Abstract:
A process for estimating the lubricant viscosity in a bearing comprises the steps of: monitoring acoustic emissions from the bearing with an acoustic emission sensor; monitoring the bearing for at least one of speed, load and temperature; demodulating the acoustic emission signal from the acoustic emission sensor; calculating the root mean square values of the demodulated acoustic emission signal and storing the calculated root mean square values; aligning the root mean square values with values of one or more of speed, load and temperature and storing the aligned values; using the aligned root mean square values, and at least one of speed, load and temperature in a model which calculates viscosity and/or viscosity ratio from the aligned values.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method for monitoring the lubrication condition in a lubricated rolling element bearing by analyzing a demodulated acoustic emission signal, and in particular to estimating the viscosity of the lubrication film in the bearing. 
       BACKGROUND OF THE INVENTION 
       [0002]    In order for a rolling element bearing to operate in a reliable way it must be lubricated adequately. Lubricant prevents metal to metal contact within the bearing and protects surfaces within the bearing against corrosion. It is therefore important to select the proper lubricant and lubrication method for each individual bearing application, as well as a correct maintenance program. 
         [0003]    In some instances, particularly concerning large machinery that is not easily accessible, such as wind turbines, it is desirable to provide a method of monitoring the lubrication condition of a bearing without affecting normal running of the bearing. 
         [0004]    A method of monitoring the lubrication condition of a rolling element bearing by means of determining a lubrication parameter indicative of the lubrication condition is described in WO2010/085971. In a first step, high-frequency structure-borne acoustic emissions are measured. The measured acoustic emissions are generated as a result of asperity contact between rolling surfaces of the bearing, and provide a measured signal. In a second step, emitted acoustic energy is extracted from the measured signal. In a third step, the lubrication parameter is determined from the emitted acoustic energy, on the basis of a power-law relationship between acoustic energy and the lubrication parameter. The lubrication parameter used to indicate lubrication condition is either specific lubrication film thickness (Lambda) or lubricant viscosity ratio (Kappa). The value of Kappa is the ratio of a lubricant&#39;s actual viscosity to a minimum required viscosity that the lubricant must possess in order to form an adequate lubricant film. The required viscosity is dependent on the size and speed of rotation of the bearing whilst actual viscosity is dependent on the lubricant and temperature. 
         [0005]    In bearing life calculations the actual viscosity in the viscosity ratio (Kappa) is presumed from the viscosity grade of the lubricant, the lubricant operating temperature and bearing speed. 
         [0006]    A bearing life model is a statistical model which says that under a certain set of operating parameters a certain percentage (for example 90%) of bearings of the same type and lubricated in the same way will last a certain number of hours before failure. ISO 281:2007 entitled, “Rolling Bearings—Dynamic Load Ratings and Life” describes such a model. There are two problems with this type of model. The first is that the model cannot predict which particular bearings will be the bearings that last the predicted number of hours and which will fail prior to the predicted life. The second is that the presumed actual viscosity used in the viscosity ratio may not be representative of the actual viscosity if the viscosity were measured. 
         [0007]    If rather than presuming the actual viscosity it can be monitored, then rather than basing a bearing life calculation on a presumption, the bearing life calculation can be based on a monitored estimated actual viscosity ratio in the bearing concerned. This would be of particular value where bearings are of high value and are difficult to access, as servicing or repair of the bearing can be scheduled prior to failure. 
         [0008]    Whilst WO2010/085971 attempts to provide a solution to the above-mentioned problem, it may be difficult to identify acoustic emission signals associated with asperity contacts occurring as the viscosity ratio falls only marginally below 0.5. 
         [0009]    Also, monitoring a raw acoustic emission signal requires sampling and processing equipment capable of sampling and processing MHz signals. Such equipment is sophisticated, expensive and generally for use in the laboratory rather than in the field. 
         [0010]    It would therefore be desirable to be able to ascertain lubricant viscosity, and hence lubricant viscosity ratio more accurately. 
         [0011]    It would also be desirable to be able to ascertain lubricant viscosity and lubricant viscosity ratio using less sophisticated equipment. 
