"We have encountered premature failures caused by wear on the friction discs of our wet brakes and cannot find the source of the fault. Do you have any idea why premature wear is occurring?"
When it comes to wet brakes, the oil serves many different purposes inside the housing, such as cooling the assembly, reducing excess wear and friction, and providing a hydraulic medium for the brakes to function properly. If the properties of the oil are matched to the needs of the equipment, the internal machine surfaces should be protected and yield a satisfactory life. In this instance, the friction discs are failing due to some sort of excessive wear mechanism. Your first step should be to determine what type of wear is taking place.
Considering the sliding nature of these discs, one prominent wear mechanism that could be involved is adhesive wear. This is characterized by larger, platelet-like chunks of material that are transferred from one machine part to another. It can be triggered by a variety of factors, but they all consist of a lack of a properly sized lubricating film. If the discs appear welded together, this is the wear mechanism at play. Even if you have the right standard of oil, the viscosity may not be correct. If the viscosity is too low, this wear pattern becomes much more pronounced.
Abrasive wear is also common in sliding contact areas. It can be caused by a rough surface gouging into a softer surface, known as two-body wear, or having a particle trapped between two sliding surfaces and cutting into them, which is three-body wear. Abrasive wear is characterized by scores or marks on the machine parts. If the failed discs have excessive scratches and cuts on them, you are dealing with an abrasive failure. To mitigate this, you must remove the solid particles. Verify that the proper filters are in place and do your best to decontaminate the oil before it goes into service.
If the wear appears as pitting or the surfaces are discolored, it may be initiated by corrosion. As oils oxidize, they produce stronger acids that in turn can damage softer machine parts. If left unchecked, these acids can result in significant damage. When the oil is changed, look for signs of oxidation, such as darkening of the oil, an increase in viscosity and a burned oil smell. These are all indicators that the oil drain interval is far too long.
Also, ensure the correct oil additives are being used based on the material from which the discs are made. Some friction discs are manufactured from alloys of softer metals. If this is the case, you should avoid any oils with extreme-pressure (EP) additives. These additives are chemically aggressive and exacerbate corrosive wear.
Finally, one underlying contaminant that can affect all three of these forms of wear is water. If the oil is contaminated with water, wear will be much more prevalent, which can result in shorter equipment life. You can sample the oil and send it to a lab for analysis or perform a simple crackle test to see if water is present. If so, you will need to either dry the oil or replace it with one that is clean, dry and healthy. Making sure all these criteria are met will help you achieve longer life for your components.