GB2415752A

GB2415752A - A method of montioring the condition of a bearing or engine

Info

Publication number: GB2415752A
Application number: GB0414518A
Authority: GB
Inventors: Ian Graham Pegg; Keith William Barter; Michael Willcock; Michael James Watts
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2004-06-29
Filing date: 2004-06-29
Publication date: 2006-01-04
Anticipated expiration: 2024-06-29
Also published as: JP2006017122A; GB2415752B; GB0414518D0

Abstract

A method of monitoring the condition of a bearing or an engine comprising supplying oil or lubricant to a bearing, determining the viscosity of the oil, determining the pressure of the oil, determining the flow rate of the oil and estimating the bearing clearance for the bearing supplied with the oil based upon the determined viscosity, pressure and flow rate and using the estimate of bearing clearance to determine the condition of the bearing or engine. The condition can be based upon a comparison of the determined bearing clearance or wear rate with a predetermined bearing clearance value or wear rate. A first sensor measures the pressure of the oil, a second sensor measures the viscosity of the oil and a third sensor measures the flow rate of the oil and wherein each sensors sends a signal to an electronic control unit which estimates the bearing clearance.

Description

l; 24 1 5752 203-0826GB

A METHOD FOR MONITORING THE CONDITION OF AN ENGINE

This invention relates to monitoring the condition of a bearing and, in particular, to monitoring the condition of an engine to detect excessive bearing wear.

It is well known that during the life of an internal combustion engine wear occurs to the engine which if allowed to become excessive can result in catastrophic engine failure. This is particularly a problem in the case of internal combustion engines used in commercial vehicles where large annual mileages are involved and where the vehicle often travels a great distance from its home depot.

It is desirable for such vehicles to have a system on the vehicle which can predict engine problems before they become serious or indicate to a user of the vehicle that a major service needs to be carried out. It is known to provide a motor vehicle with a service indicator to warn the user when a routine service is due but these devices normally only rely on counting the engine revolutions since the last service, the determinations of the miles travelled since the last service or the number of hours the engine has operated since the last service to determine when to illuminate the service indicator. Such devices therefore generally do not and are not intended to monitor the actual condition of the engine during use.

It is also desirable after manufacture of an engine to provide a means of checking that the engine has been correctly built or that the components used to build the engine are functioning normally before the engine is despatchedO It is an object of this invention to provide a method that can monitor the condition of an engine so as to 1, 8, c,, 203-0826GB 2 establish when the condition of the engine is suspect and requires investigation.

According to a first aspect of the invention there is provided a method for monitoring the condition of an internal combustion engine having a lubrication system for supplying oil to one or more bearings of the engine wherein the method comprises determining the viscosity of the oil in the lubrication system, determining the pressure of the oil lo in the lubrication system, determining the flow rate of oil through the lubrication system, estimating the bearing clearance for the or all of the bearings supplied with oil based upon the determined viscosity, oil pressure and oil flow rate and using the estimate of bearing clearance to determine the condition of the engine.

The condition of the engine may be determined based upon a comparison of the determined bearing clearance with a predetermined bearing clearance value.

The method may further comprise notifying a user of the engine if the comparison indicates that the bearing clearance is excessive.

The condition of the engine may be determined based upon a comparison of a determined bearing wear rate with a predetermined wear rate.

In which case, the wear rate may be determined based upon two determinations of bearing clearance and the time that has elapsed between the determinations of bearing clearance.

The method may further comprise notifying a user of the engine if the comparison indicates that the bearing wear rate is excessive.

t ' :: e' :: :: le: ece: 203-0826GB - 3 The condition of the engine may be determined based upon a number of estimations of bearing clearance and a determination of the change in wear rate with respect to time.

The method may further comprise notifying a user of the engine if the determination indicates that the rate of bearing wear is increasing.

lo The method may further comprise notifying a user of the engine only if the comparison indicates that the rate of bearing wear is increasing beyond an expected rate of wear.

