Machinery Lubrication magazine published by Noria Corporation
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42 January - February 2015 | www.machinerylubrication.com a high oil level may cause high fl ow velocities of oil injection directly into the gear teeth. In practice, you often see an overlap of several of these factors. While each factor on its own would not be a problem, a combina- tion of these factors can lead to increased foaming. This makes it diffi cult to identify the actual causes. Air Release Standards Several standards exist for the measurement of air-release properties: ASTM D3427, ISO 9120 and IP 313. All of these use the same test procedure (the impinger method). Air is blown into the oil sample through a valve at a precise time and pressure. The release of dispersed air bubbles is recorded until the volume remains unchanged. The air release is defi ned as the number of minutes needed to release the air dispersed in the oil to 0.2 percent volume. Air release is an important property for many applications. For example, minimum air-release requirements are included in the standards for new hydraulic fl uids and turbine oils. Modern turbine oils and hydraulic fl uids often have lower air-release values than the minimum requirements listed in Table 3. Air release is also a key property for the condition monitoring of in-service fl uids. For highly viscous lubricating oils, air release is measured at higher tempera- tures, e.g., 75 degrees C. Foaming Standards The measurement of the foaming characteristics of lubricating oil is standardized in ASTM D892 (ISO 6247 and IP 146). Air is pumped into the oil through a spherical, porous stone. Small air bubbles are created, which form an air-in-oil dispersion. These air bubbles rise to the surface where a layer of foam builds up. After fi ve minutes, the air fl ow is stopped. The volume of foam is measured immediately after switching off the air and after 10 minutes. After the fi rst test sequence at 24 degrees C, a second oil sample is measured in the same way but at 93.5 degrees C and then at 24 degrees C. The test result for each sequence consists of two numbers, as seen in Table 4. However, DIN 51517/3 includes a foot- note that seems to discredit ASTM D892 as a test procedure for foaming characteristics of industrial gear oils: "A change of the given test procedure for the foaming characteris- tics will be done if a new or modifi ed test procedure is standardized." Certainly the test equipment does not work as well with highly viscous oils in comparison to turbine oils or hydraulic fl uids. One of the reasons could be because the amount of air pumped into the oil depends on the viscosity. Highly viscous oils generate a high counter-pressure. The higher the viscosity, the lower the amount of entrained air and foaming characteristics. Air entrainment in a gearbox also functions differently than in a hydraulic system. Flender Foam Test The measurement of foaming characteristics according to Flender is standardized in ISO/DIS 12152. Inside the Flender foam test rig, a horizontal pair of spur gears rotates at 1,405 revolutions GEAR LUBRICATION AIR RELEASE FOAMING BEHAVIOR At what speed do air bub- bles rise to the surface? At what speed do ascended bubbles burst? Residence time in the tank should be higher than the air release to avoid re-intake of air bubbles. Ascended bubbles should burst fast to avoid a stable or increasing foam buildup. Large bubbles ascend faster than small bubbles. Small bubbles produce big bubbles before bursting. In higher viscosity oil, air bubbles rise less quickly. Higher viscosity oils show higher foam stability. Air release cannot be improved by additives. Foaming behavior can be improved by additives. Aspiration of air bubbles promotes pseudo cavi- tations; dieseling effect reduces the capacity. Surface foam reduces cooling capacity and promotes oil oxidation. An air-oil emulsion distributed over the entire volume of oil leads to serious problems in plain bearings or hydraulic systems, including diminished capacity, limited lubrication and cooling, declining oil life, cavitation, microdieseling, etc. TABLE 2. Air release and foaming behavior characteristics ISO VG/Type 32 46 68 100 (150) (>320) Turbine Oil DIN 51515, ISO 8068 5 5 6 - - - Hydraulic Fluid (HLP/HM) DIN 51524/2, ISO 11158 5 10 13 21 32 - Lube Oil (CLP/CKC) DIN 51517/3, ISO 12925/1 - - - - - - TABLE 3. Minimum air-release requirements according to various international standards ISO VG/Type 32 46 68 100 (150) (>320) Turbine Oil DIN 51515, ISO 8068 I II III 450/0 50/0 450/0 - - Hydraulic Fluid (HLP/HM) DIN 51524/2, ISO 11158 I II III 150/0 75/0 150/0 - Industrial Gear Oil (CLP/CKC) DIN 51517/3, ISO 12925/1 I II III 100/0 (100/10) 100/0 (100/10) 100/0 (100/10) 150/60 TABLE 4. Foaming characteristics requirements