Contamination Measurement Instruments
A Contami-Kit is portable and simple. It filters a sample of working fluid, and gathers particles on a membrane filter. The distribution and number of particles are observed with a microscope, and the level of contamination is determined by compared to the standard contamination plates. This kit has the same system of the JIS B 9930 specifications described in section 14-4.
Application Limit
General working fluid, as new oil, contains moisture in the amount of 50-80 ppm (0.005-0.008 percent). But, the ratio increases with moisturized air injected by actuators and air breathers. Moisture generates rust on the inner walls of hydraulic equipment, and enhances the deterioration of lubricants and working fluids. Measurement of moisture in working fluid is conducted by the Carl Fisher Method, which uses characteristics of a reagent that reacts only with a set amount of water, with 10 ppm detection range.
The acceptable amount of dust and moisture in working fluid depends on the equipment. Refer to Tables 14.6 and 14.7. The code of JIS B 9933 (ISO 4406) in Table 14.6 indicates a contamination level by pre-set class numbers that correspond to sizes and numbers of particles (for example, the number of particles of 5 ^m or greater and the number of particles of 15 ^m or greater). Recently, this rating system is beginning to be widely accepted.
Contamination Control
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.
Viscosity
Kinematic viscosity (absolute viscosity divided by density) [m2/s] is used to determine the value of viscosity of industrial lubricants, such as working fluid. Practically, mm square per second [mm2/s] is used, and its value is the same as the value of customary employed centistokes [cSt]. The process of viscosity calculation is specified in “Determination of Kinematic Viscosity and Calculation of Viscosity Index from Kinematic Viscosity” in JIS K 2283, which recommends the use of a thin tube and scale of [mm2/s] for the calculation. Saybolt Seconds Universal[SSU] measured by Saybolt viscosity gauge is also practically used.
The viscosity of working fluid is very important for hydraulic equipment. Working fluid with inappropriate viscosity causes poor pump suction, lubrication, and valve operations, and enhances internal leakage and heat in circuits, which eventually lead to shortened lives and can eventually lead to shorter machine life and an increased possibility of equipment failure or accidents. On the other hand, working fluid with low viscosity is suitable for saving energy by reducing piping resistance. Also, high viscosity index fluid helps to shorten warm-up time in winter. JIS K 2001 “Industrial Liquid Lubricants-ISO Viscosity Classification,” classifies the viscosity spectrum of ISO VG 2 to 3 000 into 20 grades. Figure 14.2 shows the spectrum. It is only relevant to hydraulics. Figure 14.3 clarifies viscosity-and- temperature performance of working fluid with viscosity grades of VG22- VG150.
Petroleum-Based Fluid
Working fluid with the equivalent viscosity of turbine oil in petroleum lubricant is selected. There are two types of turbine oil: turbine oil without additives (first type) and with additives (second type). The second type contains anti-rust additives and antioxidants, etc. Working fluids manufactured under the following are widely used. 1) JIS K 2213 second type: turbine oil with additives—ISO VG32, VG46, VG68, 2) specialized working fluid “R&0” type with a viscosity equivalent to turbine oil with additives, and 3)“AW (anti-wear)” type.
Selection of working fluid is based upon the general characteristics of fluid (specific gravity, color, flash point, viscosity, total acid number, etc.) and upon data from special tests (lubricity, oxidation stability, stability of shear- ability, etc.); both of the characteristics and the data are presented by manufacturers to users and manufactures of hydraulic equipment. Users need to select proper working fluid, taking into consideration the suitability of the fluid for the equipment and its durability. General characteristics are guidelines for users and assure the quality and standard value of each working fluid. These characteristics and data are also used to compare old and new working fluids; the continuous use shall be determined by comparing the degree of fluid deterioration. Table 14.2 gives examples of general characteristics of petroleum-based fluid.
Users should consult federal, state and local laws for regulations concerning the handling of petroleum based fluid. In Japan, handling of petroleum-based fluid and hydraulic equipment with specified reservoir sizes is regulated by “hazardous substance class 4” in the Fire Service Law. For instance, class 4 petroleum in hazardous substance class 4 is regulated when the total volume of the oil is 6 000 litters (1 585 gal) or more.
