Content : 4.1 Hydraulic and Pneumatic actuators. 10 Marks Hydraulic Actuators - Hydraulic cylinders (single, double acting and telescopic) construction and working, Hydraulic motors (gear and piston type) construction and working. Pneumatic Actuators - Pneumatic cylinders (single and double acting) construction and working, Air motors (gear and piston type) construction and working 4.2 Valves for Hydraulic and Pneumatic systems. 08 Marks Classifications of valves, poppet, ball, needle, throttle, pressure control directional control, sequencing synchronizing,rotary spool, sliding spool two position, multi position. Non-return valves. Proportionate valve Construction and operation of above valves. 4.1 Hydraulic and Pneumatic actuators. Page 1
Hydraulic actuators : These are the elements of hydraulic system which transform hydraulic energy into useful work, through these device we can achieve objectives like moving,gripping lifting,rotary motions with force. Single acting cylinder with single piston rod : This type of cylinder is used for comparatively lower loads than plunger or ram type hydraulic cylinder. This cylinder may be gravity return or spring return. Its applications mostly in production tooling for clamping pushing etc also in automation plant where number of cylinder perform individual operations in a predetermined sequence. Page 2
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Rotary Actuators : Rotary actuators are used to generate rotary motion by using force of hydraulic oil or compressed air so it is categories into two types Hydraulic motor and air motor. In which pressure energy converted into mechanical energy. Classification of hydraulic motors are as bellow, 1) 2) 3) 4) 5) Gear motor similar to Gear pump. Vane motor similar to vane pump. Swash plate piston motor. Bent axis piston motor. Radial piston motor. Construction of hydraulic pump and motors are similar with each other only difference is that for operation of pump there is need of prime mover which are directly coupled with pump. In which mechanical energy is converted into pressure energy. Applications of hydraulic motor : hydraulic motors are used for Industrial and Civil Engineering task for docks and moving bridges, Crane drives, Military vehicles, Excavators, Conveyors and feeder drives, Plastic injection machine, Drill machine, Trench cutter etc. Gear type hydraulic motor : Gear type motor is a rotary actuator used to rotate the shaft. It consists of two gears in mesh with each other. One gear is connected to output shaft and other is idler. Both the gears are mounted in closed casing. Pressurized fluid enters from the bottom, and pressurizes the chamber. This pressure exerts a force on teeth, These forces results in rotation of both gears. This rotary motion is further used in rotation Page 9
of output shaft. Gear motors suffer from leakage, which is quiet high at low speeds. Hence gear motors are used where medium speed and low torque are required. Page 10
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Vane type air motor : A typical vane type air motor is shown in fig. These motors are simple in construction, extremely compact. The rotating element is a slotted rotor which is mounted on a drive shaft. Each slot of the rotor is fitted with a freely sliding rectangular vane. The rotor and vanes are enclosed in the housing, the inner surface of which is offset from the drive shaft axis. The compressed air is made to pass through inlet ports and this air flows over the vanes and is exhausted through the outlet, causing the shaft to rotate. The speed of air motor can be varied by varying the air flow to the motor. Page 14
OR Construction and Working of Piston type air motor : Radial Piston Motor : In this, three pistons are fitted in cylinder block. The curve ends of pistons can rest on smooth surface of rotor. Cylinder block and rotor are rotating member of motor. If compressed air is introduced in the cylinder under pressure, piston will pushed outward in its cylinder. This principle is used in this motor. Suppose compressed air under pressure is admitted to cylinder no 1, piston will move outward in its cylinder. Now curved end of piston will slide inside the rotor with force and rotor will turn in clockwise direction. Then the cylinder no 3 will occupy the position of 1 since cylinder block also starts rotating. And same cycle will start which results in rotational motion of rotor. These motors produce high power and torque. Page 15
2) Axial Piston motor : Construction and Working : In axial piston motors, the piston reciprocates parallel to the axis of the cylinder block. These motors are available with both fixed-and variable-displacement feature types. They generate torque by pressure acting on the ends of pistons Page 16
reciprocating inside a cylinder block. The figure illustrates the inline design in which the motor, drive shaft and cylinder block are centered on the same axis. Pressure acting on the ends of the piston generates a force against an angled swash plate. This causes the cylinder block to rotate with a torque that is proportional to the area of the pistons. The torque is also a function of the swash-plate angle. The inline piston motor is designed either as a fixed- or a variable-displacement unit. The swash plate determines the volumetric displacement. 4.2 Valves for Hydraulic and Pneumatic systems. Page 17
