**1.2. Test results and discussion**

28 Performance Evaluation of Bearings

replicates at each point.

system.

**1.1. The metods of investigations** 

apparatus and the structure of the friction node.

*1.1.1. Authors' own methodology (MB –1)* 

experimentally, and this is due to the construction SMT1.

**Figure 1.** SMT-1 friction tester a) general view, b) friction node

recommended by the manufacturer (FŁT PLC in Kraśnik - Poland). Should add that all of the test, later in this chapter bearings are factory items and their designations are in accordance with PN, PN-EN and ISO standards. All tests were performed for three

Tribological tests were conducted with a widely-used friction tester SMT-1. Fig. 1 shows the

Laboratory tests were conducted in two stages. In the first stage tribological tests were conducted according to the authors' own methododology. Next the tests were conducted according to the method applied in FŁT in Kraśnik (Poland). These tests were meant to compare the two methods and evaluate their influence on test results. It should be noted that the load is strictly radial, without a longitudinal component which has been verified

In this section the following test parameters were assumed: constant normal pressure N = 350 [kG] ≈ 3433 [N], changeable rotational speed of the bearing : n1= 300 [rpm], n2= 600 [rpm], n3= 1200 [rpm]. Because of changeable rotational speed it was assumed that the number of bearing's cycles will serve as the unit of its operation. Since the tests were treated as pilot ones, at this stage tests were conducted at 20,000 cycles of bearing's operation. It should be noted that before each measurement the bearing was cooled in a stream of air for 4 hours in normal conditions. All the measurements were recorded using a computer Due to the extensive research material, the present section does not contain the results of initial tests obtained with MB-1 and MB-2 methods, it only includes their description.

As far as MB-1 method is concerned, it should be noted that its results indicate diversification of tribological characteristics of the examined elements in relation to rotational speed. There is also a certain analogy in the course of characteristics for the two groups of the evaluated bearings. In the initial stage of the process the maximum value of friction moment was obtained (for all rotational speeds). In the subsequent stages it decreases and, in the final stages, the resistance of motion becomes stable. One should note that at the beginning of the process the highest value of the moment of friction was obtained at the speed of n = 1200 [rpm] while the lowest at n = 300 [rpm]. In the final stage of the process the trend is reversed.

In the case of MB-2 method general diversification of the moment of friction was also noticed. It is logical considering different conditions of the external function. However the lowest value of the moment of friction was recorded for the lowest rotational speed n = 200 [rpm], while the highest for n = 1000 [rpm]. Further measurements, at higher rotational speeds, indicate the decrease of the value of the moment of friction. However, the most controversial conclusions are drawn from the comparative analysis of the two research methods.

Fig. 2 and 3 shows selected comparative graphs of the courses and values of the moment of friction obtained in tests according to MB-1 and MB-2 methods described above.

The graphs above provide data concerning completely divergent results in the conducted tests. The analysis of the data in fig. 2. (MB-1 method) proves that both at the speed of n = 300 [rpm], and especially n = 1200, throughout the whole of the research cycle, one notices distinctly higher values of the moment of friction for the bearings from the 134-781TNG-2RS group. At the same time there are lower values of resistance to motion for the bearings from the CBK 441TNG group. Since these bearings have different dimensions of working elements one could assume that different values of the moment of friction is a natural phenomenon. When tests were carried out according to the other method similar trends in friction characteristics were expected. However, when the results obtained with MB-2 were analysed, quite contrary to expectations, they were in direct opposition to those obtained with MB-1 method.

Performance Evaluation of Rolling Element Bearings Based on Tribological Behaviour 31

**Figure 4.** Comparison of two research methods MB-1 and MB-2 (bearings) a) 134-781TNG-2RS b)

reflect the real process of technical objects exploitation closely enough.

