Grease monitoring lengthens bearing life

Bearing life is often dependent on the condition of its lubricating grease. Paul Fanning looks at a sensor that allows it to be monitored during operation.

A grease sensor has been developed that enables the condition of the lubricant to be analysed immediately during bearing operation could ensure that the lubricant is changed long before the bearing sustains any damage. In conjunction with Freudenberg and lubrication expert Klüber Lubrication, Schaeffler Group Industrial has developed a grease sensor incorporating an electronic evaluation system that enables the condition of grease in bearings to be analysed during operation.

The condition of the grease used to lubricate bearings is often a critical factor in their failure. Indeed, the grease operating life is the decisive value during preventive maintenance if it is shorter than the bearing life. In this case, bearings are usually relubricated halfway through the so-called grease operating life. The great disadvantage of this procedure is that absolutely nothing is known about the condition of the grease.

Up to now, users could only receive information about the grease in bearings by taking samples and undertaking costly and time-consuming analyses of these samples in a laboratory. According to Peter Schuster, Schaeffler Industrial's Director for Advanced Development, there is no similar grease monitoring sensor currently available. This is due to the way in which the sensor operates. The optical near-infrared reflection method is used here.

The process, developed in conjunction with the Fraunhofer Institut for Electronic Nano Systems (ENAS) in Chemnitz, Germany, is based on the infrared process used in laboratories to measure grease quality and was adapted for online measurements in rolling bearings. The know-how involved is not only in the setup of the sensor, rather in particular in the evaluation of the measured signals.

The grease undergoes rotationally symmetrical irradiation with certain wavelengths from the sensor. The sensor head is embedded in the lubricant during this procedure. The reflected light is measured perpendicular to the grease. This enables shadow effects and surface 'inhomogeneities' or mismatches to be completely excluded. The reflected light is evaluated in terms of the quality of the grease.

Four grease parameters can be detected using the sensor: Water content, cloudiness, wear (thermal or mechanical wear) and temperature. An analogue signal (4 – 20 mA) is generated from these parameters in the electronic evaluation system, which displays the condition of the grease quickly and simply. A digital signal can be generated that indicates poor/good grease quality by setting a trigger threshold (limit value).

The sensor has to be located in the bearing via a hole put in the position best calculated to monitor the condition of the grease continuously. Currently, no wireless option is available. However, the concept has been proven on test rigs and has been deployed in pilot applications. Says Herr Schuster: "This will not be a catalogue product. It is designed as a bespoke solution." Currently only in pilot applications, the system will be available by the end of this year.

Paul Fanning

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