Abstract:
A method and apparatus for anticipating the onset of or measurement of a deleterious condition in a bearing comprising monitoring the bearing for electrostatic activity indicative of the onset of the condition using one or more electrostatic sensors, processing signals derived from the sensors and employing same to activate alarm means, alter bearing loading to reduce or eliminate the source condition or shut down the equipment to prevent further bearing damage.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and means for monitoring the performance of mechanical bearings. 
     BACKGROUND OF THE INVENTION 
     Mechanical bearings, such as ball or roller bearings, are designed to operate under predetermined load conditions. When correctly loaded bearings work well. However at loads approaching zero, slippage can occur between components of the bearing. With changing loads the bearing races can move out of their correct optimum positions. Vibration and impact can also damage bearing components. Hitherto it has been difficult to sense the onset of slippage or other conditions giving rise to bearing damage with the result that bearings are generally over-engineered relative to the loading and other conditions under which they are designed to operate. 
     When a mechanical rolling element bearing operates correctly the surface rotation speed of the rolling elements of the bearing are the same as the differential speed between the inner and outer races. The bearing elements will appear to rotate at around half the differential speed of the inner and outer components of the bearing. If a bearing slips, overheating and damage can result. The surface speed of the rolling elements will reduce and the speed of the cage in which they are mounted will be less than the differential speed of the inner and outer races. Detection of these conditions therefore provides early warning of potential bearing failure. 
     Hitherto sensing of slippage or other stress conditions in such bearings has been relatively complex. Previous proposals include the use of thermal sensors to detect overheating of the lubricant film in which the bearing operates or have required incorporation of radioactive tracers in the bearing components themselves. Such systems are complex. In addition they operate by detecting slippage and cannot detect overstressing of bearings through other causes. Previous proposals have also included proximity sensors operative to detect the speed of rotation of bearing rolling elements and races. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved method and apparatus for monitoring bearing performance. 
     We have found that bearings subject to stress or slippage generate a significant electrostatic charge, proportional to the degree of stress and a change of charge distribution under slippage. Thus it is possible to sense the condition of a bearing by sensing the resultant change and pattern of electrostatic activity. Accordingly the invention provides a method of anticipating the onset of a deleterious condition in a bearing comprising monitoring the bearing for electrostatic activity indicative of the onset of distress of the bearing or its components. 
     Preferably the method includes generation of an output signal responsive to detection of said change in electrostatic activity. Said output signal may be employed to actuate indicator or alarm means providing a visual and/or audible warning of the onset of the said condition. Alternatively or in addition said signal may be employed to rectify a condition giving rise to stressing or slippage of the bearing. Thus the output signal may be employed to alter the bearing loading to remove the source of the deleterious condition. Alternatively said output signal may be employed to terminate operation of equipment of which the bearing forms a part to prevent further bearing damage, or to limit the ‘life’ or further running of the bearing to prevent failure. 
     The invention also provides apparatus for anticipating the onset of a deleterious condition in a bearing comprising means for sensing electrostatic activity in the bearing generated in response to the onset of distress of the bearing or its components. 
     Preferably said means for measuring electrostatic activity comprises an electrostatic sensor adapted to be mounted within sensing proximity range outside the bearing or within the bearing structure and operable to detect changes in electrostatic charge on components of the bearing. 
     Preferably also the apparatus includes means for generating a signal in response to said changes in activity and means for processing said signal to produce an electrical output. 
     The apparatus may also optionally include means for detecting individual bearing elements and for processing of signal(s) derived therefrom. 
     The apparatus may include visual or audible alarm means operable in response to said output to provide a warning of the onset of said deleterious condition. Alternatively or in addition the apparatus may include means operable in response to said output to alter the loading on the bearing to reduce or eliminate said condition. The apparatus may alternatively include means operable in response to said output to interrupt operation of equipment of which said bearing forms a part to prevent further damage to the bearing. 
     The output signal may be recorded or stored to show events in the bearing life/performance as part of an engine/shaft/bearing health management system. In this way a continuous record of bearing performance over time may be produced and may be employed to predict remaining or used bearing life. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
     FIG. 1 is a diagrammatic cross-section through part of a gas turbine engine incorporating means for monitoring bearing performance according to the invention; 
     FIG. 2 a  is a diagram of the signal produced by a bearing under normal operating conditions; and 
     FIG. 2 b  is a similar diagram showing the signal produced from a bearing undergoing or about to undergo slip. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, there is shown the rear bearing compartment of a gas turbine aircraft engine incorporating a compartment housing cap  5  within which a hollow compressor shaft  6  is supported at its rear end in a mechanical rolling element bearing  7  mounted in the cap structure  5 . The bearing  7  comprises a plurality of rolling elements  7   a  in the form of balls or rollers retained between inner and outer races  7   b  and  7   c.  Lubricating oil is supplied to the bearing through oil inlet and outlet pipes  8  and  9  housed within fixed support members  10  by means of which the cap structure  5  is mounted within a surrounding plug assembly (not shown). The components of the bearing may be metallic, non-metallic or a combination of metallic and non-metallic components. 