This comparison is shown in fig.4. Fig. 4a refers to the test results for the bearings from the 134-781TNG-2RS group, fig. 4b is related to the bearings from the CBK 441TNG group. No doubt, the results for the two groups of bearings are completely different. It should be noted that, in the 134-781TNG-2RS group, the differences in the recorded values of the moment of friction range from several to twenty-odd per cent, while in the CBK 441TNG group the differences are even two-fold. Moreover, one should note that in the case of bearings from the 134-781TNG-2RS group at the speed of n = 600 [rpm] slightly higher values of the moment of friction were registered with MB-1 than with MB-2 method. In turn, at the speed of n = 1200 [rpm] the situation is reversed. That means that when tribological tests are carried out, especially on such elements, one should bear in mind the significance of the selected research method. Choosing an arbitrary method may result in, as the evidence discussed in the present section suggests, obtaining entirely different or even unexpected test results. In consequence it may lead to wrong conclusions related to the tribological properties of such elements. At the same time one should remember it is often a complex task to select the right method that can

CBK 441TNG

**Figure 2.** The course of the moment of friction in the function of time for two groups of bearings at different rotational speeds (MB1 method): a ) 300 [rpm], b) 1200 [rpm]

**Figure 3.** The values of the moment of friction in the function of rotational speed – MB-2 method

**Figure 2.** The course of the moment of friction in the function of time for two groups of bearings at

**Figure 3.** The values of the moment of friction in the function of rotational speed – MB-2 method

different rotational speeds (MB1 method): a ) 300 [rpm], b) 1200 [rpm]

**Figure 4.** Comparison of two research methods MB-1 and MB-2 (bearings) a) 134-781TNG-2RS b) CBK 441TNG

This comparison is shown in fig.4. Fig. 4a refers to the test results for the bearings from the 134-781TNG-2RS group, fig. 4b is related to the bearings from the CBK 441TNG group. No doubt, the results for the two groups of bearings are completely different. It should be noted that, in the 134-781TNG-2RS group, the differences in the recorded values of the moment of friction range from several to twenty-odd per cent, while in the CBK 441TNG group the differences are even two-fold. Moreover, one should note that in the case of bearings from the 134-781TNG-2RS group at the speed of n = 600 [rpm] slightly higher values of the moment of friction were registered with MB-1 than with MB-2 method. In turn, at the speed of n = 1200 [rpm] the situation is reversed. That means that when tribological tests are carried out, especially on such elements, one should bear in mind the significance of the selected research method. Choosing an arbitrary method may result in, as the evidence discussed in the present section suggests, obtaining entirely different or even unexpected test results. In consequence it may lead to wrong conclusions related to the tribological properties of such elements. At the same time one should remember it is often a complex task to select the right method that can reflect the real process of technical objects exploitation closely enough.

## **2. Selected energetic aspects of needle bearings work performance**

Shafts on needle bearing can work at heavy load accurately lateral. Durability of such a bearing is guaranteed by suitable selection materials which are made (roughness, microhardness, etc.) and optimum lubrication conditions. The factors specified above have influence on reliable work and life of needle bearings.

Wear equation presented in energetic form describes real processes occurring under various external influences on materials which lead to their damage and create wear products. The initial form of energetic approach to wear processes is as follows:

$$E\_{y\circ} = A\_{\text{un}} \not\!\!/ V \tag{1}$$

Performance Evaluation of Rolling Element Bearings Based on Tribological Behaviour 33

(6)

should consider the properties of materials

(7)

depends on the character of external load and assumes the

, irreversibly absorbed by the material at the instant of creating

ϑi - the average value of velocity of impact load, ϑkr - critical velocity of impact which causes damage of wear material , Aiϑi – the flux of energy which enters a material in the course of separate impact, *<sup>y</sup> E* <sup>∂</sup> ϑkr – critical density of strain energy – critical density of strain

, we obtain:

( ) \* *i kr <sup>w</sup> V ft W*<sup>=</sup>

The numerator and denominator of this equation are reduced after the singularity of a given type of wear is taken into account. The flux of external energy *w* should be expressed by the friction force in external friction. Fig. 5 shows the model of distribution of the energy of

