Method to monitor a thrust load on a rolling bearing and machinery equipped with a system to monitor said thrust load

A method to monitor a thrust load on a rolling bearing, obtaining a first parameter on the basis of at least a bearing cage rotation speed and of a first race rotation speed of a bearing, and subsequently obtaining a calculated thrust load on the basis of the first parameter and of the first race rotation speed.

FIELD OF THE INVENTION

Embodiments of the subject matter disclosed herein correspond to a method to monitor a thrust load on a rolling bearing, and to a machinery equipped with a system to monitor said thrust load.

BACKGROUND OF THE INVENTION

In the field of ‘Oil and Gas’ machinery, like turbomachinery in general, are widely used.

Those kind of machineries comprise rotating parts, like shafts, that may be mounted on rolling bearings. Rolling bearings comprise a plurality of rolling elements, which may be balls or cylinders, located between an outer ring and an inner ring. The rolling elements may rotate on an inner race and an outer race respectively formed on the inner ring and on the outer ring of the bearing. The balls or cylinders may be coupled to a cage.

In order to lubricate and refrigerate the bearings, the machinery may be equipped with a lubricating circuit, feeding lubricant to the each bearing. The lubricant draining form the bearing may gather in a sump structure surrounding the bearing and it may be recirculated in the lubricating circuit.

Rolling bearing faults or malfunctioning may be due to bearing axial thrust (or simply thrust load) overloads or underloads.

The problem of overloads and underloads is particularly felt in machineries comprising a turbine and/or a compressor mounted on the same shaft. Turbines and compressors, especially axial or radial compressors, which may be of the multistage type, may generate axial loads on the shaft that may significantly change during different operating phases of the machinery.

In underload conditions a skidding between the rotating elements (balls of cylinder) and an inner and outer race of the bearing may arise. The presence of skidding between balls and races may reduce the useful life of the bearing.

An overload in the axial thrust load may compromise the physical integrity of the bearing and therefore generate malfunctioning.

As rolling bearings are among the top offenders in the failures statistics of turbomachineries, the enhancement of monitoring capabilities of rolling bearing thrust load could be beneficial. In fact, a malfunctioning of a rolling bearing may lead to serious damages, especially in turbomachines. Here, impellers or turbines are mounted on the shaft with a minimal distance from a stator, in order to operate correctly and efficiently. If a rolling bearing fails, impellers and turbines may contact the stator leading to a severe damage of the entire machinery.

In known applications, thrust load may only be estimated by indirect calculus and its real value is not under continuous monitoring. In particular, experimental models are used for indirect estimation of the thrust load from others machinery operating parameters.

The estimation models used in known applications may not give a sufficiently accurate measure of the value of the thrust load acting on bearings.

A continuous read of the axial thrust load value was performed only for testing purposes, using load cells or strain gages.

Those systems are difficult to be installed in the bearing housing in order to accurately measure the load, and are not suitable for permanent installation.

SUMMARY OF INVENTION

Therefore, there is a general need for an improved method to monitor a thrust load on a rolling bearing that may be based on parameters directly reflecting the bearing operating status.

An important idea is to estimate the thrust load of a rolling bearing based on a first parameter calculated on the basis of at least a bearing cage rotation speed and of a first race rotation speed of a bearing, and obtaining a calculated thrust load on the basis of the first parameter and of a first race rotation speed.

First embodiments of the subject matter disclosed herein correspond to a method to monitor a thrust load on a rolling bearing.

DETAILED DESCRIPTION

The following description of exemplary embodiments refers to the accompanying drawings.

The following description does not limit embodiments of the invention. Instead, the scope of embodiments of the invention is defined by the appended claims.

With particular reference toFIG. 1, reference10indicates a machinery, in particular a turbomachinery, and more in particular a gas turbine.

The turbomachinery10comprises an axial air compressor22driven by a turbine23. The compressor22comprises a plurality of compressor blades20, and the turbine comprises a plurality of turbine blades21. Both the compressor22and the turbine23may comprise a plurality of compressor or turbine stages (not shown in the drawings). The compressor blades20and the turbine blades21are installed on a common shaft18.

The turbine blades21shown inFIG. 1, may cooperate with turbine nozzles13and stator nozzles14in order to operate correctly.

The shaft18may be supported by a rolling bearing16located between the axial compressor22and the turbine23and by further bearings (not shown) located at an inlet of the axial compressor22, and/or in proximity of the outlet of the turbine.

