RU2337348C1 - Method for determination of fatigue damage of crank shafts - Google Patents

Abstract

FIELD: measuring equipment.

SUBSTANCE: method for determination of fatigue damage of crank shafts is performed in dynamic mode. Double-mass oscillatory system is made and suspended vertically at one point on a flexible link, so that the product axis and suspension axis are put in one line, while junction section of oscillatory system and fundament is between them. Low-frequency damped natural rotary oscillation is excited in the system, damping rate is defined as indicator of accumulated damage level in crank shafts depending on the fatigue endurance limit of the shaft.

EFFECT: increased precision of internal fault control in product material.

6 dwg

Description

The invention relates to non-destructive testing of internal defects of products, namely to methods for controlling the quality of shafts, and can be used to detect accumulated fatigue damage in the crankshafts of internal combustion engines and compressors.

The crankshafts of modern machines are made of high quality cast iron and steel. During the operation of cars, tractors and compressors, shafts are subjected to periodic influences of various loads both in magnitude and direction, under the influence of these loads, bending, torsion, etc. stresses arise in the material of the crankshaft. When using hard operating conditions or when the clearance of the main and connecting rod bearings increases, the amplitude voltage values in dangerous sections of the crankshaft increase several times in comparison with normal operation modes. This leads to a gradual accumulation of internal fatigue damage in the shaft material. Over time, fatigue cracks appear in the crankshafts, the development of which can lead to a breakdown of the shaft, which leads to unforeseen expenses of time and money for bringing the engine into working condition.

There are various methods for determining damage in the material of products, consisting in the fact that a sample of the material is subjected to alternating cyclic loading at a given load amplitude, and the deformation of the sample is measured in the loading process (A. Gusenkov, “Properties of cyclic deformation diagrams at normal temperatures” in Sat . "Deformation resistance with a small number of loading cycles", M., Nauka, 1967, p. 34-40), and in another case, the strain amplitude in each half-cycle is measured, then a graph is plotted in the coordinate "Strain ratio of amplitudes in adjacent half cycles - fatigue defect sample" (.. №879383 AS A method for determining the material fatigue damage BI №41, 1981) and to change these parameters are judged on the fatigue damage.

From the description of the known methods it follows that the method allows to determine the fatigue damage of the sample at any stage of alternating cyclic loading, i.e. the method is suitable for testing samples. Under real operating conditions of machines at the same operating time, damage of various levels accumulates, in addition, during the repair period of machines, shafts of various repair size, chemical composition, hardness and geometric parameters specified in GOST and TU for a specific shaft come under control. For this reason, it will not be possible to plot the damage from the measured parameter. Given this, the above methods for determining fatigue damage in crankshafts are of little use.

Closest to the technical nature of the proposed method is a method of controlling the quality of the shafts (and.with. No. 262462. A method of controlling the quality of the shafts. B.I. No. 6, 1970). According to the description of this method, the shaft under test is fixed at one end to the vibrating table and the assembled structure is brought into motion, harmonic vibrations are reported in the horizontal direction to the vibrating table (linear-translational vibrations perpendicular to the direction of shaft vibrations), while the vibration frequency is close to the natural vibration frequency of the shaft, sequential rotation of the shaft relative to its own axis. As a result of these actions, any points located on the free end section of the shaft describe the curves by the shape of which they judge the quality of the shaft being tested.

This method is suitable for monitoring smooth shafts of certain sizes, and for controlling crankshafts it has low accuracy or is unsuitable for the following reasons:

- the presence of connecting rods (crankshaft design), geometric dimensions and weight;

- accumulation of damage is random;

shafts of various repair size, chemical composition, hardness specified in GOST and TU come to control;

- the presence of dimensions acceptable without repair, for example (shaft deflection, etc.).

The aim of the invention is to improve the accuracy of control of internal defects in the material of products, namely the control of accumulated fatigue damage to the crankshafts of internal combustion engines and compressors.

Comparative analysis with the prototype shows that the inventive method for determining fatigue damage of crankshafts, carried out in dynamic mode, is characterized by the presence of new signs:

- create a two-mass oscillatory system;

- the system is suspended by flexible communication vertically at one point;

- the axis of the product and the axis of the suspension are on the same straight line;

- the nodal section of the oscillatory system and the support is located between them;

- in the system, low-frequency free torsional vibrations are excited and the decay rate is determined, the latter being an indicator of the level of accumulated damage to the crankshafts depending on the shaft endurance limit.