(mechanical, physical, chemical and others), causing damage in given conditions of external

By presenting ϑkr as the sum of components of elastic and plastic deformation, ϑkrspr ϑkrpl

( ) \*\* \* 1 1

wear products, can be determined according to the durability or hardness graph as an area below the curve σ(ei) or H (ei), restricted on the right side by the damage deformation ep determined in given wear conditions with the method of micro-hardness directly on the

 ϑϑ

3 3 *spr pl WE E kr y kr y kr kr* == + ∂ ∂ ϑ

**Figure 5.** The model of the energy flux distribution in the deformed volume

averaged values in the worn out volume (\* the symbol of averaging).

The criterion of resistance to wear Wkr\*

influences. The value Wkr\*

respectively, we shall obtain:

The specific strain energy Eya\*

worn out surface of a material.

ω

where:

power.

plastic deformation.

Assuming Aiϑi = w and *<sup>y</sup> E* <sup>∂</sup> ϑkr = Wkr\*

where:

*<sup>y</sup> E* <sup>∂</sup> - specific energy of wear material of volume V when wear products are created, Awn - the work of external forces affecting the volume in time t or at N cycles of load.

It is assumed that:

$$A\_{\rm env} = A\_i N = A\_i \alpha t \tag{2}$$

where: Ai – external work under single load of volume V, ω - frequency of load.

Equations (1) and (2) result in wear equation in the following form:

$$V = \frac{A\_{\parallel}}{E\_{y\bullet}} \alpha t$$

Equation (3) describes the wear when wear products are produced at a uniform rate i.e. linear wear.

For non-linear wear:

$$V = \frac{A\_{\parallel}}{E\_{y\ominus}} f\left(\alpha t\right) \tag{4}$$

where: the function f (ω t) may be presented (apart from the linear one) as power, exponential, logarithmic and as other relations.

In dynamic character of load (impact, hydro-abrasive, cavitation wear) the equation (4) should consider the velocity of impact:

$$V = \frac{A\_i \vartheta\_i}{E\_{y^\flat} \vartheta\_{kr}} f\left(\alpha t\right) \tag{5}$$

where:

32 Performance Evaluation of Bearings

where:

It is assumed that:

linear wear.

For non-linear wear:

exponential, logarithmic and as other relations.

should consider the velocity of impact:

**2. Selected energetic aspects of needle bearings work performance** 

influence on reliable work and life of needle bearings.

initial form of energetic approach to wear processes is as follows:

work of external forces affecting the volume in time t or at N cycles of load.

where: Ai – external work under single load of volume V, ω - frequency of load.

Equations (1) and (2) result in wear equation in the following form:

Shafts on needle bearing can work at heavy load accurately lateral. Durability of such a bearing is guaranteed by suitable selection materials which are made (roughness, microhardness, etc.) and optimum lubrication conditions. The factors specified above have

Wear equation presented in energetic form describes real processes occurring under various external influences on materials which lead to their damage and create wear products. The

*<sup>y</sup> E* <sup>∂</sup> - specific energy of wear material of volume V when wear products are created, Awn - the

*Awn i i* = = *AN A t*

*i y <sup>A</sup> V t E* ω

∂

( ) *<sup>i</sup> y <sup>A</sup> V ft <sup>E</sup>*

∂

where: the function f (ω t) may be presented (apart from the linear one) as power,

In dynamic character of load (impact, hydro-abrasive, cavitation wear) the equation (4)

∂ ϑ

( ) *i i y kr <sup>A</sup> V ft <sup>E</sup>* ϑ

ω

ω

Equation (3) describes the wear when wear products are produced at a uniform rate i.e.

