Types of Check Valve
1. Ball
2. In-Line
3. Right Angle
4. Restriction
5. Pilot-Operated
Pilot-Operated Check Valves
1. Pilot-to-Open
2. Pilot-to-Close
Two-Way Valve
A valve with only an A and B port that represent the inlet and outlet ports. They are two-position valves.
Three-Way Valve
A valve with ports P, A, and B, with P representing the pressure port and A and B representing the different cylinders. Can be 2 or 3 position valves
Four-Way Valve
Has 4 ports, P, A, B, and T. T represent the port that heads back to the tank. Can be 2 or 3 position valves
Spring-Centered Valve
A spring holds the valve in a center position unless an acted upon by an actuator. A 3-position one has 2 actuators
Two-Position Spring Offset Valve
Normally offset to one extreme position by a spring and has one actuator that will shift the spool to the other extreme.
Three-Position Detented Valve
Has 2 actuator and is held in place by a detent mechanism in one position until shifted by actuator. It will remain in the detented position even after the actuating force is released. Detents are mechanical devices and must be maintained.
Spool Center Conditions
1. Closed Center
2. Float Center
3. Open Center
4. Tandem Center
Closed Center Spool Condition
all ports are closed
Float Center Spool Condition
Ports A and B are vented to T and P is closed off
Open Center Spool Condition
All ports are vented to the tank
Tandem Center
The pressure port is vented to the tank and A and B are closed off
Direct Acting Valves
1. Manual Actuator
2. Mechanical Actuator
3. Pneumatic Actuator
4. Hydraulic Actuator
5. Electrical Actuator
Manual Actuator
Usually a simple lever connected to the spool. Generally considered unsafe. Not as accurate or repeatable as other valves.
Mechanical Actuator
Either wheel or plunger type devices that is moved by some mechanical device such as a cylinder or cam.
Pneumatic Actuator
Uses air pressure applied to a piston to shift the valve spool.
Hydraulic Actuator
2-stage valve with a pilot stage and a main stage that is hydraulically actuated. Most often a solenoid-operated valve is used for actuation.
Electrical Actuator
Commonly called solenoids that is made up of a coil and an armature. The armature pushes on the spool, or a small rod called a pushpin as it is pulled into the magnetic field. The two designs used today are the Air Gap and Wet Armature types.
Air Gap Solenoid
Air space separates the solenoid from the hydraulic system. A problem is that the dynamic seal eventually wears and starts to leak.
Wet Armature Solenoid
All of the solenoid's moving parts operate in the system's hydraulic fluid, eliminating the need for a dynamic seal. Only two seals are used: a static seal where the core tube is screwed into the valve body and a static seal at the manual override pin tha
AC Solenoids
Has a high current draw at beginning of the stroke and a lower draw at the end of the stroke. The armature itself becomes part of the resistance as the solenoid as it is pulled into the coil, reducing the current.
DC Solenoids
Draw a constant current when energized and are designed to handle high, continuous current. Shift time is slower than AC solenoids. Generally safer than AC because they operate at lower voltages. Generally found more often in mobile applications than indu
Relief Valves
Normally closed valve connected between the pressure line (pump outlet) and the reservoir. Its purpose is to limit pressure in the system to a preset maximum by diverting some or all of the pump's output to tank when pressure setting is reached.
Cracking Pressure
The pressure at which the valve first begins to divert flow
Full-Flow Pressure
Pressure at the relief valve inlet when the valve is passing its maximum volume
Pressure Override
The different between cracking pressure and full-flow pressure.
Pilot-Operated Relief Valve
...
Unloading Relief Valve
Used in accumulator charging circuits to:
1. Limit maximum pressure
2. Unload the pump when the desired accumulator pressure is reached.
Pressure Reducing Valves
Normally open pressure controls used to maintain reduced pressure in certain portions of the system. Actuated by pressure sensed in the branch circuit and tend to close as it reaches the valve setting. Direct Acting and Pilot-Operated version are in use
Sequence Valve
Used to cause actions to take place in a system in a definite order and to maintain a predetermined minimum pressure in the primary line while the secondary operation occurs.
Counterbalance Valve
Used to maintain control over a vertical cylinder so that it will not fall freely because of gravity.