The rolling bearing16may be refrigerated and lubricated by oil flowing in a scavenge line19in direct contact with the rolling bearing16.

The bearing may comprise an outer race16O, which may be coupled to a stator, and an inner race16I, which may be coupled to the shaft18. Between the inner race16I and the outer race16O a plurality of rotating elements like bearing balls16B may be located. The plurality of rotating elements may be coupled to a bearing cage16C.

A machinery control unit40may control the operation of the machinery through a plurality of sensors. The control unit may be part of a system to control the thrust load on one or more of the rolling bearings of the machinery.

The system may also comprise, coupled to the control unit40, a speed sensor41configured to read the rotating speed of a race of the bearing16. The race of the bearing may be the inner race16I, the outer race16O, or both the inner and the outer race. In some embodiment the inner race16I of the bearing16may be torsionally coupled to the shaft18of the turbomachinery; in this case the rotation speed of the shaft may be the same of the inner race. Therefore the sensor41may be mounted in a position of the shaft that is far from the monitored bearing16.

A further sensor60may be placed close to the bearing16, in order to read the rotation speed of the cage16C (bearing cage) coupled to the rotating parts (balls or cylinders) of the bearing16.

The speed sensors41,60may be of any known type, and may be for example a keyphasor or a different tachometer.

The system may also comprise, coupled to the control unit40, a monitor50(for example a touch screen or a control panel of the machinery), which may show alerts alarms and/or any other kind of information useful to control the operating status of a machinery. Furthermore the control unit may be coupled with a debris sensor42placed in the scavenge line19.

The control unit40may be configured to monitor the thrust load on a rolling bearing16, calculated on the basis of the bearing cage rotation speed CS read from the sensor60and on the basis of a first race rotation speed SS, read from the inner race rotation speed sensor41(or from the shaft speed sensor).

In the present description, reference will be made to the steps necessary to monitor the thrust load (axial load) on the rolling bearing16, but of course the thrust load L may be monitored also on other bearings, for example on the bearing placed at the entrance of the compressor, on the bearing placed at the outlet of the turbine or anywhere else on the machinery.

Furthermore the control unit40, may show on the monitor50information regarding the health status of the rolling bearing16, obtained on the basis of the method herein described. Those information may be useful for an operator that may control the machinery.

With reference toFIG. 2, the control unit40, in order to monitor a thrust load on a rolling bearing, may perform one or more of the following steps:preliminary step F0: obtaining a value of a bearing cage rotation speed CS and of a first race rotation speed SS; it should be noted that these values may be read directly from the sensors60and/or41, or they may be calculated in any known way starting from different parameters. These values may also be estimated on the basis of a plurality of variables. In an embodiment, the first race rotation speed SS is the rotation speed of the inner race. In some embodiments the speed of the inner race may be the shaft speed.first step F1: obtaining a first parameter FP on the basis of the values of at least a bearing cage rotation speed CS and of a first race rotation speed SS of a bearing. In an embodiment, the first parameter FP, is obtained dividing the bearing cage rotation speed CS by the first race rotation speed SS. This first parameter may correspond to a Fundamental Train Frequency (FTF), or cage frequency, of the bearing.a second step F2: of obtaining a calculated value of the thrust load L (axial load) acting on the bearing, on the basis of the first parameter FP and of the first race rotation speed SS. The thrust load L may be obtained as an example form the diagram ofFIG. 4that may correlate a plurality of values of the first parameter FP with a plurality of values of the axial thrust load, for any given first race rotation speed S1, S2, Sn). Speeds S1, S2. . . Sn on the graph may be been obtained through data analysis of experimental tests campaign. As an example, given a first race rotation speed S2, and given a value of the first parameter FP1, the value of the load L may be derived graphically as represented inFIG. 4, or through any other suitable numeric approach. By way of example a suitable numerical approach may be an estimation of the load value that minimizes the distance (Euclidian or other) between the FP measured and the polynomial curves that expresses FP as function of load.

After the second step F2a third step F3may be performed, where it is verified if the calculated thrust load L is equal or above a first predetermined value LMAX. If the calculated thrust load L is equal or above the predetermined value LMAX, an overload alarm is generated (ninth step F9). The alarm may be simply displayed on the monitor50, or the control unit40may act automatically according to predetermined procedures in order to diminish the value of the load L, and even to stop the turbomachinery if it is needed.

