Hydraulic power has found uses in a plethora of applications ranging from Industrial installations to mobile equipment. The increase in efficiency of the field over the last few decades has only endeared it more to system designers. The main use for hydraulics is to provide powerful motion to systems by amplifying the input in terms of power. Since the basic need for hydraulics is motion, it is but obvious that one of its most basic components would be one to control such motion. Directional Control Valves are used to start, stop and change the direction of fluid flow which results in the control of the end actuator providing motion.
Classifications
As any hydraulic valve, Directional Control Valves have their classifications based on a number of factors. Some of them are listed below.
Internal Construction: Directional Control Valves can have varying internal constructions to allow and restrict flow paths from various inlets to various outlets. Some of the more popular ones are poppets/pistons, balls, rotary spools and sliding spools. Poppet and ball type constructions invariably have a seat within the valve to form a metal-on-metal contact and may be used to hold loads in place. They have near zero leakage from one port to another. Spool valves (either rotary or sliding) have certain amount of permissible leakages due to limitations in machining. They shouldn’t be used to hold loads in place. None-the-less they are still quite popular!
Flow Paths: The internal construction of valves provides various methods to allow and restrict flow paths. Hence the number of flow paths being governed also contributes to the valve’s classification. Valves may be two-way, three-way and four-way. Although the list is only limited by the design, valves with flow paths more than 4 are rarely used. Here the term “way” stands for a path.
Number of Ports: This is the cumulative count of all the entry and the exit passages for oil that the valve has. This number could be a humble two in the case of a simple poppet valve or more than six in the case of mobile valves or larger manifold mounted valves (CETOP8, NG22). The most common configuration is a four port valve with the pressure port, tank port and two actuator ports forming the entry and exit points. Additional flow ports are needed when there are external pilot and external drain ports or if the system has a “carry-over” configuration.
Actuation Methods: Flow paths can be selected by either external or internal actuation. Internally actuated Directional Control Valves are limited to check valves. External actuations can be by manual means (levers, buttons or foot pedals), mechanical actuators (such as cams, rollers, plungers/tracers or springs), electrical methods (either solenoids or electrical motors that get their signals from limit switches, push buttons or PLC controls) or by the application or release or hydraulic or pneumatic pressure.
Mounting of the Valve: Directional Control Valves can also be specified depending on the mounting patterns of the valve which may be flange mounted, piped thread mounted, straight thread mounted, cavity mounted or manifold interface mounted.
Size of the Valve: Directional Control Valves can also be specified depending on the size of the mounting pattern. Valve sizes can be defined by their flow capacities (given in lpm or gpm), their port or flange sizes (BSP, SAE etc.), their mounting plate size which are usually interchangeable (standard interfaces of CETOP such as 3, 5, 7, 8 or 10; NG sizes such as 03 mini, 03, 04 mini, 04, 06, 10, 16 or 22), the cavity size (SAE 08, 10, 12, 16 or 20) or manufacturer specific cavities with 2, 3 or 4 ports.
Construction
Strictly speaking Check Valves and Pilot Operated Check valves are also a part of directional control valves but for the purpose of this article, we will only consider sliding-spool, subplate-type Direction Control Valves with more than 2 positions (ways).
Most Directional Control Valves are made up of three major parts or sub-assemblies:
Valve Body: The main valve body is made of non- porous cored cast iron or steel body with internal pathways connecting the various grooves to their respective external ports. The bore where the spool slides into place is usually ground and honed/lapped. The valve body does not generally differ by much in a particular size of valve regardless of the actuation or flow paths. The body may have various features depending on the specifications such as external gauge ports, interfaces for smaller valves in hydraulically actuated valves, speed control orifices for controlling the spool switching, orificing for draining moisture in pneumatic actuated valves etc.
Spool Assembly: The spool assembly, which consists of the spool, centering springs, washers and O-rings. Spools are made of case hardened steel and are mechanically or electrically moved. The movement of the spool restricts or permits the flow, thus it controls the fluid flow. Spools come in numerous versions depending upon the configuration of flow paths and the number of positions in the valve. Each spool has its own unique features and limitations which will be dealt with in a separate section later.
Actuation Assembly: The assembly for the actuation depends on the actuation method used. There are various levels of complexity from simply push button type valves to proportional electro-hydraulic vales with on-board controllers
Manual actuation valves usually have a simple lever assembly with either spring centering or detents to hold the position in place.
Mechanical actuation is a lot simpler in terms of assembly where the spool position is changed when the mechanical device is pushed inwards due to its actuation which in turn pushes the spool. The valve is reset by a spring on the opposite side.
