Properties of Lubricants

Properties of Lubricants:-

Lubricants have several properties, some important properties are given below:-

1.      Viscosity:

                             It is the property of a liquid or a fluid by virtue of which it offers resistance to its own flow. If two layers of a liquid separated by a distance’s and moving with a relative velocity difference ‘v’ then force per unit area(f) required to maintain this velocity difference is given by-

                                                f=nv/d

Where n is the coefficient of viscosity.Viscosity is the most important property of any lubricating oil. If the viscosity of the oil is too low a liquid oil film cannot be maintained between two moving surfaces and excessive wearing takes place.

Determination of Viscosity:

The apparatus which is used to determine the viscosity is known as viscometer. In industry viscosity of lubricating oil is determined by Redwood, say bolt and Angler instrument. In the Redwood viscometer the measure of viscosity of oil is the time in seconds for 0ml of oil to flow through standard orifice under a given set of condition.

By Redwood Viscometer:  It is of two types:-   (i). Rw_{1 ,}  (ii).Rw₂

Fig.1.(a). Redwood viscometer no. 1

Working:

                The apparatus is leveled and water bath is filled with water. A thermometer is placed in water bath. The oil cup is cleaned and ball of value rod is places on the agate jet to close it. An empty cleaned kohlrausch flask is kept just below the jet. The experimental oil is listed in oil cup up to a pointer. A thermometer is also placed in oil cup read the temperature of oil. Now water bath is heated up to a certain temperature with constant stirring the water. When the oil at desired temperature heating is stopped and the ball valve is fitted and suspended from thermometer bracket. The time taken for 50ml of the oil to pass through the agate jet and collected into kohlrausch mask is noted. Now the valve immediately closed to prevent any overflow of the oil. The experiment is repeated and the mean value of time of flow for 50ml oil sample is reported as a result expressed in “Redwood No. 1 seconds” at a particular temperature then

                        n₁/n₂ =r₁t₁/r₂t₂

where n₁= viscosity of experimental oil sample

n₂= viscosity of standard liquid

r₁= Density of experimental liquid

t₁= Time of flow of experimental oil sample

Thus the value of n₁ i.e., viscosity of experimental oil is calculated in ‘poise’.

2.      Viscosity Index:-

                                The arbitrary scale which measures the variation of viscosity with temperature is called viscosity index. Generally the viscosity of an oil decreases with rise in temperature. The viscosity of a good lubricant should not change very much with the rise in temperature. If the viscosity of oil is very much affected with the rise in temperature it is called low viscosity index. Similarly if the viscosity of oil is slightly affected with the rise in temperature, it is called high viscosity index lubricant. A good lubricant should have high viscosity index.

Determination of Viscosity index: 

                                                         For the determination of viscosity index of experimental oil the viscosities of testing oil at 100ºf and 210ºf are found out. The viscosity at 100ºf of the oil under test is represented by ‘U’. Now we compare the viscosity of oil under test with two standard oil, one with the highest viscosity index (VI=100) and another with the lower viscosity index(VI=0). Pennsylvanian oils have highest viscosity index(VI=100) and Gulf oil have lowest viscosity  index(VI=0). Now viscosity index (VI) may be calculated as:-

                               Viscosity Index(VI)=  L-U/L-H*100

Where ‘U’ is the viscosity of experimental oil at 100ºf. And ‘L’ is the viscosity of low viscosity index standard oil (Gulf oil having VI=0 ) at 100ºf and also having the same viscosity of experimental oil at 210ºf. And ‘H’ viscosity of high index standard oil (Pennsylvanian oil having VI=100) at 100ºf and also having the same viscosity of experimental oil at 210ºf.

Viscosity curves:-  

Fig.2.(a).

                                       

3.      Cloud-Point and Pour Point:-

                                                                When an oil is cooled slowly, the temperature at which an oil becomes  cloudly   in appearance it is called its ‘cloud point’. The temperature at which the oil ceases to flow or pour is called ‘pour point’. Cloud point and pour point indicates the suitability of lubricants in cold conditions.

Determination of Cloud-point and Pour-point: 

Fig.3.(a). Cloud Point and Pour Point

            It is determined with the help of pour point apparatus as shown in fig(a). It consists of a flat-bottomed tube for taking lubricating oil. The tube is enclosed by a air jacket. The air-jacket is surrounded by freezing mixture (ice+Cacl₂) contained in a jar. The flat tube is half filled with experimental oil. A thermometer is placed in the oil. The oil gets start  cooling and  the temperature decrease slowly. At an interval of fall in temperature every 1ºC, the tube is withdrawn from the air jacket for a moment and examined and then replaced in ice-bath immediately. The temperature, at which cloudness is noted is recorded as the cloud-point. After this cooling is continued and the test tube is removed from the cooling bath after every 3ºC fall of temperature and tilted to observe the flow or pour of oil. The temperature at which oil does not flow in the test tube, even when kept horizontal for 5 second, is recorded as the pour point.

Significant:  Cloud point and pour point tells us a minimum temp at which oil can be used as a lubricant.

 

4.      Flash and Fire point:-

•       Flash point:  It is the lowest temperature of lubricant at which the lubricant gives enough vapours which burn for a moment, when a flame is brought near it.

•       Fire Point: It is the lowest temperature of lubricant at which the lubricant gives enough vapours which burn continuously for at east five second when a small flame is brought near to is.

Significance: Flash and fire point tells us the maximum temperature at which a lubricant can be used.

Determination of Flash and Fire point: 
                                   The Flash and fire point is determined by penskey-marten’s apparatus as shown in fig.4.(a).

 Fig.4.(a). Penskey-Marten's flash point apparatus

 

First of all clean and dry all the parts of the apparatus and experimental lubricating oil is filled up and the mark in the oil cup. The apparatus is heated with constant stirring at the rate of about 1-2 revolutions per second. The heating of oil cup is adjusted in such a way that the temperature of the oil rises at the rate of about 5ºc per minute. At every 1ºc rise in temperature test flame is introduced for the moment with the help of a shutter. The temperature at which a distinct  flash appears inside the up is recorded as the flash point of lubricating oil. The heating is continued further and the test flame introduced as before. The temperature at which the experimental lubricating oil catches fire at least 5 seconds is recorded as its fire-point.

5.      Aniline Point:-

                                 It is defined as the minimum temp. at which the lubricating oil is just miscible with equal volume of aniline.

Significance: Aniline point of any lubricant is a measure of aromatic content.

Determination of Aniline Point:

 Fig.5.(a). Aniline point apparatus

 

                          The whole apparatus may be represented as in fig. The apparatus is cleaned and dried at 100-110ºc . An equal volume of pure and dried aniline  and dried oil sample have been taken in the test tube. This test  tube is   fitted with electrically operated glass   rod stirrer and a thermometer . The test tube is  inserted into an outer air-jacket  made of heat resistance  glass. The aniline and oil sample mixture is stirred to get  a homogeneous  solution . For the  sometimes we use hot bath  and stirring is continued. The jacket is  then withdrawn from the hot  bath and the  temperature is allowed to fall at a rate below 1ºc per minutes. For this purpose we may use cold bath. The temperature, at which the two phases just separate out, is reported as the aniline point of the sample.