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HYDRAULIC FLUID PROPERTIES: VISCOSITY


We were discussing basic principles of hydraulic system and also we have seen the hydraulic fluid in our previous discussion. Today we will try to understand here few important characteristics that must be considered during the selection of suitable hydraulic fluid. We will see here first viscosity of suitable hydraulic fluid and after that we will discuss other characteristics also.

As we know that liquids are used in hydraulic units for transmitting and distributing the forces to various actuators. Liquids are used in hydraulic systems for various operations because liquids are almost in-compressible in nature. 

Hydraulic equipment manufacturers are normally provides the information about the best suitable liquid to use with their equipments for smooth operation and better service life of the equipments by considering various factors such as working environment, optimum inside and outside temperature rating of the system, pressure rating, type of services, corrosion effect etc.

But we must have to understand here that we could not use any type of liquid in hydraulic system, because there are not only qualities such as fluidity and incompressibility will be required but also there will be some other important characteristics that must be considered during selection of suitable liquid for hydraulic system.

Hydraulic fluid characteristics: Viscosity

As we know that viscosity is basically defined as the measure of internal resistance to flow. Viscosity is quite important characteristic of a hydraulic fluid. Viscosity decreases with increase in temperature and we could see it in below figure how viscosity varies with variation in temperature.
Liquid with lower viscosity such as gasoline will flow easily and liquid with higher viscosity such as tar will flow slowly. For satisfactory performance of the hydraulic units, liquid must have optimum viscosity for providing sufficient sealing at pistons, valves and pumps but should not be higher than optimum limit otherwise it will offer resistance to flow and will result the loss of power and overheating of the system.

Viscosity of fluid must not be too low also as it will not provide the sufficient lubrication between moving components and wear rate between moving components will be higher.

Viscosity measuring instrument

Viscometers are used for measuring the viscosity of liquid. Saybolt viscometer is displayed here which is used these days for measuring the viscosity of hydraulic fluid. 

Saybolt viscometer will measure the require time to pass the 60 ml of hydraulic fluid at a temperature of 1000F via a standard orifice. Time measured will be in seconds and will be used for expressing the viscosity of hydraulic fluid. Measured viscosity will be in Saybolt universal seconds.

We will discuss other characteristics of hydraulic liquids such as chemical stability, flash and fire point in our next post.

Image courtesy : Google

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