       SUMMARY OF THE INVENTION 
       [0012]    According to the invention there is provided a process for estimating the lubricant viscosity in a lubricated bearing comprising the steps of:
       i) monitoring acoustic emissions from the bearing with an acoustic emission sensor during an acquisition period and time stamping the acquisition period;   ii) monitoring the bearing for at least one of speed, load and temperature and storing in a database time stamped monitored values of said at least one of speed, load and temperature;   iii) demodulating the acoustic emission signal from the acoustic emission sensor;   iv) calculating the root mean square values of the demodulated acoustic emission signal in a data processor and storing the calculated root mean square values in the database;   v) aligning in time the values of the root mean square of the demodulated acoustic emission signal with values of one or more of speed, load and temperature and storing the aligned values in the database;   vi) using the aligned values of the root mean square of the demodulated acoustic emission signal, and at least one of speed, load and temperature in a model which calculates viscosity and/or viscosity ratio from the aligned values.       
 
         [0019]    According to a second aspect of the invention there is provided a process for creating a mathematical model of viscosity or viscosity ratio comprising the steps of:
       i) running a bearing with new lubricant at different speeds, loads or temperatures;   ii) monitoring acoustic emissions from the bearing with an acoustic emission sensor during an acquisition period and time stamping the acquisition period;   iii) monitoring the bearing for at least one of speed, load and temperature and storing in a database time stamped monitored values of said at least one of speed, load and temperature;   iv) demodulating the acoustic emission signal from the acoustic emission sensor;   v) calculating the root mean square values of the demodulated acoustic emission signal in a data processor and storing the calculated root mean square values in the database;   vi) aligning in time the values of the root mean square of the demodulated acoustic emission signal with values of one or more of speed, load and temperature and storing the aligned values in the database;   vii) calculating the viscosity of the new lubricant from a known viscosity/temperature data;   viii) storing the aligned values of the root mean square of the demodulated acoustic emission signal, and at least one of speed, load, temperature and calculated viscosity in a database;   ix) performing multi-variate regression analysis on the stored values and developing a “best fit” model where viscosity or Kappa is the dependent variable and the root means square of the demodulated acoustic emission signal and one of speed, load and temperature are independent variables.       
 
         [0029]    Preferably, the values of the root mean square of the demodulated acoustic emission signal are calculated over an acquisition period, and preferably the acquisition period is aligned in time with values of at least one of speed, load and temperature. 
         [0030]    The acquisition period is preferably between 0.5 and 30 seconds, depending on the rotational speed of the monitored bearing. Smaller acquisition periods are used with faster running bearings and longer acquisition periods with slower running bearings. For example, an acquisition period of less than 1 second, for example 0.8 seconds, may be used with a bearing rotating at 3600 rpm or greater, whereas an acquisition period of 30 seconds may be used with a bearing rotating at 5 rpm. 
         [0031]    Ideally, the acquisition period is sufficiently long to capture four rotations of the bearing. 
         [0032]    Advantageously, the model of step vi in the first aspect of the invention is a model created according to the process of the second aspect of the invention. 
         [0033]    The process for estimating the lubricant viscosity in a bearing may include the step of establishing whether the model has already been created and if not creating the model according to the process of the second aspect of the invention. 
         [0034]    The process for estimating the lubricant viscosity in a bearing may comprise the further step of using the calculated value of viscosity or Kappa in a bearing life model. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0035]    In the drawings, which are by way of example: 
           [0036]      FIG. 1 a    shows a graph illustrating for an oil lubricant the variation of lubricant viscosity ratio Kappa with the root means square of the demodulated acoustic emission signal; 
           [0037]      FIG. 1 b    shows a graph illustrating for a grease lubricant the variation of lubricant viscosity ratio Kappa with the root means square of the demodulated acoustic emission signal; 
           [0038]      FIG. 2  is a flow diagram illustrating the process of the invention; 
           [0039]      FIG. 3  is a block diagram of a signal processing system used in the invention; 
           [0040]      FIG. 4  is a table showing collected model parameters; 
           [0041]      FIG. 5  is a three axis graph representing an example of a model used in the process of the invention; and 
           [0042]      FIG. 6  is schematic representation of an instrumented bearing. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0043]      FIGS. 1 a  and 1 b    show graphs for oil and grease respectively of the typical variation in root means square (RMS) of demodulated acoustic emission signal with changing lubricant viscosity Kappa. The graphs were established by experiment and form part of the state of the art. 
         [0044]    As can be seen, for oil the RMS of the demodulated acoustic emission signal increases significantly when Kappa falls below 1.5, and for grease when Kappa falls below 1.0. 
         [0045]    The symbol “” represents the measured changes in the RMS of the demodulated acoustic emission signal with changing Kappa. For each graph the theoretical curve of variation of the RMS of the demodulated acoustic emission signal where the bearing is rotating at a notional speed is denoted by the symbol “▪”. The theoretical curve is the curve that is the “best fit” to the measured data represented by the symbol “”. 
