COMPONENTS OF HYDRAULIC SYSTEM

HYDRAULIC FLUID
Hydraulic fluid must be essentially non-compressible to be able to transmit power instantaneously from one part of system to another.At the same time,it should lubricate the moving parts to reduce friction loss and  cool thr components so that the heat generated does not lead to the fire hazards.It also helps in removing the contaminants to filter. The most common liquid used in hydraulic systems is petroleum oil because it is only very slightly compressible.the other desirable property of oil is its lubricating ability.Finally ,often,the fluid also acts as a seal against leakage inside a hydraulic component.The degree of closeness  of the mechanical fit and the oil viscosity determines leakage rate. Figure below shows the role played by hydraulic fluid films in lubrication and sealing.

LUBRICATION AND SEALING OF HYDRAULIC SYSTEM

THE FLUID DELIVERY SUBSYSTEM

 It consists of the components that hold and carry the fluid from the pump to the actuator .It is made up of the following components.

RESERVOIR
It holds the hydraulic fluid to be circulated and allows air entrapped in the fluid to escape. this is an important feature as the bulk modulus of the oil, which determines the stiffness of hydraulic system deteriorates considerably in the presence of entrapped air bubbles. It also helps in dissipating heat.

RESERVOIR

FILTER
 The hydraulic fluid is kept clean in the system with the help of filters and strainers. It removes minute particles from the fluid, which can cause blocking of the orifices of servo-valves of cause jamming of spools.

GRAPHYCAL SYMBOL OF FILTER & RESERVOIR

LINE

DIFFRENT LINES IN HYDRAULIC SYSTEM

Pipe, tubes and hoses, along with the fittings or connectors,constitute the conducting lines that carry hydraulic fluid between components. Lines are one of the disadvantages of hydraulic system that we need to pay in return of higher power to weight ratio.Lines convey the fluid and also dissipate heat.In contrast, for Pneumatic System, no return path for the fluid, which is air, is needed.since it can be directly released into the atmosphere.There are various kinds of lines in a hydraulic system.The working lines carry fluid that transmit controlling pressures to various directional and relief valves for remote operation or safty . lastly there are drain line that carry the fluid that inevitably leaks out ,to the tank.

FITTINGS AND SEALS
Various additional components are need to join pipe or tube sections, create bends and also to prevent internal and external leakage in hydraulic systems. although some amount of internal leakage in built-in, to provide lubrication, excessive internal leakage causes loss of pump power since high pressure fluid returns to tanks, without doing useful work. External leakage,on the other hand, causes loss of fluid and can create fire hazards, as well as fluid contamination. Various kind of sealing components are employed in hydraulic system to prevent leakage.A typical such component, known as the O-ring is shown below.

SEALING BY O-RING

HYDRAULIC PRESS :PRE- FILL VALVES

MODEL OF PRE-FILL VALVE

 Poppet-type pre-fill valves

Pre-fill valves operate similarly to pilot-operated check valves, but they are usually much larger. Some pre-fill valves can handle flows in excess of 6000 gpm at pressure drops of less than 4 to 8 psi. Their normal function is to fill and exhaust a large bore cylinder as it travels to and from contact with the work piece. Large, high-tonnage presses -- both vertical and horizontal -- use pre-fill valves to reduce pump size while maintaining cycle time.
The cutaway view and symbol in Figure 10-8 show the construction of a typical poppet-type pre-fill
valve. A large main-flow poppet seals the path between the tank and the cylinder ports. As the piston
advances, vacuum in the void behind it allows atmospheric pressure to push the main-flow poppet open so fluid from the tank can fill this void. On the retraction stroke, a signal to the pilot piston pushes the main-flow poppet open so fluid can return to tank. While a pilot-operated check valve’s pilot piston is larger than the poppet it opens, the main-flow poppet in a pre-fill is much larger in diameter than the pilot piston. Thus it is impossible to open the main-flow poppet against high back pressure. This keeps decompression shock from damaging pipes and components.Decompression shock occurs when large volumes of fluid at high pressure are released suddenly.
Because all hydraulic oil has some entrained air (bubbles so small they cannot be seen without
magnification), there is a 0.5 to 1% compressibility that must be dealt with when using large-bore
cylinders. On top of fluid compressibility, the cylinder tube may stretch diametrically and longitudinally.
In addition, the framework that is resisting the tonnage produced also can stretch. Summing all these
factors, a 50-in. bore cylinder with a 72-in. stroke can contain more than 25 gal of extra fluid at 3000
psi. If this trapped fluid suddenly has a large open path to atmosphere, its velocity at first release is
such that it can break fittings, blow hoses, straighten tubes or pipe bends with relative ease. Releasing this same trapped fluid in a controlled manner over a few seconds dissipates the excess energy and no damage is seen.
The plain pre-fill valve might be used on smaller cylinders or circuits that have other means for
decompressing. The pre-fill valve with decompression has a small poppet in the large poppet that is
easy to open at high pressure but will not allow the high flow that causes decompression shock. This
decompression poppet usually has a means to adjust how fast the cylinder decompresses.
Another pre-fill valve design is the sleeve type that must be externally shifted open and closed. Both
designs give the same results even though their operation is different.
See also
hydraulics
working-of-hydraulc-press 
hydraulic-directional-control-valve