Selecting Rational Method to Restore Part Worn-Out Surfaces

Key words: part restorative maintenance, wear resistance, fatigue, adhesion factors and a durability factor, self-cost, specific costs.

In the process of the machine operation the parts of assemblies and units wear out. As the research results demonstrate, more than 65% of parts are worn-out up to 0.15 mm, 30% - 0.15 to 0.5 mm and only 5% — more than 0.5 mm. [1]

To eliminate similar part defects, there are dozens of various methods of coating application. Thus, to restore the seal assemblies of tractors and agricultural machines, one applies 22 methods, spline connections — 15, fixed connection parts — 37; parts operating under the friction conditions — 41, cranked shafts— 8, caterpillar tractor track rollers — 12. Such situation can be explained by the absence or non-conduct of preliminary tests for the proposed methods to restore the parts with a single method.

This article sets a task to define the sequence of tests and choose necessary indicators for a comparative assessment of the part worn-out surface renovative maintenance.

The problem of the justification of the restorative maintenance methods is considered by a large number of papers and recommendations [1, 2, 3, 4], however, the absence of a single generally accepted method leads to biased solution taking in a number of cases.

Taking into account the aforementioned, the author proposes the following procedure on the choice and definition of a rational restorative maintenance method for worn-out surfaces. 

1. The first stage shall consists in the acquisition of the restored part:

The name and number according to the machine manufacturing plant catalogue, applied material, superficial hardness, roughness of operating surfaces, kinds and factors of the defect re-occurrence on each surface type.

After obtaining necessary technical data one needs to conduct a logical assessment of restoration methods in terms of the intended use indicator: geometrical dimensions, superficial layer parameters, mechanical and physical and mechanical characteristics of the restored surfaces. 

2. Selected restorative maintenance methods shall be assessed according to the comparative benchmark trials. Depending on the operation conditions of an associated worn-out part, it is necessary to define the wear resistance, fatigue resistance, adhesion resistance of the applied coatings.

These mechanical characteristics are defined at the test machines with the use of the existing GOST methods.

For the tests the samples are prepared, they are made of the same materials as a new part. New parts are tested with some set frequency and obtain necessary average indicators on mechanical properties. To obtain the data on the restorative maintenance methods, one applies a restorative layer of a necessary thickness onto the samples and conducts required mechanical and heat treatment. Further, the tests of prepared samples are conducted to obtain average mechanical characteristics of the renovated surfaces. The deviation of the mechanical characteristics of restored surfaces in relation to the new sample mechanical characteristics defines wear resistance, fatigue resistance and adhesion resistance values. 

3. Wear resistance factor defining.

Mechanical properties shall be defined for the samples depending on the operation of the surface being restored. One needs to prepare flat samples for the parts operated in abrasive media (blade, arms, discs, etc.) and conduct the wearing tests with the help of friction machines feeding the abrasive type IM-01.

For the parts of a shat type operated in the oil medium on needs to prepare disc parts samples and a contact part in the form of a shoe. The research is conducted at the friction machines SMT-1 and II-5018 at rotation or at the machines where the sample moves in a reciprocal way.

The wear resistance factor is defined by the formula 

,

where И_св, И_сн — average wear resistance of each restored and new surface.

4. Defining the fatigue factor.

An important indicator for the parts exposed to reversed loads is the fatigue threshold defining the fatigue factor. This indicator is generally defined for cylindrical parts at their test at fatigue-testing machines of the type MUI-6000 or UK -10 m. Depending on the existing operating loads on the part, one chooses the load configuration or symmetric bending, or cantilevered bending or twisting.

At the fatigue resistance test it is reasonable to use accelerated test methods, such as LOKATI, to reduce the test time. This method has been tried out at the test of full-scale specimen by the example of crank shafts; it provided rather accurate results of such accelerated tests.

The fatigue is defined by the formula

,

where σ-1в, σ-1н is ab average value of the fatigue limit for each restored and new surface, correspondingly.

