Proportional Control in Road Milling Machines

As developing nations look to increase their infrastructure capabilities and realise the full potential of their natural and human resources, manual labour is giving way to automated and motorised equipment that does work at a quicker pace with better results. Today efficiency isn't measured by cubic feet of material moved per liter of petrol consumed, rather by cubic feet per minute. And with the large number of infrastructure projects planned, a quintessential need is for better roads for their connectivity.

Even as new cities sprout up and the old ones swell in size, there is a tremendous need for efficient and regular maintenance and upkeep of inter-city vehicular highways and intra-city thoroughfares. Road laying equipment is getting more advanced with the advent of proportional hydraulic technology with better and more exact control of operations with minimum human intervention and error.

A Road Milling Machine

One of the interesting concepts is in a road milling machine. This has a large rotating roller equipped with hard metal cutters which mills off the surface, usually at an angle. Milling, or cold planning, is one of those tools that serve a variety of purposes. Milling is often used to remove surface distresses, maintain or correct elevation, restore roadway geometric properties such as cross-slope, and improve surface characteristics. Milling provides a supply of reclaimed asphalt pavement (RAP) that can be recycled into the new surface and generally improves the bond between layers as well.

In the case of these machines up until in the recent past, the milling depth and the angle were set with the help of push buttons by the operator (Up, Down, Left Raise/Lower and Right Raise/Lower) and continually corrected with the visual feedback he received either by observing the operation himself or by being guided by a secondary operator standing outside. This required a high level of concentration from the operator and frequently led to mistakes in the setting which invariably resulted in a less than ideal situation.

Milling Head for a Road Milling Machine

A new solution was implemented where the controlling activity is to be automated with the milling depth and milling angle to be predefined and set and complied with by means of a control system operating on a feedback loop.

Physical Structure

The roller is controlled via two cylinders that either move together to raise or lower it, or independently to vary the milling angle. The cylinders would be controlled via spool valves that are actuated through a PCB that gets its inputs from the operator. Since there is no feedback, the spool valves would usually be of the conventional type.

Schematic for the Physical Structure of the Hydraulic System

Solution Approach

With regards to the roller, the two hydraulic cylinders which set the required milling depth would remain untouched. Replacing the conventional spool valves with Proportional Controlled valve, the oil flow to the cylinder would be metered. This is measured with position control systems that provide feedback to the control electronics. Every axis has its own position control circuit which automatically complies with the predefined milling depth. An offset signal is supplied to the two position control circuits, by which the milling angle can be set.

The variable for the "Milling Depth" is provided with the help of a potentiometer at the "Analogue Input 1" and is the command variable to both position control circuits as an analogue value in the form of a voltage. On the controller card, this is used as the command value for Cylinder 1 and Cylinder 2. 

The variable for the "Milling Angle" is also provided with the help of a potentiometer but at the "Analogue Input 2" and is the analogue command value in the form of a voltage. The controller card adds this to the position command variable for Cylinder 1 and subtracts this from the position command variable for Cylinder 2. Thanks to the flexible parameterisation possibilities, the MD2 digital amplifier enables the addition / subtraction of the two command values (milling depth / milling angle).

Proportional Control Layout Showing Feedback Loop

The position of the two cylinders is provided at "Analogue Input 3" and "Analogue Input 4" as an analogue value in the form of a voltage supplied by the two position control measuring systems “Feedback value 1” and “Feedback value 2”. The predefined position of the cylinders is complied with automatically by the control system and corrected in case of any deviation. 

MD2 Amplifier Module

With the MD2 amplifier module, two channels are used internally. Both channels are operated in the controller mode “Axis Position Controlled”. Channel 1 controls Cylinder 1 through the solenoids A and B while Channel 2 controls Cylinder 2 through the solenoids C and D.

In case of both channels, the value of "Analogue Input 1" is used as the Command value 1 but the value of "Analogue Input 2" as command value 2 is used as-is for Channel 1 and inverted for Channel 2.