When analyzing patent and technical literature, the applicant did not find another identical and equivalent technical solution to the claimed one and therefore believes that the proposed invention meets the eligibility criterion of “novelty”. In addition, a new set of essential features, which allows to increase the accuracy of monitoring fatigue damage of crankshafts, does not explicitly follow from the prior art, which allows us to conclude that the solution meets the criterion of "inventive step".

The inventive method for determining fatigue damage of crankshafts is illustrated in drawings 1-6.

FIG. 1. The suspension scheme (sample) of the crankshaft in figure 2. The flowchart of the control method.

Figure 3. The main stages of free torsional vibrations in figure 4. Fatigue Damage Installation

FIG. 5. The dependence of the endurance limit (τ _-1 ) of the shaft from the decay time of free torsional vibrations t.

FIG. 6. Determination of permissible oscillation decay time.

According to the method for determining the fatigue damage of crankshafts, carried out in dynamic mode, create a two-mass oscillatory system, which, on the one hand, favors the flow of free damped vibrations, and on the other hand, covers the maximum volume of crankshaft material with a minimum number of nodal sections. In the system created in this way, the damping time of oscillations ranges from a few seconds to several hundred seconds, which in turn makes it possible to select the best sections for measuring the damping.

The created system is suspended by means of flexible coupling vertically at one suspension point 2 (Fig. 1), where 1 is the support, 2 is the suspension point, 3 is flexible coupling, 4 is an eye bolt, 5 is the bearing axis, 6 is the bearing housing, 7 - bearing, 8 - sensor, 9 - additional masses, 10 - crankshaft, MN - suspension axis, while the influence of the number of points on the measured parameter is minimized.

The system is suspended so that the axis of the product and the axis of the suspension are on the same straight line MN (Fig. 1). When the product axis deviates from the suspension axis, the accuracy of the control of accumulated damage by the measured parameter decreases.

The nodal section of the oscillatory system and the support is located between them, otherwise the sensitivity to accumulated damage deteriorates.

Then in the system (figure 2) including 10 - sample (crankshaft), 9 - inertial masses, 8 - piezoceramic sensor, 11 - suspension device, 3 - kapron thread, 13 - amplifier, 14 threshold device, 15 - power source, 16 - frequency counter-chronometer, 17-18 oscilloscopes, excite low-frequency free torsional vibrations and determine the attenuation rate, when using other types of vibrations, the system does not respond in one case, the sensitivity to damage decreases in the other, the measurement is carried out in a steady state (Fig. 3) where 19 (T _in ) - p the period of forced oscillations, 20 (T _p ) is the transition period, 21 (T _y ) is the steady-state period of oscillations, 22 (T _w ) is the noise zone of oscillations when measuring the rate of damping of oscillations.

A control block diagram has been developed (Fig. 2), a device has been designed and manufactured to determine the fatigue damage of crankshafts (samples) (Fig. 4), where 23 is an oscillatory system, 13 is an amplifier, 15 is a power source, 16 is a frequency counter-chronometer , 17 - oscilloscope, 14 - threshold device.

An example implementation of the method for determining fatigue damage in the material (sample) of the crankshaft (figure 2).

The inertial masses 9 and the suspension device 11, 12 are attached to the crankshaft 10. The support of the piezoceramic sensor 8 is rigidly attached to one of the inertial masses. Thus created two-mass oscillatory system is suspended by means of flexible connection 3 vertically at one point.

If two equal oppositely directed twisting pairs of forces are applied in the plane of inertial masses 9, and then they are suddenly removed, or a pulse impact is applied to one of the masses, then free torsional vibrations will appear in this system. In the process of oscillations, inertial masses will always rotate in opposite directions, this follows from the law of conservation of the main moment of momentum. At the initial moment, the momentum of the two masses relative to the shaft is zero and should remain equal to zero. Equating to zero the main moment of momentum requires that both masses rotate in opposite directions.

