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Electrical Method for Non-Destructive Testing of Working Surfaces of a Rolling Support K.V. Podmasteryev Orel State Technical University RUSSIA Contact

Physical essence of the method.

Reliability and safety of working machines and vehicles is one of the most important problems. For all this operating longevity and vitality of machine elements, the constructions of which have bearing units, are depended to a great extend on the quality of the bearings themselves.
The condition of a rolling support is determined by joint influence of the plenty of various factors. Among them: quality of details working surfaces, their wear degree, availability of local defects on them, deviations from the correct geometric from of rolling tracts, stipulated by technological errors of details manufacturing and operating conditions of an item, defects of a lubricant material and lubricating systems, etc.
The failure of a bearing can become the cause of damage and crashing resulted in large economic looses and risk for life and health of the people. Therefore one of the directions for increasing reliability and safety of mechanical systems is the realization of functional diagnostics and non-destructive testing of a rolling support while manufacturing, service, maintenance service and repair. In these cases it is necessary to have special methods, checking bearing condition in bearing support at the necessary moment.
There are many different methods of bearing testing. The electrical methods take an important place among them. They have no inertia, are very convenient for process of an indicated signal and have a high level of reliability as measuring information comes from friction zones of bearing elements. One of the methods of this group has been developed in Orel State Technical University. It is based on measuring of normalized integral time of elements electrical contacting (in other words - NIT parameter).
Operating surfaces roughness and presence of different defects are the cause of short-time breach of lubricant sheet (film) separating the friction surfaces. The lubricant material has high resistivity and that is why the simultaneous contacting of rolling elements with both rings causes a reducing of electrical resistance between bearing rings (Fig. 1). In practice the NIT parameter is approximately equal relative time of oil film breach. NIT is a statistical evaluation microcontacting probability (destruction of lubricating film in friction zones). Irrespective of the concrete reason the aggravation of a bearing condition results in growth of microcontacting probability and significance of a NIT parameter.
Herewith it is offered the complex of special algorithms of testing and algorithms of processing of a measuring information for a solution of various testing problems. The appropriate technical diagnosis equipments are developed. The created equipment has high metrological characteristics with high sensitivity to a modification of a unit condition. For want of range of NIT measurement from 0 up to 1, the threshold of sensitivity -10^-7 is insured.
The application efficiency of the NIT method for solving the problem of tribological researches is illustrated in the work [1]. In this work we represent the outcomes of theoretical and experimental researches of application efficiency of the considered method for non-destructive testing of working surfaces of a rolling supports. The following problems are solved:
• search for fault location of the surfaces of an operating bearing with the indication of the fault detail and its location on its surface;
• detection of a kind (ovality or lobing) and size of out-of-roundness in actual races profile of the bearing rings.


Fig 1: Resistance of bearing (Rb ) as a function of time (t): K - NIT parameter value; T - time of NIT parameter measurement; Rt - threshold of electrical resistance; ti - time of lubricant sheet destruction

Fault-detection in the rolling supports

The given task is most important at realization of maintenance service both repair of machines and mechanisms. Test NIT parameter is integral and gives an average assessment of bearing quality. However, under certain conditions of technical state inspection it gives an opportunity to solve these problems. The main physical essence of these methods concludes the following. Destruction of lubricant sheet takes place in loaded contacting zones of elements and with loading elevation the probability of destruction rises. It is clear that simultaneous destruction of lubricant sheet in contacting with both rings which is recorded by a tester is possible only in loaded rolling element. That is why the test result of NIT is determined with condition only those selections surfaces, which hit the loading zone for the time registered of NIT.
In that way, for localization of controlling zone on operating surface of bearing element under investigation it is necessary to make immobility available in the ratio of radial loading. We got the field conditions [2], completion of which makes possible to supply a minimal controlling zone limited by an angle sector ±2p/Z relative to action line of loading (Z - a number of rolling elements in bearing). Operating surfaces scanning in search of local defect is included in moving the controlling zone on the surface under investigation with average effect on diagnostic parameter quality of other elements. In this case the dependence NIT has the appearance of splash with maximum coordinate, corresponding to passing of defects line of radial action loading.
The diagnostic model of processes microcontacting in the bearing is developed. The model takes into account many various factors influencing probability microcontacting and on NIT parameter. Theoretical and the experimental researches have confirmed a hypothesis that the occurrence of local defect on a working surface of details of the bearing (cracks, hollows, cavities, marks, corrosion cracks) results in growth of controllable NIT parameter. Is established, that the increase of the sizes of defect synonymously results in increase of test parameter. It allows not only to find out defects, but also to compare them on the size. In a fig. 2 the examples of theoretical functions of controllable parameter from the sizes of defects and in fig. 3 results of experimental researches are submitted. The results are received for bearings 113 and 208.