If the thrust load value is below LMAX, a fourth F4and/or a fifth step F5may be performed.

As discussed above, after the second step F2, the fourth step F4may be performed.

If the first parameter FP falls out the first range HR a sixth step F6may be performed.

The first range HR may be defined by design characteristics of the bearing, and it is graphically represented inFIG. 4. The fact that the first parameter falls out of the first range HR, which may also be defined as healthy range of the bearing, may show the presence of a potential problem on the bearing.

In order to assess what is the nature of the problem, the sixth step F6may be performed. In this step it is verified if a variation of the first race rotation speed ΔSS corresponds to an expected variation L1, L2of the bearing cage rotation speed CS, at the calculated thrust load L.

This step is graphically represented inFIG. 5. Preliminarily it should be noted that the graph ofFIG. 5correlates a plurality of values of the bearing cage rotation speed CS to a plurality of values of the first race rotation speed SS, given a plurality of known thrust loads L1, L2, Ln. . . . This correlation may be obtained by experimental values.

According to the sixth step F6, the predicted variation is obtained by selecting, on the basis of the calculated thrust load L, one load curve L1, L2between a plurality of load curves L1, L2each correspondent to a given load (in underload condition), and by verifying if at a determined point CS1, SS1, a first slope of first a curve R describing the variation of the bearing cage rotation speed CS with respect to the first race rotation speed SS corresponds, within a predetermined interval, to the slope of the selected load curve L1in the same point CS1, SS1.

Graphically, it is possible to see that the slope of the line R1passing for the points CS1, SS1-CS3, SS3, representing the variation of CS in function of SS, do not correspond to the slope of the curve L2representing the predicted variation of cage speed with respect to the first race speed for the load L2. This means that the bearing is skidding due to anomalous bearing conditions, which, by way of example, may comprise a lubrication problem, or any other known bearing problem. In this case an alarm (seventh step F7) may be generated.

On the contrary the line R2passing for the points CS1, SS1-CS2, SS2, corresponds (within a predetermined interval) to the slope of the curve L2. In this case, the skidding of the bearing is due to underload of the bearing itself. Therefore the control unit40(or an operator) may act on the machinery in order to solve the problem, for example by correcting the thrust load L on the bearing.

By way of example a value of the load correction may be calculated as shown in the eight step F8, by comparing the calculated thrust load L with a predetermined design thrust load LD. The predetermined thrust load LDmay be the minimum load value for making the bearing work in a healthy range condition given a first race speed SS. This value may be derived directly from the design of the bearing, as it is graphically represented again inFIG. 4. Given a first parameter value of FP2and a first race speed of S2, the predetermined thrust load LDis L2.

Additionally, after the fourth step F4a fifth step F5may be performed, where a skidding percentage is calculated. The skidding percentage may be calculated comparing the first parameter FP with a predetermined design value of the first parameter FPD. In this step the ratio between the variation of CS and the variation of SS may be compared with the expected range (experimentally determined for skidding due to underload).

The skidding percentage may be used by the control unit40to trigger the thrust load L adjustment that may be available on the machinery, and it may also be displayed on the monitor50, for an operator.

According to a possible embodiment, the first parameter FP may be also obtained on the basis of a second race rotation speed ORS. This may be particularly useful if the second race rotation speed is the outer race speed. According to this configuration the outer race of the bearing is not fixed, and the shaft may be supported by another rotating element. In this case the first parameter may be calculated as the ratio between the CS and the average of inner and outer race speeds.

It should be noted that a sequence of steps of an exemplificative method was described. Of course the steps of the method may be executed in a different order or sequence, and some of the described steps may also be omitted.

It should be also noted that, in order to make the description more understandable, reference to diagrams was made. The results here described as obtained in a graphical way may of course be obtained, according to different embodiments, through any suitable numeric method.

While the disclosed embodiments of the subject matter described herein have been shown in the drawings and fully described above with particularity and detail in connection with several exemplary embodiments, it will be apparent to those of ordinary skill in the art that many modifications, changes, and omissions are possible without materially departing from the novel teachings, the principles and concepts set forth herein, and advantages of the subject matter recited in the appended claims. Hence, the proper scope of the disclosed innovations should be determined only by the broadest interpretation of the appended claims so as to encompass all such modifications, changes, and omissions. In addition, the order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments.