Investigation of Internal Processes and Parameters of Chipping and Riveting Hammers

V. I. Baburov, V. F. Gorbunov

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

Chipping hammers are intended for chipping, riveting, chasing, cleaning castings of gates, cutting out cavities in forgings, upsetting pipes, dressing seams, and other work. Hammers of the MP-4, MP-5, and MP-6 types are structurally similar to one another and differ only in the length of the barrel and of the striker. The reciprocating motion of the striker is effected by the alternate admission of compressed air into the chambers of the hammer cylinder. Switching of the air supply is performed by a flat valve.

Riveting hammers (Fig. 1) are intended for riveting rivets and are used in various branches of the machine-building, metallurgical, and construction industries. Three standard sizes of riveting hammers (KE-16, KE-19, and KE-22) are also used as chipping hammers, after preliminary re-equipment for this purpose.

Unlike chipping hammers, KE-type hammers have air distribution by a cylindrical spool and differ from one another also in the length of barrel 3 and striker 4 (Fig. 1).

The hammers were tested on a horizontal experimental stand in the laboratory of the Department of Mining Machines and Mine Transport.

The research procedure included recording the change in compressed-air pressures in both chambers of the hammer by means of strain-gauge transducers, an amplifier, and a loop oscillograph [1], recording the path of the striker over time with the use of induction transducers [3], recording vibrations of the hammer bodies with rheostatic transducers [2], and determining the final velocity of the striker by an electrical contact transducer (oscillographic tip).

The use of various methods for determining the parameters and character of the working processes of the hammers contributed to increasing the accuracy, reliability, and objectivity of the investigations.

The diagrams were recorded at air pressures in the line of 4.5 and 6 atm and at feed forces of 10, 20, 30, 40, 50, and 60 kg. These experimental conditions made it possible for us to obtain a sufficient number of observations for analysis and to determine the principal characteristics.

Analysis of the internal working processes of pneumatic hammers

can be carried out most fully only under the condition of simultaneous recording of the air-pressure diagrams and the striker travel.

Fig. 1. Riveting hammer:
1—valve chest; 2—valve; 3—barrel; 4—striker; 5—bushing.

The maximum pressure of the compressed air in the rear cavity of hammers, as a rule, is equal to the pressure in the line and is usually established at the end of the idle (return) stroke of the striker, before it stops (Fig. 2, a). The extreme rear position of the striker corresponds to the beginning of the working stroke. During the working stroke, the pressure in the rear cavity begins to decrease because of the rapid increase in the volume of the cavity, despite the continuous inflow of air from the line. The front edge of the striker soon covers the exhaust ports. For chipping hammers, which have a two-tier arrangement of the exhaust ports, at this moment the process of compressing the air in the front cavity begins, which causes the valve to shift. In riveting hammers, after the exhaust passages have been covered, the front cavity continues to communicate with the atmosphere through the passages supplying compressed air to it, and also through the annular space of the valve, connected by a passage with the exhaust windows. For this reason the striker, having covered the exhaust window with its front edge, experiences almost no counterpressure from the front cavity. The shifting of the valve is effected by the pressure difference on its shoulders, which increases sharply after the exhaust windows are opened by the rear edge of the striker. By the beginning of exhaust (Fig. 2, a), the air pressure decreases to 70–80% of the line pressure. After the blow on the shank of the working tool (the rivet set), the idle stroke of the striker begins. The maximum air pressure established in the front cavity of the hammer (Fig. 2, a) is 60–80% of the line pressure, which is explained by the presence of air leakage into the atmosphere through the clearances at the point where the tool shank mates with the hammer body, and by the additional resistance in the inlet passages of the barrel. In addition, limitation of the air pressure in the front-

in the front chamber is to some extent due to the presence of rebound of the striker after impact. During the return stroke of the striker, after the exhaust of air from the rear chamber, its compression begins. The increase in air pressure continues until the valve is shifted. In the subsequent moments the process is repeated periodically.

It should be noted that the decrease in air pressure in the cavities of riveting hammers, both during the working stroke and during the idle stroke, occurs almost according to a linear law [5]. This regularity is somewhat disturbed in hammers MR-4, MR-5, and MR-6. In Fig. 2, a, the change in air pressure in the front cavity follows a curvilinear dependence.

