Flender foam test for gear oil analyses

Year of publication: 2009


In the past, OELCHECK used only the method specified by ASTM D 892 or ISO DIS 6247 to determine the tendency of gear oils to form surface foam. This involves blowing air through a porous stone into the oil sample in order to produce foam. In this test, the formation and decay of the surface foam is observed (ÖlChecker Spring 2002). However, this method does not correspond especially well to practical conditions. For this reason, an instrument for test device for the Flender foam test has now been installed in the OELCHECK laboratory. It has the advantage of corresponding closely to practical conditions. The Flender foam test yields especially valuable information when mixtures of different types of oils or impurities cause excessive oil foaming. It is an internationally recognised test method. Leading gearbox manufacturers insist on proof of a successful Flender foam test for fresh oil before they recommend a gear oil for their drives.

A member of the Siemens group of companies, A. Friedr. Flender AG is one of the world’s leading manufacturers of components for mechanical and electrical drive systems. Its wide range of products extends from individual components to complete drive systems for virtually all industrial applications. As an expert in drive technology, Flender also focuses intensively on all issues related to tribology and lubrication technology. The Flender foam test was developed in this context, originally as an in-house test designed to serve as an important criterion for judging the foaming tendency of oils, in particular industrial gear oils. Flender and other renowned gearbox manufacturers insist on a low foaming tendency for any gear oil to be included in their list of recommended lubricants. The test has become so well established internationally that a working group is now developing an ISO standard for it.

Modern gear oils contain many additives to enable them to achieve the performance level necessary to provide adequate wear protection. Due to the use of new types of base oils, and especially due to EP additives, these oils have an increased tendency to foam formation in use. To suppress surface foam formation, foam inhibitors are added to the gear oil during production. They are usually based on high-viscosity silicone compounds. The formation of surface foam is not always a cause for alarm.

The hazard potential is low unless the surface foam forms a stable layer on top of the oil with a thickness of several centimetres and remains intact for 30 minutes or more after the gears stop moving, or the foaming is so strong that foam is expelled from vent filters and other openings. Excessive foam formation can seriously degrade the load-bearing capacity of the lubricant film between the flanks of the gear teeth or in the bearings due to the inclusion of small air bubbles in the lubricant film.

Causes of unusually strong foaming:

  • The foaming behaviour of the fresh oil is inherently close to the limit.
  • Long storage can lead to separation of the antifoam additives, since foam inhibitors are not dissolved in the oil but instead emulsified.
  • Removal of antifoam additives by filtration. These additives, which are added relatively late in the process of oil production, are microdrops that, unlike all other additives, can become trapped in 5-μm filters.
  • Insufficient oil volume, excessive pumping rate or excessive speed.
  • Too much of the previous oil with other additives remains in the system after an oil change (mixing should always be less than 10%).
  • Improper addition of an anti-foam additive to existing oil.
  • Impurities such as dust or gasket materials, fitting paste, grease, degreaser, residues of machining fluids, and paint ingredients degrade the foaming behaviour.
  • Outside air sucked in after an inspection or oil pump repair.
  • Incompatibility with other types of oil (mixing is clearly present).
  • For example, with a new gearbox the first filling of gear oil may mix with residual amounts of a flushing oil with a detergent additive.
  • Failing to drain the oil cooler when the system is changed to a different type of oil. An excessive amount of the previous oil is still present in the gearbox.

There are also other possible causes.

Problems with excessive foam formation often occur with conversion from a mineral oil to a synthetic oil, for example in mobile hydraulic systems with conversion from a mineral oil to a bio-oil or in the gearboxes of wind turbines. Excessive amounts of the previous oil can also remain in oil pockets or double-acting cylinders if the oil is changed to a different product when the equipment or system is cold (not warmed up).

Even if the manufacturer confirms that two oils can be mixed together, the compatibility of oil mixtures with regard to surface tension and additives is a different story and is usually ignored by oil manufacturers. It can happen that a mixture of two oils produces high foam levels even if both of the original oils have excellent foaming characteristics. Foam occurs when air or gas bubbles with diameters ranging from 5 μm to a few millimetres are formed by turbulence, rise to the surface in quiet regions such as the oil tank, and remain intact there. The strength of the foaming tendency of a particular gear oil and how long it takes for the foam to disperse can be tested in advance in the laboratory.

One way to do this is to use the “static” foam stone test specified in ASTM D 892 or ISO DIS 6247. However, this method for testing gear oils does not correspond to practical conditions as closely as the “dynamic” Flender foam test, which is now used in the OELCHECK laboratory. The latter test is especially suitable for testing the foaming characteristics of mixtures of different gear oils actually used in practice. The increase in volume is assessed as follows according to the Flender test:

  • up to 5%: good foaming characteristics
  • up to 10%: satisfactory foaming characteristics
  • up to 15%: marginally acceptable foaming characteristics
  • more than 15%: unacceptable foaming characteristics
  • Gearbox and bearing manufacturers assume that the load-bearing capacity of the gear teeth or bearings is adversely affected with a volume increase of more than 15%. Oils or oil mixtures with such high values are therefore not recommended by Flender or other gearbox manufacturers. Additionally, used oil with an elevated foaming tendency has a distinctly higher dielectric constant than fresh oil.

    Local sensors for measuring oil level, oil quality and moisture that operate on the principle of electrical conductivity do not give reliable results with oils that have strong foaming tendencies, and they often cause costly false alarms.

    Our advice: In case of unusual behaviour with gear oils, especially after an oil change, have the foaming characteristics of the fresh and used oil tested before your gearbox loses too much oil due to expelled foam and runs dry or you are irritated by false alarms from sensors.

    How the Flender foam test works:

  • Sample volume: 1 litre
  • The oil is left standing for at least two hours before the test, so that any entrapped air can escape.
  • The actual test fi xture consists of a closed gearbox with a transparent pane, which is marked with a scale graduated in percent.
  • A set of equal-sized gearwheels with an outer diameter of 54 mm and a module of 2 mm is fi tted at the middle of two vertical shafts.
  • The test housing is first cleaned with no residue and dried.
  • Approximately 1,000 ml of the oil under test is poured in until it reaches the middle (0 point of the glass scale) of the horizontal gearwheels and warmed to room temperature (25 °C). The colour and temperature of the sample are recorded.
  • The set of gearwheels is then rotated at 1405 rpm for five minutes. This strongly agitates the oil and entraps air in the oil. This results in foam formation.
  • Before, during and after the test, the increase in the oil volume can be read directly in percent from the graduated scale on the glass pane.
  • However, other factors can also be read from the scale after the end of the test. It is possible to distinguish between the actual surface foam and the underlying oil/air emulsion or the “pure” oil. The regression of the foam level is recorded at fixed time intervals for a period of 90 minutes In this way, the time-dependent behaviour of foam development and regression is documented. The foaming characteristics of an oil are assessed on the basis of the volume increase exhibited by the oil under test one minute after the test fixture stops moving.

    Additional information: Read also Air and foam in oil