2. Give a short summary of Text 3.2.

Text 3.3

1. Pronounce the following words:

Rate of change, viscosity index, naphthenic oils, additives, viscosity index improver, subject, tolerate, plot of viscosity.

II. Read Text 3.3 and answer the following questions:

2. 1. What depends on the viscosity index?

3. 2. What is the viscosity index effected by?

4. 3. Why is the viscosity so important?

5. 4. What can you say about the temperature during the operation?

Change of Viscosity with Temperature: Rate of change of viscosity with temperature is expressed as viscosity index, or VI. This viscosity index is calculated from actual viscosity measurements at 100 and 210 F. The higher the VI, the less the change in viscosity with temperature. Paraffinic oils as a class have higher viscosity indexes than do naphthenic oils.

In either case the VI is affected by the treatment given the oil in the manufacturing process. For example, the index can be increased by the use of additives. Disadvantages of viscosity-index improver additives include their tendency to lose effectiveness under high shear rates and their relatively high cost.

Viscosity index is important only in applications subjected to a wide temperature range. In such cases it is important that a high-viscosity-index oil be used. If not, viscosity may be so high when the equipment is started cold that the fluid won’t flow properly. Or it may be too thin at operating temperatures. If the equipment is not subjected to low temperature, and if oil temperature is controlled within reasonable limits, viscosity index is less important.

Normally oil temperature during operation should not exceed 160 F. Minimum starting temperature is determined by the maximum viscosity which can be tolerated by suction system of the pump. Minimum starting temperature can be obtained by comparing pump capability against plot of the viscosity-temperature characteristics of the oil. The most satisfactory normal operating temperature for a hydraulic system is usually about 120 F.

Change of Viscosity with Pressure: Lubricating oils increase in viscosity as pressure increases. Even though only small differences exist in viscosity of oils at atmospheric pressure, there may be large differences at high pressure. Oils which show the greatest change in viscosity with temperature also show the greatest change in viscosity with pressure. 1923

III. Give a short summary of Text 3.3. Text 3.4

I. Pronounce the following words:

Concern, expose, pour point, compressibility, undergo, specified reduction, bulk modulus, reciprocal of compressibility, stiffness servosystem.

2. Analyse the grammatical structure of the following sentences and translate them:

1. This is the lowest temperature at which when chilled under given conditions.

2. It can be expressed in terms of reduction in volume produced of a given pressure increase or in terms of increase in pressure required for a specified reduction in volume.

3. Expressed as percentage reduction in volume of oil under pressure, compressibility may be considered about 0,5 per cent for each 1000 psi pressure increase up to 4000 psi.