The method may further comprise estimating the bearing IS clearance for the or all of the bearings supplied with oil based upon the determined viscosity, oil pressure, oil flow rate and engine speed.

The bearing clearance may be only estimated after one or more predetermined engine operating conditions have been established.

The predetermined engine conditions may include a confirmation that the temperature of the oil has reached a predetermined temperature and a confirmation that a minimum period of time has elapsed after engine startup.

According to a second aspect of the invention there is provided an engine system having a lubrication system including a pump for supplying oil to one or more bearings of the engine, a first sensor to provide a signal indicative of the pressure of the oil in the lubrication system, a second sensor to provide a signal indicative of the viscosity of the oil, a means for providing a signal indicative of the flow rate of oil from the pump and an electronic control unit to receive the signals from the first and second sensors and the signal indicative of flow 1 1 1,,, , : l. : 203-0826GB - 4 - rate wherein the electronic control unit is operable to estimate the bearing clearance for the or all of the bearings supplied with oil based upon the signals received from the first and second sensors and the signal indicative of oil flow rate and use the estimate of bearing clearance to determine the condition of the engine.

The electronic control unit may be further operable to notify a user of the engine if the bearing clearance is lo determined to be excessive.

The electronic control unit may be further operable to determine a bearing wear rate from two estimated bearing clearances and notify a user of the engine if the bearing wear rate is excessive.

The electronic control unit may be further operable to calculate a bearing wear rate from a number of estimated bearing clearances and determine whether the rate of wear is increasing.

The electronic control unit may be further operable to notify a user of the engine if the bearing wear rate is increasing.

According to a third aspect of the invention there is provided a method for monitoring the condition of at least one plain bearing fed with lubricant at pressure from a lubrication system having a pump to circulate the lubricant through the lubrication system wherein the method comprises determining the viscosity of the oil in the lubrication system, determining the pressure of the oil in the lubrication system, determining the flow rate of lubricant through the lubrication system, estimating the bearing clearance for the or all of the bearings supplied with lubricant based upon the determined viscosity, lubricant pressure and lubricant flow rate and using the estimate of 1, , , 8 , , , 203-0826GB - 5 bearing clearance to determine the condition of all of the bearings.

The condition of all of the bearings may be determined based upon a comparison of the determined bearing clearance with a predetermined bearing clearance value. The method may further comprise notifying a user of the machine to which all of the bearings are fitted if the comparison indicates that the bearing clearance is excessive.

The condition of all of the bearings may be determined based upon a comparison of a determined bearing wear rate with a predetermined wear rate. in which case, the wear rate may be determined based upon two determinations of bearing clearance and the time that has elapsed between the determinations of bearing clearance. The method may further comprise notifying a user of the machine to which all of the bearings are fitted if the comparison indicates that the bearing wear rate is excessive.

The condition of all of the bearings may be determined based upon a number of estimations of bearing clearance and a determination of the change in wear rate with respect to time. The method may further comprise notifying a user of the machine to which all of the bearings are fitted if the determination indicates that the rate of bearing wear is increasing. The method may further comprise notifying a user of the machine only if the comparison indicates that the rate of bearing wear is increasing beyond an expected rate of wear.

The invention will now be described by way of example with reference to the accompanying drawing of which: Fig.1 is a diagrammatic representation of an engine system according to the invention; and :: ce:: :: e 203-0826GB - 6 Fig.2 is a flowchart of a method according to the invention.

With reference to Fig.1 there is shown an engine system 1 comprising an engine 2 and an electronic control unit 20 for controlling the operation of the engine 2.

The engine 2 is a reciprocating piston engine and has a number of pistons (not shown) attached to a crankshaft (not lo shown) having a flywheel 9 attached to one end. As is well known in the art the crankshaft is supported by a number of main bearings and each of the pistons is connected to the crankshaft by a connecting rod having small and big end bearings.

