Modem precision controlled hydraulic tools using Electro-Hydraulic Control (EHC) servo-valves and proportional valves require clean hydraulic systems to operate accurately and reliably.
Present day high precision machinery demands greater reliability and precision from its hydraulic systems. The equipment builder must therefore build machines with lubrication and hydraulic systems that have means of keeping these lubricants clean. The user, if he wants performance built into the machines, must follow a preventive maintenance program including such items as changing filters, monitoring the oil condition, and when conditions warrant, changing the oil.
Because of the many advantages of fluid power, the development of the electro-hydraulic servo-valves was pioneered by the aerospace industry. The high reliability and performance demanded of aircraft, rockets and associated equipment required clean fluids and the means to keep the fluids clean whether the fluids are for fuel, for lubrication, or for power transmission.
Government agencies, SAE, ASTM, and other groups have developed procedures for determining the amount of particulate contamination in fluids for aerospace use. As the servo-valve was a critical component in many control systems, the effects of contamination on servo-valves is the concern of many safety institutions.
Undoubtedly many of the troubles with earlier model precision hydraulic machines could be traced to a) low dirt tolerance of early types of servo-valves, b) marginal filtration equipment installed on the machines, and c) poor maintenance procedures by the user.
Fortunately through evolution, improvements have been made in all areas and better performing systems are now common.
A typical precision electro-servo control hydraulic system receives its instruction from a computer/ command signal. The printed logic circuit transfers the commands to electrical signals. These signals energize a torque motor in the servo-valve and a flapper is moved to control the flow of fluid in proportion to the strength of the signal.
This flow is amplified in one or more stages to direct the flow of oil to a fluid motor or cylinder. Again the signal is proportional to the fluid motor or cylinder responses.
The servo-valve signal sent is very low power, made to be sensitive so that its response to the input signal is rapid, proportional, and accurate.
The diameter clearances of the spool in the valve body are on the order of 0.001/0.002 inches or less. This means that the clearance per side is 12 to 25 micron. It would seem reasonable that if the maximum size of contamination particles in the hydraulic fluid were kept below the minimum side clearance, the servo-valve will perform properly.
Most manufacturers of servo-valves for machine tool use recommend a 5-micron filter just ahead of the servo-valve.
The Contamination Problem
A contamination particle might cause a spool to seize, allow movement in one direction only, or cause frictional drag. When the torque motor is energized, a valve failure will result if any of the above conditions exist within the servo. As applied to a machine tool, dynamic jamming would cause the servo-valve spool movement to lag the torque motor signal and result, for example, in a machine movement hunting for the commanded position. This can be extremely dangerous in mobile equipment with an operator on board!
When any of the above malfunctions occur, servo-valve contamination is the prime suspect and if this is the case, steps are taken to render the valve operable and to prevent the recurrence of particle contamination.
The aerospace industry currently has many servo-valves operating today where the spool-bore clearance per side is in the order of 2.5 micron. Clearances this small are required not only of the servo-valves but also other components in the hydraulic system so as to reduce leakage and consequently increase system efficiency, reliability, and response.
It can be expected that when the hydraulic industry takes advantage of this new technology, the equipment builder and user will he forced into vastly cleaner hydraulic systems.
Reduction of Particulate Contamination
The filters recommended by some servo-valve manufacturers for electro-hydraulic servo use are surface type 5-micron absolute rating, without a bypass valve. It is also recommended that the filter be placed ahead of and as close to the servo-valve as practical.
As a precautionary measure, most servo-valve manufacturers have one or more filters built within the valve body. Prior to installing servo-valves in a hydraulic system, the system must be purged to clean out contaminants.
In spite of the cleaning precautions taken during the installation of new equipment, many contaminants are present and must be removed. A typical hydraulic system may have a suction line filter ahead of the hydraulic pump, a 10 micron high-pressure filter after the pump, and a 5 micron filter just ahead of each servo-valve used. One procedure requires that the highly refined hydraulic oil must come from sealed 205 litre drums.
As soon as an equipment installation reaches the point where oil is required in the reservoir, oil is pumped from the drums into the sump through a 2 micron filter. The circulating pump on the hydraulic oil heat exchanger is started to flush the heat exchanger. An auxiliary pump draws oil out of the sump and pumps it through a 2 micron filter and then returns the oil to the sump.
