LUBRICATING OILS
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The Significance
of S.A.E. Numbers
• By E. V. PATERSON, A.M.I.Mech.E., M.S.A.E., M.I.Pet.
LUBRICATING oils for motor vehicles are frequently classified according to their ,S.A.E. numbers. Vehicle operators, generally, know which number is recommended by the engine or vehicle maker for their particular trucks, but many have little idea of what the
numbers really signify. It is often thought that these numbers have some connection with the quality of the oil, and that if an oil be bought having the right S.A.E. number it will be certain to have the necessary lubricating properties. This is quite wrong.
Fundamentally, the S.A.E. numbers, which comprise 10 grades of crankcase oil (S.A.E. 10 to S.A.E. 70) and four grades of gear oil (80, 90, 140 and 250), are a standardization of viscosity limits.
Generalization Vague
A general classification of oils by terms such as heavy, medium, light, and so on, is too broad to be serviceable and would vary between different oil manufacturers. The Society of Automotive Engineers, the American counterpart of what was our Institution of Automobile Engineers, standardized the ranges or limits of viscosity, ' and have given each range an S.A.E. number.
In short, the viscosity of any oil having an S.A.E. number must lie within certain limits at a stated temperature. It should be pointed out that the S.A.E. specifications concern viscosity only and do not consider any other properties of the oil. They A32 are not, therefore, in any way indicative of quality.
A brief, description of what is meant by viscosity will enable the reader to understand what follows. Fundamentally, viscosity is a measurement of a liquid's internal friction or, in other words, its resistance to flow. The more fluid a liquid is, the lower its viscosity. Heavy oils, or viscous oils, have greater internal friction and result in loss of power and reduced mechanical efficiency. Lightor low-viscosity oils arc more easily squeezed out from between bearing surfaces, but reduce power loss.
The golden rule is to use the lowest-viscosity oil that will satisfactorily do the job. Viscosity is measured scientifically by timing the late of flow of a sample of the oil through a specified capillary tube, under certain conditions of temperature and pressure. Commercially, it is measured by the time taken in seconds for a measured quantity of oil to flow through an orifice of standard dimensions.
The apparatus employed in the U.S1A. is the Saybolt viscometer, which times the flow, in seconds, of 60 cubic cm. of oil through a standard universal orifice of small
At 210 degrees F. dimensions. Readings are usually Minimum Maximum recorded at temperatures of 130 degrees F. and -210 degrees F. The time is known as Saybolt Universal Seconds.
In this country a Redwood viscometer is more frequently used, and this is similar, fundamentally, but employs a different volume of oil, a different size of orifice and the temperatures usually recorded are at 70 degrees F., 140 degrees F. and 200 degrees F. The readings are said to be so many Redwood seconds.
The S A.E. numbers are based upon viscosity in Saybolt seconds and are as shown in Table 1. Conversions from the Saybolt readings to Redwood are not absolutely accurate, because different temperatures are employed for each instrument, An
approximate conversion of the S.A.E. specifications into Redwood seconds is given in Table 2 and is near enough for all practical purposes.
It is seen that, for crankcase oils, there are seven oils ranging from S.A.E. 10, which is too light to be used in road vehicles in this country, to S.A.E. 70, which is too heavy for use in this country. S.A.E. 10 oil would be used in a climate where sub-zero temperatures are frequent. The S.A.E. 70 oil would be used only very rarely, even in tropical climates.
Oils for British Users
The oils most used in this country are 20, 30 and 40. S.A.E. 20 is a good winter oil and is frequently used all the year round in engines in good condition. S.A.E. 30 is the popular summer grade of oil. Some operators of hard-slogging petrol engines showing signs of wear prefer to use a 40 oil during summer, whilst occasionally 50 is used for old engines to reduce oil consumption.
A heavier oil will generally reduce consumption on an old engine, but due to its high viscosity at starting temperatures it will not circulate readily and can harm the bearings during the warming-up period.
