AUTOMATIC RAILWAY HAMMER
The oldest and most reliable railroad tool is an ordinary hammer on a long handle, which is taped by rails or wheels since the invention of the locomotive. The presence of defects is determined by hearing a characteristic rattling, which should catch the delicate hearing of the inspector. You need to know where to hit, how to bang, and what sound should be if the product is good. Among the many advantages of this method, there is one very significant drawback. The effectiveness of this method largely depends on the professional experience of man and his subjective parameters. Attempts to automate the process of searching for defects led to the emergence of a theory of oscillations, which can be the basis for the development of an electronic hammer, and a method for measuring impact parameters.
The method is based on the measurement of the frequencies of the natural oscillations of the wheels, and the construction of their spectrum. The vibrations are excited by the impact of the hammer on the wheel. In this case, the impact force is dosed by a special electromechanical device, which is available in the hammer design. The hammer developed at our enterprise has the design shown in the photo.
The problem with this method is that there is no single criterion for estimating the oscillations that are the same for different wheels. Therefore, standard tests are required to develop digital parameters for assessing the technical condition of each wheel type. Calibration can be carried out on bench (destructive) tests, when the values obtained for different degrees of wear or the presence of various defects are analyzed. In this case, the factor of increasing the time interval between the moment of excitation of oscillations (at the moment of impact) and the instant of appearance of the formed natural oscillations are taken into account. Sound passes through the thickness of the material at a certain rate, which depends on the presence of defects or physical properties of the structure. It has been experimentally established that the presence of cracks significantly slows down the speed of sound transmission. This process by its speed and by its nature is comparable with the process of passing shock waves in a solid. It is known that a shock wave in a solid consists of at least two waves. The first wave is elastic, has a high propagation velocity, while the energy loss during its passage is insignificant. The second wave is plastic, which occurs even when the impact is not very strong. Its velocity is less than the elastic wave and, when it propagates, a significant fraction of the impact energy passes into heat.
The process of energy transfer to heat requires a certain time delay. This delay can be used as a criterion for estimating the physical properties of the material under study. In the presence of a crack, a similar phenomenon is observed. Near the crack tip, even with a small amplitude of transmitted waves, there always arises a zone of plastic deformation, which consumes much more energy than elastic deformation. Accordingly, the amount of energy transferred to heat increases, which also causes a certain delay in the passage of the wave. As is known, every body in the process of deformation tends to have a minimum of internal energy. If the presence of a crack during deformation causes the walls of this crack to rub against each other, this leads to a significant increase in internal energy. Elastic deformation occurs as a result of a hammer blow on the wheel and the subsequent propagation of elastic waves from the point of impact. At the first moment, a wide spectrum of various waves propagates with a finite velocity, which is sometimes called a perturbation. The main mass of waves quickly damps, only those harmonics remain that fit in the geometric parameters of the wheel and form “standing waves”. These waves, that is, the natural oscillations, concentrate in themselves the bulk of the impact energy. They tend to translate this energy into heat. Therefore, the energy that the body receives upon impact is divided into those forms of natural oscillations that do not create shear processes in the cracks. That is, the cracks dampen certain forms of natural oscillations, and they are not excited, then one can observe a change in the propagation velocity of the waves, and, accordingly, a change in the vibration spectrum.
Summarizing the above, in a more simple language, it can be said that the sound of hitting a hammer along a rail is analogous to the sound of a tuning fork tuned to a certain frequency. If there is a crack in the tuning fork, or a part of the tuning fork is filed by the file, modeling the mechanical wear, then the tonality of the sound during the impact will be different. The more changes (defects or wear), the more the frequency of sound shifts. A numerical evaluation of the change in the physical properties of the object under study can be carried out by the method of spectral analysis of acoustic oscillations (sound). However, it should be noted here that for the measurement of the technical condition of rails, wheels and other objects, the spectrum is not suitable, since in the conditions of operation of measurement, not a highly qualified scientist but a normal surveyor exercises. Therefore, the results of measurements should be presented in the most simple and understandable form. At the same time, the accuracy of the measurements must be quite high. These problems were solved in the design of the electronic hammer, which is included in the set of the universal measuring system “Expert”, developed at our enterprise.