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					                                                 HOW TO READ
A member of th e Set Point group                 A CAN OF OIL
                                           . . . AND HOW THE
                                         STUFF INSIDE WORKS
                                                                               by John S. Evans, B.Sc.

                                                     When purchasing         centistoke is one millimetre squared
                                                     a can, drum or          per second. It is important to
                                                     tankerful of oil,       remember that as temperature
                                                     how do you know         increases, the viscosity of an oil
                                                     what you are            decreases.
                                                     buying, where you
                                                     can use it and          The SAE or Society of Automotive
                                                     what sort of            Engineers has a grading system
                                                     quality you are         that describes the viscosities of
                                                     getting for your        oils that are used in all automotive
                                                     money?                  applications from motor scooters,
                                                                             to family cars to 4x4s to buses
                                      John Evans    On every can of          and trucks and bulldozers. There
                                                    lubricating oil          are two parallel systems, one for
                                   there are (or should be) a series
                                                                             engine oils and one for gear oils.
                                   of numbers and letters that
                                                                             The numbers          and    letters
                                   describe what is inside. This
                                                                             associated with the SAE system
                                   article will look at what those
                                                                             are shown below.
                                   numbers and letters tell you.

                                   Oils have both physical and                Engine 0W 5W 10W 15W 20W 25W 20        30   40   50   60
                                                                              oils
                                   chemical properties. They consist
                                   of a ‘base stock’ which is refined
                                   crude oil blended with various             Gear    70W 75W 80W 85W 90   140 250
                                                                              oils
                                   ‘chemicals’ that impart desired
                                   p ro pe rt ie s t o t he lu br ica n t
                                   enabling it to perform its job.           The reason for two systems is
                                   Let’s look at the physical                that gear oils have very different
                                   properties first.                         chemical properties from engine
                                                                             oils as they have to perform
                                                                             different functions. You might get
                                                                             away with putting an engine oil in
                                   THE PHYSICAL                              a gearbox but you certainly won’t
                                                                             do your engine any good by putting
                                   PROPERTIES                                a gear oil in it. If it’s a big number
                                                                             (more than 60) it’s a gear oil, if it’s
                                   OF OIL                                    a small number (less than 70) it’s
                                                                             an engine oil. This is simply to avoid
                                                                             confusion. It is important, however,
                                                                             to note that both series cover the
                                   The     most important physical           same range of viscosities. An SAE
                                                                             30 engine oil is just as thick as an
                                   property of an oil is its viscosity.
                                   Viscosity is defined as a fluid’s         SAE 85W gear oil.
                                   resistance to flow, under gravity,
                                   at a specified temperature. What          You will note that some of the
                                   that simply means is: ‘how thick is       grades have a ‘W’ after them.
                                   the oil?’ Thick oils do not flow so       These are the lower or thinner
                                   easily and have high viscosities,         grades that function better at low
                                   thin oils are quite fluid and have        temperatures as all oils will be
                                   low viscosities. Think how differently    thicker when it is cold. These
                                   transmission fluids behave when           grades can also be blended with
                                   poured compared to gear oils.             other non-W grades to form what
                                   The actual property of interest is        are known as multigrade oils.
                                   called kinematic viscosity, its units     Mo nograd e o ils ha ve s uch
                                   are known as centistokes and one          designations as SAE 10W or SAE

                                                             REG IS TERED FOR I SO 9 001 :20 00 an d I SO 1 400 1: 199 6
    90 whereas the multigrades have names                    The advantage of using a multigrade oil is
    such as 20W50 or 80W90. Remember, if                     that its viscosity is more stable over a greater
    you increase the temperature of an oil you               range of temperatures. In effect, a 20W50
    will decrease its viscosity.                             will behave like an SAE 20W when it is cold
                                                             and an SAE 50 when it is hot, providing
    A temperature viscosity graph might look                 protection for your engine over a wide range
    something like this:                                     of conditions. The ‘W’, in fact, stands for
                                                             Winter.

