The following points highlight the eight main types of hydraulic systems. The types are: 1. The Hydraulic Accumulator 2. The Differential Hydraulic Accumulator 3. The Hydraulic Intensifier 4. The Hydraulic Ram 5. The Hydraulic Lift 6. The Hydraulic Crane 7. The Hydraulic Press 8. The Hydraulic Coupling or Fluid Coupling.
Type # 1. The Hydraulic Accumulator:
A hydraulic accumulator is a device for temporary storing of the energy of a liquid. This device is used to accumulate liquid under pressure delivered by the pump when it is not needed by the machine. The liquid at pressure stored in the accumulator can be supplied to the machine when needed. Many hydraulic machines like cranes or lifts do a huge amount of work in a small interval of time followed by an idle period of time. For instance, a crane or lift needs energy to be supplied to it only during the upwards motion of the load. No energy need be supplied during the downward motion.
But it may be realized that the pumps supply the water at high pressure energy continuously. Hence high pressure water, during the idle stroke of the machine, can be stored in the accumulator, and can be given out at an increased rate during the working stroke of the machine. It should further be noted that the uniform supply of high pressure water from the pumps need not be as large as that needed by the machine while performing its maximum rate of work, as the machine at such time will draw the water from the accumulator.
The hydraulic accumulator consists of a vertical hollow cylinder in which slides a sliding ram (Fig. 27.1). The sliding ram is attached to a container which is filled with heavy material like slag. Alternatively, the sliding ram can be loaded by external loads. High pressure water delivered by the pump is passed into the hollow cylinder when not needed by the machine which it is working.
The loaded sliding ram gets raised until the cylinder is full of high pressure water. At this stage the accumulator is said to have stored the maximum quantity of energy. During the interval of time when maximum work has to be done by the machine, the machine will draw high pressure water from the accumulator. When the machine draws high pressure water from the accumulator, the sliding ram will descend.
Let A = Area of the base of the sliding ram in metre2
H = Total lift of the ram in metres
Volume of water in the accumulator = AH metre3
The weight of the ram = pA Newton
Work done in raising the ram = pAH Newton metre
This is also the energy stored in the accumulator. This is also called the capacity of the accumulator.
∴ Capacity of the accumulator = pAH Nm = p x Volume of the accumulator.
Type # 2. The Differential Hydraulic Accumulator:
This device consists of a fixed vertical cylinder (also called the plunger) provided with a central vertical cavity of small diameter. The fixed plunger is encased by a brass bush or sleeve. The sleeve is encased by a sliding inverted cylinder which can be loaded.
The water from the pump mains is passed into the fixed plunger at its bottom. The water moves up the vertical cavity and ultimately exerts an upward pressure force on the horizontal annular roof of the loaded sliding cylinder. This annular area is the same as the sectional area of the brass bush.
Let the pressure intensity of the water coming from the main be p N/mm2.
Let the annular area of the brass bush be a mm2.
Let H = Total lift of the accumulator in metres.
W = Total weight of the loaded sliding cylinder in N
p = W/a
The pressure intensity can be increased by making the annular area a small, adopting a conveniently small value of the load W.
Total energy stored in the accumulator = WH Nm
As the loaded sliding ram moves down, the high pressure water flows out of the fixed cylinder through the delivery pipe.
Type # 3. The Hydraulic Intensifier:
The hydraulic intensifier is a device meant for increasing the intensity of water pressure by the energy of a larger quantity of water at a low pressure. Such a device is needed when the pressure of water supplied to a machine is not requisitely high.
Fig. 27.3 shows the hydraulic intensifier which consists of a fixed ram conveying the high pressure water to the machine. A hollow sliding ram contains the high pressure water. The sliding ram is surrounded by a fixed cylinder containing the low pressure water from the main supply. The weight of low pressure water acts on the upper end of the sliding ram and forces downwards on to the fixed ram. In this process the water in the sliding ram is compressed and its pressure is thus increased. The high pressure water is squeezed out of the sliding cylinder through the fixed ram to the machine.
The sliding ram moves down as the water is drawn out of it. Finally it reaches the bottom of its stroke. At this stage, the valve allowing high pressure water to the machine is closed. The water collected in the fixed cylinder is now opened to exhaust. The low pressure water from the main supply is admitted into the sliding ram. The sliding ram will now rise and will ultimately reach the upper limit of its stroke. At this stage the valve admitting the high pressure water to the machine is opened, and the valve allowing the low pressure water to the inside of the sliding ram is closed.
At the same time the exhaust valve of the fixed cylinder is closed and the low pressure water is admitted into the fixed cylinder. The low pressure water passed into the fixed cylinder exerts a force on the sliding ram and moves it down thus increasing the pressure intensity of water inside the sliding ram. The high pressure water is forced out of the sliding ram through the fixed ram, to the machine.
Thus, the intensifier described above supplies high pressure water during the downward stroke only. It is found that it is possible to raise the pressure of water to 70 to 80 N/mm2 by using an intensifier.
Type # 4. The Hydraulic Ram:
The hydraulic ram is a device by which a small quantity of water can be raised to a considerable height by utilizing a large quantity of water available at a small height.