The change in the pressing force on the hammer has a noticeable effect on the course of the working processes. Moreover, for chipping hammers and for riveting hammers this effect is manifested differently.

With a decrease in the pressing force in chipping hammers, during the working stroke of the striker a back pressure arises in the front cavity, which is the greater the smaller the pressing force. In riveting hammers this phenomenon is manifested to a much lesser degree.

With the aid of Fig. 3 it is possible to explain the influence of the withdrawal of the hammer body from the tool on the nature of the internal processes. It is known that the smaller the pressing force, the greater the vibration amplitude [1, 4, 5] of the hammer body. During operation of the hammer, its body performs a complex oscillatory motion composed of several types: vibration of the body with a frequency equal to the number of striker strokes per minute, and with an amplitude depending on the mass of the body and other conditions; and withdrawal, the maximum value of which, as a rule,

Fig. 2. Diagrams of the working process of the MR-6 chipping hammer, obtained at an air pressure in the line of 5 atm and a pressing force of 20 kg:
a) diagram of air pressures in the rear \((p_1)\) and front \((p_2)\) cavities of the hammer and diagram of the striker travel \((S)\) versus time; b) indicator diagrams of the forward \((p_{\mathrm{n}})\) and return \((p_{\mathrm{o}})\) strokes of the striker; c) diagram of the resultant force versus time.

is equal to the amount by which the striker stroke is limited by the shank of the tool. The latter was confirmed by the results of our experiments. The oscillatory motion of the hammer body is also caused by impacts when it seats on the collar of the tool, by impacts of the working tool when the latter rebounds from the medium being worked, by shocks at the moment of impact of the striker, and by other causes. As is evident from the foregoing, the process of hammer oscillation is very complex and has not yet been fully studied. With simultaneous recording of diagrams of striker travel, body travel, and air pressures, it is possible to establish a number of characteristics of this motion and to explain certain phenomena occurring in the working cyc-

in the hammers. The diagram of motion of the hammer body was recorded relative to zero line 1 (Fig. 3). The displacement of the curve \(S_{\text{body}}\) from this line means motion of the hammer body toward the handle. The remaining diagrams were recorded relative to line 2 (Fig. 3).

Fig. 3. Influence of vibrations of the hammer body on diagrams of striker travel and air pressure in the cavities.

Under the action of the pressure of the compressed air entering the rear cavity, the hammer body tends to move away from the medium being worked, but this is opposed by the pressing force, whose magnitude precisely determines the amount of rebound of the hammer body. By the moment the exhaust begins, the hammer body already has a reserve of a certain amount of kinetic energy, and therefore by inertia it continues on its path even after the pressure has decreased to 1 ati \((S'_{\text{body}},\) Fig. 3). After a certain interval of time, under the action of the pressing force and of the air pressure in the front cavity on the walls of the body, the latter changes the direction of its motion and begins to approach the tool collar (point \(A\), Fig. 3). However, at small values of the pressing force (7–15 kg), the hammer body does not reach the collar and stops at some distance from it \((\Delta S_{\text{body}},\) Fig. 3), since at this time compressed air (diagram \(p'_1\), Fig. 3) begins to enter the rear cavity of the hammer and again causes it to change its direction. In subsequent cycles the rebound of the body gradually increases until its magnitude becomes equal to the amount by which the striker travel is limited by the tool shank. When this moment occurs, the striker begins to deliver blows to the barrel bridge (Fig. 1), which quickly returns the body to its initial position. In the subsequent moments the process of gradual rebound of the body is repeated. The frequency and amplitude of the rebound and vibration of the hammer body are different and change with changing operating conditions of the hammers. This question is treated in greater detail in the works of B. V. Sudnishnikov [4, 5]. Here we consider this question for the purpose of establishing the influence of the rebound of the hammer body on its working process.

