DETERMINATION OF STRIKER ENERGY IN IMPACT MACHINES

P. M. Alabuzhev, O. D. Alimov

In the study of any machine, including an impact machine, with the aim of identifying optimal parameters, the principal comparative characteristics are the productivity of the machine and its economy.

Comparison of impact machines used for the extraction of minerals—pick hammers and drilling hammers—according to these characteristics can be expressed through the work performed per single blow, the number of blows, and the energy supplied to the machine.

However, this task, simple at first glance, is considerably complicated by the fact that pick hammers, drilling hammers, and other impact machines used in the mining industry operate under highly diverse conditions. In this case the productivity of the machine as a whole, as well as the work performed per single blow, depends on the rate of destruction, the ratio of the colliding masses of the striker and the tool, on a whole series of physico-mechanical properties of the material being worked, and on the mechanism of destruction; therefore it can be determined only in one or another specific case.

However, in order to create an efficient and highly productive impact machine, it is necessary to know in advance its optimal parameters. These may be identified, first of all, from investigation of the mechanism of destruction of rocks for a given method of extraction and from study of the process of energy transformation in the machine itself and in the tool.

On the other hand, after manufacture of the machine it is necessary to verify that the machine satisfies the imposed requirements and, first of all, whether it develops the required impact energy and the required number of blows per unit time. In both cases one has to use special methods and devices. Determination of the number of blows, as a rule, presents no great difficulties, since at present there is a considerable number of various tachometers, frequency meters, and other instruments. It is more difficult to determine the energy of the striker before impact.

In factory tests, when comparatively evaluating impact machines of the same type of construction, various work meters are used.

In the literature [1; 2; 3; 4; 5; 6] descriptions are given of work meters for impact machines, based:

a) on the deformation of plastic materials—lead or copper crushers (so-called upsetting work meters);

b) on the deformation of elastic elements (springs, a pneumatic buffer);

c) on overcoming frictional force, etc.

Calibration of the indicated work meters is carried out either with a standard hammer or by blows of a dropped weight, and sometimes even statically.

Possessing in most cases sufficient simplicity and productivity, the indicated work meters in most cases determine not only

the kinetic energy of the striker, but only the work expended on one or another deformation.

In view of this, a device designed and manufactured by the Tomsk Electromechanical Plant (TEMZ) for the comparative evaluation of hammers of one and the same design by the kinetic energy of the striker before impact is of considerable interest.

The apparatus is a drop tester on which hammers are tested and impressions are calibrated (Fig. 1). During testing the hammer 1 is set vertically on the support 2 and pressed against it by the pneumatic clamp 3. The hammer pick is replaced by a cylindrical rod—“holder” 4, whose spherical end, by means of a rubber ring (or pneumatic clamp) 5, is pressed tightly against a specimen of annealed steel 6. The specimen is moved in guides built into the massive cast-iron base 11 by means of handle 8, gear wheel 9, and rack 10.

Fig. 1

Fig. 2

When the hammer is operating, the striker strikes rod 4, which deforms specimen 6, leaving spherical impressions on it. The data for converting the diameters of the impressions into kilogram-meters of impact energy are obtained in the form of a calibration graph of the dependence of impression diameter on the height of fall of a weight on the drop tester (Fig. 2). For this purpose, a weight is dropped onto rod 4,

equal to the weight of the striker from various heights; four vertically tensioned strings 12 serve as guides for the striker. Since both testing and calibration are carried out under the same conditions, there is no need to take into account the rebound height of the striker, or the nature and conditions of deformation of the specimen. The diameter of the impressions serves only as a standard.

However, in this method, as in dynamometers, the hammer operates with a special steel tool. As our investigations have shown, changing the deformable material or the tool in some hammers leads to a substantial change in the machine’s operating regime, the impact energy, and the number of blows. Thus, for example, an electro-pneumatic hammer whose parameters ensured stable operation and considerable impact energy when working on steel, operated unstably when breaking rock and did not develop the required impact energy. Therefore this method can be used as an approximate one.

Conversely, a hammer that provided high productivity in breaking rock gave mediocre results when working on steel.

Therefore the most important part of investigating impact machines for the mining industry is not only determining the impact energy, but also identifying the nature of the machine process under working conditions close to production conditions. On the basis of an analysis of the data obtained in this way, it is possible to assess the machine as a whole more correctly, and to adjust the regulating mechanism, on which the stability of the working process and the productivity and economy of the machine depend.

At the Western Branch of the Academy of Sciences of the USSR, a method was developed for testing pneumatic jackhammers, based on Sudnishnikov’s theorem “On the displacement of a mass during the action of a force” [4]. The essence of the method is that, in a pneumatic hammer, the change in pressure on both sides of the striker and the magnitude of the striker displacement are measured, and then the character of the striker motion and its velocity before impact are determined by a graphical-analytical method.

