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

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.

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.

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.

The accompanying drawings, which are incorporated herein and constitute a part of the specification, illustrate exemplary embodiments of the present invention and, together with the detailed description, explain these embodiments.

In the drawings:

FIG. 1 schematically shows a machinery, and in particular a part of a turbomachinery comprised between a compressor and a turbine, in proximity of a burner;

FIG. 2 is an enlarged view of a particular of FIG. 1 , showing in a greater detail a rolling bearing;

FIG. 3 is a schematic view of a plurality of steps performed to monitor a thrust load of a rolling bearing, for example of the rolling bearing of FIG. 2 ;

FIG. 4 is a graphical representation of a step of the method of FIG. 3 ;

FIG. 5 is a graphical representation a further step of the method of FIG. 3 ;

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 to FIG. 1 , reference 10 indicates a machinery, in particular a turbomachinery, and more in particular a gas turbine.

The turbomachinery 10 comprises an axial air compressor 22 driven by a turbine 23 . The compressor 22 comprises a plurality of compressor blades 20 , and the turbine comprises a plurality of turbine blades 21 . Both the compressor 22 and the turbine 23 may comprise a plurality of compressor or turbine stages (not shown in the drawings). The compressor blades 20 and the turbine blades 21 are installed on a common shaft 18 .

The turbine blades 21 shown in FIG. 1 , may cooperate with turbine nozzles 13 and stator nozzles 14 in order to operate correctly.

The shaft 18 may be supported by a rolling bearing 16 located between the axial compressor 22 and the turbine 23 and by further bearings (not shown) located at an inlet of the axial compressor 22 , and/or in proximity of the outlet of the turbine.

The rolling bearing 16 may be refrigerated and lubricated by oil flowing in a scavenge line 19 in direct contact with the rolling bearing 16 .

The bearing may comprise an outer race 16 O, which may be coupled to a stator, and an inner race 16 I, which may be coupled to the shaft 18 . Between the inner race 16 I and the outer race 16 O a plurality of rotating elements like bearing balls 16 B may be located. The plurality of rotating elements may be coupled to a bearing cage 16 C.

A machinery control unit 40 may 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 unit 40 , a speed sensor 41 configured to read the rotating speed of a race of the bearing 16 . The race of the bearing may be the inner race 16 I, the outer race 16 O, or both the inner and the outer race. In some embodiment the inner race 16 I of the bearing 16 may be torsionally coupled to the shaft 18 of the turbomachinery; in this case the rotation speed of the shaft may be the same of the inner race. Therefore the sensor 41 may be mounted in a position of the shaft that is far from the monitored bearing 16 .

A further sensor 60 may be placed close to the bearing 16 , in order to read the rotation speed of the cage 16 C (bearing cage) coupled to the rotating parts (balls or cylinders) of the bearing 16 .

The speed sensors 41 , 60 may 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 unit 40 , a monitor 50 (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 sensor 42 placed in the scavenge line 19 .

The control unit 40 may be configured to monitor the thrust load on a rolling bearing 16 , calculated on the basis of the bearing cage rotation speed CS read from the sensor 60 and on the basis of a first race rotation speed SS, read from the inner race rotation speed sensor 41 (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 bearing 16 , 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 unit 40 , may show on the monitor 50 information regarding the health status of the rolling bearing 16 , obtained on the basis of the method herein described. Those information may be useful for an operator that may control the machinery.

With reference to FIG. 2 , the control unit 40 , in order to monitor a thrust load on a rolling bearing, may perform one or more of the following steps:

preliminary step F 0 : 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 sensors 60 and/or 41 , 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 F 1 : 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 F 2 : 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 of FIG. 4 that 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 S 1 , S 2 , Sn). Speeds S 1 , S 2 . . . Sn on the graph may be been obtained through data analysis of experimental tests campaign. As an example, given a first race rotation speed S 2 , and given a value of the first parameter FP 1 , the value of the load L may be derived graphically as represented in FIG. 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 F 2 a third step F 3 may be performed, where it is verified if the calculated thrust load L is equal or above a first predetermined value L MAX . If the calculated thrust load L is equal or above the predetermined value L MAX , an overload alarm is generated (ninth step F 9 ). The alarm may be simply displayed on the monitor 50 , or the control unit 40 may 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 L MAX , a fourth F 4 and/or a fifth step F 5 may be performed.

As discussed above, after the second step F 2 , the fourth step F 4 may be performed.

In the fourth step F 4 is verified if the first parameter FP falls within a first range HR and, if the first parameter FP falls inside the first range HR, the first step F 1 is performed again with updated or new values (obtained in F 0 , see line A, FIG. 3 ) of the bearing cage rotation speed CS and of the first race rotation speed SS.

If the first parameter FP falls out the first range HR a sixth step F 6 may be performed.

The first range HR may be defined by design characteristics of the bearing, and it is graphically represented in FIG. 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 F 6 may be performed. In this step it is verified if a variation of the first race rotation speed ΔSS corresponds to an expected variation L 1 , L 2 of the bearing cage rotation speed CS, at the calculated thrust load L.

This step is graphically represented in FIG. 5 . Preliminarily it should be noted that the graph of FIG. 5 correlates 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 L 1 , L 2 , Ln. . . . This correlation may be obtained by experimental values.

According to the sixth step F 6 , the predicted variation is obtained by selecting, on the basis of the calculated thrust load L, one load curve L 1 , L 2 between a plurality of load curves L 1 , L 2 each correspondent to a given load (in underload condition), and by verifying if at a determined point CS 1 , SS 1 , 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 L 1 in the same point CS 1 , SS 1 .

Graphically, it is possible to see that the slope of the line R 1 passing for the points CS 1 , SS 1 -CS 3 , SS 3 , representing the variation of CS in function of SS, do not correspond to the slope of the curve L 2 representing the predicted variation of cage speed with respect to the first race speed for the load L 2 . 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 F 7 ) may be generated.

On the contrary the line R 2 passing for the points CS 1 , SS 1 -CS 2 , SS 2 , corresponds (within a predetermined interval) to the slope of the curve L 2 . In this case, the skidding of the bearing is due to underload of the bearing itself. Therefore the control unit 40 (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 F 8 , by comparing the calculated thrust load L with a predetermined design thrust load L D . The predetermined thrust load L D may 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 in FIG. 4 . Given a first parameter value of FP 2 and a first race speed of S 2 , the predetermined thrust load L D is L 2 .

Additionally, after the fourth step F 4 a fifth step F 5 may 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 FP D . 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 unit 40 to trigger the thrust load L adjustment that may be available on the machinery, and it may also be displayed on the monitor 50 , 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.

Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

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.