Hydraulic systems contaminants

CLEAN ASSEMBLIES MINIMISE HYDRAULIC SYSTEM CONTAMINATION... 

Hydraulic system contamination can arise from a number of sources, including accumulation of debris that results from the hose assembly process itself. A number of methods are available to manufacturers to allow them to quantify the level of contaminants in such systems and optimise their production processes. 

Cleanliness in hydraulic systems has received considerable attention recently. Some hydraulic systems, such as aerospace hydraulic systems, are extremely sensitive to contamination. Fluid cleanliness is of primary importance because contaminants can cause component malfunction, prevent proper valve seating, cause wear in components, and may increase the response time of servo valves. Fluid contaminants are discussed later in this chapter. 

Hydraulic fluid contamination may be described as any foreign material or substance whose presence in the fluid is capable of adversely affecting system performance or reliability. It may assume many different forms, including liquids, gases, and solid matter of various composition, sizes, and shapes. Solid matter is the type most often found in hydraulic systems and is generally referred to as particulate contamination. Contamination is always present to some degree, even in new, unused fluid, but must be kept below a level that will adversely affect system operation. Hydraulic contamination control consists of requirements, techniques, and practices necessary to minimize and control fluid contamination. 

There are many types of contaminants which are harmful to hydraulic systems and liquids. These contaminants... 

Water Water is a serious contaminant of hydraulic systems. Hydraulic fluids are adversely affected by dissolved, emulsified, or free water. Water contamination may result in the formation of ice, which impedes the operation of valves, actuators, and other moving parts. Water can also cause the formation of oxidation products and corrosion of metallic surfaces. 

Solvents Solvent contamination is a special form of foreign fluid contamination in which the original contaminating substance is a chlorinated solvent. Chlorinated solvents or their residues may, when introduced into a hydraulic system, react with any water present to form highly corrosive acids. 

Maintaining hydraulic fluid within allowable contamination limits for both water and particulate matter is cmcial to the care and protection of hydraulic equipment. Filters will provide adequate control of the particular contamination problem during all normal hydraulic system operations if the filtration system is installed properly and filter maintenance is performed properly. Filter maintenance includes changing elements at proper intervals. 

The condition of a hydraulic system, as well as its probable future performance, can best be determined by analyzing the operating fluid. Of particular interest are any changes in the physical and chemical properties of the fluid and excessive particulate or water contamination, either of which indicates impending trouble. 

The most common device installed in hydraulic systems to prevent foreign particles and contaminations from remaining in the system are referred to as hlters. They may be located in the reservoir, in the return line, in the pressure line, or in any other location in the system where the designer of the system decides they are needed to safeguard the system against impurities. 

Clean hydraulic fluid is essential for proper operation and acceptable component life in all hydraulic systems. While every effort must be made to prevent contaminants from entering the system, contaminants that do find their way into the system must be removed. Filtration devices are installed at key points in fluid power systems to remove the contaminants that enter the system along with those that are generated during normal operations of the system. 

Solid impurities must be continuously removed because hydraulic systems are self-contaminating due to wear of hoses, seals and metal parts. Efforts should be made to exclude all solid contaminants from the system altogether. Dirt is introduced with air, the amount of airborne impurities varying with the environment. The air breather must filter to at least the same degree as the oil filters. 

Accuracy and repeatability of temperature/time/velocity/pressure controls of injection unit, accuracy and repeatability of clamping force, flatness and parallelism of platens, even distribution of clamping on all tie rods, repeatability of controlling pressure and temperature of oil, oil temperature variation minimized, no oil contamination (by the time you see oil contamination damage to the hydraulic system could have already occurred), machine properly leveled. 

Oils used to lubricate machinery, motors, hydraulic systems and other mechanical devices can sometimes contaminate fuel systems. These oils often carry with them low levels of the metals which wear from the lubricated surfaces of the mechanical components. Some common wear metals and possible sources of origin are listed in TABLE 4-12. 

For soil systems contaminated with Na+, kinematic viscosity is not significantly affected, thus the components controlling water flow velocity are the hydraulic gradient (A< >/AX) and soil permeability (k). The latter component (k) is influenced by clay dispersion, migration, and clay swelling. These processes may cause considerable alteration to such soil matrix characteristics as porosity, pore-size distribution, tortuosity, and void shape. 

Hiac PC4000 portable liquid particle counter is a contamination measurement tool, designed to run on-line analyses of hydraulic systems and fluids. The fully self-contained counter operates in the light-blocking mode using a laser diode and reports contamination levels at 4, 6, 10, 14, 21,38 and 70 pm at a flow rate of 60 ml min. ... 

Eleftherakis, J., A Proven Approach to Hydraulic and Lubrication System Contamination Control , Lubr. Fluid Power J. 2, 13-20, Aug 2001. 

Common cause failure Afire in a compartment might destroy aU the channels of a system mnning through that compartment. Likewise, contaminated hydraulic fluid could cause all the channels of the hydraulic system to faU, or mechanical failures in an electrical loom. 

Water contamination in hydrauhc fluids can pose a severe threat to the hydraulic system. However,... 

A second test which is even more discriminating is the so-called "contamination test, in which the rubber is soaked in the fluid for several hours at room temperature and is then subjected to hot air ageing at high temperature. This test simulates the environment of many engine components (e.g. seals, gaskets, hoses, etc.). The main rubbers commonly used in such applications are nitrile rubbers which are not appreciably swelled by hydrocarbon oils and ethylene-propylene (EP) and their ter-polymers (EPDM) which are resistant to the phosphate ester fluids (e.g. Skydrol) used in aircraft hydraulic systems. The following discussion is concerned with the performance of antioxidants under these veiry aggressive conditions. 

Objective The objective of this pump and modification is to reduce contamination that is introduced into an existing hydraulic system through the addition of new fluid and the device used to add oil to the system. 

Never touch a hydraulic filter with your hand during installation. By touching the filter you will introduce contamination to the hydraulic system. 

Another possible source of microbial contamination of hydraulic fluids may arise from water intrusion if leaks in the hydraulic system occur or when make-up water is added to maintain the proper water content. 

Contaminated hydraulic fluid could cause all the channels of the hydraulic system to fail. 

Scenario 3 contamination of hydraulic fluid could result in failures of all channels in that hydraulic system (i.e. common cause failure). 

Of all of the utility application of filtration, the filtration of hydraulic systems is probably the most important, because of the critical need to keep hydraulic fluids free of solid contaminants, and because of the very wide application of hydraulic systems themselves in almost every manufacturing operation, in almost every agricultural and constraclion machine, and in most commercial vehicles, railway stock, ships and aircraft. Hows in hydraulic systems are not great, and flow channels are... 

All hydraulic systems are contaminated, although the level of contaminant varies considerably depending on the environment and the use for which the system exists. 

In fact, in excess of 70% of hydraulic system failures are caused by contamination or poor fluid condition. Hence filters are essential in modem hydraulic systems, to provide a particular or specified level of contaminant removal. This can vary with the type of system, types of components involved, application and duty cycle. 

Filters fail because they either burst open or clog up. If a filter is left in a hydraulic system long enough, one or other of these two failnre mechanisms wiU occur. For the precise needs of hydraulic system design, a snitable scheme for representing contaminant levels is required. 

---