There are three reasons for working fluid to be replaced.
(1) Change in quality and deterioration of working fluid
(2) Dust or external elements mingled in working fluid
(3) Moisture mingled in working fluid during its operation
Table 14.3 is a reference for deterioration and quality change. Contamination by dust and moisture is common, and causes abrasion in pumps and valve malfunction. The size of particles in working fluid, ranging from a few to ten, twenty, and thirty ^m, can have a negative impact on the performance of machines equipped with actuators and precision valves such as servo valves. Because of this, the following measurements are necessary to keep the contamination within a standard range: 1) Measurement of dust and particles in working fluid based on “Determination of particulate contamination by the counting method using a microscope”, specified in JIS B 9930, 2) Calculation of the number of particles by an automatic particle counter, 3) Observation of particles by the simple measuring instrument shown in Fig. 14.4, and 4) Measurement of particle mass.
Measurement of cleanliness is conducted with equipment that filters 100 mL of working fluid. Particles are gathered on a millipore1) filter, and their numbers and sizes are measured, and classified according to Table 14.4. Contaminated working fluid is classified based on dust mass according to Table 14.5. Cleanliness of general working fluid, as a new oil, is classified into approximately class 6 to 8 shown in Table 14.4.
Note 1) Millipore filter: filter with microscopic holes, the size of 1/1 000 mm
This table gives reference for replacement or renewal of the working fluids. There are test points other than these; it is recommended to refer to working fluid manufacturers. For example, the reference value of the total acid number (or acid number), which indicates deterioration of the fluids, varies according to types or amounts of additives. For the water-glycol working fluid, the pH value is controlled as well.
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