Oils used in gear units, turbines or hydraulic systems are subject to a special testing rig in the laboratory in order to determine its foaming behaviour in practice in accordance with ASTM D 892 and ISO DIS 6247. The test looks at how long it takes before the foam disintegrates. Preheated air is discharged via a spherical, porous stone into a 410 ml sample of oil to be tested. This leads to an air in oil dispersion in the form of fine bubbles. These bubbles rise to the surface and create a layer of foam. The foam volume is measured immediately after the air is switched off and again after 10 minutes have elapsed. There are no generally valid limit values for an oil‘s foaming tendency. However, the development of the trend and the change in comparison to fresh oil do constitute criteria for assessment. The VGB guidelines for turbine oils with a limit value of 600/0 ml/ml can be used for orientation purposes. However, each case must be assessed individually.
The practical Flender foam test
The Flender foam test was developed due to the fact that determining the foaming tendency via „foam bricks“ has only limited applicability in practice. The practical test is primarily used for assessing gear oils, particularly when a combination of oil types or impurities have resulted in excessive oil foaming in the gears. In addition, leading gear manufacturers require gear oils to have passed a Flender foam test before approving it for use in their gearboxes. The testing method was originally used by A. Friedr. Flender AG as an in-house test for assessing the foaming tendency of industrial gear oils. The company merged with Siemens AG in 2010 and commenced operations under the name Siemens Mechanical Drives. Today, it is the Group‘s specialist for gears and couplings. The „Flender“ brand name has been retained. The extensive product portfolio ranges from individual components to complete drive systems for virtually all industrial applications.
Today, the Flender foam test is standardized according to ISO 12152. Furthermore, Siemens Mechanical Drives now lists the laboratories approved for the Flender foam test following an audit commissioned by Siemens. For the audit, the independent laboratories must have the requisite test benches and trained staff, must be certified or accredited, and must publish the test results in a standardised report. OELCHECK is currently one of very few laboratories officially appointed by Siemens Mechanical Drives to perform the test. In the Flender foam test, a gearbox housing is filled with 1,000 ml of oil. A gear pair with equal-sized gear wheels is used to stir the oil for five minutes at an RPM of 1405 min-1 at 25°C. The gear pair sits horizontally to the halfway point – the centre of the gear wheel – in the oil sump. The high RPM and the half-submerged gear wheels enable the oil to be stirred at a high speed, which draws in air. This leads to the formation of foam in all types of oil, and the oil volume increases as a result. The oil level can be read before, during and after the test via a graduated scale on a glass pane in the wall of the gearbox, and the change in the oil‘s volume can be stated directly as a percentage. An oil‘s foaming tendency can be assessed based on the percentage increase in volume displayed by the test oil one minute after the test rig is stopped. The oil/air dispersion volume (%) can be calculated five minutes after stopping the rig. OELCHECK directs a camera at the glass pane to record the test run and stores the data accordingly. The test results are classified as follows according to the specifications of Siemens Mechanical Drives: Rise in oil volume one minute after stopping (%)
- < 5 % good
- < 10 % satisfactory
- < 15 % acceptable
- > 15 % not acceptable
However, these values are only valid for the text gearbox and the standardised method. They are based on the experience of Siemens Mechanical Drives gained through meeting the requirements for oils in Flender gear units. The 15% stated above is not an actual limit for foam formation in gear units. Volume rise in oil/air dispersion five minutes after stopping (%)
In general, an increase of up to 10% is tolerated. This limit value of 10% free air is set by leading pump manufacturers to avoid cavitation. The five-minute time window is the result of design guidelines issued by Siemens Mechanical Drives, and relates to the minimum ratio of oil volume and pump capacity.