24 S E P T E M B E R 2 0 0 5 T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y

he variety of hydraulic equipment inthe United States is matched with a

variety of lubricants designed to make itoperate efficiently. The lubricant purchasingdecision maker is bombarded with a batteryof fluids which may be categorized as fol-lows:

• Antiwear hydraulic fluid.

• Multigrade antiwear hydraulic fluid.

• Tractor hydraulic fluid.

• Biodegradable hydraulic fluid.

• Food grade hydraulic fluid.

• Fire resistant hydraulic fluid.

• Synthetic hydraulic fluid.

To add to the confusion, there are a dozenor more viscosity grades associated withmany of these categories. Over the past 10years or so, equipment manufacturers have

acknowledged that water contamination is areality with most applications and is difficultto control.

Many mobile equipment suppliers preferthat any contaminant water be emulsifiedinto the oil; others prefer the water to beseparated from the oil.

Pump manufacturers such as Parker Han-nifin (formerly Denison) and Sauer Danfoss(Sundstrand) have established pump testswhich examine the ability of the oil to pro-tect the pump under conditions where watercontamination is present in the oil. The T6-C vane pump test, introduced by Parker inthe mid-90s, included a wet phase examin-ing the impact of water. This new Parker testreplaced the T5-D pump, which was typical-ly run without the addition of contaminantwater and which formed part of the HF-0specification. The Danfoss pump forms part

By the Afton Chemical Industrial Team

Practical Applications

Hydraulic industry under pressureHydraulic industry under pressure

T

A variety of technical issuesawait those in charge ofpurchasing lubricants forhydraulic machinery.

A fair amount ofhydraulic lubricantnow is formulatedwith Group II baseoil in the UnitedStates.

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T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y S E P T E M B E R 2 0 0 5 25

of John Deere’s tractor hydraulicspecification J20-C using the Series90 piston pump.

For many years, the industry rec-ognized that filtration performance isimpacted by water contamination.This was tested a number of waysincluding Parker’s filtration test,AFNOR filtration test and others.Typically, these tests screen the oil’sability to filter in the presence ofwater contamination.

In recent years, however, thestakes have risen. There is morestress on the oil caused by equip-ment operating under higher pres-sure as well as environmental consid-erations.

Higher pressuresGenerally, smaller pumps are beingdesigned to run at higher pressures.

Higher pressures give rise to highertemperatures, more shearing of anypolymers in the oil and hydrolysis andoxidation. The impact of water on theoil is now even more significant.

In a recent presentation given atthe 2005 STLE Annual Meeting, Park-er highlighted the impact high pres-sure has on properties of the oil suchas viscosity index, shear stability andwear protection. Parker has respond-ed to this industry trend of higherseverity with their new hybrid pump

test. This unique test, which is run ina few labs around the globe, exam-ines the impact of water on wear andfiltration performance of the oil inboth a piston and a vane pump in thesame test.

Environmental considerationsIt is no longer considered acceptablefor new hydraulic equipment to leak.

CONTINUED ON PAGE 26

Figure 1. Scratches on a pistonpump caused by particulates inthe oil.

(Photos courtesy of Philippe Parreau, Parker

Hannifin (formerly Denison))

Figure 2. Lubricant-relatedpump failure.

Figure 3. Schematic cutaway of the T6H20-C hybrid pump.

Figure 4. The T6H20-C hybrid pump.

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26 S E P T E M B E R 2 0 0 5 T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y

Leaking hydraulic oil is a safety hazard,implies poor quality and adds maintenancecost. No leaks, however, means that there islittle need for regular topping off of thehydraulic lubricant. Topping off was a con-venient way to add extra antiwear chemistry,corrosion inhibitors and antioxidants whichare present in the lubricant. This regularadditive replenishment is now missing.

OSHA noise standards have caused theindustry to focus on noise levels of hydraulicequipment. Baffles are often included in thedesign of the equipment to reduce noise.Unfortunately, such noise damping also canresult in less air flow around the pump,which may result in increased operatingtemperature of the hydraulic oil. Increasedoperating temperature accelerates anyhydrolysis and oxidation and reduces theeffective oil film thickness.

