Lube-Tech
This has just two tests, high-temperature kinematic viscosity again using ASTM D445 and low-temperature viscosity using a Brookfield viscometer. Both of these are designated as being critical specifications. Table 5 shows the allowed SAE grades and their respective limits.
The kinematic viscosity specification limits are quite wide and as ASTM D445 is a very precise test the Acceptance Limits are close to the Specification Limits and should not cause a significant problem for formulators or blenders. However, there is a separate issue with the kinematic viscosity of gear oils in that J306 states that they should still meet the kinematic viscosity limits after the product has been subjected to very high levels of shear in a test that lasts for 20 hours. In practice this only really impacts 75W-90 gear oils but means that they have to be manufactured with very shear resistant viscosity index (VI) improvers often based on polyalkyl methacrylate (PAMA). Using the sort of VI Improvers usually found in engine oils will result in gear oils that fail to meet this SAE J306 requirement.
If the ASTM D445 test for kinematic viscosity can be described as having high levels of precision the same cannot be said of the other test required by the J306 standard, that of low-temperature viscosity using the Brookfield method ASTM D2983.
In this test the specification limit is the same across all grades and is set at 150,000 mPa.s maximum. However, the test temperature varies from a modest -12o
C for a 85W grade down to -55o for a 70W product.
Taking the case of the increasingly popular 75W-90 gear oil, this has to be tested at -40o
C and at this temperature
the reproducibility of the test using a liquid bath to do the cooling is 28.5%. Calculating the Acceptance Limit produces a figure just under 125,000 mPa.s which is quite a long way from the already challenging specification of 150,000 mPa.s.
LUBE MAGAZINE NO.142 DECEMBER 2017 37 C Summary
Although the SAE standards for the viscosity limits of automotive engine and gear oils are universally accepted within the industry, some of their features appear to be less widely known. In particular the concept of critical specifications and the more stringent acceptance limits that go with them may be new to a number of readers.
Formulators need to work within the acceptance limits if their customers, the blenders, are to have any chance of meeting their obligations when new products are placed on the market.
Similarly, blenders should avoid the temptation to alter formulations in an attempt to reduce costs if this means that products will fall the wrong side of these more demanding limits.
An understanding of these requirements should bring about an improvement in lubricant product quality and hopefully this short paper will have at least gone some way to raise awareness amongst readers.
Dr. Edward ‘Ted’ Wright Following a period of research at Cambridge, Ted joined the Esso Research Centre and was responsible for electron microscopy and all aspects of elemental analysis.
Moving into lubricants his positions within Esso included quality control, marketing and procurement. For a number of years he was on assignment as Technical Manager at Comma Oil. Ted is still very much involved in the lubricants industry, especially with training, and consults on problem solving. He also serves on the Technical Review Panel of the VLS scheme.
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.113 page 5
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