         [0046]    In each graph, two plots in broken line form are shown, one above and one below the plots of theoretical and measured variation of the RMS of the demodulated acoustic emission signal with Kappa. In each graph the upper and lower plots in broken line form represent the variation in of the RMS of the demodulated acoustic emission signal with kappa for a bearing rotating more quickly and more slowly respectively than the notional speed. 
         [0047]    The prior art, measuring raw acoustic emission signals shows the detection of asperity contacts, which occur when Kappa falls to 0.5. By looking at the RMS of the demodulated acoustic emission signal, changes in Kappa well above 0.5 can be detected and hence a greater knowledge of the changing condition of the bearing can be obtained. 
         [0048]    The flow diagram shown in  FIG. 2  illustrates the process of the invention. 
         [0049]    In step  1 , an acoustic emission sensor (referred to as an AE sensor in  FIG. 2 ) is placed in contact with a bearing or bearing housing and described in greater detail with reference to  FIG. 6 . 
         [0050]    In step  2  the acoustic emission sensor generates a continuous acoustic emission signal from the bearing. The system acquiring the acoustic emission signal for subsequent demodulation will typically have a sampling rate of around 1 to 10 ksps (kilo samples per second) depending on the limitations of the acquisition system and the speed of rotation of the shaft supported by the monitored bearing. Higher shaft speeds allow for faster sampling rates with better RMS variations, but lower shaft speeds often result in lower sampling rates. This is because it is desirable that data is captured over a number of revolutions of the bearing. Looking at two bearings rotating at different speeds, the time period required for the bearing that is rotating more slowly will be greater than the time period required for the more quickly rotating bearing to rotate the same number of times. The system for acquiring the acoustic emission signal typically has a capacity for a maximum number of samples in any one acquisition. In the case of a bearing rotating slowly the combination of the time required for the desired number of revolutions of the bearing to be completed multiplied by a desired sampling rate may result in a total number of samples exceeding the capacity of the acquisition system, and hence it would be necessary to reduce the sampling rate of the system for acquiring the acoustic emission signal. 
         [0051]    In step  3  the raw acoustic emission signal is demodulated using a signal processing system as shown in  FIG. 3 . 
         [0052]    In the processing system shown in  FIG. 3  the output of an acoustic emission sensor  28  forms the input to a pre-amplifier  35  provides a required voltage gain. The amplified signal is passed through a high pass filter  36  set to 100 kHz, that is signals having frequency greater than 100 kHz pass through the filter and those having a lower frequency do not. The output of the high pass filter  36  forms the input to a low pass filter  37  set to 500 kHz, that is signals having frequency lower than 500 kHz pass through the filter and those having a higher frequency do not. The output of the low pass filter  37  forms the input to a full wave rectifier  38 , the output of which forms the input to an anti-aliasing filter  39 , the function of which is to remove noise introduced to the signal by passing the signal from the acoustic emission sensor  28  through the other components signal processing system. The anti-aliasing filter  39  outputs to a balanced driver  40 , which conditions the signal to have power and impedance characteristics that provide for transmission thereof to the data acquisition system of step  4  of the process of the invention. 
         [0053]    In addition to the acoustic emission sensor, the bearing or bearing housing is also equipped with other condition monitoring sensors, in this case a speed sensor a for monitoring the rotational speed of the bearing, a load sensor b for monitoring the load on the bearing and temperature sensor c for monitoring the bearing temperature. Instead of providing a single temperature sensor c, multiple temperature sensors may be provided, allowing the temperature of the bearing to be measured at different positions on the bearing. 
         [0054]    The performance of step  4  requires a processor and data storage. The acquisition system therefore includes a processor and a database. 
         [0055]    The outputs of the speed, load and temperature sensors form inputs to the processor of step  4  and are utilized in the processing step  4 . 
         [0056]    The demodulated acoustic emission signal from step  3  also forms an input to step  4 . 
         [0057]    All the inputs to step  4  are recorded in the database. 
         [0058]    In step  4  the root mean square (RMS) of the demodulated acoustic emission signal is calculated, typically over an acquisition time of between 0.5 and 30 seconds, depending on the rotational speed of the bearing. Ideally, the acquisition period is sufficiently long to capture four rotations of the bearing. 
         [0059]    The RMS of the demodulated acoustic emission signal represents the fluctuation in energy of the demodulated acoustic emission signal. However, the RMS of the demodulated acoustic emission signal has also proved to be proportional to the energy of a continuous raw acoustic emission signal as its fluctuations increase proportionally with its energy within the demodulation&#39;s filter band. 