     An electrostatic sensor  12  is mounted in a fixed position adjacent the bearing  7  and is connected in an electrical circuit  13  with a processor  14  operable to generate a signal in response to changes in electrostatic charge sensed by the sensor  12 . Any suitable form of electrostatic sensor may be employed, for example, sensors of the type disclosed in International Patent Specification No. WO92/02886 or U.K. Patent Specification No. 2302593. 
     Under static conditions the sensor measures ball or roller distress which bears a relationship to contact load. If the bearing  7  is subjected to overload or slippage resulting in changes of relative speed and /or direction of movement of the rolling elements of the bearing in relation to the bearing races in which they run, a measurable change of electrostatic charge is generated in the components of the bearing and sensed by the sensor  12 . This change in electrostatic charge arises as stress or slippage conditions are approached and the sensor  12  thus generates a signal indicative of the onset of such a condition. The signal is transmitted to the processor  14  which generates an output which may be used to perform any of a number of functions. 
     FIGS. 2 a,    2   b  are illustrative of the signal produced from a bearing under normal and stressed conditions respectively. The signal generated under normal operation is of generally sinusoidal form. Under stress a narrow and increased positive peak is generated as shown in FIG. 2 b,  the amplitude of which is proportional to the degree of stress in the bearing. During initial setting up, the sensor may be adjusted to measure the electrostatic charge under static conditions (and hence the initial level of ball or roller distress) on to which the variable change is then superimposed. Alternatively the sensor may be adjusted to zero reading under static conditions so as to measure only the variable electrostatic charge generated under operating conditions. 
     In one arrangement the output from the processor  14  may be employed to energize an audible or visual warning device  15  to warn of the onset of the adverse condition in the bearing. Alternatively, or in addition the processor  14  may be connected to a control unit  16  operable to alter the source of the stress or slippage as disclosed for example in co-pending United Kingdom Application No. 9912108.9. In such cases restoration of satisfactory loading conditions will restore charge levels being detected by the sensor  12  and processor  14  to normal and thereby remove the corrective action once optimum operating condictions have been restored. In this way the sensor  12  forms part of a continuous monitoring and control system serving to sense the onset of slippage or overstressing of the bearing and operable to reduce or eliminate the source condition and thereby minimize wear or damage to the bearing and increase bearing life. 
     By virtue of the use of an electrostatic sensor positioned in close proximity to the bearing to be monitored, the method and apparatus are non-invasive and less complex than techniques proposed hitherto since, as shown in FIG. 1, the sensor  12  is free of any wire electrical connection to the bearing  7 . Moreover by sensing changes in electrostatic charge generated within the bearing components as a result of the onset of a deleterious condition, the arrangement described is effective not only to sense the onset of slippage, but also other stress conditions within the bearing not hitherto detectable by previously proposed techniques. The output signal from the processor  14  may be recorded and stored in a recording device  17  to show events in the bearing life/performance as part of an engine/shaft/bearing health management system. In this way a continuous record of bearing performance over time may be produced and may be employed to predict remaining or used bearing life. 
     Various modifications may be made without departing from the invention. For example while the invention has been described by reference to a shaft bearing in a gas turbine engine, it may be employed in any situation where mechanical bearings are employed in order to continuously monitor bearing performance and provide early warning of stress or slippage and, where appropriate, initiate corrective action to reduce or eliminate the source of the problem. In some cases the signal generated by the sensor or processor may be employed through a control unit such as the unit  16  to shut down equipment of which the bearing forms a part in order to prevent further or accelerated damage. It will be appreciated also that in the case of gas turbine engines a similar sensing system may be employed in association with any or all of the bearings on which the various shafts incorporated in such engines are rotatably mounted. 
     In a further modification an additional sensor  20  may be located adjacent to the bearing to detect the presence of each rolling element as it moves past the sensor. By correlating signals from both sensors, the individual ball, roller or like bearing element giving rise to a stress or slippage signal may be identified. 
     Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.