This way, many valuable hours of complete system purge time can be saved; by pre-cleaning the hydraulic oil, the reservoir and the heat exchanger. As the hydraulic system is being installed, bypasses are put around each of the servo-valves to temporarily disconnect them from the circuitry. The 5 micron filters ahead of servo-valves are left in the circuit.
When the oil contamination level in the reservoir is within allowable limits, the main hydraulic pump is started and most of the hoses, fittings, and fluid lines are flushed. A shut-off valve is placed in each servo-valve circuit ahead of the filter to provide a means of changing filter elements without shutting the machine off. During the preliminary flush, all shutoff valves are open so that the hydraulic fluid is circulating in each servo-valve circuit loop.
With many routes to travel, the oil velocity in this flush is low. It has been found that the amount of contamination present in the new oil, the reservoir, and the heat exchanger was much larger than the contamination in the main pump and servo-valve circuit loops.
By using auxiliary filtering equipment and purging the system oil, sump, and heat exchanger prior to installation, considerable time can be saved and introduction of new contamination reduced.
The 5 micron system filters are likely to be blocked rapidly during the flushing operation. By pre-cleaning as much as possible before flushing the complete system, the filters ahead of the servo-valves did not require changing. Each filter change introduces contamination by the opening of the filter housing and even more contamination due to the filter element itself being contaminated.
After about two hours of the preliminary flushing, all servo-valve shutoff valves except one are closed. With only one loop to accept the oil volume and by raising the pump pressure to at least 1000 psi, high velocity oil is circulated through the fluid lines to further cast off any scale, dirt, etc. that adheres to the inner surfaces.
Under these conditions, care must be taken that the differential pressure across the filter does not exceed the minimum filter element collapse pressure. A two-hour flush of each servo-valve loop, taken one at a time, will usually lower the fluid contamination level in each of the loops to acceptable contamination levels.
Flushing continues until each of the loops shows an acceptable fluid contamination level after which time the servo-valve can be connected. Failure to follow a flushing procedure such as outlined can result in many hours spent to clean specks of dirt from the servos.
When the equipment builder puts clean hydraulic fluid into a machine for the first time, the fluid initially acts as a solvent and carrier for the contaminants that are present in the internal parts. After a few passes through a newly assembled machine that has no filtration equipment, a clean fluid would soon become very contaminated.
With filtering and following a purging procedure, the particulate contamination present in the fluid can be reduced to some predetermined level that is consistent with the level of filtration used.
It is believed most EHC equipment builders recommend to their customers that before a machine is made ready to run, the hydraulic fluid be filtered until the particulate contamination level is at least NAS Class 6 level or equivalent.
EHC equipment in production generates particulate contamination in the hydraulic fluid as the result of wear, cavitation, erosion, fluid breakdown, etc. Particulate contaminants enter the hydraulic system through breathers and seals.
As contaminants are added to the hydraulic fluid, the filters are acting to remove the particles to the extent of their capability while breakdown of large particles into small particles is occurring due to the shearing action of pumps, valves, and other moving components.
As the filter picks up contamination from the fluid, the filter allows fewer and fewer large particles to pass. At any time during normal operation, the hydraulic fluid particle contamination distribution curve will be somewhat "S" shaped.
The actual distribution of particle size against number of particles in a hydraulic fluid from operating equipment is not the same as the distribution found in the cleanliness level classes of SAE or NAS and that the SAE and NAS contamination levels do not apply in this situation.
Wear particles that appear only after a machine is in use are mostly iron. Estimates of the weight of iron present in the particulate contamination on new oil samples varies but seem to be about 10%. As the iron content of the contaminants increases due to the addition of wear particles, gravimetric analysis procedures show an increase in the weight of contaminants present in a sample even though the number of large particles decreases.
Maintenance of System Cleanliness
The hydraulic fluid, once put into a machine, is kept clean by the action of filters and by the chemical action of the additive package placed in the fluid. The types of filters used in most instances remove solid or gelantinous particles to the limit of the filter. These filters do not generally remove the liquid or gaseous contaminants.