Fig. 27.5 shows the components of a hydraulic ram. From a natural supply tank A (a low level channel or tank) water moves down a depth H1 through the pipe G and enters the chamber B and flows out of the waste water valve C.
As the rate of flow of water down the pipe increases the water in the chamber B will exert a dynamic thrust on the valve C. This thrust goes on increasing until finally the valve C gets closed. When this happens the entire quantity of water in the pipe G is suddenly brought to rest, thus causing a sudden increase in pressure. This large increase in pressure will force open the valve F and the water is forced into the air vessel D compressing the air in it.
The high air pressure will force the water to move up the delivery pipe into high level tank E. When the water in B loses its momentum, the valve F automatically closes and the valve C opens allowing the flow pass out to waste.
Type # 5. The Hydraulic Lift:
The hydraulic lift consists of a platform or cage supported by a vertical ram which slides in a fixed vertical cylinder. High pressure water from the mains enters the fixed cylinder at its bottom and exerts a pressure on the lower end of the sliding ram. Thus the ram is raised and the platform can be brought in level with any desired floor. The cage moves in its guides.
Modern lifts have a high velocity ratio. These lifts are called suspended lifts. These lifts derive their movement from a jigger. The cage of the lift is suspended by ropes running between guides, when the lift is raised, the ram of the jigger moves down. The weight of the jigger is carried by the cables so that there is no tendency of the ram to move independently of the lift cage. The weight of the cage is balanced by sliding counter weights.
These lifts have a high velocity ratio and can lifts at a speed 200 m per minute or more.
Total load lifted = (force on the ram/velocity ratio) x Mechanical efficiency.
Type # 6. The Hydraulic Crane:
The hydraulic crane consists mainly of two parts namely the jigger and the crane. The jigger is attached to the mast of the crane. The jigger consists of a cylinder and a ram which slides in it. A set of 2 to 6 pulleys is provided to the top end of the sliding ram, another set of 2 to 6 pulleys is provided to the lower end of the fixed cylinder. Obviously the set of pulleys at the lower end do not have any vertical movements, while the top set of pulleys carried by the ram will move with the ram.
A rope one end of which is fixed to a movable pulley is passed over all the pulleys and is then taken over the jib and the other end carries the load W. The number of pulleys fixes the velocity ratio. Generally a six-sheaf pulley block system is provided giving a velocity ratio of 6. This means the displacement of the suspended load is 6 times the displacement of the plunger.
The crane consists of a vertical mast and a jib. The mast has its own pedestal and foundation. The mast along with the jib can be revolved about its vertical axis.
Type # 7. The Hydraulic Press:
This device was first built by Ernest Bramah and is also called the Bramah Press.
The principle of this device is based on Pascal’s law of transmission of pressure by fluid. There are two cylinders of different sizes which work with corresponding pistons. The two cylinders are connected by a pipe. The fluid is subjected to a high pressure by the application of a force on the piston of the smaller cylinder.
The hydraulic press in its practical form is shown in Fig. 27.11.
It consists of two fixed platens and a movable platen. The movable platen is attached to a plunger which passes through the upper fixed platen and works in a fixed cylinder. The two fixed platens are connected by columns. Water at high pressure is supplied by the pumps into the cylinder. Consequently the plunger gets pushed down and a huge pressure is exerted on any material placed between the lower fixed platen and the movable platen.
If the water from the cylinder is removed the movable platform along with the plunger will move up by the action of the return weights.
Type # 8. The Hydraulic Coupling or Fluid Coupling:
This is a device for transmitting rotatory motion from one shaft to another. This device consists of a radial pump impeller keyed to the driving shaft A and radial flow reaction turbine runner keyed to the driven shaft B as shown in Fig. 27.12. The impeller and the turbine runner together form a casing which is completely filled with oil. Suppose the driven shaft B is at rest and the driving shaft A is rotated slowly.
A forced vortex will be produced in the impeller due to which oil will be forced outwards by centrifugal action and then projected with a definite whirl component on the stationary vanes of the turbine runner. Since the angular momentum corresponding to this velocity of whirl is destroyed, a tangential force and hence a torque is exerted on the runner blades. Since there is no centrifugal head to resist, the oil will freely find its way to the inner eye of the runner and then across to the impeller again, thus having a continuous circulation.
Suppose the speed of A is now increased, the torque transmitted on B will increase sufficiently to overcome the resistance which had held it so far at rest. If the speed of the impeller is further increased it will make the runner also to increase in its speed. Ultimately at the designed full load and speed the speed of the driven shaft B will be less than that of the driving shaft by about 2 percent. For all practical purposes we may consider the two shafts to be running at the same speed. This type of transmission avoids or dampens any torsional inequalities that the driving shaft is liable to transmit to the driven shaft.
Under the running condition even though a forced vortex exists in both the impeller and the runner, the slight excess of speed of the impeller over the running ensures the needed excess of centrifugal head to maintain a continuous flow of oil from the outer edges of the impeller to the outer edges of the running blades.