Q. Why do some hydraulic systems run for years without any filters?
A. The are a number of reasons. The most common reason is low pressure. Systems that operate below 500 psi can get away with no filters. As system pressure increases, good filtration becomes more important. Above 1000 psi, it's mandatory for good fluid, seal and component life.
Another reason is the internal clearances of the system components. Components with tight internal clearances (like most modern hydraulic components) require 10 micron filtration or finer. On an old system that's still running OK without filters, all the internal clearances have been opened up from years of running without filters. It's like running microscopic lapping compound through all the components which opens up the internal clearances and rounds off all the sharp corners. The end result is lower efficiency. The new components with the tight internal clearances have much lower internal leakage and so, much higher efficiency (and higher efficiency also results in less heat).
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