Foam in an industrial lubricant will promote wear on bearing and gear surfaces. Foam depressants are usually a part of the additive packages in many oils, especially gear oils. Additive depletion and contamination are the usual causes of foaming.
The Foaming Tendency test consists of three temperature sequences, 75° F, 200° F, then back to 75° F, using the same sample for the last two sequences. At each temperature sequence, air is blown into a cylinder containing the oil through a diffusion stone for five minutes. At the end of five minutes, the amount of foam generated is measured in mL and reported. At the end of 10 minutes settling time, the amount of foam remaining is again reported. Quality lubricants will have 0 mL foam after about 5 minutes.
The Foaming Tendency test should not be a part of your routine analysis program. It is a quality check of the lubricant and should be performed for evaluation of new oils or when a foaming problem is apparent in the reservoir or sump.