Grease Sensor in Bearing Condition Monitoring

A new sensor has been developed that enables the online condition monitoring of grease in rolling bearings. The sensor is claimed to be ideal for monitoring critical plant and machinery located in difficult-to-access areas, such as wind turbines and automated assembly lines.

Developed by The Schaeffler Group in conjunction with Freudenberg Dichtungsund Schwingungstechnik GmbH & Co. KG and lubricants specialist Kliiber Lubrication, the new grease sensor incorporates what is claimed to be a unique electronic evaluation system, which enables the condition of the grease to be monitored while the bearings are operating. The sensor is positioned directly in the rolling bearing immersed in the grease.

This is a significant breakthrough, as the schedule for replacing rolling bearing grease can now be planned precisely into maintenance schedules. Any changes in the condition of the grease are detected early, long before any damage can be caused to the bearings.

Grease operating life is key to maintenance
In preventive maintenance regimes, the operating life of the grease is critical, particularly if the life of the grease is less than the expected life of the bearing. In this case, the bearings would normally be re-lubricated halfway through the grease operating life. The disadvantage here is that the grease is replaced without knowing anything about the actual condition of the grease. For example, it may have been possible to continue using the grease for a longer time period without affecting the performance of the bearing. Conversely, perhaps damage has already occurred to the bearing due to ingress of water or high temperatures and so the grease should have been replaced earlier.

Up to now, users could only gather information about the condition of the grease inside a bearing by taking a sample and then conducting costly, timeconsuming analyses of that sample in a laboratory. However, the new grease sensor enables grease to be replaced according to the actual operating requirements of the bearing and not according to any pre-defined time periods. This is referred to as 'demand-based' rather than 'time-based' re-lubrication. The result is a reduction in the cost of lubricants, replacement parts and maintenance, while also benefiting the environment in terms of the volume of lubricant used. In addition, operational downtime of plant and machinery will be reduced, whilst process efficiencies and machine utilisation will also improve.

How does it work ?
The grease sensor, which has a diameter of just 5mm and a length of 40mm, is able to detect four parameters of the grease: water content, cloudiness (opacity), wear (thermal or mechanical), and temperature. From these parameters, the sensor's electronic evaluation system utilises complex software algorithms to generate an analogue signal (4-2OmA), which then displays the condition of the grease.

By setting alarm thresholds (limit values), digital signal outputs can also be generated, indicating whether the grease quality is `poor' or `good'. The user can decide at which point in the condition of the grease (from 100 per cent for as-new, to a theoretical 0 per cent for an unusable grease) re-lubrication or grease replacement should be carried out.

Process development - The Fraunhofer Institute
The sensor operates by using the optical, near-infrared reflection principle. This method - developed in conjunction with the Fraunhofer Institute for Electronic Nano Systems (ENAS) in Germany - is based on an infrared process used by laboratories to measure the quality of grease, but has been adapted for online measurements in rolling bearings. The know-how involved in developing the grease sensor is not only in the set-up of the sensor, but also in how measurements are evaluated.

In terms of evaluating measurements, the method involves the rotationally symmetrical irradiation of the grease at an angle of 45 degrees using certain wavelengths within the infrared spectrum by the sensor. The sensor head is embedded directly in the lubricant during this procedure. The reflected light is measured perpendicular to the grease, which enables any shadow effects or surface anomalies to be completely excluded. The reflected light is then evaluated in terms of the quality of the grease.

In terms of sensor set up, the optimum measurement point will vary depending on the application. Experienced application engineers at Schaeffler will provide advice and guidance to customers and will specify where the sensor should be positioned for each specific application. Cables are used to provide power to the sensor and to transmit signals from the sensor to the electronic evaluation system. However, if required, a wireless solution can be provided.

Schaeffler, Freudenberg and Kliiber have validated the measurement method for around 95 per cent of greases currently available on the market. Online monitoring of the condition of the grease enables users to draw conclusions and to react quickly to any changes that may suddenly occur. It also enables the optimisation of bearing design and position. A further solution is currently being developed for integration of the sensor in rolling bearing seals.
February 2012


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