V. I. BABUROV

SOME RESULTS OF A STUDY OF KE-TYPE RIVETING HAMMER DESIGNS

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

In recent years, in mechanical engineering, welding has increasingly been displacing riveting. In this connection, light KE-type riveting hammers, produced serially by the Tomsk Electromechanical Plant, have begun to be used as chipping hammers for fettling medium and large castings [2]. Since these hammers are not adapted for this purpose, production workers introduce changes into their design. In particular, they reduce the weight of the hammers by decreasing the outer diameter of the barrel, and also replace their guide bushing [2]. The weight per unit of power for KE-type riveting hammers is 13–18 kg/hp, whereas for pneumatic breakers and chipping hammers this figure is 6–10 kg/hp. A comparison of the specific weights indicates that riveting hammers are obviously less advanced, and that their designs can be significantly improved.

In 1962, employees of the Department of Mining Machines and Mine Transport of TPI, on assignment from the Tomsk Electromechanical Plant, began conducting studies of KE-type riveting hammers. The scope of the studies included investigation of the dynamics of internal processes, parameters, designs, chipping processes, and also the operating conditions of KE-type hammers, with the aim of developing recommendations for their improvement [2]. The present work gives the results of experimental studies of KE-type hammer designs. All the designs of the hammers mentioned (Fig. 1) are similar and differ from one another only in dimensions. The principle of their operation is also known [5]. The hammer dimensions required for analysis are given in Table 1 and shown in Fig. 1.

The studies were carried out on a test stand with the hammers in a horizontal position and with an air pressure in the network of 5 atm; the investigation procedure was described earlier [1, 6].

To analyze the operation of the air-distribution device, simultaneously with recording diagrams of the air pressures in the rear \(P_z\) and front \(P_p\) chambers of the KE-22 hammer, we recorded the diagram of spool-valve shifting \(D_{p.z.}\) (Fig. 2). For brevity, instead of “air pressure” we shall use only “pressure.”

As can be seen from Fig. 2, after the striker strikes the shank of the tool, the shifting of the spool valve into the extreme rear position ends.

At this moment the pressure in the rear chamber is equal to atmospheric pressure (point \(A\)), while in the front chamber it reaches almost a maximum (point \(A'\)).

The striker has begun its return stroke. At the moment the exhaust ports open, the pressure in the front chamber drops sharply, while in the rear chamber it begins to rise rapidly as a result of the compression of the air by the striker. This circumstance causes the valve to begin its forward travel (point Б), which lasts approximately 0.007–0.01 sec. The average speed of its motion

Fig. 1. KE-type riveting hammer with a tapered bushing: 1—handle; 2—valve; 3—valve box; 4—striker; 5—barrel; 6—guide bushing; 7—working tool (chisel); 8—intermediate ring.

is then 0.72–0.50 m/sec. The shifting of the valve ends at the time when the pressure in the rear chamber of the hammer has decreased somewhat (point В) and, as usual, at this moment it is equal to the line pressure.

Table 1

During the motion of the valve, the pressure in the front chamber has time to decrease almost to atmospheric (point В′) owing to the discharge of air into the atmosphere through the exhaust ports.

The position of the valve at which the working stroke of the striker takes place is maintained until the rear edge of the striker opens the chan-

*) Here the values of K for the second tier of ports “C” are given.

of “C” channels (Fig. 1). The “C” channels are located before the exhaust ports. In the KE-16 and KE-19 riveting hammers there are two “C” ports, and they are located at the same level; in the KE-22, KE-28, and KE-32 hammers there are three: the first tier of ports has two outlets into the cavity, and the second has one. As soon as the striker uncovers the second tier of “C” channels, the spool moves away from the hammer barrel and begins to move toward the intermediate ring (Fig. 1).

Fig. 2. Diagrams of striker displacement, spool-valve transfer, and air pressures in the cavities of the KE-22 riveting hammer.

The fact that the transfer of the spool begins at the moment the “C” channels open, and not at the moment the exhaust ports open, was shown by Doctor of Technical Sciences B. V. Sudnishnikov [5].

In this connection, the location of the “C” channels plays a very important role in the operation of the hammers. If they are located relatively far from the exhaust ports, then premature transfer of the spool will occur. Compressed air will enter the front cavity before the moment of collision and thereby reduce the impact energy of the striker. Conversely, if the “C” channels are located near the exhaust ports, a delay in the transfer of the valve will be observed. The hammer’s blow frequency will noticeably decrease because of the later admission of compressed air into the front cavity of the cylinder.