Most servo-valve manufacturers and equipment builders recommend that the hydraulic fluid filters be rated at 5 micron nominal particle size. Several airlines have backed away from ultra-fine filtration not because good filtration is unimportant but that a system will clean itself with high-quality filters somewhat coarser than ideal, if not disturbed.
A lubrication engineer might then ask whether effective contamination control of hydraulic fluids is possible and if so, what limits of contamination should be placed on fluids for use in electro-hydraulic servo-valve controlled equipment. Effective contamination control is not just a matter of filters.
System planning, location of filters, heat exchanger capacity, etc. are but a few of the items that have been considered in a machine's design to reduce the generation of particulate contamination.
Servo-valves installed ten years ago are probably different than servo-valves used today, even from the same manufacturer. Because of differences in design, the tolerance of servo-valves operating in contaminated fluids varies from manufacturer to manufacturer. In general, most servo-valves made today will operate in fluids with high amounts of particulate contamination. The nominal 5-micron filtration requested by most manufacturers of industrial servo-valve will keep servo-valves wear to a tolerable minimum.
When considering the warranty life of a servo-valve, the contamination level rather than the degree of filtration is the deciding factor.
In aerospace, servo-valves operate at higher pressure and under a wide range of environments. As human life may depend upon the prompt and accurate response of servo- valves, the need for the best practice in hydraulic fluid cleanliness is necessary. In regard to industrial equipment, durability is the major consideration.
As mentioned previously, when equipment is operational, particulate contamination is generated as the result of wear, cavitation, erosion, etc. These particles are generally of composed of iron and hence are abrasive.
Abrasive particles carried in high velocity oil can generate more abrasive particles. High velocity oil would be found in pumps and valves. Within servos, lands, clearances, and nozzles are particularly subject to high velocity oil and erosion can become a problem over time.
An approximation of the effects of wear by erosion as a function of particle size can he made if the particles are assumed spherical. A 10 micron particle will have eight times more honing effect than a 5 micron particle.
Normally the high capital cost of EHC equipment requires the utilization of the machine be at a maximum. It has been estimated that the normal wear and tear on EHC equipment is some three times higher than on the same type of conventional machine tool. In terms of elapsed time, the wear on hydraulic system components of EHC equipment seems high in comparison to conventional machinery.
Contamination Limits
Barring isolated instances, it is generally recognized that servo-valves will operate in fluids with a high particulate contamination level. It is also agreed that in clean fluids, servo-valves will give better performance and more reliability than in contaminated fluids and that changing filter elements at regular time intervals is not desirable, nor required.
Filter elements should be changed whenever the differential pressure across the filter exceeds the suggested maximum differential. Opinions begin to differ when a demarcation line must be drawn on the degree of filtration. The abrasive particle generation process can be reduced with the use of fine filtration or by changing the hydraulic fluid.
Many other benefits will occur by changing the hydraulic fluid. For one thing, water that may have entered through the reservoir breather or through leakage in a water-oil heat exchanger can be removed. Fluid contaminants resulting from oxidation can be flushed from the system.
Oxidation products lead to varnish and sludge deposits. The additive package placed in many fluids such as metal deactivators, rust inhibitors, foam inhibitors, oxidation inhibitors, anti-wear agents, etc. can become depleted through use. In short, many contaminants cannot be removed by filtration and to do so, the fluid itself must be replaced.
The lubrication engineer in a plant using EHC equipment must operate within the management's attitude toward maintenance. If management says "fix it when it breaks", then the best that can be hoped for is to change filters when the differential across the filter exceeds recommended limits.
Monitoring the oil condition also should include checking the water content, which should be less than 0.1 %, the viscosity, and the neutralization number. If any of these tests indicate that the fluid is becoming contaminated with liquids, the fluid should be changed. Air is another contaminant that usually cannot be removed with filters. Antifoam agents are added to hydraulic fluids to get rapid separation of air.
It should be noted that entrained air in fluids, when compressed to, e.g. 2000 psi or more, can become very hot locally. This generated heat causes the fluid surrounding the bubble to burn. As the products of combustion are both fluid and solid contaminants, more contamination can be generated.