A lubrication system for the engine 2 comprises of a pump 4 having a pickup connected to a an oil storage reservoir in the form of a sump 3 and an outlet connected via a filter 5 to various internal passageways or external conduits used to supply oil to all of the bearings associated with the engine 2. The bearings of the engine are shown diagrammatically on Fig.1 by the orifice 7 as they represent a resistance to flow of lubricant in the form of the oil which is circulated through the lubrication system.

After passing through the various bearings 7 the oil is returned to the sump 3 for re-use.

In this case the pump is a variable flow rate pump in the form of a variable displacement pump 4 driven directly by a power takeoff from the engine and in this case the pump is driven by a chain. The pump 4 includes a means for varying the flow rate of oil from the pump 4 for a given rotational speed. In this case the pump is a vane pump 4 with a variable offset however it will be appreciated that other forms of variable displacement pump could be used.

:: : :: :: : 203-0826GB - 7 - The electronic control unit 20 as shown in Fig.1 is a single unit controlling all functions of the engine 2 but it will be appreciated that several different electronic control units could be used or the electronic control unit could form part of a general vehicle electronic controller.

The only thing that is important is that a means for collecting and processing information is provided for the engine 2.

lo The electronic control unit 20 receives a number of sensor inputs and is programmed to use these inputs to determine various indicators of engine condition as will be described later herein in greater detail.

A first sensor in the form of a pressure sensor 6 is used to supply the control unit 20 with a signal indicative of the pressure of the oil in the lubrication system and in this case the pressure of the oil in a main oil gallery of the engine 2.

A second sensor 10 is used to supply the electronic control unit 20 with a signal indicative of the current viscosity of the oil in the sump 3. Such sensors are known from, for example, DE-A-101 12 433 where an oscillator is inserted into the oil and the oscillation frequency and damping of the fluid are observed and the viscosity is inferred based on the shift of oscillation frequency and/or altered damping. Further examples of devices for determining the viscosity of a fluid can be found in US Patents 3,215,185 and 6,543,274 and European Patent applications 1, 393,041 and 1,405,052 However, it will be appreciated that any suitable device for determining the viscosity of the oil could be used.

A third sensor in the form of an engine speed sensor 8 is used to supply a signal indicative of the rotational speed of the engine 2. In this case the sensor 8 is an 4, ' 4 4' 203-0826GB - 8 - inductive pick-up which is placed in close proximity to a number of teeth formed in a ring gear attached to the flywheel 9 but other forms of engine speed sensor could also be used.

A fourth input is supplied from the variable displacement pump 4 which is indicative of the current displacement per revolution of the pump. It will be

understood from the foregoing description that the

lo displacement per revolution of the pump 4 can be varied.

The signal from the engine speed sensor 8 and the feedback from the pump 4 form a means for providing a signal indicative of the flow rate from the pump 4 in accordance with the following equation: Q = ES*GR*D.

Where: ?O Q iS the flow rate of oil in litres per second; ES is the rotation speed of the engine in revolutions per second; OR is the number of pump revolutions for a single engine revolution; and D is the current displacement per revolution of the pump in litres.

Therefore for any given engine speed the electronic control unit 20 is provided with an indication of the current oil flow rate from the pump 4.

It will also be appreciated that a variable flow rate pump can take the form of an electrically or hydraulically driven pump of known displacement per revolution in which the flow rate is varied by varying the rotational speed of the pump. In this case the means for providing a signal indicative of flow rate of oil from the pump is a signal or I t 7 r e 203-0826GB _ 9 _ feedback of the actual rotational speed of the pump from which the flow rate can be simply calculated. However, it will be appreciated that any arrangement for providing a variable flow rate could be used.

The electronic control unit 20 is also connected to a device to notify a user of the engine 2 when the condition of the engine 2 is considered to be worth investigation. In this case the means used to notify the user is a warning 0 lamp 11 but it will be appreciated that other means such as an LCD display or audible warning device could be used.

Operation of the system is best understood with reference to not only Fig. 1 but also Fig.2 which shows a method embedded within the electronic control unit 20.

