2. Hydraulic Valves, Actuators and Accessories. 24 Marks

Transcription

1 2. Hydraulic Valves, Actuators and Accessories 24 Marks

2 Co related to chapter Describe working principle of various components used in hydraulic & pneumatic systems Choose valves, actuators and accessories required for simple hydraulic and pneumatic circuits.

3 Pressure-control valves Function Pressure-control valves are used in hydraulic systems to control actuator force (force = pressure area) and to determine and select pressure levels at which certain machine operations must occur.

4 Types of pressure control valves Pressure-relief valve. Pressure-reducing valve. Unloading valve Counterbalance valve. Pressure-sequence valve.

5 Pressure Relief valves The pressure relief valves are used to protect the hydraulic components from excessive pressure. It is normally a closed type and it opens when the pressure exceeds a specified maximum value by diverting pump flow back to the tank.

6 Schematic of direct pressure relief valve is shown in figure This type of valves has two ports; one of which is connected to the pump and another is connected to the tank. It consists of a spring chamber where poppet is placed with a spring force. Direct type of relief valve

7 Generally, the spring is adjustable to set the maximum pressure limit of the system. The poppet is held in position by combined effect of spring force and dead weight of spool. As the pressure exceeds this combined force, the poppet raises and excess fluid bypassed to the reservoir (tank). Direct type of relief valve

8 The poppet again reseats as the pressure drops below the pre-set value. A drain is also provided in the control chamber. It sends the fluid collected due to small leakage to the tank and thereby prevents the failure of the valve. Direct type of relief valve

9 Three-dimensional view of simple pressure-relief valve.

10 Compound relief valve or pilot operated relief valve

11 Compound relief valve or pilot operated relief valve

12 symbol VIDEO 1

13 Pressure Reducing Valve Sometimes a part of the system may need a lower pressure. This can be made possible by using pressure reducing valve as shown in Figure. These valves are used to limit the outlet pressure. Generally, they are used for the operation of branch circuits where the pressure may vary from the main hydraulic pressure lines. These are open type valve and have a spring chamber with an adjustable spring, a movable spool as shown in figure.

14 A pressure-reducing valve uses a spring-loaded spool to control the downstream pressure. If the downstream pressure is below the valve setting, the fluid flows freely from the inlet to the outlet. When the outlet (downstream) pressure increases to the valve setting, the spool moves to the up to partially block the outlet port. Just enough flow is passed to the outlet to maintain its preset pressure level. If the valve closes completely, leakage past the spool causes downstream pressure to build up above the valve setting. This is prevented from occurring because a continuous bleed to the tank is permitted via a separate drain line to the tank.

15 Three-dimensional view of a pressurereducing valve and symbol VIDEO 2

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22 Direction control valve Definition: A valve is a device that receives an external signal (mechanical, fluid pilot signal, electrical or electronics) to release, stop or redirect the fluid that flows through it. Function: The function of a DCV is to control the direction of fluid flow in any hydraulic system.

23 Direction control valve Rotary spool type: In this type, the spool is rotated to change the direction of fluid. It has longitudinal grooves. The rotary spools are usually manually operated. OUTLET-B VALVE BODY SPOOL INLET-A

24 Direction control valve Sliding spool type: In this type, the spool is slides to change the direction of fluid. A B

25 Check Valve The simplest DCV is a check valve. A check valve allows flow in one direction, but blocks the flow in the opposite direction. It is a two-way valve because it contains two ports. Figure 1.1 shows the graphical symbol of a check valve along with its no-flow and free-flow directions.

26 Ball type check valve In Fig. 1.2, a light spring holds the ball against the valve seat. Flow coming into the inlet pushes the ball off the seat against the light force of the spring and continues to the outlet. A very low pressure is required to hold the valve open in this direction. If the flow tries to enter from the opposite direction, the pressure pushes the ball against the seat and the flow cannot pass through.

27 VIDEO

28 Poppet check valve Figure 1.3 provides two schematic drawings showing the operation of a poppet check valve. A poppet is a specially shaped plug element held on a valve seat by a light spring. Fluid flows through the valve in the space between the seat and poppet. In the free flow direction, the fluid pressure overcomes the spring force. If the flow is attempted in the opposite direction, the fluid pressure pushes the poppet in the closed position. Therefore, no flow is permitted

29 Pilot-Operated check Valve A pilot-operated valve along with its symbol is shown in Fig This type of check valve always permits free flow in one direction but permits flow in the normally blocked opposite direction only if the pilot pressure is applied at the pilot pressure point of the valve. The check valve poppet has the pilot piston attached to the threaded poppet stem by a nut.

