RESTORATION OF THE EXTERNAL SURFACES OF MACHINE MECHANISMS BY ELECTROMECHANICAL TREATMENT

ABSTRACT

The article discusses the possibility of restoring the outer surfaces of machine mechanisms with electromechanical processing.

АННОТАЦИЯ

В статье рассматривается возможность восстановления наружных поверхностей механизмов машин электромеханической обработкой.

Keywords: outside surface, machine mechanisms, restoration, electromechanical processing.

Ключевые слова: наружная поверхность, механизмы машин, восстановление, электромеханическая обработка.

The practice of launching a series of new engineering products shows that their quality is largely ensured by their refinement after production and operational tests. This leads to significant losses of time and material resources from the initial design stage to the launch of new products in a series. Hence, the accumulated results of theoretical and experimental studies [1-10] have shown that static fatigue strength, contact stiffness, tightness, wear resistance, corrosion resistance of machine parts and their connections depend on the quality parameters of the surface layer formed during machining, the physical and mechanical properties of the material parts, resulting dimensional accuracy and operating conditions.

As a working tool in carrying out these experiments, tools were used - rollers made of bronze of the BRKh0.7 brand and a pseudo-alloy of tungsten carbide with copper.

The experiments were carried out using ethyl silicate (liquid glass), zapon varnish, and graphite lubricant. The additional material (powder) was mixed with the binder in a ratio of 1:1 and applied in a uniform layer on the surface to be restored.

After surfacing of 3 layers, the restored layer was strengthened by passing the tool in the same modes with the supply of LC by irrigation into the treatment zone. The values of the levels of input factors are presented in Table. 1.

Table 1.

The values of the levels of input factors for the experiment 2³

The experiment was carried out on samples with three repetitions in each experiment. The experiment planning matrix and the average values of the output factor Y (recovered size) are presented in table. 2.

The results of the studies have shown that electromechanical surfacing of additional material (powder) applied by coating, bronze and pseudo-alloy rollers allows you to effectively restore the size of the outer cylindrical surfaces (with subsequent finishing by diamond turning or grinding) by 0.1 - 0.3 mm.

With this recovery method, the magnitude of the current strength has the main influence on the magnitude of the restored size. This factor will affect the quality of the adhesion of powder particles to each other and to the base material, as well as the magnitude of the porosity of the coating.

Table 2.

Matrix of experiment on surfacing with a pseudo-alloy roller

When additional material (powder) is applied to the surface to be restored by coating, a coating with a very high porosity is obtained, and sometimes even the deposited layer peels off. This is due to the presence of the so-called "third body" - graphite lubricant, which burns out during surfacing due to high temperatures in the contact zone of the tool and a layer of additional material.

To improve the quality of the restored layer (reduce its porosity and better adhesion to the surface), it is proposed to get rid of the "third body" by applying powder by free spilling from the hopper. However, with this method of applying additional material, there is a very large overrun.

Therefore, to avoid this shortcoming, it is proposed to use an electromagnetic field to hold the powder in the surfacing zone. To conduct experiments with this method of applying additional material, a special device was developed and manufactured.

Electromechanical surfacing of powders can be carried out by preliminary application of coatings and by applying powders with an electromagnet. To obtain a more uniform layer, it is recommended to apply a thin layer of the binder on the surface to be restored, and then apply additional material by spilling it freely onto the coated surface. As the main factors influencing the value of the restored size as a result of the literature analysis, the following were taken: 1) current (I) - X1 2) tool pressing force (P) – X2 3) workpiece rotation speed (V) - X3 The values of the levels of input factors are presented in table 3.

The experiment was carried out on samples with three repetitions in each experiment. The number of deposited layers is 1. After surfacing, the restored layer was hardened by passing a tool with the same modes and with watering with LC into the treatment zone. The experiment planning matrix and the average values of the output factor Y, mm (the value of the restored size), are presented in table 4.

Table 3.

The values of the levels of input factors for the experiment 2³

Table 4.

Matrix of experiment on surfacing with an electromagnet

After the restoration operation, the surfaces of the parts are processed by finishing processing to obtain the required size. For surfaces restored by electromachining with additional metal, diamond turning, or semi-finishing and fine grinding, is recommended as a finishing treatment. The amount of allowance to be removed is 0.1-0.3 mm.

The deposited surfaces of the sample were subjected to grinding and polishing with an endless belt. Experimental studies of wear were subjected to initial samples heat-treated and non-heat-treated and restored according to the proposed technologies. The results of measurements of the friction coefficients during the wear of the samples are shown in fig. 1.

Their analysis shows that approximately 2 hours after the start of testing, the running-in process ends at the level of the microgeometry of the surface layer, i.e. the parameters of the equilibrium surface roughness are formed. As can be seen from the graphs, the coefficient of friction with the sample restored by surfacing with EMT with an electromagnet is somewhat less than that of the sample after volumetric hardening.

Figure 1. Coefficient of sliding friction for various recovery methods

This is due to the porosity of the deposited coating

These pores on the working friction surface of the sample act as oil pockets in which lubricant and wear products are retained. Sample wear curves are shown in  fig. 2.

The processing of the results made it possible to determine the amount of wear of the samples during the running-in period and the intensity of wear of the samples during the period of normal wear (table 5).

Figure 2. Graphs of wear versus time

Table 5.

Results of wear experiments

In general, the results of the studies show that electromechanical surfacing of additional material (powder) using an electromagnetic field allows you to effectively restore the size of the outer cylindrical surfaces, followed by grinding by 0.2 - 0.4 mm. An analysis of the results of comparative tests shows that the proposed restoration technologies make it possible to almost double the wear resistance of the restored part.

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