Brake Valve
Used in the exhaust line of a hydraulic motor to:
1. Prevent overspeeding when an overrunning load is applied to the motor shaft
2. Prevent excessive pressure buildup when decelerating or stopping a load.
Flow Control
Controlling the rate of flow in a hydraulic circuit makes it possible to control the speed of the actuator. Actuator's speed is determined by its size and the flow rate of the oil going into or out of it.
3 Factors that affect flow rate through an orifice
1. Pressure drop
2. Viscosity
3. Orifice size
Types of Flow control circuits
1. Meter-in
2. Meter-out
3. Bleed off
Meter-In Circuit
The FCV is placed between the pump and the actuator. It controls the amount of fluid going into the actuator. Can only be used with opposing loads.
Meter-Out Circuit
The FCV is placed on the outlet side of the cylinder to control the flow coming out. Used for loads that run away.
Bleed-Off Circuit
The FCV is installed in a branch circuit off the main line to control cylinder speed by delivering some of the flow to the reservoir. There is no flow passing the relief valve, so pump will operate at pressure needed to move the load, conserving energy. W
Orifice Properties
Flow through an orifice increases as the pressure drop across the orifice increases
Types of Flow Control
1. Nonpressure-Compensated
2. Pressure-Compensated
-Bypass
-Restrictor
Pressure-Compensated FCV
Automatically compensates for pressure changes and maintains its setting, even as the work load changes.
Bypass Type
As long as pressure differential across the throttle is less than 20 psi, the valve will remain closed and all flow will pass to the cylinder.
Restrictor Type
Restricts the inlet flow rather than bypassing it. Can work in a meter-in, meter-out, or bleed off function. As pressure differential rises above 20 psi, the compensator spool will rise and begin to restrict inlet flow until the 20 psi is maintained.
Temperature Compensation
As the temperature increases, fluids become less viscous and flow increases. Closing off the throttle slightly will compensate for the change
Proportional Valves
Provide the ability to control hydraulic flow rates electronically. The amount of out flow is proportional to the magnitude of the electrical input signal. Requires an electrical power supply and electronic controls.
Deceleration Valves
Are restrictor type flow controls and are usually used in a meter-out configuration. Most are cam-operated with tapered spools. They gradually decreases flow to or from an actuator for smooth stopping or deceleration.
Contamination
Any foreign substance that contributes to or causes harm to the hydraulic or lubrication fluid of the system, or parts of the system, in which the fluid flows.
Sources of Contamination
1. Built-In Contamination
2. Ingressed Contamination
3. Internally Generated Contamination
Built-In Contamination
Might include burrs, chips, flash, dirt, dust, fibers, sand, moisture, pipe sealants, weld splatter, paints, and flushing solutions that came with the equipment, even if it was new.
Ingressed Contamination
AKA environment. It is foreign material that is added to the system during service or maintenance, or is introduced to the system from the environment surrounding the equipment. A common source is when the system is filled with new fluid.
Internally-Generated Contamination
Created within the system by the moving parts of hydraulic components. Includes:
1. Abrasion
2. Erosion
3. Adhesion
4. Fatigue
5. Cavitation
6. Corrosion
7. Aeration
Abrasion
Particles grinding between moving surfaces. Can be a One body, Two body, or Three Body type.
Erosion
High velocity particles striking surfaces
Adhesion
Metal-to-metal contact due to the loss of a lubricating film, and a molecular attraction between them.
Fatigue
Repeated stressing of a surface
Cavitation
High pump inlet vacuum which creates a pressure below the vapor-pressure, causing the molecules to become a gas and create a "cavity" in the fluid. The outlet pressure will then cause the gas to implode and damage the system.
Corrosion
Foreign substance in fluid
Aeration
The presence of air or gas bubbles in the fluid.
Four primary functions of Hydraulic Fluids
1. Transmit power
2. Cool or dissipate heat
3. Lubricate moving parts
4. Seal clearances between parts
Mechanical Clearances in Components
Up to 5 microns for high-pressure units. 10-20 microns for low-pressure unis.
Silting
The accumulation of very fine particulate contamination.
Cartridge Valve
1. Screw-In (low flow)
2. Slip-In (high flow)
Manifold Block Systems
Unique solutions to specific hydraulic circuit or system requirements, using one or more cartridge valves installed into either an aluminium or steel block to perform control or work functions.