This method is quite simple and accurate, which made it possible for the workers of the Western Branch of the Academy of Sciences and the TEMZ to investigate the operation of jackhammers under conditions closest to service conditions. As a result of this work, the necessary conclusions were drawn for improving the designs of jackhammers for the coal industry. However, as the author himself notes, this method can be applied only when the friction of the striker against the cylinder walls is so small that its influence does not exceed the errors of calculation and experiment; that is, for machines with a free striker when tested in the vertical position.

In drilling hammers, where the resistance to motion of the striker is very large, this method cannot give reliable results. In the investigation of drilling hammers, mechanical and mechano-optical methods are currently used; in these methods the displacements are recorded by means of levers and spokes rigidly connected with the striker or piston. This very simple method for investigating slow-running machines—for example, forging hammers—requires exceptional care in setting up the experiment when investigating high-speed impact machines, since the large inertia forces acting at the moment of impact often lead to destruction of the recording devices, while vibrations of the actuating levers reduce the accuracy of the record. The problem becomes still more complicated if it is necessary simultaneously to record the travel of the striker, the piston, and the body of the machine directly during drilling.

This is the problem we encountered when carrying out an investigation of an electro-pneumatic rock drill.

After rather lengthy investigations, a fairly simple optical method was developed and mastered for simultaneously recording, during investigation, the travel of the striker, the piston, and the displacement of the machine body.

The method developed is based on transillumination of the working cylinder of the machine and simultaneous recording of the displacement of the striker, the piston, and the machine body on photographic film or paper.

Here, Fig. 3 shows a diagram for recording the travel of the striker, piston, and machine body on a stand for testing an electric perforator (hammer) with an air connection between the striker and the piston. For transillumination, through holes 2 are drilled in the walls of cylinder 1 of the machine over a length equal to the travel of all moving parts. The number and diameter of the holes are determined by the required accuracy of the experiment. Threaded plugs made of transparent plexiglass are screwed into the holes. The use, for transillumination, of holes with screwed-in plexiglass plugs, rather than a continuous slot, eliminates the possibility of large air leaks. On one side, reflector 4 with electric lamps 5 installed along the cylinder is attached to the cylinder. On the other side of the cylinder, the photographic device 6 is installed.

Fig. 3

The device may be installed fixed in place or may move together with the machine. In the first case, the travel of the striker, the piston, and the displacement of the machine body are recorded simultaneously; in the second, only the travel of the striker and piston is recorded.

The photographic device 6 consists of lens 7, shutter 8, and drum 9, on which the photographic film is fastened. For filming, an ordinary motion-picture camera may also be used.

The drive of the drum with the film may be effected either through a reducer from a synchronous electric motor for recording with respect to time, or through flexible shaft 10 from crank 11 of the machine for recording with respect to the angle of rotation of the crank in electromechanical machines. To record the travel of striker 12 and piston 13, lamps 4 are switched on, sending light through plexiglass inserts 2 into lens 7 onto the photographic film moving perpendicular to the axis of the cylinder. As they move, the striker and piston cover and uncover the holes; as a result, discontinuous lines are exposed on the film, characterizing the motion of the striker and piston. The image obtained on the film (Fig. 4) may be enlarged to the required dimensions. By processing the records, one can determine the nature of the process in the machine, the velocity and acceleration of the moving parts at any point, and, as a special case, the energy of the striker before impact.

Of course, this method too is not without drawbacks: the integrity of the machine cylinder is compromised, and careful preparation and execution of the experiment are required. At the same time, however, we obtain a picture closer to reality of the course of the working process in the machine directly

Fig. 4

during drilling. The method described can be used for research and factory testing of electric-pneumatic impact machines, and of pneumatic breakers and drilling hammers in the coal-mining, mining-ore, and construction industries.

REFERENCES

1. Ternigorev A. M., Demidov P. M., Protodyakonov M. M. Mining machines for extracting bedded minerals. Ugletekhizdat, Moscow, 1949.

2. Davidenkov N. N. Dynamic testing of metal. Gosizdat, Moscow–Leningrad, 1929.

3. Control of machines and instruments for mechanical testing of metals (collection of instructions and guidelines). Metallurgizdat, Moscow, 1949.

4. Proceedings of the Mining-Geological Institute, West Siberian Branch of the Academy of Sciences, No. 8, Novosibirsk, 1950.

5. Dobroborsky S. I. Pneumatic drilling hammer. OM—506, Leningrad, Ugletekhizdat, 1950.

6. Buchnev V. K. Drilling-and-blasting operations. Ugletekhizdat, 1950.