Lube-Tech


Then there are the two high-temperature tests which mainly apply to the grades which don’t contain the letter “W” and are often used on their own to describe monograde lubricants i.e. those that contain no viscosity index improver.


The simplest of these tests is kinematic viscosity which is run at 100o


C by ASTM


D445 and measures the rate of flow of the oil under the influence of gravity. Most oil laboratories, even the most rudimentary, will run this test which is also used for transmission fluids and, with modifications, industrial oils as well. However, its simplicity is probably its most attractive feature as in a real engine situation the only time when the oil is flowing under gravity at a temperature around 100o


C is when it is draining back


to the sump, and this is hardly the most stressful of its operations.


Of significantly more importance is the other high-temperature test known as the high-temperature high-shear-rate (HTHS) viscosity. This is measured by a number of different methods but all at 150o


C which is a more realistic figure for the working parts of an engine and subjects the oil to the sort of high shear rates which are likely to be found in


PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE


critical parts such as the bearings and between piston rings and cylinder walls under severe operating conditions.


The HTHS viscosity is an extremely important parameter to the engine designer. Too low a figure and the oil will not keep moving metal surfaces apart which can lead to very high rates of wear and early engine failure. Too high a value will result in the efficiency of the engine, and hence the fuel economy, being reduced. Much of the recent improvements in fuel economy have come about due to the development of special oils with low HTHS values.


Taken together these tests completely define products in the SAE J300 standard which is shown in Table 1.


Up to now, nothing has been said about the Critical Specifications mentioned in the title of this paper. So, what are they? They are defined as being specifications which due to some aspect of the product characteristic or end-use of the product, or both, require that the user has a high degree of assurance that the true value of the product property actually meets, or exceeds, the quality level indicated by the specification limit value.


No.113 page 2 So, what does this mean in practice?


Let’s take the case of a blender making a SAE 30 oil. This should have a kinematic viscosity at 100o


C measured by ASTM D445 of between 9.3 and 12.5 mm2 /s.


Suppose the blender tests the product and gets a result of just above the specification minimum, say 9.301 mm2


/s.


Now the blender could argue that the product met the specification and therefore release it for sale. But, kinematic viscosity is defined in the SAE J300 as being a critical specification so there should be a high degree of assurance that the true value is within specification. Generally, the high degree of assurance equates to a 95% confidence level or being right 19 times out of 20.


Even if the blender’s equipment for measuring viscosity is perfectly calibrated and exhibits no bias there will always be some imprecision in the test results. For a Critical Specification the blender should only release the material if the result obtained is far enough inside the specification to take into account this inherent variation. This more conservative value is known as the Acceptance Limit.


The SAE J300 standard directs the reader towards another standard (ASTM D3244) when it comes to dealing with these issues and how far from the Specification Limit the Acceptance Limit lies. Basically they can be very close together for laboratory tests which have a high degree of precision. Fortunately kinematic viscosity by D445 is such a test and whereas the Specification Limits for a SAE 30 oil are 9.3 to 12.5mm2


9.376 to 12.40mm2


/s, the Acceptance Limits are /s.


Table 1. The SAE J300 Standard for Automotive Engine Oils 34 LUBE MAGAZINE NO.142 DECEMBER 2017


In practice a blender is unlikely to release a batch of material that is really close to the specification limit because it allows no tolerance for the viscosity to change as a result of the packaging process. In the case of kinematic viscosity, as the Acceptance and Specification Limits are close together, the fact that this is a critical parameter is not likely to be a


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