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Knowledge Center Gear Oils: Part 4 – Foaming Characteristics of Gear Oils Learn how to identify the numerous causes and cures of foam formation in a gearbox.

  • Understand the causes and effects of foam formation inside a gearbox.
  • Know which areas to check when foaming occurs.

In gears and oil circulation systems, undesirable foam may form, or air may be finely dispersed in the oil. Therefore, it is important to know whether the problem is due to surface foam or air mechanically incorporated into the oil. These two are often confused and quite difficult to distinguish. Here are some basic facts you need to know:

1. Before using an antifoam additive, the nature and the causes of foaming have to be determined. In some cases, a foam test will help in analyzing the phenomenon. An antifoam additive can reduce or avoid surface foam, whereas it cannot prevent, but will even enhance the inclusion of air in the oil.

2. The foaming characteristics and air shedding capacity of gear oils are largely determined by the viscosity of the oil. Normally, the foam of low-viscosity gear oils disperses faster than the foam formed in highly viscous products. Varnish and carbon build up can be removed by means of cleaning oil and manual cleaning, as far as accessible. The cleaning oil is added at a concentration of 10% and drained after 24 to 48 hours of operation. Any remaining residues can be removed mechanically.

3. Excessive foaming is rarely due to the oil used. Possible reasons for foaming are:

    With circulation lubrication:

  • Antifoam additive removed by filter
  • Oil cycles too high
  • Feed pressure of pump too high
  • Cross sections of oil lines too small
  • Intake of air
    With immersion lubrication:

  • Wrong filling quantity
  • Inadequate geometry of the oil reservoir/gearbox
    General causes:

  • Gear oil mixed with another brand or type of oil
  • Gear oil contaminated by bearing grease
  • Gear oil contaminated by dust
  • Gear oil contaminated by water

Find out more by downloading the white paper.

Read Part 3 on gear oil change and flushing procedures.

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