            Viscosity/temperature relationship               Well, that has dealt with the physical
          200
                                                             properties of the oil, but what about the
          180                                                chemicals that are added to the base stock
          160
                                                             in order for the oil to do its specific job? In
                                                             other words, if I buy a can of oil, how good
          140
                                                             is it?
          120


          100


          80


           60


           40
                                                                         THE CHEMICAL
           20                                                              PROPERTIES
    -20
            0
                0   20        40     60     80         100                      OF OIL
                         Temperature ºC


    On a very cold winter’s morning in Bethlehem,
                                                             The most important thing to realize at this
    the temperature might be as low as -10
                                                             point is that if you buy oil from any reputable
    degrees but at operating temperatures the
                                                             oil company there is no such thing as a ‘bad
    engine will have heated the oil to 90 degrees.
                                                             oil’. You can certainly get cheap oil and you
    Ideally what you need is a fairly thin oil that
    will flow at low temperatures but doesn’t                can pay up to R200 a litre if you really want
    thin out too much as the engine reaches                  to; you get what you pay for but there are
    operating temperature. Multigrade oils are               no ‘bad oils’.
    formulated to perform under exactly these
    conditions as they thin out less than                    So, how do you know how much bang you
    monograde oils when they are heated. The                 are getting for your buck? Engine oils (and
    following graph is an exaggerated illustration           to a lesser extent gear oils) are graded
    of how the two types of oils behave:                     according to performance specifications.

                                                             There are any number of international bodies
                                                             that are equipped and certified to carry out
              Mono and Multigrade oils
                                                             testing to confirm whether a certain product
          Viscosity/temperature relationship
          200
                                                             meets internationally accepted performance
                                                             criteria.
          180
                                          Monograde
          160
                                          Multigrade         The one that most people are familiar with
          140                                                is the API or American Petroleum Institute
          120                                                although Europe has ACEA, Japan has JAMO
                                                             and even our own SABS has specifications
          100
                                                             for lubricants. There is only space here to
          80
                                                             look at the API which grew out of the US
          60                                                 military classification system.
          40


           20
                                                             Systems such as the API came into being
                                                             w h e n en g i n e m an u f a c t u r e rs s ta r t e d
            0
    -20         0   20        40     60     80     100       communicating their needs to the oil
                     Temperature ºC                          companies. When an engine oil is blended,
                                                             many chemicals (additives) are added to the




2
oil. These additives impart certain properties                  The S designates petrol engines and stands
to the oils enabling them to function in a                      for spark as these are spark ignition engines,
desired manner. Below is a list of some                         the C stands for compression as diesel
typical additives and what their functions                      engines employ compression ignition. Basically,
are:                                                            the further you go in the alphabet, the ‘better’
                                                                the oil is. What this actually means is that
    ADDITIVE                     FUNCTION                       an SM oil has passed a greater variety and
Anti-wear                Reduces friction thereby
                                                                more stringent performance tests than an
                         reducing wear                          SG oil.

Extreme Pressure agent Prevents scoring and seizure
                                                                Most engine oils will generally meet both an
Corrosion inhibitor      Prevents rusting, particularly         S and a C classification, for example SL/CF4.
                         during storage                         The diesel specification is usually lower as
Detergent                Keeps surfaces free of
                                                                the combustion of diesel puts much higher
                         deposits                               demands on the lubricant; the combustion
                                                                by-products of diesel are much more
Dispersant               Keeps deposits in suspension
                                                                aggressive than for petrol.
Friction modifier        Alters the frictional
                         properties of the oil                  In order to grade engine lubricants, engine
Pour point depressant    Permits oil to flow at low
                                                                manufacturers have designed a series of
                         temperatures                           tests to be carried out under scientifically
                                                                controlled conditions. Most major engine
Seal swell agent         Prevents seals from leaking
                                                                manufacturers are involved and, in order for
Viscosity index improver Ensures viscosity stability            an oil to achieve an API specification, all tests
                         over a wide temperature                must be passed to the level stipulated by
                         range
                                                                the manufacturers.
Anti-foamant             Reduces foaming
                                                                These performance tests do not look at
Anti-oxidant             Retards oxidation of the oil
                                                                overall performance, but specific
Metal deactivator        Inhibits the catalytic oxidation       characteristics of the oil under test. For
                         of the oil by some metals              example, the Caterpillar IG2 test is used to
                                                                determine the ability of the oil to protect
The right combination of the right additives                    against ring sticking, wear and accumulation
in the right amount are blended with a refined                  of deposits under high speed turbo-charged
crude oil (base stock) in order for an oil to                   conditions. The Peugeot TU3 test is used
do its job. The API grades these oils on how                    to measure the ability of the oil to protect
well they can do that job. Most API
                                                                against valve train scuffing.
specifications are of the format Sx/Cy where
x and y are letters of the alphabet; the first
half of the designation refers to petrol                        All tests are defined by the ASTM (American
engines and the second half to diesel engines.                  Society for Testing and Materials) which
The full range is shown below:                                  works closely with the API and the SAE. The
                                                                SAE defines the need for a new specification
   Petrol engines             Diesel engines                    (in conjunction with oil companies and additive
                                                                manufacturers), the ASTM develops the
            SA                         CA                       tests and the API defines the new oil category.
            SB                         CB                       This is a very lengthy, complicated and
            SC                         CC                       consultative process as all stakeholders need
                                       CD                       to be involved.
            SD
            SE                         CE
                                                                This also explains why a simple oil analysis
            SF                         CF4                      laboratory cannot determine if an oil meets
            SG                         CG4                      a particular specification or not. Chemical
            SH                         CH4                      and physical tests cannot replace actual
            SJ                         CI4                      performance testing and the costs to an oil
            SL                         CI4+                     company trying to get a new product certified
                                                                runs into millions of dollars. Whether a
            SM
                                                                product meets a specification or not is