Customer benefit

The proportional system brings in a new dimension to the equipment and is easily plugged in to the existing framework of the machine. Road Milling is of course only one of the many uses for this particular layout which finds a place in any system requiring tandem as well as independent cylinder control with feedback looping. In conclusion, the benefits of proportional technology in construction equipment are varied and far reaching with 

• Automated, economical working sequences
• Low effort for the implementation
• Solution from a single hand

Screw-in cartridges for Higher Volume flows in Hydraulics

The constructional possibilities of modern machine engineering lead to ever larger and higher performance machines in the most diverse industrial fields. Movements have to be carried out more rapidly and efficiently. Part of this is also that machines and devices should provide more possibilities regarding functionality, while, if poossible, they should not increase in weight. It must be the objective of the designer to achieve functionality and nonetheless use materials economically, in order to keep the investments within limits and to maintain the necessary drive energy as low as possible. Energy consumption always means financial expenditure. Finally, for the investor the „Return on Investment“ has to be right.

In the case of large hydraulic consumers the requirement of hydraulic liquid is correspondingly high. The valves must be of the suitable size, so that the volume can flow through the various bores and control openings in the hydraulic control unit and in the valves with as little energy loss as possible. For the operator of the plant energy loss always entails costs. It is possible, that a certain energy loss is accepted, so that a certain task can be carried out more rapidly. If the hourly rate of the machine is higher than the costs of the energy loss, this is an advantage for the operator. Nonetheless the energy loss has to be kept within limits.

The requirement for a compact construction in mobile tools is increasing. For this reason, the Wandfluh company has complemented the screw-in valve range in the ISO-sizes M33 and M42 with a spool valve cartridge.

The construction of valve functions in blocks made of steel or aluminium enables an individual adaptation to the available space, without having to accept losses in the flexibility of the functions. The well proven screw-in technology here provides the possibility of constructing flexible solutions adapted to the application. Because of the internal connections between the individual valves, the block construction has the advantage of less leakage points in comparison with the solution of aligned and flanged valve elements. This block construction simplifies the decentralisation of the control functions, as a result the designer can place them in the machine exactly where they are required.

The block construction provides the designer with a lot of flexibility in selecting the suitable valve size per function. With this, on the one hand a technical solution precisely adapted to the requirements is achieved, and on the other hand it is possible to optimise the space requirements and the costs.

The new spool valve cartridges correspond to the requirements defined at the beginning, namely to design the systems as efficient and economical. In addition, part of the valve range of the sizes M33 and M42 in addition are pilot operated pressure relief valves and pressure control valves with manual as well as proportional adjustment, manual and proportional throttles as well as pressure-compensated flow control valves in 2-way and 3-way versions, also with manual as well as proportional adjustment.

For applications, where a load has to be securely held, pilot operated, leakage-free poppet valves are available. For demanding load holding and lowering functions, tight-seating, proportionally adjustable and load–compensated flow control valves have proved successful. The large selection of screw-in valves is designed for operating pressures up to 420bar. Corresponding to the size of these valves, depending on the valve type, volume flows of up to 400l/min can be controlled. A very good controllability of the screw-in valves in the fine range is the prerequisite for operating the system of the customer with high precision and the demanded efficiency.

The type screw-in spool valve does not concern an independent element, it has to be externally pilot operated with a hydraulic pressure. Therefore additionally driving elements or pilot operation valves are required. For the 4/3-way function (4 connections, 3 switching positions) no bores are foreseen in the ISO cavity system. For this reason, Wandfluh has elaborated a proprietary cavity type. The size of the valve cartridge is determined by the size of the volume flow and the required lateral bores for the inlet – and outlet tubes.

The pilot operated valve can be arranged directly at the spool valve cartridge, but does not have to be. The pilot control can be located anywhere and connected to the control block with small control lines.

Various valve types are suitable for the pilot operation. In case of the example of a crane it is a small pressure reducing valve, with which the control pressure can be steplessly controlled in the range of up to 20bar. Following the control pressure the piston in the spool valve cartridge moves against the spring force. The more control pressure is acting on the piston, the more the valve opens and the flow changes. Corresponding to the volume flow and the system pressure the hoist motor rotates.