The rebound of the body contributes to an increase in the travel of the striker relative to the tool, as well as relative to the body. In Fig. 3 the diagrams of striker path \(S_{\text{str}}\) and body travel \(S_{\text{body}}\), and of the air pressure \(p_1\) and \(p_2\), characterize the course of the working processes at large values of the pressing force (30–60 kg), while the diagrams \(S'_{\text{str}}\), \(S'_{\text{body}}\), \(p'_1\), and \(p'_2\) characterize it at small values (10–30 kg).

As is seen from Fig. 3, the full travel of the striker and the travel of the striker to the exhaust ports increase, with decreasing pressing force, owing to the rebound of the hammer body from the tool collar. The farther the hammer body is from it, the longer is the path of the striker from the point

impact with the shank of the tool until the exhaust ports are opened by the front edge (during its return stroke). This leads to an increase in the kinetic energy of the striker during the return stroke (the area of diagram \(p'_2\) is greater than the area of diagram \(p_2\)). The transfer of impulse to the striker by the compressed air of the front chamber is greater, the smaller the pressing force. This proposition is true both for the riveting hammers and for the chipping hammers of the TEZ plant. The absolute values of the increment, naturally, depend on the design of the hammer. Thus, for example, for the MP-4 hammer the impulse of the resultant force acting on the striker from the front chamber during the idle stroke (area \(F_3\), Fig. 2, c) at a pressing force of 10 kg is equal to \(0.147\ \text{kg}\cdot\text{s}\); for the MP-5 it is \(0.164\ \text{kg}\cdot\text{s}\), and for the KE-22 riveting hammer the impulse is \(0.36\ \text{kg}\cdot\text{s}\) at the same pressing force. As the pressing force increases, the indicated impulse decreases. For example, for the MP-4 hammer at a pressing force of 10 kg it is \(0.164\ \text{kg}\cdot\text{s}\), at 30 kg—\(0.1\ \text{kg}\cdot\text{s}\), and at 50 kg—\(0.06\ \text{kg}\cdot\text{s}\) (line pressure 5 atm).

The increase in impulse under consideration, with a decrease in pressing force, introduces significant changes into all the parameters of the hammers, as will be indicated below.

The absolute values of the maximum pressure of the compressed air in the front chamber of the hammer, as the pressing force is increased from 10 kg to 40 kg, decrease: for chipping hammers by 9–22%, and for riveting hammers by 2–17%. With decreasing hammer weight this reduction is manifested more noticeably. The decrease in the maximum air pressure in the front chamber of the hammers with increasing pressing force is explained by the reduction of its volume due to the decrease in the recoil of the housing.

The oscillatory movements of the hammer housing also introduce changes into the pressure diagram of the compressed air in the rear chamber of the hammer. These changes consist mainly in an increase, by 5–15% for riveting hammers, in the time during which the compressed air acts on the striker as the pressing force decreases (diagram \(p'_1\), Fig. 3), as a result of which the kinetic energy of the striker increases toward the end of its working stroke (Fig. 4, a). The tests showed that the magnitude of the maximum air pressure in the rear chamber of the hammer, at constant line pressure, does not depend on the pressing force. Nor were changes observed in the pressure value at the moment of exhaust.

We have already noted that a change in the pressing force affects the character of the oscillatory motions of the hammer housing and their internal working processes, which, in turn, influences the magnitude of the basic parameters. At the request of the TEZ plant, our task also included determining the hammer parameters for various values of the pressing force. Tables 1 and 2 contain the hammer parameters according to the plant data and those obtained as a result of laboratory tests (pressing force 20–30 kg).

The blow frequency per minute was determined from the marks of the vibrator of the MPO-2 loop oscillograph. It is necessary to note here that the presence of a metal rod running from the striker through the shank of the tool (the set) to the induction sensor for recording the striker-stroke diagram reduces the blow frequency by 5–16%. Table 2 contains the values of the blow frequencies calculated without the presence of the rod. As a result of the tests it was established that in some cases this hammer parameter is higher than the plant data (KE-22, KE-32, MP-5), and in others lower (MP-4, KE-19, KE-28) (Tables 1 and 2). Figure 4 gives the dependences of the main parameters of the KE-22 hammer on the pressing force. It is evident from it (Fig. 4, b) that the blow frequency of the hammer, as well as the other parameters, with decreasing pressing force and increasing pres-

air pressure in the network varies over wide limits. These changes, as we have already mentioned, are a reflection of changes occurring in the operating cycles. An increase in the blow frequency with an increase in operating pressure is characteristic of all types of pneumatic hammers and is also observed in the chipping and riveting hammers of the TEZ plant.

