Hydraulic fluid: Pay now or pay later

Choosing the optimum fluid for an application can minimize operating problems and save money

Whether they're used in mobile or stationary equipment, hydraulic fluids give new meaning to the term multitasking. Hydraulic fluids must transmit power as fluid energy, lubricate components, cool the system by transferring heat, offer little resistance to flow, provide a viscous seal, withstand wide and frequent pressure cycles, minimize wear, reduce friction, prevent rust and corrosion, and keep system components deposit free.

Catastrophic failure of an axial piston pump reveals pieces of pistons and the shoe plate in the housing. Fine metal particles coat the internal pump surfaces. This type of failure can be caused by cavitation, fluid contamination, pump over pressurization, using an improper hydraulic fluid, or any combination of these.

That's a lot to ask, and no two fluids offer the same level of performance for every parameter across the board. Therefore, don't expect optimum performance and longevity from a hydraulic fluid unless you have discussed details of the specific application with your fluid supplier. Only then can the supplier recommend a fluid best suited to the application.

The big three
When discussing your specific needs with a supplier, many factors must be considered. However, three have the greatest impact on your selection.

Base oil used — Traditionally, hydraulic fluids have been produced from solvent neutral mineral oils (called Group I base oils). But the trend is toward the use of hydroprocessed and hydrocraked base oils (called Group II and Group III). As the group number increases, so does the cost. However, the higher base oil group number also provides longer life for equipment due to lower volatility, better oxidation and thermal resistance, and, sometimes, better demulsibility. In addition, the higher numbers contain less sulfur and have higher saturate levels, both of which are better for your equipment.

Additive package — Hydraulic fluids are made up of about 99% base oil and 1% or less of additives. The components of that 1% are critical to your operation, particularly when the trend is to use longer-life fluids. A variety of hydraulic fluid additive packages can be used in fluids, and using the right additive combination is critical. A fluid supplier can help identify what mix will provide the best performance for your equipment.

Excessive heat has caused varnish to build up in the reservoir of this hydraulic system.

Factors that influence additive selection include its performance, compatibility, color, odor and economics. With the trend toward smaller hydraulic systems, the fluid stays in the system's reservoir for a shorter period of time. This reduces the time to release air from and cool the fluid, so the equipment tends to run hotter. In smaller systems, the additives in the fluid must work harder because they have less time to perform in the presence of contaminants, such as dirt, metal particles, and water.

Less time in the reservoir means that additives may be less effective at demulsifying the water that is inevitably contained in hydraulic fluid through condensation or leakage. Water in the fluid impacts fluid performance by plugging filters and resulting in corrosion and pump wear. Demulsifiers separate water from the fluid so the water can be drained from the system. Furthermore, corrosion inhibitors and anti-wear additives protect surfaces from contamination that can harm the equipment.

The additive package also needs to be concentrated enough to remain effective in case operating conditions become more severe over time, maintenance becomes overdue, or a mechanical problem occurs.

Viscosity — Assuming a fluid with the appropriate type of base oil and additive package is used, viscosity is the most important property to consider when choosing a specific fluid for an application. Viscosity is the strength of the cohesive force in the fluid. It determines the amount of fluid friction and the drag exerted by moving parts, which draws the fluid between metal surfaces. The viscosity of the fluid at the equipment's operating temperature determines the bearing friction, the rate at which the fluid will flow through a bearing and the load-carrying capacity of a bearing.

Excessive pressure caused this piston shoe to break at the piston joint.

Fluid of the correct viscosity is readily distributed to moving surfaces — a key to long pump life. Fluid with lower than recommended viscosity can result in high internal pump leakage, which increases temperature. Conversely, fluid with higher than recommended viscosity will result in excessive pressure drop and, again, higher temperature. Following fluid recommendations of component manufacturers helps prevent these problems.

