The MEHF performance level can be achieved with fluid formulations that include high VI (>150) in combination with good shear stability.

This can most economically be achieved with the use of mineral oil (Group I, II, or III) base stocks.  Synthetic (PAO, PAG, POE) basestocks can also be considered if other performance features are desired (high temperature stability, fire resistance, extended life, etc.) 

High VI is most economically achieved with polyalkylmethacrylate (PAMA) viscosity index improver additive technology.  Standard PAMA viscosity index improver additives have good solubility in all mineral oil basestocks, and are also available in modified versions that are soluble in synthetic fluids as well.   This polymer chemistry offers extremely good VI boost, and may also offer some functionality as a pour point depressant.  For additional information on PAMA additive technology, click on the link to www.rohmax.com.

PAMA additives are available in a wide range of molecular weight ranges, tailored to the shear rate of the application.   Engines and transmissions are low shear environments, and therefore higher molecular weight polymers are selected to minimize additive treat cost in engine oils and ATF.  Hydraulic fluids and gear oils are subjected to very high shear rates due to the pressures and mechanical forces in these systems.  Therefore, lower molecular weight PAMA additives are the best choice. 

If the formulator wants to design an HV fluid with good low temperature fluidity, then PAMA additives with moderate shear stability are acceptable (PSSI ≥ 30).  In order to achieve the MEHF performance level, the formulator must select a PAMA viscosity index improver with excellent shear stability (PSSI ≤ 20).  The use of low molecular weight PAMA additives will enable the fluid to meet the shear stability requirements of the major global specifications, such as:

Denison HF-0       <15% viscosity loss in the KRL shear test (CEC L-45-A-99)

JCMAS HK            <10% viscosity loss in the Sonic shear test (ASTM D 5126)

SS 155434            Meet minimum viscosity value after KRL shear test (CEC L-45-A-99)