Fig 2: The NIT parameter increases at magnification of depth (a) and extent (b) of local defect

Fig 3: Circular diagrams, NIT parameter diagrams and NIT regressive functions from dimensional parameters of different imperfections: a - four cavities by a diameter of 0,5 mm and various depth - 1, 3, 4 and 16 mm (site: 32°, 140 °, 225° and 310°); b - four cavities by depth 15 mm and various diameter - 200, 500, 600 and 1000 mm (site: 20°, 115°, 200° and 290°); c - four marks by depth 4 mm and various width - 100, 150, 200 and 300 mm (site: 60°, 150°, 230° and 315°); d - four hollows of various depth - 2, 6, 8 and 10 mm (site: 0°, 90°, 180° and 270°)

Thus while diagnosing we exposed both used and new bearings with artificially put imperfections. The average NIT values on the graph are obtained on 50 realizations. The data processing with application of the statistical analysis have allowed to state importance of influence of dimensional performances of local defects on diagnostic parameter. It confirms by established dependencies at high values of correlation ratio (0.75 - 0.97).
For practical realization of search of local defects by the considered principle the algorithms and means of the control [3-7] are developed. Some methods use special loading devices (test control). Other methods use the information on loading, which is in the bearing at its functioning (functional control). With the help of special researches for each method the verification modes are reasonable, at which the best results [2] are provided.
One of the new methods of fault-detection is being brought off with the help of original device, producing rotating radial loading on a bearing [4] (fig. 4).

Fig 4: Technical diagnosis equipment:1 - bearing; 2 - bearing unit shaft; 3 - power source, 4 - contact point; 5 - comparator; 6 - contact point; 7 - loading device; 8 - transducer of angular loading vector position; 9 - integrator; 10 - differential amplifier; 11 - voltage compensator; 12 - display; 13 - mode switch; 14 - transducer of phase difference; 15 - transducer of angular shaft position.

While testing of one of the rings of a bearing in a unit the loading vector slowly turns relative to this ring making a controlling loading zone on its surface. Form and frequency controlling signals loading device supplies constancy vector modulus of radial loading and required angular velocity of vector rotation. Vector rotating brings to slow displacement of controlling zone along the whole race and all surface points fell sequentially within this zone.
Falling within the controlling zone of defected section is located with increasing of NIT value, determining with electron section of the device. NIT is determined as a function of an angle loading vector turning relative to the ring controlled and a coordinate of parameter splash makes it possible to detect a local defect along the race.
Vector of radial loading slowly turns relative to the ring controlled and rotates fastly relative to the other ring and separator. During the time of every measurement all the points of the uncontrolled ring and all the rolling elements fell multiply under loading. That is why the effect of quality uncontrolled operating surfaces on the result of detecting parameter of NIT will form the additive component of the measuring results. This component is a complex assessment of a bearing quality. Changing of the value of NIT parameter is determined by variable component, which is characterized by quality of race. In that way, algorithm and technical diagnosis equipment are carried out parallel with complex quality assessment of bearing in unit, the search of local defects along the race of its inner ring and outer ring with defect location.
Let's consider other examples. We suggested another method, also furnishing to detect the local defects of the bearing operating surfaces in particular in units. It is concluded in selective surfaces scanning [3, 5, 6].
The main idea of this method is as follows. Circulation radial loading is used. It is created with help of unbalance the shaft for the stationary ring or constant loading element for rotating ring. In that case the frequency of displacement of loading vector relative to the ring controlled is equal to the frequency of rotating bearing ring.
On the race we detect schematically the control section, displaced according to each other on a scanning step y_sc, determined by the accuracy required for detecting defect location. NIT parameter measurements of bearing for every section are carried out and with that every measurement is brought off in the moments of passing loading vector of angle sector ±2p/Z from the center of controlled section of testing ring. In that scanning principle the electronic diagnostic device is automatically turned on at given time moments, corresponding to a certain relative location of the element controlled and loading vector.
In that case the defect detecting on the race of the ring controlled may be determined as a section center, which corresponds to the most parameter value. The defect location is determined with the accuracy to accepted scanning step y_sc.

