Control of fluid power elements. Direction, Pressure, Flow Control Valves
- 1. UNIT 3 Control of Fluid Power Elements
- 2. Directional control valves (DCVs): • 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. The function of a DCV is to control the direction of fluid flow in any hydraulic system. A DCV does this by changing the position of internal movable parts.
- 3. Functions of DCV 1. To start, stop and change the direction of motion of a hydraulic actuator. 2. To permit the free flow from the pump to the reservoir at low pressure when the pump’s delivery is not needed into the system. 3. To isolate certain branch of a circuit.
- 4. • Based on fluid path, DCVs can be classified as follows: 1. Check valves. 2. Shuttle valves. 3. Two-way valves. 4. Three-way valves. 5. Four-way valves.
- 5. 1. Check Valves: • 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. • 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. Ball type check valve
- 6. • 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 Poppet type check valve
- 7. Pilot-Operated check Valve • 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. • The light spring holds the poppet seated in a no-flow condition by pushing against the pilot piston.
- 8. 2. 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. • 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
- 9. 2/2-Way DCV (Normally Closed) • A spool valve consists of a cylindrical spool that slides back and forth inside the valve body to connect or block flow between the ports. • This particular valve has two ports labeled P and A. P is connected to the pump line and A is connected to the outlet to the system. • Figure (a) shows the valve in its normal state. The valve is held in this position by the force of the spring. In this position, the flow from the inlet port P is blocked from going to the outlet port A. • Figure (b) shows the valve in its actuated state. The valve is shifted into this position by applying a force to overcome the resistance of the spring. In this position, the flow is allowed to go to the outlet port.
- 10. 2/2-Way DCV (Normally Opened) • The spring holds the valve in a position in which ports P and A are connected as shown in Figure (a). • When the valve is actuated, the flow is blocked from going to A as shown in Figure (b).
- 11. 3/2-Way DCV (Normally Closed) • A three-way valve has three ports, namely, a pressure inlet (P),an outlet to the system (A) and a return to the tank (T). • Figure shows the operation of a 3/2-way valve normally closed. In its normal position, the valve is held in position by a spring as shown in Fig. (a). In the normal position, the pressure port P is blocked and outlet A is connected to the tank. • In the actuated position shown in Fig. (b), the pressure port is connected to the tank and the tank port is blocked.
- 12. 3/2-Way DCV (Normally Opened) • Figure shows a three-way two- position DCV (normally open) with push button actuation and spring return. • In the normal position, shown in Figure(a), the valve sends pressure to the outlet and blocks the tank port in the normal position. • In the actuated position, the pressure port is blocked and the outlet is vented to the tank. (Figure b)
- 13. Application of 2/2 DCV
- 14. Application of 3/2 DCV
- 15. Application of 3/2 DCV
- 16. Four-Way Direction Control Valves • Four-way DCVs are capable of controlling double-acting cylinders and bidirectional motors. • A four-way has four ports labeled P, T, A and B. P is the pressure inlet and T is the return to the tank; A and B are outlets to the system. • 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 and port B connected to tank T. • In four-way DCVs, two flows of the fluids are controlled at the same time, while two-way and three-way DCVs control only one flow at a time.
- 17. Application of 4/2 DCV
- 18. 4/3 DCV • Many cylinder and motor applications require a third DCV position or neutral in which the actuator is subjected to pump pressure. • Four-way three-position circuits are therefore used in many hydraulic circuits. Many types of neutrals are available; • the most common of them are as follows: Closed neutral. Tandem neutral. Float neutral. Open neutral. Regenerative neutral.
- 19. Application of 4/3 DCV (closed neutral) for controlling a double-acting cylinder: • The valve shown here is spring centered, which means that it always returns to the neutral position automatically when not actuated. • For closed neutral, the pump line is blocked so that the flow must pass over the pressure relief valve. • This is wastage of energy because it generates power in the form of pressure and flow, but does not use it. • The wasted energy in the system goes as heat. • This is undesirable because the hydraulic fluid becomes thinner (less viscous) as it heats up.
- 20. Application of 4/3 DCV (tandem neutral) for controlling a double-acting cylinder: • The pump flow is allowed to flow back to the tank through the DCV when it is in the neutral. • This is a very desirable situation because only pressure in the pump line is due to the flow resistance of the lines and DCV. This keeps the pressure low when the valve is in the neutral. • In this situation, the system is said to be unloaded because the power consumption is reduced. • This wastes much less energy than does a closed central neutral that forces the fluid over the pressure relief valve at a high pressure. • The cylinder is held in position with a tandem neutral because the outlet port is blocked.
- 21. Application of 4/3 DCV (float neutral) for controlling a bidirectional motor: • The pressure port is blocked so that the pump flow is forced over the pressure relief valve. • Because both the outlets are connected to the tank, the motor floats or spins freely when the DCV is in the neutral. • This type is used in motor circuits because it allows the motor to spin to a stop when the valve is shifted to the neutral. • This is often preferable to shifting to a closed position because motors often build up a great deal of momentum. Shifting the valve closed in this situation causes a large pressure hike in the outlet line because the motor tends to keep spinning and tries to push the fluid into its outlet. This is known as shifting shock.