The small pressure reducing valve can be directly operated manually with a hydraulic joystick or, equipped with a suitable solenoid, proportionally remote-controlled by electronics. From a control point thus the operator can hydraulically trigger a function from a distance or remote-control it with an electronic system by a computer or joystick. When an operator remote-controls the valve, he can see, what is happening at the winch and implement corrections when needed. The winch system then operates in a closed circuit, controlled control circuit. Without visual contact of the operator the winch would operate in an open control circuit, undesirable movements and incidents would then be possible.

In other applications the pilot operating valve can be replaced by an on/off valve with fixed control pressure relief, then the spool valve cartridge also has an on/off function.

The driving with a pressure limitation is also possible, but is not efficient, because control oil continuously flows to the tank and correspondingly energy is lost.

For the spool valve cartridges, pistons for two switching positions (4/-2-way) and three switching positions (4/3-way) are available. In case of two switching positions, only one control oil connection is under pressure. The other control oil connection has to be connected with the tank, otherwise the piston does not switch. In case of three switching positions both connections are supplied with control oil.

The spool valve cartridge is foreseen for high volume flows, M33 up to 150 l/min, M42 up to 250 l/min. In order to master the high flow forces and to assure a stable functioning, a high setting force of the pilot control and a correspondingly strong return force of the spring are necessary. The spring is designed for an unlimited continual strength in case of the foreseen operating path. The permissible system pressure is indicated as 315bar.

For the valve manufacturer the particularity of the large cartridges is the long, thin-walled valve components, which call for a high degree of experienced in their manufacture.

Thanks to the embedded installation, the risk of leakage towards the outside is very low with the compact block construction with screw-in cartridges in comparison with screwed-on valves. Because only the screw-in part projects from the block in case of the spool valve cartridge, the surface protection is easy to ensure. The valves are zinc coated and therefore satisfy the general requirements, they can additionally, together with the block, be provided with a colour coating. Also an enhanced surface protection such as a Zn/Ni-coating or external components protected against rust is conceivable.

With the spool valve cartridges in particular and with the other screw-in valve types from the Wandfluh programme, the block-builder has acomprehensive selection of valves at his disposal, which enables a compactness of design of the hydraulic control blocks, which has never been known previously.

by Paul Häberli, Wandfluh AG, Frutigen, Switzerland

Paul Häberli, since 1988 design engineer at the head office of Wandfluh AG, Hydraulics + Electronics, Frutigen, Switzerland

Digital Mobile Amplifier Module (MD2)

The digital amplifier module MD2 is characterised by its robust and compact design. As a result, it is excellently suitable for applications in the mobile field. The module is available as basic - and as enhanced amplifier. Both versions of the MD2 do not differ in their construction. The main differing characteristics are the number of solenoid outputs, the number of digital inputs as well as the number and the resolution of the analogue inputs. The basic amplifier has 4 solenoid outputs, 2 digital – and 2 analogue inputs (10-bit). The enhanced amplifier has 8 solenoid outputs, 4 digital – and 4 analogue inputs (16-bit). In addition, in the case of both versions 2 digital outputs are available. 

Further characteristics of the MD2 are the high number of solenoid outputs, which can be used not only for driving proportional solenoids, and the broad voltage range, which enables the driving of 12 VDC- and 24 VDC devices.

Customer-specific requirements are easily implemented.

The parameterisation takes place through a USB interface by means of the menu-controlled, user-friendly parameterisation and diagnostics software PASO. This provides some new features. Every channel can be called-up in a window. In doing so, the whole signal flow from the input to the solenoid output with all adjustable and set parameters is evident. In this manner, the parameters can be changed and stored in memory in a simple way. 

NG4-Mini Proportional Valve

Proportional directional control valve
with integrated spool position control 
NG4-Mini from Wandfluh AG

Direct controlled proportional directional control valve with integrated amplifier electronics and spool position control in flange construction NG4-Mini. The valve possesses an integrated position control. With the spool position sensor (LVDT), the actual position of the valve spool is continuously recorded and brought into line with the set-point value transmitted in an analogue manner. Apart from an analogue interface the valve is also available with a field bus interface (CANopen or Profibus DP). The parameterisation takes place through a USB- interface by means of a menu-controlled parametrisation- and diagnostics software. The data are stored in a non-volatile memory. Settings once elaborated can be reproduced and transferred without any problem, also following an electric power failure.