30 Pilot-Operated check Valve The light spring holds the poppet seated in a no-flow condition by pushing against the pilot piston. The purpose of the separate drain port is to prevent oil from creating a pressure build-up at the bottom of the piston. The dashed line in the graphical symbol represents the pilot pressure line connected to the pilot pressure port of the valve. Pilot check valves are used for locking hydraulic cylinders in position.

31 Shuttle valve A shuttle valve allows two alternate flow sources to be connected in a one-branch circuit. The valve has two inlets P1 and P2 and one outlet A. Outlet A receives flow from an inlet that is at a higher pressure. Figure 1.5 shows the operation of a shuttle valve. If the pressure at P1 is greater than that at P2, the ball slides to the right and allows P1 to send flow to outlet A. If the pressure at P2 is greater than that at P1, the ball slides to the left and P2 supplies flow to outlet A.

32 2/2 DIRECTION CONTROL VALVE: This valve has two ports and two positions of spool. The two ports are inlet port-a and outlet port-b. Sliding spool type 2/2 DCV: The figure shows a sliding spool type spring return type Normally Closed 2/2DCV. In normal position of the spool, the ports are closed; fluid cannot flow from port-a to port-b When the palm button is pressed, spool moves to open the passage from port-a to port-b. VIDEO A B

33 Rotary spool type 2/2 DCV: OUTLET-B VALVE BODY SPOOL INLET-A The figure shows a rotary spool type 2/2DCV. In first position of the spool, the ports are closed; fluid cannot flow from port-a to port-b When the spool is rotated through 90O, it opens the passage from port-a to port-b, VIDEO

34 3/2 DIRECTION CONTROL VALVE: This valve is used to operate single acting cylinders. It has three ports namely, Pump port or inlet port P, Cylinder port A and Tank port or exhaust port T. 3/2 sliding spool type valve: The figure shows spring return type sliding spool valve. It has a springloaded spool inside the valve body. In figure, it is palm-operated type of valve. A In spool position as shown in figure, there is connection from port-a to port-t. Inlet port port-p is closed. In spool position as shown in figure, there is connection from port-p to port-a. The tank port or exhaust port, that is, port-t is closed. VIDEO P T A P T

35 3/2 rotary spool valve: The figure shows a 3/2 rotary spool valve. It has rotary spool inside the valve body. The spool is rotated through 120O to operate the valve. In spool position as shown in figure, there is connection from P to A. Fluid flows from pump to single acting cylinder. Hence the cylinder extends. The tank port or exhaust port, that is, port-t is closed. When the spool is turned to 120O, that is, as shown in shown in figure, there is connection from port-a to port-t. Fluid flows from single acting cylinder to tank. Hence the cylinder retracts. The inlet port port-p is closed. VIDEO valve body A P T spool A P T

36 4/2 DIRECTION CONTROL VALVE: This valve is used to operate double acting cylinders. It has four ports namely, Pump port or inlet port P, Cylinder port A, Cylinder port B and Tank port or exhaust port T. 4/2 sliding spool valve: In spool position as shown in figure-a, there is connection from P to A and B to T. Fluid flows form pump to port-a and form port-b to tank. When the palm button is pressed, the spool position is as shown in figure-b, there is connection from P to B and A to T. Fluid flows form pump to port-b and form port-a to tank. VIDEO A P A P B B T T

37 4/2 rotary spool valve: The figure shows a 4/2 rotary spool valve. It has rotary spool inside the valve body. The spool is rotated through 90O to operate the valve. valve body T T B A A B spool P P In spool position as shown in figure-a, there is connection from P to A and B to T. Fluid flows form pump to port-a and form port-b to tank. Hence the double acting cylinder extends. When the spool is turned to 90O, that is, as shown in shown in figure-b, there is connection from P to B and A to T. Fluid flows form pump to port-b and form port-a to tank. Hence the double acting cylinder retracts. VIDEO

38 5/2 DIRECTION CONTROL VALVE: This valve is used to operate double acting cylinders. It has 5 ports namely, Pump port or inlet port P, Cylinder port A, Cylinder port B Tank port or exhaust port T1 and Tank port or exhaust port T2. 5/2 sliding spool valve: In spool position as shown in figure, there is connection from P to A and B to T2. Fluid flows form pump to port-a and form port-b to tank. Hence the double acting cylinder extends When the palm button is pressed, the spool position is as shown in figure-b, there is connection from P to B and A to T1. Fluid flows form pump to port-b and form port-a to tank. Hence the double acting cylinder retracts. A P T1 T2 B A T1 VIDEO B P T2