The method starts at step 100, this can be start-up of the engine 2 but is preferably when one or more engine operating conditions have been met to ensure that the oil in the lubrication system is stable and not subject to large changes in viscosity. In this case the method starts when a predetermined time has elapsed since engine start-up and uses the test 'Is T > Tmin' where T is the measured time since start-up and Tmin is a predetermined period of time established to be sufficient for the viscosity of the oil to have become relatively stable. If the answer to the test is Yes then the method starts and proceeds to step 110 but if the answer is No then the method does not start but cycles back though step 100 to recheck the elapsed time.

An alternative operating condition is the temperature of the oil in the sump and in this case the test would be Is Tp >Tpmin' where Tp is the temperature of the oil and Tpmin is a predetermined oil temperature above which changes in viscosity are less dramatic. As before, if the answer to the test is Yes then the method starts and proceeds to step 1 ' 1 1 1 1 1 $ 1 J I I r I I 1 e I I 203-0826GB - 10 but if the answer is No then the method does not start but cycles though step 100 to recheck the temperature.

It will be appreciated that both tests could be used such that if either of the tests is satisfied the method proceeds. It will be appreciated that the purpose of this start procedure is to simplify and improve the accuracy of all subsequent determinations because, as is well known in the art, the viscosity of the oil will change dramatically lo from the time when an engine is started from cold ambient conditions to the time when the engine is close to its normal running condition. However, the method could be used from immediately after start up if required.

In step 110 the viscosity of the oil is determined from the signal sent from the sensor 10 and various algorithms stored in the electronic control unit 20.

The method then proceeds to step 120 where the oil pressure is determined from the signal received from the oil pressure sensor 6.

The method then proceeds to step 130 where the flow rate of oil is determined from the signals received from the engine speed sensor 8 and the pump 4 as previously discussed.

It will be appreciated that the steps 110 to 130 can be performed in any order or can be performed at the same time all that is required is that values for viscosity, pressure and flow rate are calculated or derived from look-up charts stored in the electronic control unit 20.

In step 140 an estimate of current bearing clearances is calculated based upon the values of viscosity, pressure and flow-rate already produced This estimate is performed using algorithms generated by the use of design of tle l' e' . et:: ce ce.e 203-0826GB - 11 - experiment techniques in which various parameters are measured for differing bearing clearances. It will be appreciated by those skilled in the art that the most significant bearings are the mains and big-end bearings and so that F provided a range of tests are conducted for various clearances for these bearing, the algorithms produced are relatively representative of bearing clearance for a range of viscosities, pressures and flow-rates.

lo Alternatively, a number of three dimensional maps could be produced and stored in the electronic control unit 20 and these could then be used to estimate the bearing clearance by interpolation between the various three dimensional maps.

It will be appreciated that such interpolation is required because the relationship between bearing clearance and viscosity/pressure/flow rate is a four dimensional one.

It will be appreciated that because the oil flows through several bearings and passageways in the engine 2 the result produced in step 140 is only an estimate of the likely main and big end bearing wear and is not an actual calculation of that wear.

After the estimate of bearing clearance has been produced in step 140 it is compared with a reference value to determine whether the bearing clearance is excessive If the bearing clearance is not considered to be excessive that is to say the estimated wear is less than the reference value the method proceeds to step 160. If the bearing wear is excessive, then the method proceeds to step 170 in which a user is notified by illuminating the warning lamp 11A In step 160 the method determines whether the wear rate is excessive by comparing the current bearing clearance with a previous estimate of bearing clearance and determining the wear per unit time. This is then compared to a predetermined predicted rate of wear for normal engine use 1 203-0826GB - 12 to determine whether the wear rate is excessive. It will be appreciated that the wear rate could also be a rate of wear per revolution of the engine 2.

If the wear rate is determined to be excessive, that is to say greater than that predicted, the method proceeds to step 170 in which a user is notified by illuminating the warning lamp 11.

lo If the wear rate is determined not to be excessive then the method is repeated to once more estimate the bearing wear, this may take place immediately or may be delayed for a predetermined period of time or number of engine revolutions.