Mechanical vibrations of the system are perceived by the sensor 8, which is in mechanical contact with the inertial mass 9. The sensor converts them into electrical vibrations with a frequency equal to the frequency of the mechanical system. Electric vibrations are amplified by an amplifier 13 and fed to a threshold device 14, which, when the oscillation amplitude is reduced to a predetermined upper threshold value, includes the start of measuring the decay time of the recording device 16. For visual inspection, there are oscilloscopes 17, 18 in the electrical part of the installation. a predetermined lower level threshold device sends a signal to the recording device about the end of the countdown of the oscillation decay time (for example, in stve indicator decay rate of free torsional oscillation damping time taken while reducing their amplitude twice). The readings of the recording device are taken and, depending on the endurance limit of the shaft, a conclusion is made about the suitability of the controlled crankshaft.

When carrying out the control process according to the proposed method, the effects of hardness, chemical composition and shaft diameter (their values were set within TU and GOST) were evaluated on the studied vibration parameters.

As follows from the analysis of the results obtained, only the diameter of the shaft neck has a significant effect on the frequency of natural vibrations of the system. The maximum change in the diameter of the neck of the shaft causes a corresponding change in the natural frequency by + 1.4%.

All studied factors turned out to be significant in measuring the decay time. In this case, the hardness of the test sample has the greatest influence. A change in the hardness of the sample by 1% leads to a change in the decay time by 1.1 ... 1.3%.

Thus, the use of the method allowed us to quantitatively evaluate the influence of the studied factors on the main parameters of free torsional vibrations of a two-mass system.

To solve the problem of identifying the effect of accumulated damage on vibration parameters, ensuring periodic measurement of vibration parameters during the accumulation of fatigue damage in the sample material, a fatigue test installation and a damage control unit were developed and used.

A methodology was developed for fatigue tests (laboratory samples and full-scale parts), the tests were carried out according to the classical scheme and the method of step loading. In the process of fatigue testing, at certain intervals of time (loading), the frequency and decay rate of free torsional vibrations were determined. Thus, laboratory samples and field samples (compartments of crankshafts) were tested before they were destroyed.

An analysis of the work performed allows us to draw the following conclusions that the accumulation of fatigue damage in the sample material leads to a slight change (2-3%) in the vibration frequency and to a significant decrease (80-90%) in the decay time of these vibrations, i.e. the greater the level of accumulated damage, the faster the decay rate. The decay time of free torsional vibrations is the most sensitive parameter to the level of accumulated damage in the shaft material.

и пределом выносливости τ_-1 (фиг.5), кроме этого на основании этих испытаний и с применением метода минимального среднего риска (при этом используется наибольшее количество информации: плотности распределения вероятности состояний диагностируемого объекта f(t₁), f(t₂), издержки R от ошибок первого и второго рода, затраты на диагностирование и др., что обеспечивает более высокую достоверность полученного результата) было установлено допустимое значение контролируемого диагностического параметра (фиг.6). Предел выносливости отобранных валов по этой методике не ниже предела выносливости валов без трещин последнего ремонтного размера и, следовательно, они пригодны к дальнейшей эксплуатацииThe results of fatigue tests with alternating torsion and bending showed the presence of a close mathematical relationship between accumulated damage

and endurance limit τ _-1 (Fig. 5), in addition, based on these tests and using the minimum average risk method (the largest amount of information is used: the probability density distribution of the conditions of the diagnosed object f (t ₁ ), f (t ₂ ) , the costs R from errors of the first and second kind, the costs of diagnosis, etc., which provides a higher reliability of the result), an acceptable value of the controlled diagnostic parameter was established (Fig.6). The endurance limit of the selected shafts by this method is not lower than the endurance limit of the shafts without cracks of the last repair size and, therefore, they are suitable for further operation

An operational check of the reliability of the shafts repaired in this way confirmed their trouble-free operation during the overhaul interval.

Claims (1)

A method for determining fatigue damage of crankshafts, carried out in dynamic mode, characterized in that they create a two-mass oscillatory system, which is suspended by flexible coupling vertically at one point so that the axis of the product and the axis of the suspension are on the same straight line, and the nodal section between the oscillatory system and support - between them, low-frequency free torsional vibrations are excited in the system and according to the decay rate, depending on the shaft endurance limit, they judge the level of damage tions.

Images