HYDRAULIC FLUIDS

Hydraulic fluid is the medium for power transmission in a hydraulic system.The life and performance of the hydraulic components depends on the kind of hydraulic fluid used in the system.The selection of hydraulic fluid is very important in any hydraulic system design.The main purpose of hydraulic fluid in the hydraulic system is  transmission of power but along with that it serves many purpose like lubrication,sealing and heat transfer medium.

PURPOSE OF HYDRAULIC FLUID 

POWER TRANSMISSION :-  As hydraulic fluids  are slightly compressible,whenever force is applied the response to the force is instantaneous.For easy transmission fluid must flow easily through the passage.Too much resistance to the flow creates the power loss.


LUBRICATION :-  Lubrication to the internal components is provided by the hydraulic fluid.So anti wear additives are added to hydraulic fluids which increases the service life of the components.


SEALING :-  Fluid acts as the sealing agent in the high pressure components like pump cylinder etc. because it has a viscous characteristics which creates a film in the leakage area.But high viscosity of the fluid is not recommended.close mechanical fit and viscosity of fluid determines the leakage rate.


COOLING :- Fluid helps in heat transfer as it flows heat through it .

CLEANING :- Hydraulic fluid helps in cleaning the system by flowing the dust and dirt or the system created particles outside the system.When these particles flow in the system either they settle at the bottom of the reservoir by gravity or they get cleaned through the return line filter.

CHARACTERISTICS OF HYDRAULIC FLUIDS
The demands placed on hydraulic systems change as industry requires greater efficiency and speed at higher operating temperatures and pressures.

Selecting the best Hydraulic fluid requires a basic understanding of each particular fluid's characteristics in comparison with an ideal fluid.

An ideal Hydraulic fluid would have these characteristics.

1.Thermal stability. 

2.Hydrological stability.

3.Low chemical corrosiveness.

4.High anti wear characteristics.

5.Low tendency to cavitation.

6.Long life

7.Total water rejection

8.Constant viscosity regardless temperature. 

9.low cost.

Although no single fluid has all the characteristics but the selection requires the knowledge of the system in which it will be used.

like.

1. Maximum and minimum operating and ambient temperature.

2. Type of pump used.

3. Operating pressure

4. Operating cycles

5. Loads encountered by various components

6. Types of control and power valves. 

The table below lists the major functions of a hydraulic fluid and the properties of a fluid that affect its ability to perform that function

Function	Property
Medium for power transfer and control	Low compressibility (high bulk modulus) Fast air release Low foaming tendency Low volatility
Medium for heat transfer	Good thermal capacity and conductivity
Sealing Medium	Adequate viscosity and viscosity index. Shear stability
Lubricant	Viscosity for film maintenance Low temperature fluidity Thermal and oxidative stability Hydrolytic stability / water tolerance Cleanliness and filterability Demulsibility Antiwear characteristics Corrosion control
Pump efficiency	Proper viscosity to minimize internal leakage High viscosity index
Special function	Fire resistance Friction modifications Radiation resistance
Environmental impact	Low toxicity when new or decomposed Biodegradability
Functioning life	Material compatibility

 VISCOSITY :-

Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms (and for fluids only), viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity. Put simply, the less viscous the fluid is, the greater its ease of movement (fluidity).

VISCOSITY IS A COMPROMISE FOR A HYDRAULIC FLUID AS HIGHER VISCOSITY CAUSES THE RESISTANCE TO FLOW AND LOWER VISCOSITY CAUSES THE WEAR AND TEAR TO THE COMPONENTS.

TOO HIGH VISCOSITY INCREASES FRICTION RESULTING IN:-

1.HIGH RESISTANCE TO FLOW.

2.INCREASED POWER CONSUMPTION DUE TO FRICTIONAL LOSS.

3.HIGHER TEMPERATURE CAUSED BY FRICTION

4.HIGHER PRESSURE DROP DUE TO RESISTANCE.

5.SLOW OPERATION.

6.DIFFICULTY IN SEPARATING AIR FROM OIL IN RESERVOIR.

TOO LOW VISCOSITY RESULTS IN :-

1.INTERNAL LEAKAGE

2.EXCESSIVE WEAR OF INTERNAL COMPONENTS DUE TO BREAKING OF FLUID FILM BETWEEN THE MATING PARTS.

3.PUMP EFFICIENCY MAY DECREASE DUE TO SLOW OPERATION OF ACTUATOR.

4.LEAKAGE LOSSES MAY INCREASE THE TEMPERATURE.

 

PRINCIPLES OF FLOW & PRESSURE IN HYDRAULICS

In the previous post i had explained you about the difference between hydrodynamics and hydrostatics.Hydrostatics is nothing but hydraulics in common language.Pressure and flow are two big terms in hydraulics.In this post i will explain pressure & flow in details.

Pressure explained :

Basically pressure is created in a hydraulic system when there is a resistance to the flow in the system.If there is a load or force which is opposing the free flow of fluid in the system(friction if neglected) then only pressure is created i.e equal to the force divided by the area perpendicular to the opposing force.


pressure = force / perpendicular area in contact with force.


if there is no opposition to the flow then pressure is equal to zero.

pressure head comes from the weight of the fluid.
e.g. A column of air of one square inch as high as atmosphere gives a pressure of one atmosphere at see level i.e 76 mm of mercury or 14.7 psi. 


a pressure gauge is used to measure the pressure but it shows the gauge pressure which is the difference between absolute pressure minus the atmospheric pressure.

gauge pressure =absolute pressure - atmospheric pressure 

psig  = psia - 14.7

* Liquid always seeks a level depending on pressure.

*according to Bernoulli's theorem the sum of pressure energy kinetic energy and the potential energy is constant for a system.

*If the fluid levels are same then the potential energy will be cancelled out hence we have kinetic energy inversely proportional to the pressure energy.

principles of flow :-

Flow is an action in the hydraulic system that gives the actuator its motion.Pressure gives the actuator it's force but flow is essential for the movement.The flow is created by pump.

There are two ways to measure flow rate one is velocity of flow ( feet per second ) and flow rate (liter per minute ).

velocity is the average speed of fluid particles past a particular point in unit time.Flow rate can be defined as the volume of fluid passing a particular point in unit time.

large volumes are measured in GPM (gallons per minute) or LPM (liters per minute) and small flow rates are measured in CC per min.

1 gpm = 231 inch^3 per minute

1 lpm = 1000 cc^3 per minute.

whenever a liquid is flowing then there must be an unbalanced force to cause motion.When a fluid flows in a streamlined path in a straight path then also there is a pressure difference between the start point and downstream.That pressure drop is due to friction and friction causes the pressure drop causing the flow of fluid in the pipe.