Screw-In Cartridges
For low flow applications. 100% of the circuit is contained inside the block.
Slip-In Cartridges
For high flow applications. Typically only have 40% of the system contained in the manifold block.
Advantages of Manifold Block Systems
1. Greater design flexibility
2. Lower installed cost
3. Smaller package size
4. Alleviated external leakage
5. Easier troubleshooting
6. Easier maintenance
7. Better organized plumbing
8. Lower noise levels
Stack Valves
Unique way of assembling a number of operational functions onto a DCV. Functions such as relief, sequence, check, and flow control can be mounted, or "sandwiched", underneath a DCV to provide a complete control circuit for an actuator.
Difference b/w Conventional and Proportional Solenoids
The proportional solenoid delivers a constant force, regardless of armature position, during the working stroke, unlike the Conventional solenoid, which has a large amount of draw at the beginning of the stroke but the force decrease during the stroke
Control Amplifier Functions
1. Deadband Compensation
2. Gain
3. Dither
Electronic Control Amplifier
Used to control Proportional solenoid valves and provide the power necessary to operate the valve and perform additional functions.
Deadband Compensation
Deadband is primarily caused by the spool overlapping the valve ports.
Gain
Gain=A=Output V/Input V. It is a measure of the ratio between the amplifier's small input signal and its large output current flow to the valve.
Dither
It is a low frequency, low amplitude AC signal used to offset the effects of a condition known as hysteresis.
Hysteresis
The difference between the response of a unit or system to an increasing signal and the response to a decreasing signal. Caused by friction between a proportional valve spool and bore, flow forces acting against the spool, residual magnetism in the armatu
Symmetrical Proportional Spools
Restricts the two flow paths equally, which enables the valve to both meter in and meter out fluid
Nonsymmetrical Proportional Spools
The main restriction is in the B to T or the A to T flow path, so the valve will meter out fluid only.
Sharp Edge Orifice Equation
Q = Cd x A x Square root (2xdeltaP/q)
Q = flow rate
Cd = Orifice discharge coefficient
A = orifice area
P = pressure drop across orifice
q = fluid density
Flow rate through valve is...
proportional to the square root of the pressure drop across the valve
LDVT
Linear Variable Differential Transformer.
Proportional Pressure Control Valves
1. Nozzle-type relief valves
2. Poppet-type relief valves w/ LDVT feedback
3. Plate-type relief valves
Proportional Flow Control Valves
1. Nonfeedback Throttle Valves
2. Throttle Valves with Feedback
Nonfeedback Throttle Valves
Has a directional valve spool and body. When it is not energized, the valve spool blocks all the ports to prevent any flow through the valve.
Feedback Throttle Valves
Used for a particular applications that requires less hysteresis, better repeatability,faster response time and greater power capacity.
Specific Speed Calculation
N_s = (N * SqR(Q)) / (H^(3/4))
N_s = specif speed
N = RPM
H = ft of head
Q = Flow rate
Specific Speed
Determines the general shape or class of the impeller. It is dimensionless. Defined as the speed in RPM at which a geometrically similar impeller would operate if it were such a size as to deliver 1 GPM against 1' of head.
Discharge Pressure
P_D = P_s + DeltaP
DeltaP = h_A * Specific Weight
h_A = pump head
Stuffing Box Pressure
Pressure acting on the stuffing box which must be sealed. It is a function of pump impeller design and the presence or condition of wear rings.
System Curve
Represents the relationship between flow and hydraulic losses in a system. It is parabolic in shape. Is the change in flow with respect to the head of the system. Developed by user based upon the conditions of services
Brake HP
BHP = (Q
H
Sp. Gr / (3960 * Eff.)
Efficiency is the pump's mechanical efficiency
Operating Point
The intersection of system curve and pump curve at given impeller size.
3 Function of Valves
1. Direction
2. Flow
3. Pressure
Volute Pumps
1. Radial Flow
2. Mixed Flow
3. Axial Flow
Diffuser Pumps
1. Vertical Turbine
2. Horizontal
Concentric Volute Pumps
1. Regenerated Turbine
2. High Speed
3. Radial Vane
4. Recessed Impeller
Centrifugal Pump Construction
-Impeller
-Seal Chamber
-Power End
-Baseplate