Understanding hydraulic fluid

Updated May 30, 2013
Hydraulic fluid at left clearly separates out, or de-emulsifies, the water that accumulates in the system. Engine oil on the right emulsifies the water making it less efficient as a hydraulic fluid.Hydraulic fluid at left clearly separates out, or de-emulsifies, the water that accumulates in the system. Engine oil on the right emulsifies the water making it less efficient as a hydraulic fluid.

Engine lubricating oils get a lot of attention in heavy equipment maintenance schedules and for good reason. They must be changed every 250 to 500 hours, they bear the brunt of all the heat, soot and a pressure cooker environment inside today’s emission compliant engines.

But hydraulic fluids are also important and deserve your attention even if you only change them every 2,000 hours or more.

Unlike engine oils, there is no universal spec for hydraulic oil. Each OEM specifies the type of hydraulic fluid and viscosity grade suitable for their machines, says Alex Smith, heavy-duty off-road, natural gas segment manager for Castrol. The key factors needed to select the right fluid include pump and equipment specification, the load put on the fluid or component, pump type, operating conditions and applications.

There are monogrades and multigrade hydraulic oils, but the most important variable in selecting a hydraulic fluid is viscosity and specifically, maintaining viscosity over the entire operating temperature range of the equipment, says Jeff Snyder, industrial brand specialist for Chevron Lubricants.

Multigrade products are typically used when a piece of equipment will be used in freezing or subfreezing temperatures in winter and high ambient temperatures in summer.


Keep it clean

Technicians and fleet managers also need to understand that good maintenance practices are critical to hydraulic fluid health and cleanliness. “For most hydraulic systems, the number one enemy is contamination,” Smith says. “The tolerances in valves and pumps are tighter than those in diesel engines, creating less separation between moving parts and a hydraulic system that is more prone to wear.”

Smith recommends using an ISO cleanliness chart and oil analysis to determine the cleanliness of your hydraulic fluid. Some OEMs specify hydraulic fluid cleanliness levels.

A complete hydraulic contamination control program will include proper maintenance of seals and hoses, proper storage of hydraulic fluid, use of filter carts, used oil analysis and prevention of leaks. Remember, if you see dirt accumulating on your hydraulic cylinders it is almost certain that that dirt if getting sucked back past the seal or wide and ingested by your hydraulic system.

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If you run a small fleet or just a few pieces of equipment, your best bet is to change the hydraulic fluid and filters at least as often as recommended in your owner’s manual, or more often in dusty or harsh environments. Larger fleets may benefit from using a “kidney loop” machine that siphons off the hydraulic fluid, cleans it and puts it back.


Lube oil as hydraulic fluid

Some fleet managers will use low viscosity engine oils in their hydraulic systems and some manufacturers offer an oil that will do both. Engine oils have certain advantages and help consolidate product choices in the shop, Smith says. But there are fundamental differences between the two types of fluid.

10W engine oils are designed to emulsify water and have a high level of ZDDP (a zinc-based, anti-wear additive), attributes that are desired by some OEMs, Snyder says. Many hydraulic fluids have 1,000 parts per million of ZDDP. Dedicated hydraulic fluids, on the other hand, de-emulsify water and typically have low levels of ZDDP, approximately 300 to 400 ppm.

Accordingly, engine oils hold water in suspension so water doesn’t go through the pump by itself and cause wear. The problem here is that the water may change the viscosity of the fluid and react with the ZDDP making the fluid less efficient. Water in the engine oils may also cause foaming, which also degrades efficiency. By contrast, hydraulic fluids drop the water out of the system and allow the product to perform as designed. Many hydraulic fluids also contain anti-foaming agents. The water that does accumulate in a hydraulic system can be easily drained away, not so with engine oils.

And water will accumulate in hydraulic systems more readily. Diesel engine oils are designed to operate at high temperature due to combustion, whereas hydraulic systems are designed to operate at 140 degrees or below, says Smith. This water tends to boil off from engine oils but accumulate in hydraulic systems.

Another drawback to engine oil in a hydraulic system is loss of pump efficiency, says Smith. For a pump requiring ISO 46 or ISO 68, the heavier viscosity 15W-40 will pump less efficiently than an designated hydraulic fluid. This causes the machine to expend more energy for the same amount of work and increases fuel consumption.

Drive train fluids are also sometimes used in heavy equipment hydraulic systems, says Snyder, for product consolidation or when there are cross-contamination concerns.


Work with your OEM

Given the variety of equipment on the market, Smith says there are a variety of specifications and requirements between each OEM regarding hydraulic fluids. Some require an ashless (zinc-free) formulation, multi-viscosity and different viscosities. There are also environmentally sensitive places that require contractors to use biodegradeable hydraulic fluids. It is important to work with your OEM and application to find the fluid that best fits your needs, he says.