By Troy Lutz
Troy Lutz is Chief Engineer – Hydraulics Operations for Eaton Corporation.
It’s easy. All you have to do is start with clean fluid and keep it that way. Dirty fluid really causes 80% of hydraulic system failures. But there is a big difference between knowing the cause of a problem and eliminating it.
The price of eliminating 80% of your hydraulic problems is a consistently implemented, in-depth cleanliness strategy that impacts your fluids from the time they’re delivered until they’re removed. That strategy begins with a clear understanding of just what is meant by the word “clean” when discussing hydraulic fluids.
How to measure cleanliness
In a perfect world we would use perfect technology and remove all contaminants from all system fluids. Unfortunately, we don’t live in a perfect world, so we need to consider factors like the practical limits of filtration technology, and, of course, cost when developing a cleanliness strategy.
Today’s technology can clean fluid so well that contamination won’t cause the failure of any component that has not reached the end of its practical, useful life. This is a sensible and achievable goal. Reaching it begins with setting a target cleanliness level based on all the relevant factors affecting the operation of a specific hydraulic system.
In this context, “cleanliness” is a very precisely defined quantitative value specified by ISO standard 4406 and based on the results of an approved laboratory particle-counting procedure. While the procedural details are complex, bottom line is that this procedure provides a cleanliness code in the form of two numbers. Those numbers, such as 14/12, represent the number of particles of a specific size present in the sample.
A professional laboratory service can determine particle counts for your systems. For example, Eaton Hydraulics provides the Vickers Fluid Analysis Service to give such a professional measure of cleanliness. Vickers engineers have found that ISO 4406 tends to under-represent very fine particles in a sample. Thus, Vickers adds a third number to its test results to account for these particles. This produces a cleanliness code such as 20/14/12 in which the boldface numbers represent the ISO 4406 values.
It takes a systematic procedure to determine the appropriate cleanliness level for your system. Suppliers produce charts and guidelines to assist in identifying the most contamination-sensitive components in your system and defining the cleanliness levels necessary to maximize their useful life.
Once you have established cleanliness goals, it’s time to develop a strategy to implement them. There are two phases; contamination control, which seeks to keep contaminants from getting into your fluids in the first place; and, contamination management, which seeks to remove contaminants that sneak into your system before they can damage it. Contamination control and management are equally important, and an effective system rigorously implements both.
The first step in keeping dirt out of hydraulic fluids is to understand where it comes from. There are four basic sources:
New oil: If you think new oil is clean, think again. Most manufacturers produce fluids under relatively clean conditions, but even the best fluids travel through many hoses and pipes
before they reach your facility.
By the time they reach your system, fluids are almost certainly contaminated with metal and rubber particles from hoses and metal flakes and scale from drums, truck tanks, and other storage containers.
Never put any fluid into a hydraulic system without filtering it on the spot. A portable transfer cart with high-efficiency filters is the ideal tool for the job.
New equipment: If new oil is not clean, you can be sure that new machines aren’t either. A builder’s system flush may, or may not, remove harmful contaminants. It all depends on the filers’ effectiveness, plus a number of other variables including the temperature, viscosity, velocity and “turbulence” of the flushing fluid.
Never put a piece of new or re-built hydraulic equipment into service without first performing an off-load “run-in.” It is absolutely essential, even if the builder does a good job with the standard flushing.
The environment: Like it or not, your hydraulic system is a dirt magnet, and environmental contamination is constantly ingressed into the fluid. The best you can hope to accomplish is to minimize the opportunities for contaminants to enter the system through the four major entry points:
• Reservoir vent ports (breathers),
• Power unit or system access plates,
• Components left open during maintenance, and
• Cylinder seals
The system itself: Oddly enough, the most dangerous contaminants you have to deal with are usually those generated by the system. Particles stripped off component surfaces are “work hardened” by the process itself. This makes them much harder than the surfaces from which they came, and very aggressive in causing further wear in the system. Even worse, if you don’t capture them very quickly, elevated contamination levels will generate a “cascade” effect that accelerates the production of more particles.
Like all the other contamination sources, the best results are obtained when you start with clean (freshly filtered) fluid and a clean (fully flushed) system.
Because we do not live in a perfect world with perfect technology, contamination will find its way into your hydraulic fluid no matter how rigorous and effective your contamination control program may be. So, phase two of a cleanliness strategy is focused on filtration to make sure those inevitable particles are removed quickly to minimize damage they may cause. These are major considerations in developing a filtration strategy:
Design for maintenance
The hydraulic systems you design are highly dependent on proper maintenance. Satisfaction with your system will be influenced by how you specify proper maintenance procedures. Here are some guidelines to include in you specifications.
Filter element service life: First, remember the filter element protects system components by sacrificing itself. The particles it catches can’t damage the system, but each particle caught reduces the filter’s service life.
Obviously, it’s a very effective trade-off, and you should always specify filter element changes at the first indication. But, even here there is a cost/benefit calculation to be made, and maximizing filter element life makes good business sense. The best way to achieve this is by aggressively preventing contaminant ingression.
• Filter all reservoir vents,
Then, keep all fluids at or below your target cleanliness level. Clean fluid has few contaminants to clog filters. Dirty fluid accelerates wear and shortens filter life.
And, finally, apply a practical standard to determine the “dirt capacity” of a filter element. Laboratory conditions don’t come anywhere near duplicating the environment in which your system operates, so don’t depend on test results to tell you how much dirt a filter will hold in your application. Element area is a much more practical measure of the real-world performance your can expect from a filter. Choosing larger elements is highly cost-effective in most cases.
Build on maintenance feedback: One unexpected benefit of a rigorous contamination-control strategy is the amount of useful data it generates. Accurate information on system performance allows more effective preventive-maintenance strategies. Good data supports real-world maintenance decisions about when to change fluids or replace system components before they fail.