ω

*E AV y wn* <sup>∂</sup> = (1)

(2)

= (3)

= (4)

<sup>=</sup> (5)

ϑi - the average value of velocity of impact load, ϑkr - critical velocity of impact which causes damage of wear material , Aiϑi – the flux of energy which enters a material in the course of separate impact, *<sup>y</sup> E* <sup>∂</sup> ϑkr – critical density of strain energy – critical density of strain power.

Assuming Aiϑi = w and *<sup>y</sup> E* <sup>∂</sup> ϑkr = Wkr\* , we obtain:

$$V = \frac{w\_i}{W\_{kr}^\*} f\left(\alpha t\right) \tag{6}$$

The numerator and denominator of this equation are reduced after the singularity of a given type of wear is taken into account. The flux of external energy *w* should be expressed by the friction force in external friction. Fig. 5 shows the model of distribution of the energy of plastic deformation.

**Figure 5.** The model of the energy flux distribution in the deformed volume

The criterion of resistance to wear Wkr\* should consider the properties of materials (mechanical, physical, chemical and others), causing damage in given conditions of external influences. The value Wkr\* depends on the character of external load and assumes the averaged values in the worn out volume (\* the symbol of averaging).

By presenting ϑkr as the sum of components of elastic and plastic deformation, ϑkrspr ϑkrpl respectively, we shall obtain:

$$\mathcal{W}\_{kr}^{\*} = \frac{1}{3} \boldsymbol{E}\_{y\boldsymbol{\vartheta}}^{\*} \boldsymbol{\vartheta}\_{kr} = \frac{1}{3} \boldsymbol{E}\_{y\boldsymbol{\vartheta}}^{\*} \left(\boldsymbol{\vartheta}\_{kr}^{\circ \boldsymbol{\vartheta} \prime} + \boldsymbol{\vartheta}\_{kr}^{\circ \boldsymbol{\vartheta}}\right) \tag{7}$$

The specific strain energy Eya\* , irreversibly absorbed by the material at the instant of creating wear products, can be determined according to the durability or hardness graph as an area below the curve σ(ei) or H (ei), restricted on the right side by the damage deformation ep determined in given wear conditions with the method of micro-hardness directly on the worn out surface of a material.

The elastic component ϑkrspr in the equation (7) is defined with the velocity of the elastic wave in deformation c0 and elastic deformation espr:

$$\mathcal{O}\_{\rm lr}^{\rm spr} = \mathbf{c}\_0 \mathbf{c}\_{\rm spr} = \sqrt{\frac{E}{\rho\_M} \mathbf{c}\_{\rm spr}^2} \tag{8}$$

Performance Evaluation of Rolling Element Bearings Based on Tribological Behaviour 35

*kr <sup>b</sup>* ≈ ≈ (12)

criterion.

criterion will assume very simple forms. For instance it can

(13)

and its components Ekr and

D

d1

d2

l2 17 15

l1=36

14 16

 σ

*pl kr*

 ρ

*M M E E*

\*33 *W const const kr* ϑ

The particular dependencies (12) are of significant practical interest with a view of predicting the use of materials and coating. By substituting equation (10) for (7) we shall

\* \* 1 2 2

ρ

Epl can be determined according to a microhardness (endurance) graph. If the graphs of hardness or endurance cannot determined, then the evaluation of resistance of materials to

Still, it is observed that when the stiff impact stress is reduced and when external influence

be expressed in reduction of indicator in the power at ϑkr and σb or hardness (12), [5, 6, 8].

Fig. 6 shows the scheme of the tribotester for the estimation of needle bearing. In this case,

12 6

1

**Figure 6.** The scheme of the tribotester: 1- frame, 2- cover, 3- investigated shaft, 4- reducer sleeve, 5 cylinder, 6- toothed gear, 7- transmission belt, 8- electric motor SZJe 3,5 [kW], 9- wedges, 10- oil, 11 needle bearing K28x33x13, 12- needle bearing K40x45x17, 13- screw gear, 14- thermoelement NiCrNi,

Parameters of investigation:surface stresses, p = 950 [MPa], rotational speed, n = 1500 [rpm].