Electrical Actuation is usually affected by solenoids although some applications call for servo motors as well. The most basic form of solenoids is of the “on- off’ type. This is because there is no mid-way control of the solenoids. They are completely off without a signal and on receiving the electrical signal, they are fully on. The version of solenoid valves that allow gradual variation in current are called proportional valves and will be dealt with in a separate article. The electrical actuation assembly for “on-off” type solenoid valves generally includes an armature of varying designs also colloquially called the “iron core” and an AC or DC solenoid coil.
Electrical Actuation is usually affected by solenoids although some applications call for servo motors as well. The most basic form of solenoids is of the “on- off’ type. This is because there is no mid-way control of the solenoids. They are completely off without a signal and on receiving the electrical signal, they are fully on. The version of solenoid valves that allow gradual variation in current are called proportional valves and will be dealt with in a separate article. The electrical actuation assembly for “on-off” type solenoid valves generally includes an armature of varying designs also colloquially called the “iron core” and an AC or DC solenoid coil.
Pneumatic Pressure actuation methods use air pressure to switch spool positions. The signals are received from external pneumatic systems operating simultaneously with the hydraulic system. Valves of pneumatic actuation need their body material to be of a non-corrosive nature so as to prevent rusting by the moisture content in the air. Internal assemblies of the valve are quite critical and rusting may cause jamming or sticking of the spool inside the bore. . In certain applications, due to the requirements of explosion resistance such as mines or oil rigs, the main spool is moved pneumatically. The solenoid valve, operated electrically, is kept safe in a place far from the explosive environment.
Hydraulic Pressure actuation is usually seen in the larger directional control valves where the power of the coil may not be sufficient to switch the spool due to large flow forces. In the case of mobile valves with bigger flows, a remotely operated joy stick is used. Joysticks are essentially one, two or four pressure reducing valves controlling different directions. At reduced pressure, the spring shifts the spool. Upon actuation, the spool shifts proportionally to the movement of the joystick. Hydraulic pressure actuation can also be obtained by using “double- decker” valves which are called so owing to the fact that a smaller directional control valve is mounted on top of the larger one. Pilot flow usually has to have a minimum pressure of 5 bar (70 psi) to switch the spool positions.
More on Spools
The main advantage using a spools over a poppet in Directional Control Valve is that spool movements are immune to pressures within the valve. When a port is pressurised, the pressure acts in equal and opposite directions on the lands, this nullifies the overall effect of the pressure. Hence they can be shifted with a constant force by manual, mechanical, electrical, pneumatic or hydraulic means regardless of the operating pressure of the valve. Poppet Valves on the other hand face pressure imbalances due to pressure on one side and only light springs on the other. Hence premature movement of the poppet is possible when the port is pressurised.
Typically the spool is closely ground and matched with the valve body and is made from hardened steel, hardened to around 60 HRC or chrome plated and ground steel. The need for low leakages across the spool over a long service life necessitates the requirement for minimal diametrical clearances (ideally 5-10 μm) while geometrical tolerances of circularity, cylindericity and concentricity are to be exceptionally fine (2 μm). The spool has lands which block the oil passages and circular recesses which permit the flow.
The lands have oil grooves which keep the spool “floating”. Without a hydrostatic oil film, the spool at rest will touch the sleeve causing abrasion, erosion and the creation of debris. If the pressure were to suddenly increase, the spool will be pressed against the bare surface of the sleeve causing metal on metal contact. With the oil grooves, there is a uniform film of oil maintained around the spool with transmits pressure equally around the circumference of the spool causing it to, in effect, float (Figure 5.b). The oil groves are generally square or ‘V’ shaped and about 0.5 - 0.8 mm deep. Square grooves with sharp corners prevent dirt particles from getting stuck in between the spool and the bore.
The number of oil groves per land is completely the designer’s choice. Higher number of groves gives a smoother movement but offers a higher leakage rate across the land which may not be appreciated in certain cases. Land length is also an important feature. Shorter the length of the land, higher is the chance of excessive leakage.
Spool positioning:
Spools that have not been shifted by the actuator(s) have to shift back into their original or dead positions. This is usually done using springs (except in the case of detented valves where the position is obtained by using the actuator).
Spring-centered: In Directional Control Valves with two actuators on opposite sides, spring centered valves are used. These valves have springs on either sides. When one actuator is activated, the spring on the opposite end gains potential energy by being compressed. When the actuator is released, the spring expands, shifting the spool back. Springs on both sides balance each other out to adjust the spool to its centre position.
Spring-offset: Spring-offset in Directional Control Valves is seen in two position valves. There is only one spring present on the opposite side of the actuator that pushes it to the extreme position when the actuator is not active. There may or may not be a centre position in the spool, but it only comes into effect for a brief moment when the spool moves through it.
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