It will be appreciated that steps 150 and 160 could be reversed.

After step 170 has been reached the method automatically proceeds to step 180. This is because once a notification signal has been sent to the user the signal is continuously generated until it is reset by the use of an external electronic service device. This ensures that a user of the engine 2 is continuously reminded to seek help in investigating the cause of the warning signal thereby encouraging the taking of early remedial action and hence reducing the risk of unnecessary engine damage.

In a further refinement of the method a further step may be included between steps 160 and 170 in which not only the rate of wear is determined but also whether the rate of bearing wear rate is increasing. This is achieved by comparing a number of determinations of bearing wear rate and determining how the wear rate has changed over time. If the bearing wear rate is increasing then a user can be notified in the manner previously discussed or the user can be notified only when the rate of bearing wear is increasing 203-0826GB - 13 - beyond a predetermined expected rate of increase of wear.

If the wear rate is not increasing then the method would revert to step 100.

As yet a further refinement to the method the bearing clearance can be estimated based not only on the pressure, viscosity and flow rate but also on the engine speed as there is a small change in the relationship between bearing clearance and the other three parameters depending upon the lo actual rotational speed experienced by each bearing.

It will be appreciated that some engines utilise features such as piston cooling jets and bypass filtration which will affect the aforementioned relationships. Two methods can be used for dealing with these types of situation either the method can include further tests in step 100 to prevent the estimation of bearing clearance when such features are active or indications of when these features are active can be supplied to the electronic control unit 20 and the electronic control unit 20 can then be programmed to take these features into account so as to produce a reliable estimate of bearing clearance.

Although the invention has so far been described with reference to a system in which a variable flow rate pump is used it will be appreciated that it could be used for a system using a conventional oil pump and pressure relief valve arrangement. However, its use for such an arrangement is limited to circumstances when the pressure relief valve is not open if the relationship between engine speed and flow rate is to be used, as once the pressure relief valve opens the pressure limits the flow rate and so the flow rate is not related directly to engine speed. That is to say the method can only be used at low engine speeds when the oil is relatively warm for outside of these conditions the relief valve will be open. This disadvantage could be overcome by using an oil flow rate sensor in the engine to measure the i: : : : . 203-0826GB - 14 oil entering the main gallery but this will add further complexity and cost to the system.

Although as described above the invention utilises the estimation of bearing clearance to determine when the condition of the engine requires investigation it will be appreciated that the method is actually estimating changes in resistance to oil flow through the engine which are then used to provide an indication that the engine is not lo functioning correctly. It will be also appreciated that further analysis of the engine can be achieved by using more than one of the specific tests. For example, if the test for wear rate indicates that this is low but the bearing clearance test indicates that the clearance is excessive, this would indicate that the bearing clearance is likely to be normal but that some other problem exists with the engine.

Therefore in summary the invention provides a method that can be used to determine during use of an engine whether its condition is deteriorating and provide early warning of potential failure to a user of the engine or be used to indicate to a user that a major engine service is required. It will be further appreciated that the method could also be used in a manufacturing environment to establish at the end of line or pre-delivery stage whether the engine is correctly assembled and whether the lubrication system is functioning correctly For example, if an end of line test indicates that the bearing clearances are excessive this is unlikely to be the case as no significant wear will have taken place. Such an indication will therefore indicate some additional problem with the engine such as a faulty oil pump, a leak in the lubrication system or perhaps incorrect assembly of the engine. The use of such a method would therefore reduce the risk of faulty engines being despatched.

. :. tee. e: :. cete 203-0826GB - 15 It will be appreciated that the invention could also be applied to other situations in which the condition of plain bearings forming part of a machine need to be assessed during use.

For example and without limitation, the method can be applied to a machine having at least one shaft rotatably supported by one or more plain bearings fed with lubricant such as oil by a lubrication system having a pump. Examples lo of such machines includes, but is not limited to, gearboxes, machine tools, rolling machines and rotary printing presses.