                                                            3
actually an act of faith as it is not within the    is a cost involved. Synthetic oils are
means (in terms of cost or facilities) of the       very good but they are also very expensive.
man in the street to have it checked.
                                                    Sadly, a landmark legal battle between Mobil
The gear oils are graded in a similar way by        and Castrol about five years ago has meant
the API. These oils will have what is known         that the definition - synthetic oil - has become
as a GL rating, GL stands for Gear Lubricant.       so blurred that it really has very little meaning
The GL rating measures the level and                anymore. When purchasing a synthetic oil
effectiveness of the EP (Extreme Pressure)          just be very careful that you know what you
additive in the oil. EP additives prevent scoring   are buying.
and scuffing of gears under high load
applications.

The scale runs from GL1 to GL6 (although
GL6 is technically obsolete), GL1 having the                            HOW DOES
lowest EP level and GL6 the highest. Oils
typically found in automotive applications are                          OIL WORK?
GL4 and GL5. GL4 oils would normally be
used in a manual gearbox application and
GL5s in the differential.
                                                    So, we have dealt with the physical and
As environmental legislation puts heavier           chemical properties of oils and their
and    heavier    demands        on    engine       performance specification but how does the
manufacturers, the engine manufacturers             stuff actually work?
are putting heavier and heavier demands on
the lubricants. Rather than put the muck in         Depending on which book, manual or learned
the atmosphere, let’s put it in the oil. Engine     journal you read, a lubricant can have as
oils work very much harder than they did            many as two dozen functions. These can be
ten years ago.                                      reduced to four very fundamental ones with
                                                    anything subsequent, more or less being a
One group of oils that hasn’t been discussed        su bd ivisio n of on e of th ese g rou ps .
needs to be touched on just briefly and that
is synthetic lubricants.                            An oil m ust reduce friction, rem ove
                                                    contaminants, cool and act as a structural
                                                    material. An oil cools by absorbing heat and
                                                    carrying it to the radiator. It keeps the engine
                                                    clean by carrying contaminants to the filter
SYNTHETIC                                           for removal. It has viscosity and ‘body’ so
                                                    can act as a structural material (think of oil
LUBRICANTS                                          as making a seal between the ring and the
                                                    bore). But how does oil reduce friction? That
                                                    is the prime function of a lubricant.

Crude oil is removed from the ground, refined       Friction is a ubiquitous part of our daily lives,
and then additives are blended to produce           we spend large amounts of energy just trying
the finished product. The refining process          to overcome it whether it be walking, driving
removes undesirable entities from the crude         to work or opening a door. Although friction
oil until a finished product is arrived at that     is often thought of as a negative mechanical
will do the job. This does not remove all           characteristic (20% of a car’s energy is used
the nasties, just the ones we really can’t          to overcome it) it must be remembered that
live with.                                          without it you would not be able to walk and
                                                    the brakes on your car wouldn’t work.
A synthetic oil is a base stock that has been
designed, in a laboratory, with all the desired     Friction is usually seen as a mechanical
properties and none of the undesirables.            problem and the force required to overcome
This makes synthetic lubricants vastly              it has long interested engineers. When man
superior to mineral ones. However, there            invented the wheel it enabled him to move




                                                                                                        4
    weights around far more easily than dragging
    them from A to B. What he had in fact done                                      LOAD
    was to replace sliding friction with rolling
    friction which has a much lower value.

    Surfaces that appear smooth and shiny to
    the naked eye will show peaks and furrows
    on closer inspection. This does not mean
    that the component has been poorly
    machined but components manufactured to
    high tolerances still have rough surfaces at
    a microscopic level.