Hydraulic Fluid Selection
Hydraulic equipment is comprised of many kinds of hydraulic appliances, and does its job by transmitting energy through a non-compressible medium. In general, petroleum-based fluid is used as a medium, and is called working fluid. Hydraulic equipment works under conditions of high pressure and high speed. Also, many kinds of materials are used for the equipment, and temperature of the fluid under operation and other conditions may change the performance of the working fluid. When these conditions are considered, the following characteristics are demanded of the working fluid.
(1) Viscosity is stable despite temperature fluctuations.
(2) Stable flow-ability must be reached under low temperature.
(3) Change in quality and performance is minimal under high temperature.
(4) Desirable oxidation stability is met.
(5) Shear-ability needs to be invariable.
(6) It does not corrode metals.
(7) It needs to prevent rust.
(8) Rubber and coating inside pipes must not be eroded.
(9) Fluid is non-compressible.
(10) Ability to elimination foam is high.
(11) It must have the maximum possible resistance to fire.
JIS specifications for hydraulic working fluid do not currently exist. But, specialized hydraulic working fluid with viscosity equivalent to turbine oil with additives in petroleum lubricant is most widely accepted.
Synthetic working fluid and water-based working fluid are selected when danger of fire is present because of leakage or blowout of the fluid. These working fluids have different characteristics from petroleum-based fluid; caution is needed for practical use. Also, because of the increasing emphasis on socially-sustainable materials, biodegradable, vegetable oil based working fluids can be employed. Figure 14.1 shows the classification of working fluid used in hydraulic equipment. Table 14.1 presents characteristics of each working fluid.
Maintenance and management of the hydraulic system
Perform the following actions for maintenance and management of the hydraulic system.
1. Keep the working fluid clean (see Chapter 14 HYDRAULIC FLUIDS).
2. Make sure that operating conditions are correct, and keep the system in an order that allows appropriate action to be quickly taken if required. The following values should be known for system maintenance and management.
(1) Saturation temperature in a reservoir (comparison with the room temperature).
(2) Input power supplied when a fixed displacement pump is unloaded or when a variable displacement pump is fully cut off (ampere).
(3) Input power at maximum load (ampere).
(4) Drain rate for a variable displacement pump (L/min (U.S.GPM)).
(5) Pump noise level [noise level at unloading and maximum loading (dB(A)) and noise quality] These values increase with lower pump efficiency and more internal leakage in valves.
The actuator (cylinder) is not in working order
1. Knocking is caused by air in the working fluid.
Remove the air from the fluid.
2. Knocking is caused by packing resistance.
Apply molybdenum disulfide coating to the piston or replace the packing.
3. Knocking is caused by the bump at the load side.
Perform the centering of the actuator and make the load line smooth.
4. Knocking is caused by a poor condition of the inner surface of the tube.
Repair the tube.
5. Knocking is caused by improper operation of the control valve.
Inspect and eliminate the failure cause.
6. Air in the working fluid causes thrust reduction.
Remove the air from the fluid.
7. Insufficient pressure causes thrust reduction.
Inspect and eliminate the failure cause.
8. Internal leakage causes thrust reduction.
Repair the failure (replace the packing).
The pressure control valve is not in working order
1. The pressure falls below the setting level.
a. The valve is not properly seated.
b. The orifice is clogged with dust.
c. Stick-slip is taking place because of dust.
d. The valve spring is damaged.
•Overhaul the valve.
•Replace the valve and seat.
•Replace the worn or damaged parts.
2. The pressure oscillates (caused by other than the factors above).
a. The working fluid contains air.
b. The capacity of the bent line is too large.
c. The valve resonates with other valves.
d. Flow is excessive.
a. Remove the air from the fluid.
b. Make the bent line thin or short or squeeze it.
c. Replace the valve spring to change the characteristic frequency.
d. Adjust the flow to a proper level or place a larger valve.
The solenoid directional valve is not in working order (burnout of the coil)
1. Spool operation is failed because of dust.
Clean the spool and replace the coil.
2. The voltage is too high or low.
Set the voltage at a proper level and replace the coil.
3. Insulation failure is caused by water.
Eliminate the failure cause and replace the coil.
4. Direction control is failed due to an excess flow.
Adjust the flow rate at a proper level or place a larger valve.
5. Direction control is failed due to a hydraulic lock.
Install a filter or replace the existing filter to a seat type.