         [0060]    In a further processing step, the calculated values of the RMS of the demodulated acoustic emission signal are aligned with the speed, load and temperature signals. That is, the time stamping of the speed, load and temperature signals is aligned with the time stamping of the acquisition period, the acquisition period being the period for acquiring the acoustic emission signal for subsequent demodulation. In this way changes in the RMS of the demodulated acoustic emission can be correlated with variations in the other monitored parameters. 
         [0061]    The process uses a model for the particular application of the monitored bearing. The model is created by operating the bearing and monitoring under controlled conditions with new lubricant. This allows a database of values of the RMS of the acoustic emission signal under different loads, speeds and temperatures to be built up when the actual viscosity is known, because the lubricant is new and can safely be assumed to have the viscosity indicated by the lubricant manufacturer. 
         [0062]    In step  5  the process asks whether viscosity estimation model has already been created or not. 
         [0063]    If the viscosity estimation model has not been created, the estimation routine is run. The speed a, load b, temperature c, demodulated acoustic emission signal from step  3 , the RMS of the demodulated acoustic emission signal from step  4  are measured and the viscosity is calculated in step  1   a  for each measured temperature using the viscosity grade/temperature chart from the lubricant manufacturer. 
         [0064]    In step  1   b  the parameters described above are collected. 
         [0065]    Table 1 in  FIG. 4  presents an example set of collected model parameters. In Table 1, the RMS of the demodulated acoustic emission signal is of an AC coupled circuit, temperature is measured in Celsius, Visc represents actual viscosity and V1 required viscosity, both in mm 2 /second and load is measured in kN. 
         [0066]    The time in hours, minutes and seconds is the time stamp for the parameters in a respective row, the acquisition period in this example for each sample being 3.2 seconds, to capture four rotations of the bearing rotating at 80 rpm. 
         [0067]    The acquisition period used in creating the model is then used in the model when the process is used to monitor the same type of bearing in the field. 
         [0068]    As can be seen for the table, the variable in the example is load, with nine or ten samples being taken for each load of 275 kN, 550 kN and 1100 kN. The nominal speed is essentially unchanged, being 80 or 81 rpm for all loads. 
         [0069]    A database of values of the RMS of demodulated acoustic emission signal is built up by testing under different loads, speeds and temperatures. 
         [0070]    In step  1   c  a multivariate regression analysis is performed where viscosity (Kappa) is the dependent variable and speed, load and temperature and RMS of demodulated acoustic emission are the independent variables. 
         [0071]    The outputs are the equation and its constants, which together represent the model. 
         [0072]    The model is stored in step  1   d  for future in field use. 
         [0073]      FIG. 5  is a three axis graph illustrating the multivariate regression analysis. In this case the “best fit” equation, the model, describing the three axis curve is y=a+b/x 1 +c/x 2 , where x 1  is the ratio of measured load to nominal load, x 2  is the RMS of demodulated acoustic emission signal and a, b and c are constants. 
         [0074]    With the model the RMS of demodulated acoustic emission signal may be measured and the equation of the model solved for viscosity or the viscosity ratio (Kappa), which is done in step  7 . 
         [0075]    In step  8  the calculated actual viscosity or viscosity ratio (Kappa) value is then used in the bearing life model. The bearing life model used is based on the ISO 281, “Rolling Bearings—dynamic load ratings and rating life”. 
         [0076]    In the flow diagram of  FIG. 2 , the dashed flow lines represent paths and actions only utilised during the initial creation of the viscosity or Kappa model. 
         [0077]    The process of the invention provides an estimated actual value of viscosity and/or viscosity ratio (Kappa) based on monitoring the condition of a bearing in use in a piece of equipment, instead of the actual lubricant viscosity or Kappa value when the lubricant was new. The initial Kappa value may be 4 for example, but the estimated actual value may only be 1.5 indicating a much shorter bearing life. With this knowledge, action may be taken, for example the lubricant may be changed, or the bearing scheduled for replacement at an earlier time than would have been indicated using the Kappa value when the lubricant was new. 
         [0078]      FIG. 6  shows an example of a system comprising a bearing to be monitored and a lubrication condition monitoring device that is arranged to execute the method according to the invention. The bearing in this example is a radial rolling element bearing  20 , comprising an inner ring  22 , an outer ring  24  and rolling elements  25  disposed therebetween. Thus an outer diameter of the bearing inner ring  22  defines an inner raceway  26  for the rolling elements  25  and an inner diameter of the bearing outer ring  24  defines an outer raceway  27 . Further, the bearing  20  is mounted in a housing  21  and is lubricated with an oil (not shown). 