39 5/2 rotary spool valve: The figure shows a 5/2 rotary spool valve. It has rotary spool inside the valve body. The spool is rotated through 72O to operate the valve. T1 T1 T2 T2 valve body valve body B A B A Figure-A spool spool P Figure-B P In spool position as shown in figure-a, there is connection from P to A and B to T2. Fluid flows form pump to port-a and form port-b to tank. Hence the double acting cylinder extends. When the spool is turned to 72O, that is, as shown in shown in figure-b, there is connection from P to B and A to T1. Fluid flows form pump to port-b and form port-a to tank. Hence the double acting cylinder retracts. VIDEO

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41 METHODS OF ACTUATION

42 METHODS OF ACTUATION

43 Types of different center positions.

44 Flow control valve Flow-control valves, as the name suggests, control the rate of flow of a fluid through a hydraulic circuit. Applications : Typical application include regulating cutting tool speeds, spindle speeds, surface grinder speeds, and the travel rate of vertically supported loads moved upward and downward by forklifts, and dump lifts.

45 Functions of Flow-Control Valves Flow-control valves have several functions, some of which are listed below: 1. Regulate the speed of linear and rotary actuators: They control the speed of piston that is dependent on the flow rate and area of the piston: 2. Regulate the power available to the sub-circuits by controlling the flow to them: 3. Proportionally divide or regulate the pump flow to various branches of the circuit: It transfers the power developed by the main pump to different sectors of the circuit to manage multiple tasks, if necessary.

46 Classification of Flow-Control Valves Flow-control valves can be classified as follows: 1. Non-pressure compensated. 2. Pressure compensated.

47 Non-Pressure-Compensated Valves Non-pressure-compensated flow-control valves are used when the system pressure is relatively constant and motoring speeds are not too critical. A non-compensated flow control passes more or less fluid as pressure increases and decreases. This is because more fluid can pass through a certain size orifice when pressure drop (pressure difference) across the orifice increases.

48 Non-Pressure-Compensated Valves The disadvantage of these valves is discussed below. The inlet pressure is the pressure from the pump that remains constant. Therefore, the variation in pressure occurs at the outlet that is defined by the work load. This implies that the flow rate depends on the work load. Hence, the speed of the piston cannot be defined accurately using non-pressure-compensated flowcontrol valves when the working load varies. This is an extremely important problem to be addressed in hydraulic circuits where the load and pressure vary constantly.

49 Non-pressure-compensated needletype flow-control valve

50 Non-pressure-compensated needletype flow-control valve Schematic diagram of non-pressure-compensated needle-type flow-control valve is shown in Fig It is the simplest type of flow-control valve. It consists of a screw (and needle) inside a tube-like structure. It has an adjustable orifice that can be used to reduce the flow in a circuit. The size of the orifice is adjusted by turning the adjustment screw that raises or lowers the needle. For a given opening position, a needle valve behaves as an orifice.

51 Non-pressure-compensated needletype flow-control valve Sometimes needle valves come with an integrated check valve for controlling the flow in one direction only. The check valve permits easy flow in the opposite direction without any restrictions. As shown in Fig. 1.4, only the flow from A to B is controlled using the needle. In the other direction (B to A), the check valve permits unrestricted fluid flow.

52 Pressure-Compensated Valves Pressure-compensated flow-control valves overcome the difficulty caused by non-pressure-compensated valves by changing the size of the orifice in relation to the changes in the system pressure. This is accomplished through a spring-loaded compensator spool that reduces the size of the orifice when pressure drop increases. Once the valve is set, the pressure compensator acts to keep the pressure drop nearly constant. It works on a kind of feedback mechanism from the outlet pressure. This keeps the flow through the orifice nearly constant.

53 Pressure-Compensated Valves

54 Pressure-Compensated Valves A pressure-compensated flow-control valve consists of a main spool and a compensator spool. The adjustment knob co trols the ai spool s position, which controls the orifice size at the outlet. The upstream pressure is delivered to the valve by the pilot line A. Similarly, the downstream pressure is ported to the right side of the compensator spool through the pilot line B.

55 We want to keep pressure drop (pressure difference on both side of knife edge orifice) is 100 to 150 psi. The compensator spool is held open by a 100- to 150-psi compression spring that sets pressure drop across the knifeedge orifice. Flow from the inlet goes through the compensating orifice, past the compensator spool, and out through the knife-edge orifice.

56 A drilled passage ports Inlet fluid to the left end of the compensator spool, which forces the spool to the right when pressure tries to go above 100 to 150 psi. After pressure reaches or goes above 100 to 150 psi, the compensator spool moves to the right and restricts flow to the knife-edge orifice flow control.