φ

Fig. 7 shows the influence of energetic criterion on the wear of a shaft neck in the function of

7 8

2,5 [mm], l2= 13 [mm], 18- bearing head

<sup>∂</sup>

= +

3

*kr y*

*W E*

In this equation, the total energy-consumption of a material Eya\*

wear at external impact is possible as a result of analogy to Wkr\*

used the bath lubrication. Bearing designation are given in Fig. 6.

4 18

resemble static conditions, Wkr\*

**2.1. The method of investigation** 

2 3

10 9 5

15- shaft neck d= 28 [mm], 16- hub, 17- needle roller

change of material and lubricating parameters.

**2.2. The results of the investigations** 

obtain:

11 13

where:

E – the module of elasticity, ρM - density of a material being used. Plastic component ϑkrpl in (7) depends on the velocity of plastic deformation wave cpl and total plastic deformation accumulated in the course of wear:

$$\sigma \partial\_{kr}^{pl} = \int\_{\epsilon\_{qr}}^{\epsilon\_p} c\_{pl} de = \int\_{\epsilon\_{qr}}^{\epsilon\_p} \left(\frac{d\sigma/de}{\rho\_M}\right)^{0.5} de\tag{9}$$

where: dσ/de - local slope of tangent to a curve of stretching determined with stress – strain coordinates (σ - ε).

By expressing the right-hand side of equations (8) and (9) as the energy of brittle Ekr and plastic Epl damage we shall obtain:

$$
\partial\_{\nu} = \sqrt{\frac{2E\_{\nu}}{\rho\_{M}}} + \sqrt{\frac{2E\_{pl}}{\rho\_{M}}} \tag{10}
$$

where: Ekr = <sup>2</sup> 2 *kr b E E* =σ , ( )<sup>2</sup> 2 *E D pl b T* = − σ σ , σ *<sup>b</sup>* , σ *<sup>T</sup>* - strength of a material and the limit of plasticity, D – module of consolidation (tg of inclination angle of tangent to a curve σ(e) within the range of plastic deformations, with linear character of deformation consolidation of a material. The critical velocity of impact, connected with singularities of spreading in materials undergoing wear, elastic and plastic deformation waves, features resistance to material wear at dynamic, high-velocity stress and it can be expressed by durability characteristics of worn-out volumes, as shown in equations (8) – (10).

Test data analyses proves that in conditions of stretching at impact (impact on the front surface of a rod) there exists an involution dependence (between ϑ*kr* i σ*<sup>b</sup>* ):

$$
\sigma \vartheta\_{kr} = \text{const} \sigma\_b^u \tag{11}
$$

where: n = 2,5 for steel with 500 σ*<sup>b</sup>* ≤ [MPa]

const – a constant determined empirically, considering the specific test conditions or the proportional coefficient between the discussed characteristics.

Taking into account (11) the criterion of resistance to wear for iron-based alloys with 500 σ*<sup>b</sup>* ≤ [MPa], it will prove that ϑkr and σb cubed are proportional:

Performance Evaluation of Rolling Element Bearings Based on Tribological Behaviour 35

$$\boldsymbol{W}\_{\boldsymbol{k}r}^{\*} = \boldsymbol{\hspace{0.5cm}} \boldsymbol{\sigma}\_{\boldsymbol{k}r}^{3} = \boldsymbol{\hspace{0.5cm}} \boldsymbol{\sigma}\_{\boldsymbol{b}}^{3} \tag{12}$$

The particular dependencies (12) are of significant practical interest with a view of predicting the use of materials and coating. By substituting equation (10) for (7) we shall obtain:

$$\mathcal{W}\_{lr}^{\*} = \frac{1}{3} E\_{\psi\phi}^{\*} \left( \sqrt{\frac{2E\_{kr}}{\rho\_M}} + \sqrt{\frac{2E\_{pl}}{\rho\_M}} \right) \tag{13}$$

In this equation, the total energy-consumption of a material Eya\* and its components Ekr and Epl can be determined according to a microhardness (endurance) graph. If the graphs of hardness or endurance cannot determined, then the evaluation of resistance of materials to wear at external impact is possible as a result of analogy to Wkr\* criterion.