Fig 5: Characteristic diagrams of NIT parameter for the bearings with various local defects


Selecting scanning algorithms are used for detecting of defect rolling elements [7].
We also formulated a number of criteria, which make it possible to find optimum selection of diagnostic conditions. For practical realization of these criteria we suggested the whole package of applied programs, imputing parameters of which are geometrical characteristics of the bearings controlled and outputting - the value of the number revolutions of the rotating ring at the control time of one of the section of the surface investigated.
The experimental investigations conducted, corroborated serviceability of the devices offered and efficiency of diagnostic algorithms designed. In particular it was established that the structural level of detecting local defects is up to a separate rolling element and to defected section of the race. In that case we detect the regulated by standard technical documentation, local cracks, hollows, lines, marks, corrosion and metal fatigue. For example, we can detect a 2mm hollow and a 20mm mark while acceptable values are relative to 3 and 150mm. Fig. 5 illustrates tolerance value of diagnostic parameter under existing different local defects on the surfaces of the rolling bearing which were under field conditions.

Testing of out-of-roundness in actual races profile of the bearing rings

The given task represents the greatest interest during realization of assembly works at manufacturing both repair of machines and mechanisms. At assembly of units the basic sizes of details of the bearing can essentially change. It occurs owing to out-of-roundness in profile of the surfaces of the shaft and case of unit, which incorporate to rings of the bearing, skew and displacement of axes of these surfaces etc. In result actual races profile of the bearing rings is formed. This structure strongly influences the operational characteristics of the bearing and on his(its) durability. Beforehand to predict theoretically this structure it is impossible. Therefore after assembly of units it is necessary to supervise out-of-roundness in actual races profile of the bearing rings.
We develop original algorithms and means for the decision of the specified task. At the control NIT parameter is used. The basic idea of a method consists in the following. At work of the bearing the rotation of its ring results in the appropriate rotation separator. It causes periodic redistribution of loading perceived by the rolling elements. In result for the bearing with ideal race (without form deviations) the probability of microcontacting periodically changes according to the certain law. Conformity to the same low a controllable test parameter is changed. The hypothesis was put forward that, the occurrence out-of-roundness in actual races profile of the bearing rings results in change of fluctuation character of NIT parameter. This hypothesis is placed to a basis of the method.
The diagnostic model of microcontacting processes in the bearing is developed. The model takes into account many various factors influencing probability microcontacting and on NIT parameter, in particular characteristics form deviations of the race of rings of the bearing. The analysis of model is carried out (spent) by a numerical method with the help of a package of the applied computer programs.
The theoretical researches have confirmed the put forward hypothesis. Is established, that various kinds out-of-roundness in actual races profile of the bearing rings results in law, specific only for this kind, of change NIT parameter from time. Thus in function K(t) contain certain (determined) harmonic components on frequencies appropriate to a kind out-of-roundness. The expressions for characteristic information frequencies look like [8]:

fu = fkM	fu = fkLM + fcZ


where fk fc - frequency of rotation of the ring and separator; M - factor, equal 2 for ovality and 3,4 for lobing of the appropriate order; L - factor, equal 1 or 2.
Is established that the changes of out-of-roundness meanings do not result in change of structure of a spectrum, and only redistribute the contribution separate harmonic components in a spectrum. In view of the given property of function are offered test parameter allowing to identify a kind and to estimate meaning dominant out-of-roundness of race.
As diagnostic parameter factor Km, describing contribution of capacity (s_{P kf})² of the appropriate frequency component to capacity of variable making function is offered (s_{P cp})²:
Km = (s_{P kf})²/ (s_{P cp})²
On fig .6 the diagram of function Km (Q) for the bearing of a type 208 is by way of illustration submitted at presence ovality (Q) of the inner ring race.

Fig 6: Dependence of parameter Km(Q) for harmonic component with M = 2, L = 2 at presence of ovality of the inner ring race of the bearing.

Experimental researches are carried out, they have proved the theoretical analysis, serviceability and reliability of a method. The researches were carried out on special equipment [9]. As an example in a fig. 7 and 8 the examples of spectrums for bearings with known parameters of out-of-roundness of the race of inner rings are submitted. For an illustration in figures the circular diagrams of the races of controllable rings are represented also. The analysis shows that various kinds out-of-roundness there correspond to various spectrum of function K (t).

Fig 7: Spectrum for the bearing of a type 208 at presence three-vertex lobing of the inner ring race.

Fig 8: Spectrum for the bearing of a type 208 at presence ovality of the inner ring race.

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