Table 1

Technical characteristics of chipping and riveting hammers of the TEZ plant
(according to plant data)

At small pressing forces, the duration of individual cycles changes depending on the magnitude of the body recoil. An increase in body recoil is accompanied by a lengthening of the striker travel and, consequently, by a reduction in the blow frequency. The higher the operating pressure in the network, the greater the magnitude of the body recoil and the greater the pressing force required in order for the operation of the hammer to stabilize and for the parameters, including the blow frequency, to become constant (Fig. 4, b). For lighter hammers, a change in the air pressure in the network causes larger changes in blow frequency in absolute magnitude. In percentage terms, however, the increase in blow frequency with an increase in air pressure in the network by 1 ati is the same for all hammers and is mainly 7–10%.

For all riveting hammers of the TEZ plant, the influence of pressing force on blow energy is expressed by the same regularity, shown in Fig. 4, a. For chipping hammers, increasing the air pressure by one atmosphere (within the range 4–6 ati) causes an increase in blow energy of 0.24 kgm for the MP-4 and MP-5 hammers and of 0.39–0.40 kgm for the MP-6 hammer. For riveting hammers this increase also rises with increasing weight: KE-16—0.34 kgm, KE-19—0.38 kgm, KE-22—0.59 kgm, KE-28—0.77 kgm, and KE-32—0.80 kgm. The different increase in blow energy is explained by the different magnitude of the volume of the cavities.

hammers and, correspondingly, different values of the impulses of the acting forces.

Table 2

Parameters of chipping and riveting hammers obtained as a result of tests

A change in the pressing force is reflected differently in the value of the impact energy for chipping and for riveting hammers. For chipping hammers, a decrease in the pressing force leads to a decrease in the energy of a single impact, while for riveting hammers it leads to an increase, as shown in Fig. 4, a.

Construction of the indicator diagram of the resultant force (Fig. 2, v) acting on the striker under different operating conditions of chipping hammers made it possible for us to establish the causes of this decrease. From these diagrams it was found that, at small values of the pressing force, back pressure is created in the front cavity of the hammer at the moment preceding the impact. The impulse of the back pressure (area \(F_4\), Fig. 2, v) is the greater, the longer the path of the striker from the exhaust openings to the moment of impact. The shifting of the flat valve takes place immediately after the exhaust openings are uncovered by the striker. The compressed air, filling the front cavity of the hammer, exerts pressure on the striker at the end of the working stroke and reduces its kinetic energy. The greater the rebound of the hammer body, the earlier the admission of compressed air into the front cavity begins. The back pressure apparently also absorbs that additional reserve of kinetic energy of the striker which it acquires as a result of the increased stroke length. It should be noted that, at small values of the pressing force (10–20 kg), the energy of a single impact in different cycl-

different and varies within wide limits (Fig. 4, a). Increasing the pressing force leads to stabilization of the impact energy (under these conditions, the change in its values lies within the calculation error).

Fig. 4. Effect of pressing force on the principal parameters of the riveting hammer KE-22.

The change in the length of the rear air cushion (Fig. 4, c) as a result of fluctuations in the values of the impulses acting on the striker from the side of the front cavity is also confirmed by analysis of the diagrams of striker travel recorded relative to the hammer body by induction sensors.

As a result of the tests, for steady operating regimes of the hammers at an air pressure in the network of 5 atm, we obtained the following values of rear dead spaces: MP-4 — 4–5 mm; MP-5 — 23.5–26 mm; KE-19 — 18 mm, KE-22 — 25 mm, and KE-32 — 24 mm.

The impact power of a hammer, as a rule, at lower pressing forces has somewhat smaller values than at higher forces. However, in individual cases during testing, almost no change in impact power was observed. The change in this parameter as a function of pressing force occurs within very small limits, ±2–10%, and the effect of the air pressure in the network on the impact power is clearly seen from Table 2.