Toward the end of the movement of the spool to the extreme rear position (the transfer time is equal to 0.0078–0.011 sec), the pressure in the rear cavity has time to fall to atmospheric pressure (point \(A\), Fig. 2). At this time the pressure in the front cavity rises rapidly owing to the inflow of compressed air from the supply network (point \(A'\)). As can be seen from Fig. 2, in both cases compressed air from the network begins to enter the hammer cavities while the spool is still moving (being transferred). And the faster the spool is transferred, the higher the blow frequency of the striker.

In the design of the KE-22 riveting hammer, the total time of spool movement in one cycle is 0.0170–0.0188 sec, and is commensurate with the duration of the complete cycle (29.4–26.6%).

It follows from the above that, when designing hammers, one must strive to obtain the fastest possible transfer of the valve of the air-distribution device.

Closely associated with the operation of the air-distribution device is the distance traveled by the striker from the exhaust ports to the end of the tool shank (the section \(\Delta l\), Fig. 1).

To clarify this relationship and the influence of the length of the section \(\Delta l\) on the principal parameters of the hammer, the author carried out corresponding investigations on KE-22 and KE-32 riveting hammers.

In conducting the experiments, the length of the section \(\Delta l\) was set stepwise by increasing the shank length of the working tool by 5 mm for the KE-22 hammer and by 10 mm for the KE-32.

The investigation yielded very interesting results. Thus, it turned out that the maximum impact power of the hammer is attained at section lengths of 15.5 mm for the KE-22 and 21.5 mm for the KE-32 (Fig. 3), while the operation of the hammers is calculated with \(\Delta l\) equal to 22.5 mm and 31.5 mm, respectively (Table 1). The character of the change in impact power \(N\) as a function of the value of \(\Delta l\) is the same for both hammers. At first the absolute values of the parameters increase, and then, after passing the optimum point, they decrease (Fig. 3).

Fig. 3. Influence of the length of the striker travel from the center of the exhaust ports to the tool shank on the impact power and the length of the rear air cushion of the KE-22 and KE-32 riveting hammers.

For the hammers tested, when \(\Delta l\) is less than 11.5 mm, the impact power drops sharply (Fig. 3). This is explained by the fact that the pressure of the compressed air located in the rear cavity does not have time to decrease to atmospheric pressure and remains rather high by the beginning of the return stroke of the striker. The striker stroke is shortened, since the length of the rear dead space \(B_3\) increases. At the same time, naturally, the impact energy decreases.

With increasing \(\Delta l\), the length of the rear inactive space decreases almost directly proportionally. This is associated with an increase in the magni-

the magnitude of the impulse of the force acting on the striker during the return stroke from the side of the front chamber. However, despite a certain deterioration in the utilization of the cylinder length, shifting the point of impact closer to the exhaust ports by 10 mm in the KE-22 hammer and by 14–17 mm in the KE-32 leads to an increase in impact power.

The investigations carried out made it possible to draw up recommendations for changing the design of the front part of riveting hammers. We recommend the following values for the travel of the striker from the center of the exhaust ports to the point of impact:

Riveting hammers, because of their specific operating conditions and the greater recoil of the body, have a guide bushing length of 70 mm, i.e., 10 mm longer than chipping hammers. And since they—the KE-16, KE-19, and KE-22 hammers—are used mainly as chipping hammers, the length of the bushing can therefore be reduced from 70 mm to 60 mm. This will considerably reduce the weight of the hammers.

The Elektrostal Heavy Machine-Building Plant, which manufactures KE-16 hammers for its own needs, has already carried out such an operation, reducing the weight of the hammer from 7.3 kg to 6 kg, i.e., by 18%. It is also advisable to further reduce the weight of KE-type hammers by reducing the outside diameter of the barrel, without reducing the diameter of the striker.

Since tapered shanks of chisels are at present very widely used (Fig. 1), we investigated the operation of the KE-22 hammer with such a shank (taper 1:12.5).

The results of the investigation showed that the use of tapered chisel shanks leads to a 10–15% increase in compressed-air consumption because of leakage from the front chamber of the hammer. In addition, the presence of air leakage into the gap formed during hammer operation between the guide bushing and the shank of the tool (Fig. 1) sharply worsens the working process itself, disrupts the stability of the hammer operation, and reduces its productivity. The parameters of the hammer decrease and vary over wide limits. A stable operating regime of the hammer occurs only at high pressing forces (50–60 kg), which the worker is practically unable to produce. We therefore believe that it is more expedient to use non-tapered bushings.

