Hydraulics

By Rick Russo, Senior Lubrication Engineer, ExxonMobil Lubricants & Specialties -- Plant Engineering, 12/1/2006

For many plant managers, achieving hydraulic system efficiency is an enigma. Growing sophistication in system components and increasing complexity of maintenance strategies have left them puzzled. During this time of technological and procedural innovation, it is imperative to acknowledge that the basic hydraulic systems principles haven't changed.

Maintenance professionals can achieve hydraulic system efficiency by focusing on the fundamentals. Concentrating on contamination control, system cleanliness and equipment wear protection enables companies to improve hydraulic system life and streamline production while simplifying maintenance.

Contamination includes air, water, dirt and other particulates. Air is detrimental to the life of the hydraulic system. It inhibits a fluid's ability to protect moving components, causing parts to wear quickly, foam to develop, operating temperatures to increase, premature pump failures and erratic actuator performance. The two most common air-related hydraulic contamination conditions are aeration and cavitiation.

Aeration occurs in the preliminary stages of a hydraulic system. When the pump sucks the oil from the reservoir, the created vacuum moves the oil through the pump and into the hydraulic pipeline. If the oil level is low or pipe fittings are loose, air enters the pump and aeration occurs. A noisy pump, a spongy pressure gauge or oil foaming are telltale signs of aeration.

Ensure the reservoir is filled to the proper fluid level. Test inlet connections by covering them with shaving cream. If bubbles implode toward the union, repair the problem and bleed the air from the system.

Mineral oil and synthetic-based fluids contain about 8% of dissolved air. When this air is freed from the oil, cavitation occurs. Common causes of cavitation include the vacuum draw on the oil exceeding 4.5 mm Hg; a clogged or collapsed inlet strainer; or inlet piping with a diameter that is too small. These factors can lead to foam formation, premature pump failures, accelerated component wear, increased operating temperatures and unscheduled maintenance. To prevent cavitation, maintain optimal hydraulic fluid levels in the reservoir, and monitor hydraulic pressure so it remains within the acceptable range.

A key indicator of a hydraulic fluid's antiwear performance is the ability of hydraulic oil to retain it properties over the long haul. Typically, three to five years is an acceptable lifespan of hydraulic fluid. Higher-performing fluids can last up to seven years.

Performance characteristics are best achieved when the oil is clean. Typical hydraulic systems require oil cleanliness in the ISO 16/13 or 14/11 range. To ensure cleanliness, maintenance professionals should install a filter before the pump, after the pump, a separate servo valve filter and/or a return-line filter. The strainer protects the pump; the pressure filter protects the actuator; the servo filter ensures proper servo operation; and the return-line filter ensures the reservoir stays clean.