hydraulics fluids and properties chapter

hydraulics and pneumatics notes useful for diploma students

1. UNIT: 2: HYDRAULIC FLUIDS AND PROPERTIES

2. List out and explain types of fluids Ideal fluid . An ideal fluid (also called Perfect Fluid) is one that is incompressible and has no viscosity. Ideal fluids do not actually exist, but sometimes it is useful to consider what would happen to an ideal fluid in a particular fluid flow problem in order to simplify the problem. Real fluid. All fluid comes under real fluid because all the fluid having viscosity, compression and surface tension in its properties with a little amount of force. Example of REAL FLUID: water, petrol, air etc Newtonian fluid. A Newtonian fluid is a fluid that follow Newton's law of viscosity , Non-Newtonian  fluid. A non-Newtonian fluid is a fluid that does not follow Newton's law of viscosity, i.e., constant viscosity independent of stress

3. Incompressible fluid. Incompressible fluid: are the fluids with constant density. They could be liquids and gases. Although there is no such thing in reality as an incompressible fluid, Compressible fluid. Incompressible means that the effects of pressure on the fluid density are zero or negligible. So the density and the specific volume of the fluid do not change during the flow.

4. The general requirements of fluids in power transmission are:  Low-cost  Non-corrosive  Have infinite stiffness  Good lubrication properties  Store well without degradation  Non-toxic  Non-inflammable

5. List and explain Properties of fluids Viscosity: Viscosity is a measure of a fluid's resistance to flow . Specific volume: The specific volume of the fluid is defined as the volume of the fluid occupied by the unit mass of the fluid . Specific volume of the Fluid is reciprocal of the density of the fluid. Specific weight: The specific weight, also known as the  unit weight , is the  weight per unit volume of a material. Specific gravity: Specific gravity of the fluid is defined as the ratio of the density of the fluid to the density of the standard fluid. The flash point: of a volatile material is the lowest temperature at which its vapors ignite if given an ignition source. The fire point: of a fuel is the lowest temperature at which the vapor of that fuel will continue to burn for at least 5 seconds after ignition by an open flame of standard dimension. surface tension: Surface tension is the tendency of liquid surfaces to shrink into the minimum surface area possible.

6. List and explain problems caused by gases in fluids Loss of horsepower: When an air pocket is present in an actuator, it is alternately compressed and relaxed as the actuator is cycled. Since the air pocket must first be compressed before the fluid can cause the actuator to move, power is consumed .  Loss of bulk modulus —  Free or entrained air in the hydraulic system reduces substantially the effective bulk modulus of the system. That is, an air-oil mixture appears to increase the compressibility of the fluid, making the system spongy. Loss of system fluid —  One of the most serious conditions that can occur in a hydraulic system is the loss of reservoir fluid. The fluid level must be kept high enough to insure enough fluid for the pump intake, otherwise cavitations begins.

7. Foaming —  Foaming normally occurs in the reservoir because of liquid impinging on the fluid surface, entraining air bubbles. Foaming also results from dissolved air being released because of an increase in fluid temperature.  Temperature Effects —  A temperature rise in a system containing air affects the bulk of modulus of the air-oil mixture. The bulk modulus, which may be much lower than, desired because of free and entrained air, new drops further. An increase in temperature also tends to liberate more dissolved air. Erosion and cavitations —  Cavitations is the formation of a cavity, as a partial vacuum in a fluid or as a gas-filled space in a liquid.