57 Pressure at the outlet is ported to the compression-spring chamber and increases the spring force. The compensator spool assures that pressure drop across the knifeedge orifice flow control stays at a constant 100 to 150 psi. With a constant pressure drop, flow stays the same regardless of inlet or outlet fluctuations.

58 Actuators Hydraulic actuators are devices used to convert pressure energy of the fluid into mechanical energy. Depending on the type of actuation, hydraulic actuators are classified as follows: 1. Linear actuator: For linear actuation (hydraulic cylinders). 2. Rotary actuator: For rotary actuation (hydraulic motor). 3. Semi-rotary actuator: For limited angle of actuation (semi-rotary actuator).

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60 1.2 Types of Hydraulic Cylinders Hydraulic cylinders are of the following types: Single-acting cylinders. Double-acting cylinders. Telescopic cylinders. Tandem cylinders.

61 1.2.1 Single-Acting Cylinders A single-acting cylinder is simplest in design and is shown schematically in Fig.1.1. It consists of a piston inside a cylindrical housing called barrel. On one end of the piston there is a rod, which can reciprocate. At the opposite end, there is a port for the entrance and exit of oil.

62 1.2.1 Single-Acting Cylinders Single-acting cylinders produce force in one direction by hydraulic pressure acting on the piston. (Single-acting cylinders can exert a force in the extending direction only.) The return of the piston is not done hydraulically. In single-acting cylinders, retraction is done either by gravity or by a spring.

63 1.2.2 Double-Acting Cylinder There are two types of double-acting cylinders: Double-acting cylinder with a piston rod on one side. Double-acting cylinder with a piston rod on both sides.

64 Double-Acting Cylinder with a Piston Rod on One Side Figure 1.4 shows the operation of a double-acting cylinder with a piston rod on one side. To extend the cylinder, the pump flow is sent to the blank-end port as in Fig. 1.4(a). The fluid from the rod-end port returns to the reservoir. To retract the cylinder, the pump flow is sent to the rod-end port and the fluid from the blank-end port returns to the tank as in Fig.1.4(b).

65 Double-Acting Cylinder with a Piston Rod on Both Sides A double-acting cylinder with a piston rod on both sides (Fig.1.5)is a cylinder with a rod extending from both ends. This cylinder can be used in an application where work can be done by both ends of the cylinder, thereby making the cylinder more productive. Doublerod cylinders can withstand higher side loads because they have an extra bearing, one on each rod, to withstand the loading.

66 1.2.3Telescopic Cylinder A telescopic cylinder (shown in Fig. 1.6) is used when a long stroke length and a short retracted length are required. The telescopic cylinder extends in stages, each stage consisting of a sleeve that fits inside the previous stage. One application for this type of cylinder is raising a dump truck bed. Telescopic cylinders are available in both single-acting and double-acting models. They are more expensive than standard cylinders due to their more complex construction.

67 Telescopic Cylinder They generally consist of a nest of tubes and operate on the displacement principle. The tubes are supported by bearing rings, the innermost (rear) set of which have grooves or channels to allow fluid flow. The front bearing assembly on each section includes seals and wiper rings. Stop rings limit the movement of each section, thus preventing separation. When the cylinder extends, all the sections move together until the outer section is prevented from further extension by its stop ring. The remaining sections continue outstroking until the second outermost section reaches the limit of its stroke; this process continues until all sections are extended, the innermost one being the last of all.

68 Tandem cylinder A tandem cylinder, shown in Fig. 1.7, is used in applications where a large amount of force is required from a smalldiameter cylinder. Pressure is applied to both pistons, resulting in increased force because of the larger area. The drawback is that these cylinders must be longer than a standard cylinder to achieve an equal speed because flow must go to both pistons.

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73 Rotary actuators-hydraulic motors Hydraulic motors are rotary actuators. However, the name rotary actuator is reserved for a particular type of unit that is limited in rotation to less than 360. A hydraulic motor is a device which converts fluid power into rotary power or converts fluid pressure into torque

74 Pumps and motors

75 Classification of Hydraulic Motors 1. Gear motors. 2. Vane motors. 3. Piston motors: A) Axial piston-type motors. B) Radial piston-type motors.

76 Gear Motors:

77 Vane motors-unbalanced type Figure 1.2 shows an unbalanced vane motor consisting of a circular chamber in which there is an eccentric rotor carrying several spring or pressureloaded vanes. Because the fluid flowing through the inlet port finds more area of vanes exposed in the upper half of the motor, it exerts more force on the upper vanes, and the rotor turns counterclockwise. Close tolerances are maintained between the vanes and ring to The displacement of a vane hydraulic motor is a function of eccentricity. The radial load on the provide high efficiencies. shaft bearing of an unbalanced vane motor is also large because all its inlet pressure is on one side of the rotor.