                                                                       ASPERITY



                                                       When one body slides over another, the force
                                                       of resistance is known as sliding friction. If
                                                       a ball were to roll over a metal surface, the
                                                       relative velocity of the points of contact are
                                                       actually zero and this results in rolling friction.
                                                       H ow ev er, th ere wi ll al wa ys b e s om e
                                                       deformation of the contacting surfaces so
                                                       some sliding friction is always involved.
                                                       Tests have shown that rolling friction is not
                                                       influenced by the presence or absence of a
          A roller element magnified 200 times.        lubricant, however, the deformation spoken
                                                       about and the presence of sliding means that
    The rough bits that stick up from the surface      the oil is a very necessary component of a
    are known as asperities. These peaks touch         ball bearing.
    and very high temperatures and pressures
    are achieved which can cause momentary             The friction that exists between two bodies
    welding of the surfaces to take place. When        in relative motion can involve an appreciable
    the surfaces slide over each other these           energy loss that needs to be minimised. This
    welds are torn apart.                              is achieved by feeding a lubricant between
                                                       those surfaces in order to replace solid
    W he ne ve r f ri cti on is o ve rc om e, the      friction with fluid friction. Unfortunately
    dislocation of surface material generates          friction can never be eliminated as there will
    heat which can be highly destructive.              be internal viscous drag (fluid friction) of the
    Additionally, when there is solid friction, wear   oil. In ideal situations the oil will cause the
    will take place.                                   surfaces to separate completely. Where this
                                                       is not possible, the oil can still keep friction
    Material is lost due to the cutting action of      to manageable proportions.
    opposing asperities and the shearing of
    microscopic welds. In extreme cases the            Three parameters need to be considered
    combination of high frictional temperatures,       when looking at the various lubrication
    welding and shearing can result in complete        regimes. They are: the speed at which
    seizure.                                           the surfaces are moving relative to each
                                                       other, the load placed on those surfaces
    The harmful effects of friction cannot be          an d th e vi sc os ity of th e o il. Id ea lly,
    overemphasised. The job of a lubrication           there should be high speed, low load
    engineer is to control it; lubrication reduces     and high viscosity; under these conditions
    friction by replacing solid friction with fluid    the surfaces would always be separated.
    friction.                                          Unfortunately we do not live in an ideal world.




5
In the case of a plain bearing and journal                                       placed under pressure or load. Oil is drawn
there will always be a small amount of oil                                       into this contact but because this takes
between the mating surfaces. However, at                                         place over a vanishingly small area, huge
start up, the speed will be zero and the                                         pressures are generated of the order of
lubricant will not keep the surface completely                                   one and a half million kPa. At these pressures
apart – some of the asperities will touch.                                       the lubricant takes on the consistency of
This is known as boundary lubrication. In this                                   window putty (reversibly) and is capable of
situation it is the additives in the oil that                                    keeping the sliding friction to manageable
help to minimise friction. As the speed of                                       proportions in this type of situation.
the journal increases, pressure differentials
cause oil to be sucked between the bearing
                                                                                       Friction versus speed for hydrodynamic
and the journal. This is known as mixed film                                           lubrication - viscosity and load constant
lubrication. At this point the surfaces are
not quite separated but are supported by a
full fluid film.
                                                                                                                        Hydrodynamic
Finally, the speed becomes fast enough for                                                                               lubrication

the surfaces to become totally separated                                                              Mixed film
and the solid fiction is replaced, totally, by                                                        lubrication
fluid friction. This is known as hydrodynamic
lubrication. Unlike solid friction, fluid friction                                       Boundary
is dependent on speed and will gradually                                                lubrication
increase. Friction is actually at a minimum
during the mixed film regime. The graph                                                                            Speed
alongside illustrates this.
                                                                                 It is not easy to cram so much information
Finally there is another regime that goes                                        into such a small space but it is hoped that
under the rather grand name of                                                   this gives a little bit of background on what
elastohydrodynamic lubrication. So far we                                        oils are available and how they work. The
have only considered mating surfaces that                                        important thing to remember is: use the
have the same geometry, sort of the way                                          right product, in the right place, at the right
that spoons stack together. But what about                                       time and in the right quantity. Also, keep
a roller bearing where the mating surfaces                                       your fuel, lubricant, air and coolant clean -
are not of the same shape?                                                       and most of your maintenance problems will
                                                                                 be solved.
As already mentioned, pure rolling friction
should not require lubrication but the                                           John Evans is diagnostic manager for
contacting surfaces deform elastically when                                      Wearcheck Africa.




                      Copies of previous Technical Bulletins can be accessed
                        on Wearcheck’s web site: www.wearcheck.co.za




                                                           A member of the Set Point group




      Publications are welcome to reproduce articles or extracts from them providing they acknowledge Wearcheck Africa, a member of the Set Point group.

                                                                                                                        Felicity Howden Public Relations 9/2005
                                                                            6

				
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