Contamination Control in Gearbox Applications
Contamination means any substance that enters a system and affects or interferes with the function of the system’s fluid and/or the operation of its components. Solids, water and various gases (primarily air) entering or existing in a system can have mechanical or chemical interactions on the oil and/or the equipment. Fluids must be protected and monitored from such contaminants by a comprehensive contamination-control program incorporating prevention of fluid contamination, removal of contaminants, and fluid-system condition monitoring.
Knowing the contaminants and their origin will provide clues as to how they might be excluded, removed, or their effects neutralized. Contaminants can be built-in due to manufacturing/maintenance processes, or they may enter a system while parts of it are open during construction or repair. Also, they can be internally generated as a result of system operation, such as wear debris, compounds of chemical reactions, or substances resulting from decomposition of the fluid or its additives. The most common entrance of contaminates from the atmosphere is either through breather caps, imperfect seals, or other unplanned openings during normal operation of the equipment. This would include the addition of fluid during initial fills or top-ups.
Original Equipment Manufacturers will provide their own equipment specific requirements for targets and limits of contamination in their maintenance manuals or service bulletins. In most cases, the end user will not obtain these targets unless a further investment into contamination control equipment is provided.
Breathers Can Prevent Infiltration of Contaminants
As system temperatures or environmental temperatures change, the gearbox or bearing housings will have a movement of air. Whether the housing is inhaling or exhaling, the movement of air is always trying to equalize the temperature difference from the inside to the outside of the housing or in reverse direction. Exhaling in many cases will carry a fine oil mist to the outside environment, while inhaling carries the industrial environment along with nature’s environment into the gearbox or bearing housing. The area above the lubricant level but inside the housing is classed, as “Headspace” and managing the quality of the environment in this headspace is one step in controlling the contaminants entering the lubricant.
Countless articles have been published, and new research and oil testing techniques are always trying to prove the effects and the destructive nature of contamination on lubricants. Controlling contamination that enters the headspace environment can reduce and in some cases eliminate many of the potential root causes of lubricant failure. Most housings have a planned method or location for breathing; otherwise a build up of pressure would cause seals to fail resulting in external leakage of the lubricant. Since many of the breather systems on gearbox housings are either just a small hole in the cap or a very poor quality strainer style breather, they typically will not prevent the required contaminants from entering the system to maintain the OEM fluid requirements.
Upgrading breathers that control contaminant ingress starts with understanding the environment around the housing. An effort to stop the ingression of water in a continuous hot dry environment will result in an investment, which would not have any financial return on that investment. Stopping only water ingression in an environment of high humidity and excessive airborne contamination will not successfully stop the ingress of the destructive powers of the airborne contamination. Oil sampling and testing, coupled with investigating the surrounding environment should provide a direction in selecting the correct contamination retention method, a method that provides the most economical and effective retention of the known contaminants from reaching the headspace of a specific piece of equipment. In cases of extreme environmental contaminants or considerable air movement within the housing, there could be a need for an external bladder system. The attached chart lists some of the breather exclusion methods along with the contaminants that they restrict from entering the housing headspace.
At some point, all styles of breathers that are not maintained will become plugged with debris resulting in an increase of internal pressure of the housing and the ultimate failure of the seals. Leaking seals in gearbox housings have become a common complaint in industry and the end result is typically the rework of the shafting material and the replacement of the seals due to the hard aggressive wear created by the abrasive particles attracted to the leaking lubricant. In too many cases the root cause (blocked or plugged breather) is not identified as the condition of the breather and is not monitored or replaced on a planned maintenance schedule.