         [0079]    The lubrication condition monitoring device comprises an acoustic emission sensor  28 , a temperature sensor  29  recording temperature at a specific position on the bearing, a rotational speed sensor  30 , a load sensor  31  and a signal processor  32  that is arranged to record and process the acoustic emission signal, temperature, rotational speed and load signals, detected by the sensor according to the process steps set out in  FIG. 2 . In the example of  FIG. 1 , the acoustic emission sensor  28  is mounted on the bearing outer ring  24 , which ring is non-rotational during bearing operation. The acoustic emission sensor  28  can also be positioned on the housing  21 , or on some other fixed structure to which the bearing is mounted. In applications where the bearing inner ring  22  is non-rotational in operation, the acoustic emission sensor  28  may be mounted, for example, in a notch in an inner diameter (bore) of the inner ring  22 . 
         [0080]    During bearing operation, an outer surface  25   s  of the rolling elements  25  is in rolling contact with the inner raceway  26  and the outer raceway  27 . These rolling contact surfaces  25   s ,  26 ,  27  have a certain roughness due to asperities. Depending on the lubrication condition in the bearing and in particular the lubricant viscosity, the oil film separating the rolling contact surfaces may not be sufficiently thick to prevent asperity contact between the rolling element surfaces  25   s  and the bearing raceways  26 ,  27 . Asperity contact starts to occur when Kappa falls below 0.5. This asperity contact (metal-to-metal contact) within the bearing  20  generates acoustic emission. The level of acoustic emission detected by an acoustic emission sensor mounted in direct or indirect contact with a bearing ring is therefore indicative of the degree of asperity contact between the rolling element surfaces and the bearing raceways, and is thus indicative of the lubrication condition within the bearing. 
         [0081]    To ensure proper transmission of the propagated stress waves to the acoustic emission sensor  28 , it is important that there is good mechanical contact between the acoustic emission sensor  28  and surface of the bearing or bearing housing to which the sensor  28  is mounted. This can be achieved in different ways. One way is to machine a flat surface in the outside diameter of the bearing outer ring  24  and mount the acoustic emission sensor on the flat surface. Since the acoustic emission sensor may be quite small, this mounting technique may also be used relatively small bearings. Another way of mounting the acoustic emission sensor to the bearing is to manufacture the acoustic emission sensor such that its detecting surface has the same radius of curvature as the bearing surface with which it will be in contact. This will also enable a mating contact between the acoustic emission sensor and the bearing. For larger bearings, it is also possible to machine a notch in the outer ring in which the acoustic emission sensor can be mounted. As will be understood, any machining in a bearing ring should be performed such that the function of the bearing is not impaired. 
         [0082]    In addition to ensuring a mating fit between the contacting surfaces of the bearing or bearing housing and the acoustic emission sensor, the transmission of acoustic waves to the acoustic emission sensor can be improved by using grease, e.g. vacuum grease, or an oil as a coupling medium between the sensor and the surface to which it is mounted. 
         [0083]    The acoustic emission sensor  28  can be mounted at any angular position of the bearing. The mounting position will depend on the available space in the machine in which the bearing is used. A preferred mounting position for the acoustic emission sensor is a position that is in line with the direction of the load on the bearing. Hence, for a radially loaded bearing, the acoustic emission sensor  28  should be radially mounted. In the case of thrust bearings, for example, which are subjected to axial loading, the acoustic emission sensor should be axially mounted, i.e. on a side face of the bearing ring that is non-rotational in operation. Furthermore, the mounting position is preferably in line with the bearing loaded zone and in line with the region of rolling contact. Since the acoustic waves propagate from the contact surface between the rolling elements  25   s  and the raceways  26 ,  27 , the signal quality will be best directly opposite the contact region. This will reduce interfering acoustic waves caused by reflections from the measured signal and from acoustic waves from other sources in the machinery. It will also improve the signal-to-noise ratio of the signal. 
         [0084]    Using the RMS of the demodulated acoustic emission signal allows less expensive data acquisition and processing equipment to be used. Also, the degree of expertise required to analyse the signal produced by the process of the invention is significantly less than that required to analyse raw acoustic emission data. 
         [0085]    Also, analysis according to the process of the invention allows all values of lubricant viscosity to be measured not just viscosities indicating asperity contacts. The process of the invention may therefore provide information indicating that lubricant viscosity is trending towards 0.5. This information is valuable in terms of equipment operation and maintenance.