5. Defining the durability factor for coating adhesion.

For the gas and thermal, polymeric and galvanic methods of coating application the important indicator is the adhesion durability factor defined for the adhesion between a main part and the coating and the cohesion between the layers. These characteristics are determined by the pulling and shear tests with the use of a “pivot” method. The nature of the method consists in a pull of a pivot included in the matrix with the applied coating at tensile testing machines. The details of the adhesion test method are provided in the paper of Mikhalchenkov A. N. [6]

The adhesion durability factor is defined by the formula

,

where q_в is an average value for the coating adhesion durability for each restored part according to the test results.

The following values are recommended to be taken as reference values depending on the part surface operation conditions:

— for external steel surfaces bearing reversal loads — 500 MPa;

— for external steel or cast-iron surfaces not bearing reversal loads — 200 MPa;

— for internal bearing seating surfaces not bearing reversal loads — 50 MPa;

— for external or internal steel and cast-iron surfaces not bearing significant reversal loads and operated under flood lubrication conditions. — 40 MPa.

The analysis of literature references, results of the research on various worn-out surface restorative maintenance methods allowed obtaining approximate factor values [5]

Table 1. Approximate values of wear resistance, durability, cohesion factors 

After the definition of mechanical characteristics for each considered method and for each surface one shall define a durability factor by means of the following formula:

Кq = К_u х К_у х K_сц

The method with the highest durability factor value is selected for application.

6. Technical and economic criterion. 

After the selection of a restorative maintenance method by means of a durability factor it is necessary to define a technical&economic criterion for the considered method. It can be defined with the help of the following equation:

,

where К_э — technical and economic criterion;

С_вi — self-cost of the restorative maintenance of the i surface with the considered method;

К_q — restored surface durability factor.

One of the key economic indicators characterizing the technological process sophistication is the restorative maintenance self-cost.

In general terms, the part restorative maintenance self-cost (rubles) at a specific plant is defined using the formula:

С_в = С_из + З_пл + С_об + Н_u + Н_з+ С_бр + С_м,

were С_из and С_м — costs of the worn-out part and materials, correspondingly;

З_пл — salary; С_об — expenses for equipment maintenance and operation; Н_u and Н_з — workshop (general production) and general plant (general economic) expenses, accordingly; С_бр — waste losses.

At the aggregated estimations of the self-cost to define an economic viability of such restorative maintenance, one can apply the following expression:

,

where d — number of the restored part surfaces; С_уд — cost of the restoration of a square unit (for example, length in case of cracks) for the i — surface by a selected method, rub./dm² (rub/dm); Si — square (length) of the i — surface, dm² (dm); K_{пд i} — defect reoccurrence factor for the i — surface; D_п — factor taking into account the expenses for the preliminary works at part restoration (at the part restoration for auxiliaries D_п = 1,03 at the centralized restoration D_п = 1.1); Ц_н — new part cost, rub.

The given work stages to select a rational method allowed obtaining unbiased data on the efficiency of the part worn-out surfaces restoration.

Lyalyakin V.P., senior researcher
FBGNU GOSNITI, Dr. Tech. Sci., professor

References

1. Kakuyevitskiy V.A. Restorative Maintenance of Bus Parts at Specialized Plants. — M. : Transport, 1982. — 147 p.
2. Chernoyivanov V.I., Lyalyakin V.P. Organization and Technology for Machine Part Restorative Maintenance. — M. 2003. — 488 p.
3. Procedure for Technical and Economic Justification of Machine Part Restorative Maintenance Methods.- М.: GOSNITI, 1988. — 30 p.
4. Shadrichev V.A. Basics of Selection of Rational Method for Car Part Restorative Maintenance with Metal Coatings. — M. — Leningrad: Mashgiz, — 1963. — 295 p.
5. Machine Repair Technology / edited by Prof. Ye.A.Puchin. — M.: Kolos, 2007. — 487 p.
6. Mikhalchenkov A.M., Biryulina Ya.Yu., Famin Yu.I., Yermakova T.A. Critical Examination of Methods for Testing Coating Adhesion Durability by Pulling Method. // GOSNITI Writings, v.121,2015, С 260–265.