Rigorous laboratory analysis of fluids, performed on a regular schedule, is the best way to obtain that kind of information. Quality laboratories provide sampling kits and detailed instructions for taking representative samples. They then provide detailed reports on fluid health and recommendations for remedial actions. The Vickers Fluid Analysis test reports also track cleanliness over time so that history can be built for a particular piece of equipment. Changes in particle count, water content, viscosity and wear-metal ratios help to identify where corrective action is needed.
Environmental considerations have substantially increased the disposal cost of used hydraulic fluids. A regular testing program, coupled with an aggressive cleanliness and maintenance strategy, can extend fluid life by six times or more, which is an added benefit of a cleanliness strategy.
It starts with a comprehensive fluid-cleanliness program that includes contamination control, an effective filtration strategy for contamination management, and continuous monitoring of the results with regular sampling and analysis.
Dirty hydraulic fluid is the root cause of 80% of all hydraulic system problems, year in and year out. Keeping system fluids clean is the most cost-effective way to eliminate those problems.
Initial filter element efficiency: The Multipass Filter Performance Beta Test (ISO 4572) is the international standard for rating the efficiency of a hydraulic filter. Test results are reported as a Beta ratio that compares the number of particles greater than a designated size upstream of the test filter with the number of same size particles downstream.
Vickers engineers have found that there is little correlation between multipass efficiencies and system cleanliness needs. When specifying filters, remember that the goal is properly cleaned fluid and not just very high Beta ratios and dirt capacity.
The most important information you need to know is the system cleanliness you can expect when a particular filter and media are properly installed in your system. It follows, then, that filter media ought to be selected based on the system cleanliness level the filter is expected to achieve, assuming:
1. The filter sees full system flow,
2. It is the primary system filter, and
3. Ingression of atmospheric dirt will be diminished through good maintenance practices and adequate air breathers.
Filter element efficiency under system stress: Filters are subjected to many stresses they never experience in the laboratory, including:
• Rapid and frequent flow rate changes and pressure pulses,
• Decompression shock waves,
• Low-temperature starts, and
• Many other variables. All of these factors degrade filter performance.
In practical terms, the best way to evaluate a filter’s likely durability is simply to look at the construction and feel the element pleats. Are they well supported? Do they flex under hand pressure? Any element that fails these simple tests will almost certainly fail to maintain efficiency and integrity, and a failed filter absolutely will not maintain the targeted cleanliness level.
Next, examine the pack construction to make sure steel-wire mesh has been used to support the pleats. Without steel-wire mesh the pleats are likely to flex and fail through fatigue quickly.
The final construction detail you should look for is downstream wire mesh. This delivers last-chance protection in case of media rupture, and is so important that elements without downstream wire mesh are not recommended for use in systems subjected even to mild stress.
Ingression prevention: It is easier to remove dirt from air than to remove it from oil. That’s why all air entering a reservoir or port vent should be filtered to remove any particles larger than 3 _m.
Filters perform three critical functions in a hydraulic system. They prevent ingression, maintain system cleanliness, and isolate critical components. Each function requires a special filtration technology. One size definitely does not fit all.
Where to locate filters
There are three critical locations for contamination control filters used to maintain system cleanliness:
• Pressure line(s),
• Return line(s), or
• Recirculating loop.
The full outlet flow of any fixed-volume pump operating over 2250 psi and any variable-volume pump operating over 1500 psi should pass through a pressure filter before it reaches any other system component. The pump’s rotating parts are a mixture of sliding and rolling contact surfaces stressed both by high pressure and changing pressure during operation. They are always generating wear debris.
Systems with servo or proportional valves should always be equipped with a high-pressure filter regardless of pump type or pressure.
Only pressure-line filters that see maximum pump flow during more than 60% of the machine-duty cycle should be considered as total-system contamination-control devices. But if there is no return filter installed, dirt returned from the system passes through the pump before it is filtered out causing increased wear.
A return line that sees at least 20% of system volume each minute is an excellent location for the main contamination-control filter. Consider adding a supplementary recirculating pump and filter when this is not the case.
Systems with cylinders having a 2:1 or greater differential area between the piston area and rod-side piston area frequently experience flow amplification problems when using return-line filters. These cylinders can generate double or more the pump flow during part of the duty cycle. Recirculating loop filters are often the best choice in any system with very high or severely pulsating flows.
Component isolation: Pumps have a finite life and as they fail they generate contaminants that travel downstream. So an upstream filter — or at least a strainer — should protect any safety or function-critical valve.
A good rule: any time a primary failure can cause a secondary component failure with unacceptable consequences, an isolation filter or strainer should be placed upstream of that component.
Because of their very precise tolerances, performance of the spools in servo and proportional valves can be degraded by even small quantities of very fine contaminants. It is good practice to protect individual valves or banks of valves with a non-bypass filter to remove silt and chips that could enter a system during maintenance.
Large servo and proportional valves with external pilot flow can be protected with an inexpensive non-bypass filter placed in the pilot line while the system filter protects the main valve. It is important not to use component isolation filters finer than the system filter in such applications because this makes the isolation filter perform the bulk of the system clean up and greatly shortens its life.
Special considerations for closed loop systems: The significant cleanliness measurement for closed loop hydraulic systems is the “in-loop” value. These systems can usually maintain required cleanliness levels using only a high-efficiency filter in the charge-pump line.
Hydrostatic transmissions running at or near maximum pressure are an exception, however. They require in-loop filters with reverse-flow valves. A side benefit is that the filters also protect the motor if the pump fails.
Eaton Hydraulics Inc.
Filed Under: Fluid power, Hydraulic equipment + components