The investigation of the influence of striker length on the hammer parameters and the internal working processes was carried out on the KE-22 hammer. The striker length was varied from 67 mm to 128 mm.

With a reduction in the striker length, its full stroke \(S\) increases; however, the stroke to the exhaust ports \(S_k\) decreases (Fig. 4). This shows that the increase in full stroke occurs only because of an increase in the travel of the striker from the exhaust ports to the end face of the tool shank. The length of the rear dead space \(B_z\) increases at the same time, since the shifting of the spool takes place, as was shown above, immediately after the exhaust ports open (idle stroke of the striker). The compressed air entering the rear chamber of the hammer stops the striker considerably earlier. The velocity of the striker, as its length is reduced, increases both during the forward and the return stroke, which is explained by the reduced weight of the ram and by the growth of the impulse of the force acting on the striker from the side of the front chamber. The increase in striker velocity (Fig. 5) raises the blow frequency of the hammer, but does not lead to an increase in impact energy. Reducing the striker length \(l_{\mathrm{ud}}\)

Fig. 4. Effect of striker length on its displacement characteristics (KE-22 riveting hammer).

Fig. 5. Effect of striker length on the energy parameters of the KE-22 riveting hammer.

with a constant tool-shank length increases the length of the section $\Delta l$. The interval in time between the beginning of the spool’s transfer to the rear position and the moment of impact increases. Under these conditions, part of the kinetic energy of the striker is expended in overcoming the resistance of the compressed air, which begins to enter the front cavity of the hammer considerably before impact. With a striker length of 67 mm, all the principal parameters, except for its velocity before impact, decrease sharply because of the disruption of the fundamental connections (relations) between the dimensions of the striker and the arrangement of the air-supply ports. The same sharp decrease in parameters is also observed with excessively large striker lengths of 118 mm and 128 mm. The length of the section $\Delta l$ becomes so small that the compressed air does not have time to escape into the atmosphere from the rear cavity of the hammer.

Thus, those enterprises that reduce the striker length by 10–15 mm for the KE-22 hammer, although only slightly, nevertheless reduce the parameters of the hammers and impair the utilization of the cylinder length, since this increases the length of its idle space.

Conclusions

1. The impact power of KE-type hammers can be increased by shortening the striker travel from the center of the exhaust ports to its extreme forward position. Reducing this distance to the values recommended in the present work will increase the impact power of the hammers by 5–10%.

2. The weight of riveting hammers can be reduced by 15–20% by changing the design of the front part and reducing the outside diameter of the barrel while maintaining the striker diameter.

3. Since the KE-22, KE-19, and KE-16 hammers are used as chipping hammers, we recommend reducing the length of the guide bushing from 70 mm to 60 mm and its inside diameter from 31 mm to 20–22 mm.

4. The use of tapered bushings during hammer operation is inadvisable, since they permit leakage of compressed air from the front cavity of the hammer, which leads to an increased consumption of compressed air by 10–15%, and also disrupts the stability of the internal working processes, reducing the parameters and productivity of the hammers.

5. Reducing the length of the strikers of KE-type riveting hammers causes a decrease in the principal parameters and worsens the utilization of the barrel length, since the amount of dead space increases.

LITERATURE

1. O. D. Atimov, I. G. Basov, V. F. Gorbunov, D. N. Malikov. Drilling Machines. Gosgortekhizdat, 1960.

2. V. F. Gorbunov, V. I. Baburov. On the unification of designs and the streamlining of production of pneumatic chipping-and-riveting hammers. Article in the present collection.

3. V. I. Baburov, V. F. Gorbunov. On a method for calculating the principal parameters of chipping-and-riveting hammers. Article in the present collection.

4. G. I. Sidorenkov. Theoretical and Experimental Study of the Operation of Valved Pneumatic Hammers and a Method for Calculating Them. Candidate dissertation, Leningrad, 1962.

5. B. V. Sudnishnikov. Approximate theory of the distributing device of a pneumatic hammer. Collection Impact-Action Machines, Novosibirsk, 1953.

6. B. V. Sudnishnikov and N. N. Esin. Experimental study of the working process of pneumatic hammers. Collection Impact-Action Machines, Novosibirsk, 1953.