Improving the quality of piston-type hydraulic machines

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One of the most common types of hydraulic machines in practice are piston pumps. Typical representatives of this class of machines are drilling pumps of the UNBT type. The service purpose of this equipment is to ensure the circulation of technical fluids during well development.

The main unit that ensures the machine performs its service purpose is the contacting pair piston-cylinder liner. In works [1-3], an analysis of the technical causes of failure of UNBT pumps was performed, and a risk assessment with decomposition by units was carried out. The results of these studies show that the cylinder liner is the biggest problem in terms of ensuring the quality of pump operation.

The main reason for the failure of the cylinder liner is wear. The drilling fluid contains particles with high abrasive capacity, which, combined with a fairly high speed of movement of the moving parts of the machine, leads to a low service life of the parts: 100-200 hours.

Leading equipment manufacturers recommend using a composite bushing design (see Figure 1) [4, 5].

1 – Cylinder bushing; 2 – Rod; 3 – Protective casing; 4 – Fastening nut;

5 – Seal; 6 – Core

Figure 1 – Unit design

The bushing (pos. 1) in Figure 1 is assembled from two parts: the outer part (housing), which is made of steel, and the inner working part (sleeve), which is made of wear-resistant material. Wear-resistant grades of cast iron, primarily ICHKh-22 cast iron, are used as the material for manufacturing the working part in industrial practice. However, a number of disadvantages of this solution can be noted: cast iron of this type has degraded casting properties and increased cost. There is also a need to provide for a bulk hardening operation, which leads to distortion of the part.

As an alternative, it is proposed to manufacture the working part from gray cast iron. The surface of the working part of the bushing is hardened on both sides by means of chemical-thermal treatment. Hardening is performed in two stages. At the first stage, the surface of gray cast iron is saturated with alloying elements while simultaneously removing the carbon of the base. This makes it possible to further harden the parts. The second stage is surface hardening. The use of surface hardening makes it possible to reduce the deformation of the part (in comparison with the basic version).

The purpose of this study is to substantiate the possibility of improving the quality of piston-type hydraulic machines by improving the design of the cylinder unit. In order to achieve this goal, it is necessary to compare the basic version (for a bushing made of high-chromium cast iron) and the proposed version according to a number of criteria:

– wear resistance;

– tightness;

– strength of the part;

– rigidity of the part;

– manufacturing cost.

Considering the fact that the factors under consideration are of different nature, we will use qualimetric analysis to perform the comparison. We choose differential analysis as the analysis method [6].

main part

The wear rate of the working part was determined using the following calculated dependence [7]:

where σ0 is the stress value that causes the bushing material to fail in one load cycle, MPa; E is the modulus of longitudinal elasticity of the material of the working part of the bushing, MPa; ty is a parameter that takes into account the nature of the frictional fatigue curve of the material of the working part of the bushing; α is the coefficient of completeness of contact of the surfaces of the piston and the bushing; Ktv is a coefficient that shows the number of load cycles at which wear products begin to be removed from the working surface of the bushing; αG is a coefficient that takes into account hysteresis phenomena; is the value of the molecular component of the friction coefficient of the "cylinder bushing-rod" pair; τ0, β are the characteristics of the operating conditions of the "cylinder bushing-rod" pair; HB is the hardness of the parts on the Brinell scale; k is a parameter that depends on the quality of the surface layer of the parts; p – pressure in the contact zone of the pair “cylinder sleeve – rod”, MPa.

The pressure in the contact zone of the pair is determined by the formula [7]:

where Fn – the acting force in the contact zone “cylinder sleeve – rod”; kd – coefficient, which allows to take into account the dynamic nature of the acting loads; Rpr – the reduced contact radius of the pair “cylinder sleeve – rod”.

The tightness parameters were determined by taking into account the amount of leaks in the contact zone of the parts “cylinder sleeve – rod”. For the calculation, the formulas presented in the literature were used [8, 9]:

where d, l – the length and diameter of the contact zone of the pair “cylinder sleeve – rod”; u – a constant, which characterizes the pocket (pocket constant); Rz, Wz – roughness and waviness parameters of the actuator surfaces of the pair “cylinder sleeve – rod”; – a correction coefficient, which takes into account the dynamics of the transmitted medium; y – deformation value in the contact zone, taking into account the elastic component (yуп) and plastic (yпл).

The deformation value is determined taking into account the properties of both parts of the pair [8, 9]:

The calculation of the tightness parameters was performed in the MathCad software environment. An example of the calculated dependence of the leakage value in the connection on the value of the arithmetic mean deviation of the profile is shown in Figure 2.

Figure 2 – Example of calculations

The strength and rigidity of the bushings was studied using the finite element method. An example of the calculation is shown in Figure 3.

Figure 3 – Example of the result of the finite element calculations for the basic option

The cost criterion was determined by the value of the reduced costs for the manufacture of a cast iron bushing according to the basic option (using chromium cast iron and bulk hardening) and the proposed option (using gray cast iron and thermal diffusion alloying). The calculation method is given in the source [10].

Table 1 and Figure 4 present the results of calculations of the comparison coefficients by the differential method. The values ​​of the comparison coefficients were determined using the formulas [10]:

Expression (1) is used in the case of evaluation of negative indicators, i.e. the parameter has a negative effect on the service purpose of the unit and it is necessary to strive to ensure that it is less for the evaluated sample than for the base one. Expression (2) is used to calculate positive indicators that have a positive effect on the service purpose of the unit.

Table 1 - Values ​​of the comparison coefficients for the options for manufacturing a cylinder liner from chromium cast iron (base option) and hardened gray cast iron (proposed option).

Figure 4 – Diagram of the results of comparison of variants

conclusions and findings

Based on the calculation of the comparison coefficients for the basic and proposed variants of manufacturing the pump unit, the resulting value of the comparison coefficient can be determined:

Analyzing the obtained results, it can be said that according to the considered criteria in terms of fulfilling the service purpose (wear resistance and tightness of the connection), the proposed variant is not inferior to the basic one. However, in terms of the cost of production, the use of modified gray cast iron is significantly more effective.

It should be noted that the analysis of the variants was carried out without taking into account technological aspects, such as processing productivity and the percentage of defects. As a prospect for further research, these criteria should also be considered.

However, even taking into account all the shortcomings of the differential evaluation method (which assumes the equivalence of the influence of all criteria on the goal), it can be concluded that the use of modified gray cast iron is preferable to high-chromium cast iron, since according to the results of calculating the comparison coefficients, the proposed variant is better than the basic one.

About the authors

Victor Ovsjannikov

Author for correspondence.
Email: vik9800@mail.ru
ORCID iD: 0000-0002-7193-7197

Профессор кафедры технологии машиностроения

Roman Yurievich Nekrasov

Email: nekrasovrj@tyuiu.ru

Arseniy Sergeevich Gubenko

Email: gubenkoas@tyuiu.ru

References

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