Oil viscosity numbers

Perhaps this is common knowledge to some, but I often wondered what do all those numbers and letters mean when an oil is defined? We sort of know that a SAE 40 oil is thicker than SAE 20 etc but what does the number really define.

SAE is short for Society of Automotive Engineers, an American body who define standards which are recognised throughout the world. The number refers to time in seconds!

The viscosity of an oil is determined by pouring a known quantity through a calibrated aperture and the time it takes is an indication of the viscosity, thicker oils take longer to pour than thinner oils. A standard instrument that contains a calibrated volume of oil is used with a calibrated aperture.

SAE standards cover 20, 30, 40, 50 and 60 and the temperature the oil is tested at is +100C. The thinner grade oil such as 10, 20 which have a W suffix is tested at -10C. So a 10W - 50 oil has a pour point viscosity of 10 at -10C and a viscosity of 40 at +100C, that is it is modified to become thicker as the temperature rises.

The things you learn from books!

Er, would that be 10W - 40 then or 10W - 50 and a viscosity of 50 at +100C.
BobH

OOPs - sorry typo. Mind wandering as the ears were on the GP!

John

I don't think there are any oils that have an increased viscosity as the temperature rises. A 10W-50 behaves like a 10 grade at low temperature, but behaves like a 50 at working temperature. It still gets 'thinner', but not as much as a mono grade oil.

Norm

And viscosity and lubricity are not the same thing, so "thickness" does not necessarily indicate "goodness" at higher temps.

Long chain Polymers involved in the change of viscosity as the oil is heated! Additives modify the oil to hold ash in suspension. (Detergent oils). Low friction additives also allow the oil to stay in place instead of draining down.

Clive

Hi Norm,

Point taken. Perhaps my poor choice of words, maybe change "become" to "remain" would have been a better choice.

Interestingly I dug out a book on the Jaguar V12 engine development and it showed that flow rates at +90C and +120C gave a higher flow rate at the higher temperature using 20W-50 oil. I suppose that makes sense as the hotter oil would be a bit thinner and easier to pump?

From memory, the SAE numbers allocated to vehicle oil each designate a fairly wide band of viscosity. The numbers themselves are just numbers, they do not have units and there is no such thing as SAE 42.5, for example. A bit like the seasons, really as long as it is between the equinox and the summer solstice it is Spring.

The numbers were allocated by the SAE, John, and they relate to the time in seconds, but not directly in any mathematical sense. I remember from the 1970s, a nice man from Duckhams explaining that the numbers were made so simple that even those who bought new American V8s could understand them ...

Cheers, Tim

They also conform to DIN standards and if you look in your car service book most are to DIN 9000, though recently they have been updated as my new Passat has a different rating than the old one.

Doesn't the DIN 9000 in Clive's post refer to the quality procedures that the firm is prepared to be audited against? Also known as ISO 9000?

The numbers quoted for viscosity do have meaning. I have carried out many thousands of viscosity measurements by many differnt methods over the past fifty years. You can look-up the meanings of the various SAE figures on the web.

There is whole body of instructional and disputational text on viscometry which makes this a subject for the academic. However the practical applications are relatively simple in industry where robustness of method and results are subject to time constraints. One such example is in the varnish kitchen where the viscosity of the varnish being 'cooked' can change whilst the measurement is being done.

The basic and simplest methods are measurements of flow rate, that is the time taken for a specified volume to flow, for example, through a specific sized hole. The apparatus used is generally a so called flow-cup. You can visualise these methods by imagining punching as hole in a bucket, filling the bucket with the liquid in question and timing how long the bucket takes to empty. For comparative purposes the temperature need to be known for each measurement. The results are reported as so many seconds at the test temperature. There are legion of cups in use, in the gravure printing industry the Zahn cups ( small bucket on a stick) are most favoured, in the car painting business in the US the Ford No. 4 cup held sway, and in the UK the BS.B4 cup, nominally equivalent to the Ford 4, in practice these could only be compared when one or other had a conversion factor applied.

Of course this sort of method only works well for Newtonian liquids. ( search Google for an explanation of Newtonian!) Non-Newtonian materials gels, thixotropes, dilatants, semi-fluids, foodstuffs, etc. etc. have to be tested in different ways often by methods which involve rotation of a paddle in the liquid and measuring the amount a spring is either stretched or compresessed. The classic instrument for this is the Brookfield Viscometer.

Jerry

Hi Jerry,

Thanks for your lucid answer. So it seems the textbook I read had the story correct. They do mention standard cups with calibrated holes.