- 22. Application of 4/3 DCV (open neutral) for controlling a double- acting cylinder: • Flow always follows the path of least resistance, so the pump flow goes back to the tank. • Because the outlets are also connected to the tank, the cylinder floats when this valve is in neutral. • This is desirable in a circuit in which some external force must position the cylinder when in the neutral.
- 23. Application of 4/3 DCV (regenerative neutral) for controlling a double-acting cylinder: • A regenerative term is used to describe a system in which the waste is fed back into the system to supplement the input power. • In this neutral, the pressure port is connected to both outlets and the tank port is blocked. • When this valve is shifted to the neutral, the pump pressure is applied to both sides of the piston. • Because the piston area in the rod side of the cylinder is smaller than that on the blind side, there is a net force applied to extend the piston rod. • As the piston extends, it forces the outlet flow from the rod side back into the valve, where it combines with the pump flow and goes to the blind end of the cylinder. • This causes the considerable increase in cylinder speed.
- 24. Solenoid-Actuated Valve • Solenoid-Actuated Valve A spool-type DCV can be actuated using a solenoid as shown in Figure. • When the electric coil (solenoid) is energized, it creates a magnetic force that pulls the armature into the coil. • This causes the armature to push on the push pin to move the spool of the valve.
- 25. Solenoid-Actuated Valve • There are two types of solenoid designs used to dissipate the heat developed in electric current flowing in the coil. • The first type dissipates the heat into surrounding air and is referred to as an “air gap solenoid.” • In the second type “wet pin solenoid,” the push pin contains an internal passage way that allows the tank port oil to communicate between the housing of the valve and the housing of the solenoid. • Wet pin solenoids do a better job in dissipating heat because the cool oil represents a good heat sink to absorb heat from the solenoid. As the oil circulates, the heat is carried into the hydraulic system where it can be easily dealt with.
- 26. Pilot-Operated Direction Control Valves • Pilot-operated DCVs are used in a hydraulic system operating at a high pressure. • Due to the high pressure of the system, the force required to actuate the DCV is high. • In such systems, operation at a high pressure uses a small DCV that is actuated by either a solenoid or manually. This pilot DCV in turn uses the pressure of the system to actuate the main DCV as shown in Figure.
- 27. 5/2 DCV • The separation of the exhaust outlets for extension and retraction (3 or 5) makes it possible to regulate the exhaust flows separately using restrictor valves, allowing the cylinder to extend and retract at different speeds.
- 28. Pressure Control Valves (PCV) • Limiting maximum system pressure at a safe level. • Regulating/reducing pressure in certain portions of the circuit. Unloading system pressure. • Assisting sequential operation of actuators in a circuit with pressure control. • Any other pressure-related function by virtue of pressure control. • Reducing or stepping down pressure levels from the main circuit to a lower pressure in a sub-circuit.
- 29. Types of pressure control valves • Pressure-relief valve. • Pressure-reducing valve. • Unloading valve • Counterbalance valve. • Pressure-sequence valve.
- 30. Pressure-Relief Valves • Pressure-relief valves limit the maximum pressure in a hydraulic circuit by providing an alternate path for fluid flow when the pressure reaches a preset level. • All fixed-volume pump circuits require a relief valve to protect the system from excess pressure. Fixed-volume pumps must move fluid when they turn. • A relief valve is essential when the actuators stall with the directional valve still in shifted position
- 31. Simple Pressure relief valve • It is normally a closed valve whose function is to limit the pressure to a specified maximum value by diverting pump flow back to the tank. • A poppet is held seated inside the valve by a heavy spring. • When the system pressure reaches a high enough value, the poppet is forced off its seat. • This permits flow through the outlet to the tank as long as this high pressure level is maintained. • The external adjusting screw, which varies spring force and, thus, the pressure at which the valve begins to open (cracking pressure)
- 32. • The pilot-operated pressure-relief valve has a pressure port that is connected to the pump line and the tank port is connected to the tank. • The pilot relief valve is a poppet type. • The main relief valve consists of a piston and a stem. • The main relief piston has an orifice drilled through it. • The piston has equal areas exposed to pressure on top and bottom and is in a balanced condition due to equal force acting on both the sides. It remains stationary in the closed position. • The piston has a light bias spring to ensure that it stays closed. • When the pressure is less than that of relief valve setting, the pump flow goes to the system
- 33. • If the pressure in the system becomes high enough, it moves the pilot poppet off its seat. • A small amount of flow begins to go through the pilot line back to the tank. • Once flow begins through the piston orifice and pilot line, a pressure drop is induced across the piston due to the restriction of the piston orifice. • This pressure drop then causes the piston and stem to lift off their seats and the flow goes directly from the pressure port to the tank. • Pilot-operated pressure-relief valves are usually smaller than direct-acting pressure-relief valves for the same flow and pressure settings.