Still, it is observed that when the stiff impact stress is reduced and when external influence resemble static conditions, Wkr\* criterion will assume very simple forms. For instance it can be expressed in reduction of indicator in the power at ϑkr and σb or hardness (12), [5, 6, 8].

### **2.1. The method of investigation**

34 Performance Evaluation of Bearings

accumulated in the course of wear:

plastic Epl damage we shall obtain:

σ

where: n = 2,5 for steel with 500

500

σ

where:

coordinates (σ - ε).

where: Ekr = <sup>2</sup> 2 *kr b E E* =

wave in deformation c0 and elastic deformation espr:

The elastic component ϑkrspr in the equation (7) is defined with the velocity of the elastic

*kr spr spr*

E – the module of elasticity, ρM - density of a material being used. Plastic component ϑkrpl in (7) depends on the velocity of plastic deformation wave cpl and total plastic deformation

0.5 *p p*

*d de c de de* σ

ρ

*ce e*

0

*spr spr*

*kr*

ϑ

2 *E D pl b T* = − σ σ,

characteristics of worn-out volumes, as shown in equations (8) – (10).

*<sup>b</sup>* ≤ [MPa]

*<sup>b</sup>* ≤ [MPa], it will prove that ϑkr and σb cubed are proportional:

surface of a rod) there exists an involution dependence (between

σ

proportional coefficient between the discussed characteristics.

*e eM*

where: dσ/de - local slope of tangent to a curve of stretching determined with stress – strain

By expressing the right-hand side of equations (8) and (9) as the energy of brittle Ekr and

ρ

*M M E E*

> σ *<sup>b</sup>* , σ

of plasticity, D – module of consolidation (tg of inclination angle of tangent to a curve σ(e) within the range of plastic deformations, with linear character of deformation consolidation of a material. The critical velocity of impact, connected with singularities of spreading in materials undergoing wear, elastic and plastic deformation waves, features resistance to material wear at dynamic, high-velocity stress and it can be expressed by durability

Test data analyses proves that in conditions of stretching at impact (impact on the front

*kr b*

const – a constant determined empirically, considering the specific test conditions or the

Taking into account (11) the criterion of resistance to wear for iron-based alloys with

ϑ

*n*

 σ

 ρ

= =

*e e*

*spr*

ϑ

*pl kr pl*

ϑ

<sup>2</sup> <sup>2</sup> *pl kr*

, ( )<sup>2</sup>

2

= = (8)

(9)

= + (10)

ϑ*kr* i σ*<sup>b</sup>* ):

= *const* (11)

*<sup>T</sup>* - strength of a material and the limit

*M E*

ρ

Fig. 6 shows the scheme of the tribotester for the estimation of needle bearing. In this case, used the bath lubrication. Bearing designation are given in Fig. 6.

**Figure 6.** The scheme of the tribotester: 1- frame, 2- cover, 3- investigated shaft, 4- reducer sleeve, 5 cylinder, 6- toothed gear, 7- transmission belt, 8- electric motor SZJe 3,5 [kW], 9- wedges, 10- oil, 11 needle bearing K28x33x13, 12- needle bearing K40x45x17, 13- screw gear, 14- thermoelement NiCrNi, 15- shaft neck d= 28 [mm], 16- hub, 17- needle roller φ 2,5 [mm], l2= 13 [mm], 18- bearing head Parameters of investigation:surface stresses, p = 950 [MPa], rotational speed, n = 1500 [rpm].