Function of valve : (any four) 1. 2. 3. 4. 5. Blocking or stopping of flow. To release the flow of working medium. To give the Direction of flow. To maintain the Pressure of working medium for safety purpose. To maintain Flow quantity as per requirement. Poppet valve :- It consists of simple disc, cones or balls are used in conjunction with simple valve seats to control flow. When a push button is depressed, it lifts ball to its seat and allow fluid to flow from port P to port A. When button is released, spring & fluid pressure forces the ball up again closing the valve. A poppet valve quickly opens relatively larger orifice in a shorter travel to permit full flow of air thus it is fast response time. These valves are simple, cheap and insensitive to dirt. Spool Valve : 1. Sliding spool : Page 18
It consist of a spool which moves axially within the valve body to control the direction of flow. The body has inlet & outlet port. The inlet port is connected to pump & outlet to actuators. The spool is moved by actuating mechanism or push button. Normally valve is in closed condition. When push button is pressed the spool moves to left side & land on the port P. so that the valve is in open position. When the push button is released spring forces spool to return to closed condition. Sliding spool type Direction control valves are the following types- 2/2, 3/2 and 4/2. 2. Rotary spool Valve : A rotary spool valve has a rotary spool which has internal oil passages to carry out the direction control element. As shown in the figure the valve body has four passages pressure (P), return(r) and two connections to load (A & B). Figure shows the three positions of the spool and respective port connections. As compared to sliding spool the rotary spool valve is less motion to operate, but it may need more operative force to switch the position. OR Rotary spool Valve : Page 19
It consist of a rotating spool which aligns with a hole in the valve casing to give required operation. Openings in casing are connected to compressor, and two sides of cylinder i.e. port A and port B. When rotary spool is rotated the compressor port is connected to cylinder port A causing pressurized air to move the piston at the same time port B is open to atmosphere. When spool returns to original position pressure port and cylinder port is blocked and valve is in neutral position. Rotary valves are compact, simple & have low operating forces. They are low pressure devices & hence used for hand operation in pneumatic systems. Leakage control is rather difficult as rotary valves require close contact between port plate and spool. Leakage is important in high pressure. Hence they are better suited for low pressure applications. Direction control valve (DCV) : Direction control valves are used for direction the pressurized fluid to obtain different positions. 3 X 2 DC valve : Page 20
Construction : 3 X 2 means 3 ports & 2 positions. In 3x2 DC spool type valve, spool reciprocates in a finely finished bore. The spring is attached to front spool. Working : Position 1:(Actuated position) Figure shown is actuated position. Push button is pressed in the port P is connected to port A and no flow through R. When spool is pressed in spring is compressed. Position 2 (Normal Position) Now push button is released, spring will expand and spool will move towards right. Now port A will be connected to port R and Port P will be disconnected so that flow of used oil will start from port A to R and then to oil reservoir. 4/2 DC valve : Construction : It consists of a shaft sliding in a bore which has large groove around the circumference. The spool is sealed along the clearance between moving spool and valve body. The grooves guide the fluid flow by interconnecting or blocking the ports. Spring is fitted in bore to bring the spool back to original position. A four-way has four ports labeled P, T, A and B. P is the pressure inlet port. T is the tank; A and B are outlet ports to the system. Page 21
Working of 4/2 DC valve: It has two switching positions. In the normal position, pump flow is sent to outlet B. Outlet A is connected to the tank. In the actuated position, the pump flow is sent to port A. Port B connected to tank T. As soon as actuating force is released from spool, compression force of spring brings the spool back to original position. 4 / 3 DC valve : Symbol Actuated position (Pulling action) Page 22
Normal position Actuated position (Pushing action) Construction: Here 4/3 D.C. valve is shown in the fig. There are four ports, A and B are consumer ports(ports going to actuator). P is pressure port through which pressurized oil goes in. R is return port through which used oil is return to oil reservoir. Valve is push button operated ( manually ) valve regains by spring expansion. There are total three positions which are represented by three square block. Out of which central one is closed position and other two are actuated positions. Working : When we have shifted the spool manually in such a manner that all ports are close to each other. No flow from port P to A or B and no flow from port A and B to R. When D.C. valve attains this position, pressurized oil returns to reservoir via pressure relief valve. While closed centre position is in active mode then flow will not takes place. When sliding spool moves towards right side then pressure port P connected to consumer port B and consumer port A connected to return port R. Similarly when sliding spool moves in a left hand side manually then pressure port P connected to consumer port A and consumer port B connected to return port R. Page 23
