Compatibility of lubricants with gear box seals is required to avoid both lubricant leaking from the gear box, as well as to prevent external contaminants entering the gear box. Both static seal immersion tests and dynamic rotating shaft seal evaluations are performed to ensure lubricant compatibility, on a wide variety of seal materials. Commonly nitrile (NBR) and fluoroelastomer (FKM) materials are found in the gear box, and tests focus attention on these materials.
STATIC SEALS TESTING
Static seal tests reflect the lubricant interaction with a non-dynamic seal such as a casing gasket. A typical test method used for static seals testing both by OEMs and in industry standards is ISO 1817. In this test method, test samples of a specified shape and thickness are first cut from a sheet of seal material, and examined for:
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Tensile strength
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Elongation at break
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Volume
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Hardness
Following immersion in lubricant for a specified temperature and duration, the seal material is re-examined and the change in these parameters reported.
Most specifications define maximum and minimum change limits for these seal characteristics.
DYNAMIC SEAL TESTING
Dynamic seals testing examines compatibility of lubricants with radial shaft seals and is designed to examine seal compatibility in an environment closely related to that of a gear box input and output shafts. An example test method for dynamic seals testing is DIN 3761, the test method used by seals manufacturer Freudenburg. In this test method either two or three radial shaft seals (Simmerrings) are tested depending on the seal type under examination.
The test chamber is filled with lubricant to half way up the shaft. The shaft is rotated, at a speed, and time period with the oil held at a set temperature dependent on the seal and lubricant type under examination. Shaft speed is typically in the range of 2000 – 3000 rpm, with test duration being between 768 hours and 1008 hours. Typical test temperatures are 80°C for NBR seals and 90°C to 110°C for FKM seals.
At the end of the test the seal will usually undergo a visual examination of the radial shaft seal lip by microscope, as well as documentation if leakage has occurred, and measurement of running track width at sealing edge, depth of shaft run in, radial force, and interference.
Each OEM has their own specific limits and requirements for this test and in general results are best discussed with the OEM.
A good additive system will enable an industrial gear lubricant to protect both dynamic and static seals from damage and prevent excessive swelling or shrinkage that could lead to lubricant leakage during use.