#### **2.2. The results of the investigations**

Fig. 7 shows the influence of energetic criterion on the wear of a shaft neck in the function of change of material and lubricating parameters.

Performance Evaluation of Rolling Element Bearings Based on Tribological Behaviour 37

**3. The influence of geometrical parameters on the friction process in the** 

The construction of the rolling bearing was initially based on the assumption that the friction loss during the bearing work is significantly smaller than during the sliding. However, during the work of the bearing in the operating conditions there exists both the bearing and the sliding friction. Different factors result in appearance of resistance to motion


When bodies are deformed in the operating conditions, the phenomenon of pure bearing exists if the cooperating elements possess the same diameter, length, the properties of the material and parallel axes. Also, the roughness of the cooperating surfaces should be minimal. In such a case, when there is no lubricant, there appear only some losses caused by hysteresis of deformations [15]. This stems from the fact that the difference between the length of the contact arc and the corresponding arcs before deformations is identical for both

If the curvatures of the cooperating surfaces are different, in the elastic deflection the length of the contact arc for both bodies is identical, whereas before the deflection it was different. Consequently, the deformations in the contact area are accompanied by microsliding; if, however, the speed of both cooperating surfaces is identical, their mutual movement is

In a typical situation, in the process of bearing of two bodies with different peripheral speed there occurs the bearing with sliding; such a situation is the subject of examination here. The aim is to estimate of the extent to which the sliding friction matters in the overall balance of

In the place of contact of two elastic bodies pressed against each other with some force, some contact stresses within a certain field of mutual contact occur. They reach significant values even in the situation when the pressing force is relatively small, which, as a consequence, may lead to exceeding the acceptable limit of the material effort. This is of paramount importance during the work of needle bearings which are under considerable load. Figure 8b shows a situation when the axis of the needle and the axis of the shaft neck are parallel. The stresses that occur (Fig. 8 a) are evenly spread along the length of the needle, and the area of contact between the two elements equals the field of the ellipse of the length which is the same as the

**3.1. The analysis of contact areas of the needle bearing elements** 

length of the needle and the width 2b calculated by means of Hertz's solution:


bodies. As a result, there is no mutual sliding of the surfaces in the deformation area.

**needle bearing** 

while the bearing is operating: - Hysteresis of deformations; - Interior friction in lubricant;


called "bearing".

motion resistance.

movement caused by the gyroscope moment;

**Figure 7.** Wear intensity of a shaft neck – energetic criterion: Ntw – relative density of friction power; Wkrw – relative critical density of strain power ; a shaft neck: 1 – 16MnCr5, 2 – 20MoCrS4 + carburising; welding : 3-80G; 4-35X5M1,5; 5-50X10GC1,5; P3 – addition to Acorox 88 oil, needle bearing K 40 x 45x17

The analysis of test data shows that at general resistance to wear criterion for materials at external friction there occurs average critical density of friction power Wkr\* , in worn out volumes.

If chemical reactions, surface-active substance and other factors have significant influence on the processes of creating various secondary structures and products, then determining adequate reliability of criteria of resistance to wear proves to be a difficult problem, for instance at low-intensity oxidizing wear of a tribo-coupling. Resistance criteria in such conditions might be: dislocation density in the thin surface layer of a material, activating energy of chemical reactions and durability characteristics [5 ,9, 10].

The list of materials resistance to wear criteria – invariants of universal criterion Wkr\* (critical density of strain power) indicates huge variety and complexity of processes of materials surface damage.

It ought to be noted that the properties which characterize deformed areas of materials at the moment of their destruction (the initial stage of creating wear products) show reactions of these areas to external influence. In order to point those reactions in the desired direction, in this case – to ensure high resistance to abrasion (endurance) of its elements, it is necessary to make the right choice of known materials or produce new ones, having structure of the highest resistance to cracking resulting from the influence of external (exploitation) factors. This means that materials science involving problems of tribotechnology should be based on the analysis of microstructures.
