Clean approach to hydraulic fluid
May 1st 2009

Vane-type pumps and motors are among the most commonly used components in mobile and industrial hydraulic systems.They are efficient and reliable but, like all mechanical devices, they are neither maintenance-free, nor indestructible. John Taylor of Eaton Fluid Power comments

Keep the fluid clean! Dirty fluid is the root cause of 80% of problems with hydraulic systems and it's the first thing to check when a problem occurs. Four key causes of contamination are: Contaminated new oil, built-in contamination, ingress contamination, and internally generated contamination.

Contaminated new oil

Although hydraulic fluids are produced under clean conditions, they are at risk of contamination during transfer or storage, be it rubber or metal particles or flakes/metal scale. Contaminants should be removed using a high efficiency filter.

Built-in contamination

New machinery harbours contamination. The amount removed during the system flush depends on the filters used and the temperature, viscosity, velocity and 'turbulence' of the flushing fluid. There should always be a 'running-in' period.

Ingressed contamination

Fluid contamination can result from surrounding conditions. The key is to minimise the risk. The four major contamination routes are: Reservoir vent ports (breathers) power unit or system access plates, components left open during maintenance and cylinder seals.

Generated contamination

The most dangerous contamination risk is the system itself. Particles are work-hardened to a greater hardness than the surfaces from which they came, and are very aggressive in causing further wear. If not removed, contamination levels will accelerate. The best way to prevent this is to start with a clean (fully flushed) system and keep it clean.

How clean is clean?

Designing and maintaining an optimal filtration system is beyond the scope of this article. A number of good references are available from filtration, fluid and equipment suppliers, for example: Eaton's Guide to Systemic Contamination Control (http:// hydraulics.eaton.com/products/filtration.htm.)

Troubleshooting

Because 80% of all hydraulic system problems are contaminated fluid related, it makes sense to learn how to identify contaminant damage. This is generally quite easy. For example, if you see marked or 'frosted' vanes, grooves cut into the shaft seal diameter, or a 'chopped' ring (Fig. 1), you can be confident the culprit is particulate contamination. Eliminate the source before repairing pump or motor and flush the system.

Cavitation

Cavitation occurs when bubbles form and collapse within the fluid. Bubbles generate high temperatures that eventually erode the internal surfaces. They might be from air trapped in fluid or from fluid being literally pulled apart by forces within the system to form voids that then collapse. Evidence of such damage is surface pitting (Fig. 2).

Typical causes: Faulty suction line, shaft seals worn, reservoir problems, low oil level, poor baffling and unsuitable fluid. Damage caused by fluid void implosions can be severe.

Causes of this kind of damage include: Inlet velocity too fast, reservoir /system design, clogged inlet strainer/filter, restricted flow, high water content and pump over speed.

Erosion

Although some damage can look a lot like cavitation, it might be caused by particulate contamination in a high-speed oil stream.

This kind of damage is typically found at metering edges or critical surfaces and tends to produce less damage than cavitation. The causes include contaminated oil or oil above the recommended ISO level. (Fig. 3).

Catastrophic damage

There are many other ways to damage a pump or motor: over-pressurisation, air locks, misalignment, improper modifications, component misapplications and incorrect assembly procedures.

Pump repair

An advantage of vane-type technology is its simple in-the-field repair qualities.Most modern designs place the critical operating components in a cartridge. In most cases, an overhaul consists of disconnecting the pump or motor from the equipment, removing a few bolts, and replacing the cartridge. Ideally, to ensure performance/reliability, anything beyond a cartridge replacement should be performed at an authorised repair centre.

Pump manufacturers provide rebuild kits for their products and instructions for the operation. A typical example can be found at http://hydraulics.eaton.com/products/pdfs/i3 143s.pdf. Unless pump or motor has failed catastrophically, a field overhaul with genuine OEM parts will quickly return it to service with performance equal to a new unit. The key to avoiding failure is maintenance, and the golden rule of pump maintenance is keep the fluid clean!

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