Why pumps fail

Using the wrong type of fluid in a system certainly can cause premature pump failure, but keep in mind that most pump failures don't occur because the wrong fluid was specified. More than 90% of hydraulic pump failures can be attributed to one or more of these causes:

Improper viscosity results in excessive bronze transfer on the piston pump swashplate.
Contamination — any foreign material in the fluid that affects its performance — dirt, metal particles, water, air, etc. Contamination can cause abrasive scratching, corrosion, wear, buildup of deposits, or any combination of these.

Aeration — the presence of dispersed air bubbles in the system's hydraulic fluid. Aeration can result in severe erosion of pump components when the bubbles collapse as they suddenly encounter high pressure when entering the discharge area of the pump.

Cavitation — occurs when the pump is starved of hydraulic fluid. When this happens, air enters the pump and causes damage similar to that of aeration. Cavitation usually can be distinguished from aeration by the sound. Aeration produces an intermittent sound, whereas the sound of cavitation is more constant.

Excessive heat — temperature above a specified limit that reduces fluid viscosity. It causes fluid degradation, resulting in acid, sludge, gum, resin, and varnish formation.

Over pressurization — subjecting the pump to higher pressures than it is designed to accommodate can result in catastrophic failure.

Improper fluid viscosity — fluid with too high a viscosity can cause premature wear and excessive heat. Too low a viscosity can have the same detrimental effects but lead to pump cavitation as well.

Premature pump failure can be attributed to any of several causes, most frequently, fluid contamination. Improper assembly and alignment account for more than a quarter of failures.

Partner with your fluid supplier
The most important practice you can do to maintain the integrity of a hydraulic system and save money is to consult closely with your fluid supplier. Discuss each specific application and the environment in which your equipment is operating.

Does your equipment run in wet conditions? If so, your fluid probably needs an additive package that provides corrosion protection and contains a demulsifier to separate water from the fluid. Are high temperatures an issue? If so, you'll need an additive package that provides good thermal and oxidation stability. Various environments and applications call for a different mix of additive components.

In addition to the smaller systems and longer drain intervals previously mentioned, several other hydraulic equipment trends impact the demand on fluids and additives. Higher pressures and temperatures increase the chance of varnish deposits and sludge. Higher power density and shorter cycle times also push the limits of additives. However, a greater focus on the cleanliness and filterability of the fluid helps reduce the detrimental effects of contamination.

Other trends have evolved from such environmental sensitivities as biodegradability, recycling, reclaiming, and reconditioning the fluid, and fluid disposal concerns.

Pay now or pay later

Using anything other than the best fluid formulation for an application can result in excessive wear, shortened fluid life, oxidation, corrosion, contamination, degradation of equipment, and warranty issues. Also be prepared for serious financial consequences: labor to drain the old fluid; labor to replace the fluid; the fluid itself; lost productivity while equipment is down; fluid disposal.

Obviously, then, collaborating with your fluid supplier to select the best fluid formulation for each application is the best way to have a positive impact on your bottom line. You can pay initially for the right fluid and protection for your application and environment. Or you can pay for the downtime and repair costs associated with using

the incorrect fluid. Considering the high cost of your hydraulic equipment, the fluid is not the sensible place to cut corners. You have the power to prevent the problems and costs associated with using the wrong fluid in your equipment.

Base oil categories

Group Description % saturates % aromatics % sulphur Viscosity index
I Solvent-refined mineral oil < 90 > 10 > 0.03 80 to < 120
II Hydroprocessed > 90 < 10 < 0.03 80 to < 120
III Hydrocracked > 90 < 10 < 0.03 120+
IV Polyalphaolefins        
V All others        

Oil drain intervals
Construction equipment manufacturer Oil drain interval - hr
Previous interval Current interval
Komatsu 2000 5000
Hitachi 2500 4000
Caterpillar 2000 4000
Sumitomo 2000 5000
Volvo 2000 4000
JCB 1000 3000
Requires all-year, multigrade oil

For more information, contact David Oesterle at [email protected]