78 Balanced type vane motor The radial bearing load problem is eliminated in this design by using a double-lobed ring with diametrically opposite ports. Side force on one side of bearing is canceled by an equal and opposite force from the diametrically opposite pressure port. The like ports are generally connected internally so that only one inlet and one outlet port are brought outside. The balanced vane-type motor is reliable open-loop control motor but has more internal leakage than piston-type and therefore generally not used as a servo motor.

79 Swash plate piston motor 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 reciprocating inside a cylinder block. Figure 1.4 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 fixedor a variable-displacement unit. The swash plate determines the volumetric displacement

80 Bent axis motor A bent-axis piston motor is shown in Fig.1.6. This type of motor develops torque due to pressure acting on the reciprocating piston. In this motor, the cylinder block and drive shaft mount at an angel to each other so that the force is exerted on the drive shaft flange.

81 Pipes and Pipe Fittings

82 Necessity of pipes and hoses In hydraulic system the hydraulic energy is carried by means of closed conduits/pipes. These pipes are designed to carry the high pressure fluid and to provide flexibility and strength. Fluid power lines classified as: a) Pipe (Rigid) b) Tubes ( Semi Rigid) c) Hoses (Flexible)

83 Characteristic features of Pipes They are rigid and suitable for very high pressure application. They are not generally bent. Used for permanent application. These have maximum strength out off all three pipes. These are specified by the nominal bore size and wall thickness i.e. for a given ID it can have various OD. Pipes are generally made of metals.

84 Characteristic features of Tubes They are made of comparatively flexible materials. They can be bent to any shape and are easy to work. It can be used over and over again. Tubes are specified based on their outer diameter(od) Tubes are suitable for light weight applications.

85 Characteristic features of Hoses Hoses have same strength as pipe. Relative movement is possible between the points it is connected. Easy to install and dismantle.

86 Properties of fluid Power Conduits Mechanical strength: It should have sufficient strength to withstand the subjected pressure. Supporting strength: To support the components and fitting mounted on the piping Terminal points: To facilitate dismantling and assembling during servicing /repair Damping capability: Should dampen out shock waves which may develop in the system accidentally. Smoothness of surfaces: The inner surface of the pipe should be sufficiently smooth to reduce friction losses.

87 Selection of Pipes,Tubes and Hoses It is based on Required strength in the system Pressure needed in the system Requirement of flexibility of the actuator Availability of materials for maintenance. Ease of replacement.

88 Pipe materials Steel : Due to Low cost, easy to bend, easy to connect. For high pressure application Copper: Due to high resistance to corrosion. For low pressure application. Aluminium : Light in weight, good resistance to corrosion. For air craft and missile hydraulic system Stainless steel: Corrosion free, Good strength. Due to high cost suitable for particular application.

89 Selection of Pipe dimension Three imp. Dimensions of pipes are considered: Outside diameter (OD) Inside diameter (ID) Wall thickness Pipes are specified by their nominal ID and wall thickness. Selection of ID: Based on flow rate of fluid Permissible frictional losses.

90 Schedule of Pipes Pipes are classified based on their wall thickness Standard pipes Extra strong ( Extra heavy)pipes Double extra strong ( Double extra heavy ) pipes The wall thickness is expressed as a schedule number and is specified by ANSI(American National standard Institute) Standard: Schedule 40 Extra strong : Schedule 80 Double extra strong : Slightly thicker than Schedule 160 For given ID greater the wall thickness greater will be the bursting pressure

91 Tubes and Tubings They are semi Rigid. Tubing size : Tubes are designed by its actual OD e.g. 5/8 inch tube means having and OD of 5/8 inch. Tubes are given numbers in increments of 1/16 inch e.g. No. 6 tube means if will have an OD of 6/16 inch. Standard pipes and tubes are available between sizes from 1/16 to 2 inch nominal size.

92 Hoses

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94 Construction of Hoses It is divided into three layers: a) Inner tube b) reinforcement c) outer covering a) Inner tube :This is the passage through which the oil would be flowing.it is made of a suitable hoses material.selection of ID of this tube is done as in case of pipes. b) Reinforcement: The inner tube is covered with reinforcement material to provide sufficient strength to it. It is made up of fiber or steel wire braiding. Depending on pressure,two or three layers may be given.

95 c) Outer covering : To avoid reinforcement from getting damaged by the impact or sharp objects outer protective cover is given. It also covers reinforcement form abrasion,corrosion nad other damages.