To provide a classification for oils that are particularly fluid at 0 degrees F., ind therefore assist starting in very cold weather, two winter grades-10W and 20W—have been added to those mentioned. Some good-quality oils meet both the 10 and 10W or the 20 and 20W specifications. The lOW and 20W oils have maximum limits to their viscosity readings at 0 degrees F., and a little thought will serve to prove the value of an oil that can be double-branded 10 and lOW or 20 and 20W.
Easy Starting Having decided that, say, a 20 oil is the one required for all-the-yearround running, if it also meets the 20W specification then easy starting in the winter is assured.
The S.A.E. numbers 80, 90, /40 • and. 250, apply to transmission lubricants. One requirement is that these . oils shall not channel at certain tern-peratures. If channelling occurred during service all the oil would be thrown to the side of the axle casing and would not fall back into the : track of the rotating gears, which, consequently, would be starved of oil. It is very important to avoid channelling In greases, and it will be appreciated that it must not occur in gear oils either.
The two grades. of transmission oil most frequently used in this country are 90 and 140, for winter and summer respectively. Hypoid gears require only an 80 or 90 oil all the year „round, although this oil must be reinforced with extreme pressure additives. Mild extreme pressure additives are frequently incorporated in most axle oils, but the hypoid gear necessitates the use of a full complement of these. chemical additives. This is to ensure lubrication under severe conditions of high rubbing and sliding velocities, in conjunction with considerable pressures 'between the gear teeth. It need hardly be pointed out that the S.A.E. numbers give no information regarding the inclusion of extreme pressure additives.
Sump Temperatures
It will be observed that the two temperatures at which viscosities are given in the S.A.E. classifications130 degrees F. and 210 degrees Fare those between which an engine oil operates while contained in the sump. Possibly the Redwood temperatures of 140 degrees F. and 200 degrees F. more clearly represent operating conditions.
The S.A.E. classifications are con sidered by many to allow too great a latitude. The viscosity of an oil falls very rapidly with temperature rise. An oil's viscosity at 140 degrees F. by Saybolt or Redwood viscometer may be only one-fifth Of that at 70 degrees F. and it is this rate of falling-off that is of importance to the lubricating properties of an oil.
At 210 degrees F. there is not a great deal of difference in viscosity between a heavy or a light•motor oil, approximate average Redwood figures being 80 and 55 secs. for S.A.E. 50 and 20 oils respectively. When it is realized that the respective viscosities at 70 degrees F. would have been about 2,600 and 800, it will be appreciated that oils tend towards a common viscosity at high temperatures.
If one could compare viscosities of two oils at two differing temperatures, say 70 degrees F. and 140 degrees F., it would be easy to see which is more likely to stand up to engine temperatures the better. The S.A.E. classification concerns itself with only one temperature, except in the case of S.A.E.-40 oil, and even then the limits are extremely wide.
So long as it is remembered that S.A.E. numbers are merely a classification of heavy, medium and light oils, and have no bearing upon
quality, the classification can be of service. It is of interest to compare the S.A.E. numbers with the B.S.!. classification of internal-combustion engine oils in Table 3. This classifies oils into grades according to viscosity ranges and names them heavy, medium. light. etc. The chart shows the corresponding S.A.E. grades.
Viscosity Index Most engine manufacturers issue their own specification for recommended lubricating oils. These usually give reasonably narrow limits for viscosity readings at 70 degrees F., 140 degrees F. and 200 degrees F., in order to ensure that the oil used shall have the least possible fall of viscosity with temperature rise. A method has been evolved for estimating this degree of falling-off and oils are given numbers known as their viscosity index.
The worst oil, having a very high viscosity when cold and a very low viscos4t.y when hot, would have a zero viscosity index, whilst an oil that maintained its viscosity better than other oils would have a viscosity index of 100 or over. Oils can now be produced having viscosity indices up to 200, but these are not used as motor oils under normal conditions.