features, which are essentially advantages of the hammer design under consideration.

The elastic link separating the handle from the barrel may be placed directly in the cavity of the handle body, as shown in Fig. 1, or in the side part of the hammer. The latter spring arrangement was used in previously developed hammer designs [2]. When the springs are installed in the handle, the length of the cavity \(L\) is determined from the expression:

\[ L = l + k, \]

where \(l\) is the length of the compressed spring;

$k$ is the design-specified displacement of the handle relative to the barrel; it is taken equal to 25–30 mm, depending on the magnitude of the vibration amplitude.

For a cylindrical spring (Fig. 2, a)

\[ l = d \cdot n, \]

where $d$ is the diameter of the spring wire;
$n$ is the number of spring coils.

For a conical spring, whose coils, when compressed, are arranged in a flat spiral, the value $l$ is equal to the diameter of its wire. Thus, the use of a conical spring makes it possible to reduce the length of the cavity in which the elastic element is placed. If one takes into account that the lateral arrangement of springs [2] increases the weight of the hammer and worsens the aesthetic design of the machine, then the use of conical springs has a number of obvious advantages over cylindrical springs.

To soften the impacts of the striker against the barrel, which cause oscillatory motions with large displacement and lead to destruction of the hammer, at present essentially one method is used, namely: an air buffer is created in the front cavity. However, this protective measure is not always effective because of the poor machining of the rubbing surfaces: the barrel–striker, the shank of the working tool—the end bushing [1].

Reduction of the vibration of the hammer arising from collisions of the barrel with the working tool is in most cases accomplished by installing shock absorbers between the barrel and the limiting collar of the tool. Undoubtedly, a certain effect is achieved in this way. Taking the above into account, we also developed a special vibration-damping bushing (Fig. 1), whose rubber shock absorbers help reduce the vibration caused by collisions of the body with the tool and the striker, and protect the barrel from destruction.

Fig. 2. Arrangements of the spring (a) and of the supply of compressed air to the distributing device (b):
1 — handle; 2 — spring; 3 — barrel.

When designing pneumatic hammers with elastic handles, considerable difficulties arise in transmitting compressed air from the inlet valve, located in the handle, to the distributing device. This observation applies to chipping-riveting hammers. Two methods of supplying air are known: through the structural elements of the handle and barrel with sealing of the mating surfaces, and by means of a flexible element (hose).

In the first case, the air supply can be implemented structurally in different ways (see [2] and Fig. 2, b). According to the scheme shown in Fig. 2, b, the cavity length is equal to

\[ L = 2k + C, \]

where $C$ is the amount of overlap of the mating elements of the handle and barrel (the intermediate link).

The value of \(C\) is selected from the sealing conditions of the mutually moving elements. The disadvantages of the air-transmission scheme considered are:

a) the presence of additional friction between the handle and the barrel, which reduces the vibration-damping effect;

b) an increase in the length and weight of the hammer being designed;

c) the complexity of manufacture and the unreliability of the operation of the joint.

In connection with the above, we abandoned this scheme and began designing hammers with compressed-air transmission through a flexible hose (Fig. 115). It should be noted that experience in creating such hammers already existed. The Central Design Bureau for Mechanized Tools had developed small-sized riveting hammers with a similar air supply into the pre-spool chamber. In our hammer design the flexible hose is located on the outside and is not protected against possible damage.

A disadvantage of the adopted air-transmission scheme is that, in this case, the aesthetic appearance of the machine is impaired.

When working with riveting, chipping, and in some cases scaling hammers, the worker controls them while supporting the front part with the left hand. To reduce the effect of vibration on this hand of the worker, a false barrel is provided [1, 2]. In addition, the false barrel protects the inner barrel from accidental external damage. The above makes it possible to reduce the weight of the barrel by decreasing its outside diameter.

The hammer (Fig. 1) is made on the basis of the MP-6 hammer, but, despite the presence of additional vibration-damping devices, it weighs 5.7 kg (Table 1). This was achieved by introducing the following changes into the design.

In barrel 5 (Fig. 1), apart from two diametrically located channels through which air enters the front cavity, there are no other longitudinal channels. In connection with this it became possible to mill the hammer barrel on both sides, which reduced the weight of the machine (Table 1). The exhaust holes are drilled directly along the radius of the barrel, without longitudinal channels, as is the case in hammers of the MP and KE types manufactured by the Tomsk Electromechanical Plant [3].

