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Despite their complexity, hydraulic systems are forgiving in nature. In most cases, they can perform well for a long time before any major or catastrophic failure occurs. Unfortunately, this “creep to failure” quality can promote apathy in production and maintenance attitudes toward failure prevention, efficiency optimization, and service-life management of these workhorse systems.

The most important component in any hydraulic system is, unequivocally, its hydraulic- fluid medium. Hydraulic fluid is an engineered component designed to simultaneously perform three major tasks: 1) act as a solid to transfer and amplify power; 2) lubricate a system’s moving parts; 3) absorb and dissipate frictional heat buildup within the system.

If the hydraulic system is to perform efficiently, the hydraulic fluid must be kept scrupulously clean and contamination-free. Knowing and taking action on this simple requirement provides the maintenance planner with an easy laser-focused strategy with which to manage and prevent most hydraulic-system failures for literally pennies a day.

Hydraulic-fluid contamination can present in three forms, i.e., as a solid particle, water, or air, all of which can seriously affect the fluid’s effectiveness and damage the components it serves. Maintaining hydraulic-fluid in an optimum condition requires a preventive strategy focused on measuring, controlling, and preventing the introduction of all three forms of contamination.

Hydraulic systems are simply not “dirt” tolerant. Cylinders and spool-valve assemblies are designed with precise operating tolerances that can be closer than .0002”, or 5 microns in size (the size of a human red blood cell). For comparison, a human hair is approximately .0035” in diameter, or 80 microns. The majority of solids manifest as grit or dirt of 100 microns or more in size that, if allowed to enter a hydraulic system, will damage machined surfaces and hydraulic seals.

In a close-tolerance environment, solid particles set up in a three-body abrasion state that will easily score mated machined surfaces, creating rapid bearing and component surface wear. This leads to undesirable fluid bypass, causing reduced hydraulic operating efficiency. Solid contaminants can cause a variety of problems. They include: valve stiction, in which valves can become “sticky” and inefficient due to static friction. increased fluid viscosity that results in sluggish operation (due to excess fluid thickness); and unwanted fluid leakage, where “nicked” and scored cylinder seals and wipe surfaces allow the contaminant to be pushed, or “weep,” past the seal.

If equipment is new or freshly rebuilt, all hoses, lines, and fittings must receive an internal cleaning. This is best accomplished using a compressed-air-projectile cleaning tool/system that shoots cleaning-wad material through the hoses and fittings to remove all dirt and swarf prior to connecting all lines and performing an initial system flush. Once cleaning and flushing is complete, the system filters can be changed out and the reservoir and system filled with correct-viscosity hydraulic oil ready for service.

Solid contamination can also find its way into new hydraulic fluid prior to delivery by the supplier or manufacturer, especially if bulk transferred with dirty transfer hoses and equipment. When receiving new oil—especially bulk oil—always perform an oil-analysis test to detect for solids and water contamination.

New oil for use in high-pressure hydraulic systems should test, at least, at an ISO cleanliness level of 16/14/11 or better. To prevent receipt of unacceptable hydraulic fluid, set up a cleanliness contract with your oil supplier, based on a contracted minimum-cleanliness level. The supplier would then be bound to provide proof of cleanliness upon delivery.

Other preventable main sources of maintenance-introduced solids contamination include:

  • improperly stored oil (loose or open lids, incorrect orientation of container, etc.)
  • non-dedicated, or dirty transfer equipment used to transfer the oil into the equipment reservoir
  • reuse of dirty “leaked” oil
  • “open to air” reservoir due to missing fill-port or missing reservoir breather
  • lack of filter maintenance, causing dirty oil to bypass into the system
  • incorrect filter rating (a 100-micron filter rating provides zero system protection)
  • poor housekeeping practices (cleanliness is godliness when it comes to handling lubricants and hydraulic fluids).  

