Viscosity and viscosity index, Chemical Engineering

Q. Discuss the significance of viscosity in lubricating oil. How the viscosity is determined by Redwood viscometer?


Write short note on viscosity index.

Ans. Viscosity: Viscosity is most important and representative property of lubricants. Viscosity is the property of the lubricants by virtue of which it offers resistance to its own flow. It is mainly due to forces of cohesion between the fluid molecules.


1.      Viscosity of lubricating oil determines its performance under operating conditions.

2.      If the viscosity of liquid is too low, the fluid lubricant film cannot be maintained between the moving surfaces, as a result of which excessive wear may take place.

3.      If the viscosity of the lubricating oil is too high, excessive friction may take place.

Viscosity index: The rate at which viscosity of a lubricant changes with temperature is measured by any arbitrary scale is known as viscosity index. Viscosity of liquid is decreases with increasing temperature so that lubricating oil become thinner as the operating temperature is raised, and lubricant can be squeezed out from the subbing parts.

 Significance: Higher V.I. lesser is the variation of viscosity with change in temperature. Thus a good lubricating oil should possess high V.I.

Redwood viscometer: It consists of the following parts:

1.      Oil cup: It is silver plated brass cup, which is open at the upper end. The bottom of the cylinder is fitted with an agate jet that can be kept closed or open by a value rod fitted with a boll made of brass. The lid of the cup is fitted with a thermometer which indicates the oil temperature.

2.      Heating Bath: oil cup is surrounded by a cylindrical cooper bath containing water having an outlet for talking out water a stirrer with four blades and thermometer.

3.      Kohlrausch flask: It is a special type of receiver for receiving the oil. Its capacity is 50 ml to the mark in its neck.

Working: Water is filled in the bath and the brass ball is kept in position to seal the orifice. Test oil is filled in the oil cup up to the tip of the indicator. An empty Kohlrausch flask is kept just below the jet. The desired temperature at which the viscosity is to be measured is maintained by the stirring the water and oil in the heating bath and the oil cup respectively. The ball value is lifted and the time is noted for 50 ml of the oil to collect in the flask and the value is immediately closed to stop the overflow of the oil.

Posted Date: 7/21/2012 8:10:11 AM | Location : United States

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