The technical characteristics of the hammer with vibration-damping devices, which was given the designation MPV-16, in comparison with the MP-6 hammer, are given in Table 1. The indicators correspond to operation of the hammers at an air pressure in the network of 5 atm gauge.

Assembly of the hammer is carried out as follows. Rubber bushings 9 and 10 are put onto bushing 8 (Fig. 1). The bushing is inserted into the cavity of the front part of the hammer, after which nut 11 is screwed on. The air-distribution device 4 is assembled and, with the aid of intermediate link 3 and pins (not shown in the figure), is fastened to the barrel by screwing. The intermediate link is fixed by stopper 13 in the same way as the handle of MP- and KE-type hammers. A rubber ring 16 is put onto the barrel until it rests against the shoulder of the barrel. Having set the handle in a vertical position with the shank downward, the conical spring is inserted into the cavity with its large base downward, and the intermediate link with the barrel is inserted so that the retainer 14 welded to the link enters the longitudinal cutout of the handle body. The false barrel is put on over the barrel from above and is screwed together with the handle. At its base the false barrel also has a longitudinal cutout for the retainer of the intermediate link.

As it is screwed in, the inserted working tool compresses the spring so that the retainer does not enter the longitudinal slot of the false barrel and thus does not interfere with screwing-in.

Table 1

Fig. 3. Dependence of the vibration amplitude of the barrel and handle on the pressing force.
---------- 6 atm; ——— 5 atm; —·—·— 4 atm.

After all the operations listed above, the flexible hose is connected. This final assembly operation presents no difficulties. Disassembly of the hammer is carried out in the reverse order.

According to the described hammer design (Fig. 1), two specimens of an MPV-6 type hammer were manufactured at the experimental section of the Tomsk Electromechanical Plant and were investigated in the laboratory of pneumatic machines of the institute. On the basis of these studies the MPV-6 hammer was improved and subsequently named MPV-16 (Table 1). Figure 3 presents the results of the investigation of the MPV-6 hammer.

In the upper part of Fig. 3 are shown the amplitude–force dependences [4] for the barrel, and in the lower part—for the handle. The character of the change in the vibration amplitude of the barrel as a function of the pressing force is similar to the same dependences established for production hammers [1, 4]. However, the presence of shock absorbers in the front part of the hammer (9 and 10, Fig. 1) makes it possible to reduce the range of barrel oscillations in the limiting principal operating mode and from the action of rebound blows of the tool. At an air pressure in the network of 5 ati (upper middle curve), the limiting principal mode occurs at a pressing force of 18–20 kg, and the maximum force of rebound blows was recorded at a pressing force of 29 kg. The vibration amplitude of the handle in the working zone of the hammer (pressing force 18–30 kg) is much smaller than the vibration amplitude of the barrel and is 0.2–0.3 mm. In comparison with the MP-6 hammer, on the basis of which the machine under study was created, the vibration amplitude of the handle has been reduced by a factor of 8–10; thus, a fairly significant vibration-damping effect has been obtained. At present the authors are continuing to improve the designs of the hammers being developed.

Conclusions

1. To reduce the effect of vibration on the human body, a new fundamental design of a hammer with vibration-damping devices has been created. Hammers manufactured according to this design are fairly simple in construction, and their weight does not exceed the weight of serially produced hammers with the same energy characteristics (Table 1).

2. The distinguishing features of the developed hammer designs are:

a) the use of a conical spring as an elastic element installed between the handle and the hammer barrel, which makes it possible to create hammers of smaller length and weight;

b) the use of a special vibration-damping bushing, which makes it possible to reduce the vibration of the barrel caused by impacts with the working tool;

c) the use of a false barrel to protect the worker’s left hand from the action of vibration and to protect the inner barrel from external damage; taking the latter into account, it appears possible to reduce the weight of the machine being created.

1. Test results showed that the introduction of the proposed vibration-damping devices makes it possible to reduce the vibration amplitude of the handle by a factor of 8–10.

References

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

2. V. F. Gorbunov, V. I. Kopytov, I. F. Vysotskii. Some results of the investigation of an experimental specimen of a breaker hammer with an elastic handle. Izv. vuzov. Mashinostroenie, No. 10, 1963.

3. V. I. Baburov, V. F. Gorbunov. Investigation of internal processes and parameters of chipping and riveting hammers. Izvestiya TPI, vol. 123, 1963.

4. V. F. Gorbunov, V. I. Baburov, Yu. A. Oparin, L. S. Redutinskii. Evaluation of the vibration characteristics of chipping-and-riveting hammers according to sanitary standards. Izvestiya TPI, vol. 129, 1965.