Hydraulic fluid is naturally hygroscopic (water retentive) and can entrain moisture in the fluid until its saturation point. For most hydraulic fluids, the saturation point is reached around 300 ppm, or at a 0.04% concentration level. Water is a universal contaminant that can deplete vital oil additives and react with them to create corrosive acids that can attack system components. In addition, water can reduce lubricant film strength and its ability to release air, thus increasing the chances of wear, corrosion, and cavitation (see “Air Contamination” section below). In high-heat applications, water can also “boil off “ and create significant inefficiency in the hydraulic-power transfer motion. 

Typical preventable water contamination sources include:   

  • incorrect outdoor lubricant storage hot-cold cycle condensation
  • “open to air” reservoir allowing ingression of wash down and/or process water.

Water can be present (and visually detected) in a free state (separated from the oil in an unstable form), and an emulsified state (in a stable form, fluid appears cloudy). It must be dealt with by using a polymeric-style filter media designed to absorb the water as it passes through the filter; through vacuum distillation to “boil off “ the oil; and/or through dehumidification of the air in the reservoir headspace with use of desiccant breathers.

Air contamination causes problems when entrained in the fluid or oil. In this form, air bubbles less than one millimeter in diameter are dispersed throughout the fluid, which can reduce fluid viscosity and result in a lack of full film strength causing premature component wear. Entrained air can also reduce oil’s bulk modulus, causing a lack of efficiency and control due to the sponginess of the oil condition; and increase the heat load, resulting in fluid deterioration and system erosion or cavitation wear.

As for air bubbles, those greater than one millimeter can create foam, which can quickly deplete any hydraulic fluid antifoam additive causing the fluid to oxidize.

Typical preventable-air-contamination causes include:

  • over/under-filled lubricant reservoir(s)
  • clogged inlet/suction filters
  • clogged reservoir breather
  • restricted inlet line
  • loosely clamped inlet lines
  • pump-shaft seal failure.

Fortunately, contamination problems are easily preventable and, again, they are inexpensive to implement. The following recommendations can be used as a checklist or starting point for an improved PM approach to hydraulic system preventive maintenance.

  • Implement a fluid cleanliness standard.
  • Store all lubricants in a cool, dry place and practice FIFO (First in-First Out) stock rotation.
  • Practice good housekeeping, and ensure reservoirs are kept clean of all dirt and debris.
  • Cap all hydraulic hose and manifolds during handling and maintenance.
  • Wad-clean all lines prior to initial system startup.
  • Flush all lube systems prior to startup, and change oil and filter(s).
  • Use a dedicated (one for each hydraulic-fluid type) filter cart with quick connect fittings to transfer/clean hydraulic fluid before it enters a reservoir.
  • Install external sight gauges marked with a high- and low-fluid markers on reservoirs to check for water contamination and correct fluid levels (leakage detection).
  • Have machine operator regularly check for system reservoir levels and leaks.
  • Repair all system leaks as soon as they are discovered.
  • Use polymeric oil filters and desiccant reservoir-style air breathers.
  • Specify cylinder-rod wipers, and replace all worn actuator seals.
  • Regularly perform oil-analysis testing in hydraulic fluids to test cleanliness and additive efficacy.


Don’t forget about the hardworking hydraulic systems in your plant. They are the lifeblood of many manufacturing operations. And their effectiveness is not just a financial concern: It’s also a safety concern. With basic care, common sense, and little expense, you can ensure these system are efficient, reliable, and long-lived.TRR


Ken Bannister has 40+ years of experience in the RAM industry. For the past 30, he’s been a Managing Partner and Principal Asset Management Consultant with Engtech industries Inc., where he has specialized in helping clients implement best-practice asset-management programs worldwide. A founding member and past director of the Plant Engineering and Maintenance Association of Canada, he is the author of several books, including three on lubrication, one on predictive maintenance, and one on energy reduction strategies, and is currently writing one on planning and scheduling. Contact him directly at 519-469-9173 or kbannister@theramreview.com.

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