In this case the method comprises determining the viscosity of the oil in the lubrication system, determining the pressure of the oil in the lubrication system, determining the flow rate of lubricant through the lubrication system, estimating the bearing clearance for the or all of the bearings supplied with lubricant based upon the determined viscosity, lubricant pressure and lubricant flow rate and using the estimate of bearing clearance to determine the condition of all of the bearings. If there is only one bearing then the result is an indication of the condition of that bearing but if there are several bearing the result is a determination of the overall condition of all of the bearings being monitored.

The condition of all of the bearings is determined in several ways, firstly based upon a comparison of the determined bearing clearance with a predetermined bearing clearance value. If the comparison indicates that the bearing clearance is excessive a user of the machine is notified.

Secondly, the condition of all of the bearings is determined based upon a comparison of a determined bearing wear rate with a predetermined wear rate. In which case, the wear rate is determined based upon two determinations of el.' .. 'se. .:: eee.

203-0826GB - 16 - bearing clearance and the time that has elapsed between the determinations of bearing clearance. If the comparison indicates that the bearing wear rate is excessive then a user of the machine is notified.

The condition of all of the bearings is also determined based upon a number of estimations of bearing clearance and a determination of the change in wear rate with respect to time. If the determination indicates that the rate of lo bearing wear is increasing then a user of the machine is notified or is only notified if the comparison indicates that the rate of bearing wear is increasing beyond an expected rate of wear.

The method therefore provides a convenient way of monitoring bearing degeneration and alerting a user before the situation becomes critical and can also forewarn of failures in the lubrication system itself.

It will be appreciated by those skilled in the art that although the invention has been described by way of example with reference to a number of specific embodiments it is not limited to these embodiments and that various alternative embodiments or modifications to the disclosed embodiments could be made without departing from the scope of the invention.

Claims

1 I, 203-0826GB - 17 Claims 1. A method for monitoring the condition of an

internal combustion engine having a lubrication system for supplying oil to one or more bearings of the engine wherein the method comprises determining the viscosity of the oil in the lubrication system, determining the pressure of the oil in the lubrication system, determining the flow rate of oil through the lubrication system, estimating the bearing lo clearance for the or all of the bearings supplied with oil based upon the determined viscosity, oil pressure and oil flow rate and using the estimate of bearing clearance to determine the condition of the engine.

is

2. A method as claimed in claim 1 wherein the condition of the engine is determined based upon a comparison of the determined bearing clearance with a predetermined bearing clearance value.

3. A method as claimed in claim 2 wherein the method further comprises notifying a user of the engine if the comparison indicates that the bearing clearance is excessive.

4. A method as claimed in any of claims 1 to 3 wherein the condition of the engine is determined based upon a comparison of a determined bearing wear rate with a predetermined wear rate.

5 A method as claimed in claim 4 wherein the wear rate is determined based upon two determinations of bearing clearance and the time that has elapsed between the determinations of bearing clearance.

6. A method as claimed in claim 4 or in claim 5 wherein the method further comprises notifying a user of the . 2 O 3-08 2 6GB - 18 engine if the comparison indicates that the bearing wear rate is excessive.

7. A method as claimed in any of claims 1 to 6 wherein the condition of the engine is determined based upon a number of estimations of bearing clearance and a determination of the change in wear rate with respect to time.

lo

8. A method as claimed in claim 7 wherein the method further comprises notifying a user of the engine if the determination indicates that the rate of bearing wear is increasing.

9. A method as claimed in claim 8 wherein the method further comprises notifying a user of the engine only if the comparison indicates that the rate of bearing wear is increasing beyond an expected rate of wear.

10. A method as claimed in any of claims 1 to 9 wherein the method further comprises estimating the bearing clearance for the or all of the bearings supplied with oil based upon the determined viscosity, oil pressure, oil flow rate and engine speed.

11. A method as claimed in any of claims 1 to 10 wherein the bearing clearance is only estimated after one or more predetermined engine operating conditions have been established.