Non return valve is also called as Check valve or one way valve. Construction : This valve consists of valve body with inlet and outlet ports having valve element like cone, ball or spherical poppet. The valve element is in corporate with specially designed spring. Working : When pressurized oil comes in through port A it will lift up the cone by overcoming spring force and flow will start from port A to port B.When flow from A stops spring will expand and cone will block the flow hence only one direction of flow is possible. Flow Control valve : (FCV) It is used to regulate flow of pressurized fluid by which we can easily control the speed of actuators. It is normally placed between direction control valve and hydraulic or pneumatic actuators. Flow control is by following method : i) ii) iii) Throttling It is nothing but passing of pressurized fluid through fixed restricted path, like through Orifice Diversion By diverting or bypassing part of pressurized fluid so that actuator receives only portion perform its task. Flow divide - Page 24
Non-compensated flow control valves : The non-compensated flow control valves create an orifice in a pipe to restrict the flow. The control element of the valve may be needle, globe type as in figure. The non-pressure compensated type is used where the system pressure are relatively constant and motoring speeds are not too critical. It works on the principle that the flow through an orifice will be constant if the pressure drop remains constant. Without compensation, flow through these simple valves can very at a fixed setting with changes in pressure or temperature. Fig. shows a non-pressure compensated flow control valve with its symbol. It consists of a check valve, which permits free flow in the direction opposite to the flow control direction. If the load on the actuator changes, then the system pressure will change appreciably and also the flow rate through a non-pressure compensated valve will change for the same flow rate setting. OR Page 25
Pressure compensated flow control valves : Page 26
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Ball Valve : Construction: Ball valves control Flow & Pressure of the fluid inside the Hydraulic systems which regulates & controlled linear & rotational movements of the applications. Rotary ball valve are very similar to rotary spool valves but with more limited porting configurations. Although these are sliding element valve their good sealing is obtained by the use of seals which is shown in fig.they may be categorized as full or reduced passage depending on size of the hole through the ball relative to the valve ports. Ball valves are available in Steel, Stainless Steel, and Brass Materials. Ball valves can be made in Cast body / Forged body & also in Solid bar stock body. Pressure control valve : (PCV) It maintain desired pressure level in the various parts of hydraulic circuit. Function : 1) 2) 3) 4) Set a pressure limit to protect the system. Regulating the pressure in certain portion in the system. Unloading the system pressure. Pressure related functions are performed. Page 28
Directly operated pressure relief valve : Function : It's function is to limit the pressure in the system & thus protect the individual component and lines from high pressure and danger of bursting. If the pressure in the system increased above a pre-set level, these valve open to release the fluid out of the system, so that the system pressure comes to normal. Page 29
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Sequence valve : Whenever more than one operations will perform in a sequential order then for providing the flow to second operation when pressure level at a particular operation has reached. It is used for sequencing operations one after another. Construction and Working : Sequence valve is nothing but pilot operated relief valve. It has a special spool having specially drilled oil passage with internal orifice drain is directed to main drain. In normal position sequence valve is closed when the operation of consumer 1 is completed pressure starts building and when reaches set value of pilot relief valve fluid flows through spool to drain/ tank. As the fluid flows through spool the orifice causes pressure difference between spring side and spool side. This pressure difference results in differential force which lifts the spool causing it to uncover the port A thus supplying fluid to another consumer A. Page 31
Question Bank 4.1 Hydraulic and Pneumatic actuators : 1) Give classification of hydraulic actuator. 2) State the types of hydraulic actuators. Describe construction and working of single acting cylinder with neat sketch. 3) Explain with neat sketch gear type hydraulic motor. 4) Explain working of gear type hydraulic motor with neat sketch. 5) Explain construction and working of piston type air motor with neat sketch. 6) Explain with neat sketch vane type air motor. 4.2 Valves for Hydraulic and Pneumatic systems. 7) Write classification of control valves. 8) Give the classification of valves used in hydraulic systems. Write four functions of valve. 6m 9) Which are the various valves used in Direction Controlling the air in pneumatic circuit? Explain construction and working of any one valve. 10) Explain the function of relief valve in hydraulic circuit and sketch the same one. 11) Explain the construction and working of non-compensated flow control valve. 12) Explain constructions of ball valve with neat sketch. 13) Draw a neat sketch of non return valve and explain it. 14) Explain rotary spool valve with neat sketch. 15) What are the direction control valves? Explain construction of 3/2 direction control valve. 16) Explain construction and working of sliding spool type 4/3 direction control valve. 17) Draw a labeled sketch of sequence valve and describe it s working. 18) Write construction and working of nonreturn valve with neat sketch. 6m 19) Explain construction and working of 4/2 DC valve which is used in hydraulic systems. Page 32