Англо- русский словарь

А

B

C

D

E

F

G

H

I

J

K

L

M

N

O

P

Q

R

S

T

U

V

W

X

Y

Z

Text 3.2

I. Pronounce the following words:

Gravity, flash point, carbon residue, neutralization, initial suitability, specify, sample, standardized, capillary, exceed, suction

II. Read and translate the text and answer the following questions:

1. Which basic factors must be considered in selecting a fluid for an industrial hydraulic system?

2. What does viscosity mean?

3. How can we determine the relative viscosity?

Selection Factors

Basic factors which must be considered in selecting a fluid for an industrial hydraulic system include viscosity, viscosity index and pour point. Other properties such as gravity, flash point, carbon residue and neutralization number have little significance in determining the initial suitability of an oil, but are useful in checking on the condition of used oil to determine its suitability for continued service.

VISCOSITY: This is a measure of resistance to flow. It is the most important single property of a hydraulic fluid.

Pumps are the most critical part of hydraulic systems with regard to viscosity. Most pump manufacturers specify the viscosity required. If the exact viscosity requirement for a pump is not known, the approximate viscosity requirements for various types of pumps and motors can be determined from the Table .

There are numerous accepted methods for determining the relative viscosity of oils, but the one most commonly employed for industrial hydraulic oils is the Saybolt Universal method. The unit of measurement is time in seconds (Saybolt seconds, Universal or SSU) required for 60 ml of the oil to flow through a standard orifice under a standard head and at a given temperature normally 100 or 210 F.

A second method in fairly common use, for determining and reporting viscosity, is the kinematic method. Time is measured for a fixed volume of sample to flow through a standardized capillary under an accurately reproducible head of liquid and at closely controlled temperature. Kinematic viscosity in centistokes is calculated from the measured time.

A third unit , absolute viscosity, is independent of specific gravity. The unit of absolute viscosity is the centipoises, and can be calculated by multiplying the kinematic viscosity in centistokes by the specific gravity of the oil.

For any specific application viscosity of oil chosen should represent a balance between requirements of power transmission and those of lubrication and sealing. Low viscosity oils transmit power more efficiently. On the other hand, lubrication and sealing require that the hydraulic oil be of comparatively high viscosity. However the oil must not be so viscous that exceeds the suction capability of the pump.

Optimum viscosity of the hydraulic oil is therefore a compromise between conflicting requirements. In general minimum viscosity acceptable for satisfactory lubrication is optimum for the hydraulic system. 2448