- 34. Pressure-Reducing Valve • This type of valve (which is normally open) is used to maintain reduced pressures in specified locations of hydraulic systems. • It is actuated by downstream pressure and tends to close as this pressure reaches the valve setting. • 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. Note that there is an internal passageway from the outlet which transmits outlet pressure to the spool end opposite the spring.
- 35. • When the outlet (downstream) pressure increases to the valve setting, the spool moves to the right to partially block the outlet port. • Just enough flow is passed to the outlet to maintain its preset pressure level. • Reverse free flow through the valve is only possible if the pressure exceeds the valve setting. The valve then closes, thus making reverse flow impossible. Therefore, pressure-reducing valves are often equipped with a check valve for reverse free flow.
- 36. Application of a pressure-reducing valve.
- 37. Counterbalance Valve • These normally closed valves are primarily used to maintain a back pressure on a vertical cylinder to prevent it from falling due to gravity. • They are used to prevent a load from accelerating uncontrollably. • This situation can occur in vertical cylinders in which the load is a weight. • This can damage the load or even the cylinder itself when the load is stopped quickly at the end of the travel.
- 39. • The pressure setting is slightly higher than that required to keep the load from free-falling. When the pressurized fluid flows to the cylinder’s cap end, the cylinder extends, increasing pressure in the rod end and shifting the main spool in the counterbalance valve. This creates a path that permits the fluid to flow through the secondary port via the directional control valve and to the reservoir. As the load is raised, the integral check valve opens to allow the cylinder to retract freely.
- 40. Application of a pressure-reducing valve
- 41. Flow-control Valves • Flow-control valves control the rate of flow of a fluid through a hydraulic circuit. • Flow-control valves accurately limit the fluid volume rate from fixed displacement pump to or from branch circuits. • Their function is to provide velocity control of linear actuators, or speed control of rotary actuators. • 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. • Flow-control valves also allow one fixed displacement pump to supply two or more branch circuits fluid at different flow rates on a priority basis.
- 42. Flow-control Valves • Typically, fixed displacement pumps are sized to supply maximum system volume flow rate demands. • For industrial applications feeding two or more branch circuits from one pressurized manifold source, an oversupply of fluid in any circuit operated by itself is virtually assured. • Mobile applications that supply branch circuits, such as the power steering and front end loader from one pump pose a similar situation
- 43. Functions of Flow-Control Valves • 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 • Regulate the power available to the sub-circuits by controlling the flow to them: • 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.
- 44. • A partially closed orifice or flow-control valve in a hydraulic pressure line causes resistance to pump flow. • This resistance raises the pressure upstream of the orifice to the level of the relief valve setting and any excess pump flow passes via the relief valve to the tank. Important parameters of flow-control valve 1. Cross-sectional area of orifice. 2. Shape of the orifice (round, square or triangular). 3. Length of the restriction. 4. Pressure difference across the orifice (Δp). 5. Viscosity of the fluid.
- 45. • The law that governs the flow rate across a given orifice can be approximately defined as Q is directly proportional to ΔP • This implies that any variation in the pressure upstream or downstream of the orifice changes the pressure differential Δp and thus the flow rate through the orifice.
- 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. • The operating principle behind these valves is that the flow through an orifice remains constant if the pressure drop across it remains the same. • In other words, the rate of flow through an orifice depends on the pressure drop across it.
- 48. The disadvantage of Non-Pressure-Compensated Valves • 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 flow-control 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 Valves • 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 tubelike structure. • It has an adjustable orifice that can be used to reduce the flow in a circuit.
- 50. Non-Pressure-Compensated Valves • 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. Usually, charts are available that allow quick determination of the controlled flow rate for given valve settings and pressure drops
- 51. Flow-control valve with an integrated check 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., 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. • A pressure-compensated flow- control valve consists of a main spool and a compensator spool. • The adjustment knob controls the main 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.
- 54. • The compensator spring biases the spool so that it tends toward the fully open position. • If the pressure drop across the valve increases, that is, the upstream pressure increases relative to the downstream pressure, the compensator spool moves to the right against the force of the spring. • This reduces the flow that in turn reduces the pressure drop and tries to attain an equilibrium position as far as the flow is concerned.
- 55. • In the static condition, the hydraulic forces hold the compensator spool in balance, but the bias spring forces it to the far right, thus holding the compensator orifice fully open. • In the flow condition, any pressure drop less than the bias spring force does not affect the fully open compensator orifice, but any pressure drop greater than the bias spring force reduces the compensator orifice.
- 56. • Any change in pressure on either side of the control orifice, without a corresponding pressure change on the opposite side of the control orifice, moves the compensator spool. Thus, a fixed differential across the control orifice is maintained at all times.