ASTM D6022 Calculation of Permanent Shear Stability Index

ASTM D6022 provides a procedure for determining the Permanent Shear Stability Index (PSSI) of an additive by using viscosities before and after a shearing procedure. PSSI is a calculation that can assess the impact of a single blend component on finished lubricant blends.

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Introduction

ASTM D6022 test measures the permanent shear stability index of lubricating oils. It estimates the performance of lubricating oils at high temperatures and shear rates. In other words, it attempts to find out whether a lubricant could withstand harsh operating conditions, holding its viscosity while maintaining such circumstances so that additional protection is provided to the components of the engines or machinery. This test method is commonly employed to evaluate the performance of automotive and industrial lubricants that experience steady shear conditions – mainly encountered in gearboxes, hydraulic systems, and internal combustion engines. This would allow them to be evaluated through the permanent shear stability index computation of the lubricant’s long-term performance under the thinning effects caused by shear stress and its ability to deliver adequate lubrication for a long time, even when conditions become extreme.

Scope

ASTM D6022 is a test method specifically developed to determine the Permanent Shear Stability Index for lubricating oils. It applies to multigrade oils, such as those that makeup motor and gear oils, which are designed to function well under a range of temperatures and shear conditions. The test method entails applying high shear conditions on the oil in a standard shear apparatus, then measuring the viscosity before and after shear to calculate PSSI. It provides valuable information on the shear stability of lubricant, which is an essential criterion for judging how good an oil may be in preserving its viscosity and performance in service life, especially at conditions such as high temperatures and shear forces where the oil begins to experience poor conditions.

ASTM E1221 Test Procedure

The following table presents the step-wise test procedure for ASTM E1221:

Steps	Description
Step 1	Prepare the lubricating oil sample by ensuring it is clean and contaminant-free.
Step 2	Set up the shear apparatus, which typically consists of a high-speed rotor that simulates shear stress.
Step 3	Introduce a measured amount of the oil sample into the shear apparatus.
Step 4	Subject the oil to shear conditions by running the apparatus at a specified temperature (usually around 100°C) for a defined duration (e.g., 1 hour).
Step 5	After shearing, measure the oil’s viscosity using a standard viscometer or rheometer.
Step 6	Compare the post-shear viscosity to the original viscosity to calculate the Permanent Shear Stability Index (PSSI).
Step 7	The PSSI is calculated by determining the percentage change in viscosity due to shear. A higher PSSI indicates better shear stability.

Sample Size

The degree of thickening should be at least 1.2. Viscosities should be measured at the same temperature and shear rate conditions and reported in the same units using the same method.

Result Analysis

If the degree of thickening is greater than or equal to 1.2, PSSI can be calculated using the equation:

PSSI = 100 ×(V0 -Vs)(V0 −Vb)

Where: PSSI = Permanent Shear Stability Index,

V0 = viscosity of the unsheared oil,

Vs = viscosity of the sheared oil, and

Vb = viscosity of the base ﬂuid

Conclusion

ASTM D6022 is a procedure related to the shear stability of lubricating oils under high shear conditions, an essential element in determining the performance of a lubricant over time in actual situations. With this method, the Permanent Shear Stability Index, or PSSI, calculated through this test method, is an essential indicator of the ability of an oil to maintain its viscosity and provide adequate lubrication, even with extreme operating conditions. Moreover, a higher PSSI value will result in a more stable and higher viscosity breakdown resistance for the lubricant, better engine protection, longer service life, and improved performance under demanding applications.

FAQs

What is the Permanent Shear Stability Index (PSSI)?

The Permanent Shear Stability Index (PSSI) measures how well lubricating oil maintains viscosity under high shear conditions.

Why is shear stability important for lubricants?

Shear stability is critical for lubricants because it determines how well the oil can maintain its protective viscosity under operating conditions where shear forces, high temperatures, and pressures can cause it to thin.

What types of lubricants are tested using ASTM D6022?

ASTM D6022 is typically used for multigrade lubricants, such as motor oils, gear oils, and other industrial lubricants, which must maintain their viscosity across various temperatures and under high shear conditions.

What does a high PSSI value indicate?

A high PSSI value indicates that the lubricant is highly shear stable, meaning it maintains its viscosity better under high-shear conditions.