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    2. Maintenance & Repair

    How to Defend Hydraulics from Dangerous Contaminants

    Sept. 28, 2010
    cex0902_prevent02

    Construction Equipment staff

    Dust, dirt, water, metal: What do they have in common? According to Doug Jahnke, product marketing manager at Eaton Hydraulics, they could all doom a piece of heavy equipment. Even in small quantities, if these seemingly innocuous particles enter the hydraulic fluid of an excavator, backhoe loader, or one of your other valuable machines, components such as pumps and valves could begin behaving erratically and shorten equipment life.

    There are three types of hydraulic failure. Catastrophic failure occurs when a large particle enters a pump or valve causing a vane to jam. Intermittent failure happens when contaminants sometimes settle on a poppet valve, preventing it from closing. Degradation failure is usually the result of abrasive wear, corrosion, or surface fatigue on a component. For instance, when particulates get in between the clearance of the surfaces of a moving pump, they can rub up against the surfaces causing stress and cracks and even breaking parts of the pump into metallic particles.

    “Because the metal is hard, it potentially could cause more abrasion and seriously damage a pump,” says Jahnke. “If the pump is operating at high pressures and high speeds, the metal particles could quickly cause it to go bad.”

    What Not to Do

    1. Do not insert new oil without first running it through a filter or transfer cart. “Filter carts should be used always,” says Jahnke, adding that this is rarely the case among fleet maintenance crews. “Most oil manufacturers don’t give you clean oil in the 55-gallon drum.”

    2. During down times after maintenance or repairs, do not leave caps off of reservoirs and do not leave motors, valves, pumps or other internal components exposed. “The system should, as much as possible, be kept sealed so that you don’t get air depositing dust and humidity into the system,” says Jahnke.

    Contaminants invade a hydraulic system in a number of ways. During the manufacturing phase of, say, a skid steer, the hydraulic system may have been improperly flushed, resulting in built-in contamination. Ingression, another source of contamination, is when particles enter the system from the outside via the insertion of new oil that is already contaminated; or when dirt, dust or water sneak through seals or reservoirs topped with inadequate breathers.

    Perhaps the most dangerous form of contamination is generated contamination, which occurs when hard particles strip off of internal components and damage surfaces composed of weaker materials. If maintenance staff fails to filter out these contaminants, a cascade effect could occur in which freed particles continue to crash into components, thus creating more particles with the potential to do further damage.

    One way to make sure contaminants are eliminated from the hydraulic system is to regularly monitor filter elements and replace them when they are clogged. And while manufacturers sell equipment with a reservoir breather, they are not always equipped with a breather filter. Replacing the standard breather with one that has a built-in filter helps enormously to keep out contaminants.

    Fluid sampling, says Jahnke, is necessary to determine what kind of contaminants have invaded the hydraulic system. For instance, if dust, dirt, or silica is found, this usually means ingression had occurred. In this scenario maintenance staff should install a breather filter on the reservoir. If dirt is found inside the components of a new machine, running the fluid through an offline filter or kidney loop should clean it out.

    Hydraulic fluid should be sampled and analyzed on a regular basis. Depending on how frequently a piece of equipment is used as well as the pressure of its hydraulics, Eaton advises maintenance staff to take samples anywhere between two and six months. In order to determine whether the hydraulic fluid meets the target cleanliness, take a representative sample from the return line before the filter and send it to a qualified laboratory.

    (1) Ingressed: When particles enter the hydraulic system from the outside, they are called ingressed contaminants. Contaminants enter via inadequate reservoir breathers, faulty cylinder seals, or already-contaminated new oil.
    (2) Built-in: During the assembly process, the hydraulic system sometimes is improperly flushed before the machine is sold. This could mean that contaminants are already inside the reservoir, components, or fluid conductors when you buy the machine.
    (3) Generated: Corrosion of components and movement of pumps, motors and cylinders can create harmful metallic particles inside the hydraulic system. If left unfiltered, a chain reaction could occur in which generated contaminants interact with components’ moving parts and break the surface into more metallic particles.
    Clearance-size particles can get lodged between moving parts in the hydraulic system causing abrasive wear. First, the abrasive interaction between the particle and the surface of the component creates stress risers on the surface. Then, micro cracks develop near the surface when the component moves. If there are several cracks, they can join to form larger ones. Finally, surface material breaks away in the form of metallic particles that have the potential to do additional damage.
    Contaminants will inevitably enter your machine, whether from the outside or generated from within. Most equipment includes a sophisticated filtration system, often comprising a suction filter to catch medium-sized particles between thereservoir and the pump; return-line filter to collect dirt as oil returns to the reservoir; breather filter to prevent atmospheric contaminants from entering the reservoir; and the pressure filter, also known as the “last-chance” filter, which cleans oil directly after the pump to protect other components downstream.
    It is up to your maintenance staff to ensure that the filtration system is working optimally by monitoring it on a regular basis. When electronic or mechanical indicators alert you that filter elements are dirty or clogged, replace the elements immediately. 
    If the cylinder is not fitted with effective rod seals, dirt could get through and damage valves before it is caught by the return-line filter. It is normal for rod seals to wear out. As they shear oil off the rod or cylinder barrel, they become more rounded resulting in a leaky cylinder that could let dust and dirt in. Before leaking begins to occur, the rod seals should be replaced.
    Breathers enable sufficient air movement inside the hydraulic reservoir while keeping out atmospheric contaminants such as dust and water particles. The ones that come standard in heavy equipment aren’t always fitted with a filter-equipped breather. To maximize elimination of harmful particles, a filter-equipped breather should be installed.
    How Donaldson’s T.R.A.P. Breather works: 1. The circuit breathes in air containing moisture vapor. 2. The breather strips moisture from the incoming air, allowing only dry air to enter the circuit. 3. During the exhalation cycle, the breather allows unrestricted airflow outward.4. The outflow of dry air picks up the moisture collected by the breather during intake and blows it back out, fully regenerating the breather’s water-holding capacity.