The challenge continues…Tests such as the Parker hybrid pump testallows the formulator to ensure that there issufficient robustness in the oil. This robust-ness may be measured in terms of:

• Antiwear properties.

• Air-release effect.

• FZG properties.

• Hydrolytic stability.

• Anti-oxidation properties.

• Filterability.

• Seal performance.

Antiwear properties are measured usuallywithin the vane pump by changes in thevane and cam ring weight changes. This is afairly realistic measure which is believed tocorrelate well in the field. One challenge,however, is that the pump tests available inthe industry tend not to run at as high pres-sure as some mobile applications. The FZGtest is an attempt to take the pressures high-er, albeit under a different configuration. Thehigh level of sliding motion designed in theFZG test gears give some simulation of thesehigher pressure conditions in vane pumps,though the correlation has not been wellestablished.

At higher pressures the air-release prop-erties of the oil become even more impor-tant. The possibility of cavitation of a pumpis greatly increased under higher pressureoperating conditions. Air-release propertiesare influenced heavily by the type and

CONTINUED FROM PAGE 25

Figure 5. Impact of poor antiwearproperties of an oil T6H20-C hybridpump.

Figure 6. Impact of poor air releaseproperties of an oil T6H20-C hybridpump.

As pressures gethigher, oils gethotter and requirehigher performinglubricants focusingon viscosity indexand shear stability.

30%

40%

50%

60%

70%

80%

90%

100%

110%

65 80 100

Temperature (°C)

Rel

ativ

e Fi

lm T

hick

ness

VI = 159

VI = 136

Table 1. Importance of Viscosity Index to film thickness at highertemperatures.

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T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y S E P T E M B E R 2 0 0 5 27

amount of additive present in the oiland in, particular, are influenced bythe selection of foam inhibitor.

Higher temperatures give rise tohigher reaction rates such as hydroly-sis (the reaction with contaminantwater). Hydrolysis is measured by ahydrolytic stability test which meas-ures both the build up of acidity andthe tendency for corrosion. Thisparameter is very important to oilsused in piston pumps containing yel-low metal and is very significant insystems running under high pressure.

The industry has seen some pumpfailures in mobile equipment whichare believed to involve a corrosivemechanism and water. The additivesand polymers selected for use inhydraulic oils are generally tailoredto reduce this tendency.

It is interesting that higher pumppressures lead to a need for higherlevels of wear protection and higherlubricant film thickness. In turn, high-er wear protection and higher filmthickness result in higher additiveand polymer levels. It is these addi-tives and polymers, which if notselected carefully, lead to hydrolysisand corrosion problems.

Viscosity IndexIn addition to the environmental andmechanical changes that are under-way in the area of hydraulics, we arealso experiencing the evolution ofbase oils. A fair amount of hydrauliclubricant now is formulated withGroup II base oil in the United States.The higher pump pressures and otherinfluences that tend to result in high-er operating temperatures putgreater stress on the seals. Ideally,hydraulic fluids formulated withGroup II base oil will have the sameseal performance as their Group Icounterparts. Unfortunately, there isyet to be much focus in this area inthe industry.

As pressures get higher, oils gethotter and require higher performinglubricants focusing on viscosity index

and shear stability. The number ofmobile hydraulic applications usingstraight grade hydraulic oil is likely todecrease as the higher pressures inthese applications demand higherfilm strength and viscosity index.This is an interesting time in thehydraulic oil industry. <<

For more information or if you have ques-

tions about this article, contact Ian

Macpherson, marketing manager-industrial

additives for Afton Chemical Corp. in Rich-

mond, Va., at Ian.macphersron@

aftonchemical.com.

For more information, including samples ofSynergexTM Premier and SynergexTM T Plus

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SynergexTM Premier and SynergexTM T Plus are trademarks belonging to Arkema Inc.©2005 Arkema Inc.

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