SYMPOSIA PAPER Published: 01 January 2001

The Importance of Shear Stability in Multigraded Hydraulic Fluids


Multigrade hydraulic fluids are known to undergo loss of viscosity during service. The mechanism is a function of severe shear forces generated in high pressure operations, which affect polymeric Viscosity Index Improvers (VII) present as additives. The level of shear degradation experienced is related to several factors: the molecular weight and chemical structure of the polymer, the pressure drop and physical design of the hydraulic system, and fluid temperature. Viscosity Index Improver polymer molecular weight will determine thickening power, but also impact the critical parameters of shear stability and low temperature viscometrics.

New data generated in a Sundstrand axial piston pump test stand provide further insight into the relationship between polymer molecular weight and shear stability.

Previous field test work has documented operating conditions encountered by multigrade hydraulic fluids in forestry equipment. The shear stability data gathered in this study can be used to predict fluid performance in high-temperature field applications.

Author Information

Hyndman, CW
RohMax USA, Horsham, PA
Kinker, BG
RohMax USA, Horsham, PA
Placek, DG
RohMax USA, Horsham, PA
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Developed by Committee: D02
Pages: 523–534
DOI: 10.1520/STP38299S
ISBN-EB: 978-0-8031-5567-1
ISBN-13: 978-0-8031-2883-5