12A A method as claimed in claim 11 wherein the predetermined engine conditions include a confirmation that the temperature of the oil has reached a predetermined temperature.

13. A method as claimed in claim 11 or in claim 12 wherein the predetermined engine operating conditions At; * 203-0826GB - 19 include a confirmation that a minimum period of time has elapsed after engine startup.

14. An engine system having a lubrication system including a pump for supplying oil to one or more bearings of the engine, a first sensor to provide a signal indicative of the pressure of the oil in the lubrication system, a second sensor to provide a signal indicative of the viscosity of the oil, a means for providing a signal lo indicative of the flow rate of oil from the pump and an electronic control unit to receive the signals from the first and second sensors and the signal indicative of flow rate wherein the electronic control unit is operable to estimate the bearing clearance for the or all of the bearings supplied with oil based upon the signals received from the first and second sensors and the signal indicative of oil flow rate and use the estimate of bearing clearance to determine the condition of the engine.

?0

15. A system as claimed in claim 14 wherein the electronic control unit is further operable to notify a user of the engine if the bearing clearance is determined to be excessive.

16. A system as claimed in claim 14 or in claim 15 wherein the electronic control unit is further operable to determine a bearing wear rate from two estimated bearing clearances and notify a user of the engine if the bearing wear rate is excessive.

17. A system as claimed in any of claims 14 to 16 wherein the electronic control unit is further operable to calculate a bearing wear rate from a number of estimated bearing clearances and determine whether the rate of wear is increasing.

':' c' '. t' 203-0826GB - 20

18. A system as claimed in claim 17 wherein the electronic control unit is further operable to notify a user of the engine if the bearing wear rate is increasing.

19. A method for monitoring the condition of an engine substantially as described herein with reference to the accompanying drawing.

20. An engine system substantially as described herein lo with reference to the accompanying drawing.

21. A method for monitoring the condition of at least one plain bearing fed with lubricant at pressure from a lubrication system having a pump to circulate the lubricant through the lubrication system wherein the method comprises determining the viscosity of the oil in the lubrication system, determining the pressure of the oil in the lubrication system, determining the flow rate of lubricant through the lubrication system, estimating the bearing clearance for the or all of the bearings supplied with lubricant based upon the determined viscosity, lubricant pressure and lubricant flow rate and using the estimate of bearing clearance to determine the condition of all of the bearings.

22. A method as claimed in claim 21 wherein the condition of all of the bearings is determined based upon a comparison of the determined bearing clearance with a predetermined bearing clearance value

23. A method as claimed in claim 22 wherein the method further comprises notifying a user of the machine to which all of the bearings are fitted if the comparison indicates that the bearing clearance is excessive.

240 A method as claimed in any of claims 21 to 23 wherein the condition of all of the bearings is determined 1 r ; :' : 203-0826GB - 21 based upon a comparison of a determined bearing wear rate with a predetermined wear rate.

25. A method as claimed in claim 24 wherein the wear rate is determined based upon two determinations of bearing clearance and the time that has elapsed between the determinations of bearing clearance.

26. A method as claimed in claim 24 or in claim 25 lO wherein the method further comprises notifying a user of the machine to which all of the bearings are fitted if the comparison indicates that the bearing wear rate is excessive.

27. A method as claimed in any of claims 21 to 26 wherein the condition of all of the bearings is determined based upon a number of estimations of bearing clearance and a determination of the change in wear rate with respect to time.

28. A method as claimed in claim 27 wherein the method further comprises notifying a user of the machine to which all of the bearings are fitted if the determination indicates that the rate of bearing wear is increasing.

29 A method as claimed in claim 28 wherein the method further comprises notifying a user of the machine only if the comparison indicates that the rate of bearing wear is increasing beyond an expected rate of wear.

30. A method for monitoring the condition of at least one plain bearing fed with lubricant at pressure from a lubrication system substantially as described herein with reference to the accompanying drawing.