Laboratory investigations of hammers from the TEZ plant showed that their factory technical specifications were, in the main, compiled correctly and quite reliably reflect the course of the working processes. The discrepancies in the parameters are apparently explained by the fact that the objects of our tests were specific specimens of hammers. With the aid of the data obtained, it is possible to evaluate individual types of hammers.

According to the test data, the indicators of some hammers duplicate one another. Thus, the impact frequencies of KE-22 and KE-28 are almost equal; the impact powers of the hammers KE-28, KE-19, and MP-6 vary within the limits of 0.56–0.58 hp.

Analyzing the lengths of the rear air cushions, it should be noted that they turn out to be quite considerable (18–26 mm). A number of researchers (B. V. Sudnishnikov, N. N. Esin, and others) note that, for the best designs of pneumatic hammers, the length of the rear air space should range within 6–13 mm. Depending on the operating conditions of the hammer, these values, as was shown above, vary. Chipping and riveting hammers are operated under the most varied conditions. The working air pressure in the line fluctuates within the range of 3.5–6.0 atm and sometimes even higher. The pressing forces are also varied. Apparently for this reason the designers of hammers at manufacturing plants provide increased lengths of the rear air buffers. In our opinion, taking into account the variety of operating conditions, the cushion length in certain types of chipping and riveting hammers can be considerably reduced—to 5–7 mm for light hammers and 7–10 mm for heavier hammers.

The first three standard sizes of riveting hammers (KE-16, KE-19, and KE-22) are used in production mainly as chipping hammers; for this purpose, production workers make certain modifications to the hammers. Thus, for example, they replace the end bushing. At the Elektrostal Heavy Machine-Building Plant, hammers of the KE-16 type of a lightweight design (by approximately 1 kg) are manufactured in-house. We also recommend that the plant review the designs of the hammers and their standard sizes with the aim of lightening and improving them. Thus, for example, the three standard sizes named above should immediately be manufactured at the TEZ plant as chipping hammers, with a chipping end bushing installed and its length reduced to 55–60 mm. It should also be noted that KE-type riveting hammers, in terms of their characteristics, are much worse than modern pneumatic hammers. This is demonstrated, in particular, by the ratio of their weight per unit of power (Table 1).

Demolition hammers of the same plant have this indicator equal to 8 kg/hp, chipping hammers—7–10 kg/hp. Modern rock drills also have a comparatively small weight per unit of power (8–11 kg/hp).

Conclusions

1. In the course of investigations of chipping and riveting hammers, the nature of the change in the internal working processes as a function of the possible operating regimes was revealed.

2. A substantial influence on the course of the internal working processes is exerted by the impulse of the force acting on the striker from the front cavity. This impulse changes greatly with a change in the pressing force.

3. The technical specifications for chipping and riveting hammers compiled by the TEZ plant are correct and quite reliably reflect the course of the working processes.

4. KE-type riveting hammers require considerable improvement by reducing the lengths of the rear air cushions, lightening, modernizing the front part of the barrel, changing the bushing design in some standard sizes, etc.

LITERATURE

1. Alimov O. D., Basov I. G., Gorbunov V. F., Malikov D. N. Drilling machines. Gostoptekhizdat, 1960.

2. Gorbunov V. F., Gusarov V. M., Berezin L. S., Electric [[unclear: continuation cut off at bottom of page]].

method of recording vibrograms of pneumatic hammers. Izvestiya vuzov, Gornyi zhurnal, No. 12, 1962.

1. Ryashentsev N. P., Timoshenko E. M. Induction sensor for investigating the working process of impact-action machines. Izvestiya vuzov, Gornyi zhurnal, No. 7, 1960.

2. Sudnishnikov B. V. On the theory of recoil of hand-held impact-action machines. In: Impact-Action Machines. Novosibirsk, 1953.

3. Sudnishnikov B. V. The influence of forward-stroke parameters on the power and recoil of a pneumatic hammer. In: Percussive-Rotary Drilling. Impact-Action Machines. Novosibirsk, 1956.