Model Code: BRW.4
♦ Flange construction NG4-Mini
♦ Operating pressure pmax = 315 bar
♦ Maximum volume flow Qmax = 20 l/min
♦ Volume flow levels: QN = 4/8 l/min
♦ Nominal voltage 24 VDC
♦ With integrated spool position sensor (LVDT)
♦ With integrated amplifier electronics (DSV)
♦ Protection class IP 67 

Advantages of the spool position control (LVDT)
♦ Minimal hysteresis
♦ Improved dynamic characteristics

Advantages of the integrated amplifier electronics (DSV)
♦ Intelligent
♦ Compact
♦ Plug & Play

Applications
► Both in industrial - as well as in mobile hydraulics
► Where a high resolution, minimal hysteresis and very good dynamic characteristics are of concern
► Adjustment of the rotor blades of wind power generators
► Machine tool - and paper production machines
► In case of position control systems
► Forestry - and earth moving machines
► Robotics


Further Information
You will find further technical information on the data sheet <<click here>> or on our website. We will be happy to advise you in the selection of the suitable components for your application.

Courtesy: Jürg Schneider, Wandfluh AG.

Directional Control Valves - Part 3

More on Coils

Solenoids are devices that are capable of changing electrical energy into mechanical, or linear, energy. The simplest type of solenoids assemblies used in hydraulic valve applications relies on two main aspects for their function: an insulated (or enamelled) wire, shaped into a tight coil, and an armature of varying designs also colloquially called the “iron core” which contains a solid rod or pin of either iron or steel. The iron or steel pin is ferromagnetic, a property that allows it, when exposed to electrical current, to function as an electromagnet. The solenoid uses the magnetic field created from an electrical current as the trigger for the production of a push or pull that drives mechanical action into the pin. Solenoids that rely on electrical current fall into two main categories-- solenoids that rely on AC (alternating current) as the source of power and solenoids that rely on DC (direct current) as the power source. 

While AC and DC solenoids use different types of current, they both work in the same basic manner. When the insulated, coiled wire of the solenoid receives electrical current, the magnetic field produced strongly attracts the pin which pushes the spool inside the valve to change the flow path (“on”). The spool is attached to a compression spring on the opposite side which is compressed till the current is stopped. When the current is turned off, the compressed spring forcefully snaps the pin and spool back into its original position (“off”).

Armature Design:

Two common designs for solenoids are (a) the air gap design; and (b) wet armature design. In the air gap design, the two sections (armature and spool) are isolated from each other using dynamic seals.The downside of this design is that the seal wears off eventually causing leakages from the valve into the armature. The wet armature design is more common with the entire armature assembly submerged in oil from the valve. The solenoid magnetically moves the armature while it is submerged in oil. This design provides a lower leakage due to the absence of wearable dynamic seals. The only sealing in wet armature designs is the O-ring that seals the threaded connection between the iron core and the body and, if present, the one on the manual over-ride push pin which, although a dynamic seal, only acts when the valve has to be manually reset once in a while. (Figure 16) Another advantage of this design is that the movement is naturally dampened by the presence of oil. This provides a smoother, more quite movement with a longer service life. The viscosity of

the oil, however, means that the wet armature design needs 60% more power to actuate. 

Voltage Selection:

There are four main types of voltages used with solenoid valves: 12 VDC, 24 VDC, 110-115 VAC and 220-230 VAC. Although the 115 VAC finds hardly any usage outside North America, the remaining three are quite common in the rest of the world. Now, though the type of current (Direct or Alternating) is the prerogative of the designer, the voltage rating of the solenoids entirely depend on the power required to shift the spool. 

Both the categories (AC and DC) have their own set of advantages and disadvantages. DC solenoids are quieter and require less maintenance than AC coils. On the other hand, they function more slowly than AC  solenoids and are also less powerful than AC solenoids. In AC solenoids, the current that runs through the solenoid starts with a first rush of extremely strong current, then drops to a lower, normal level as the solenoid gap reduces. Thus if the spool gets stuck in the open (full-current) position for too long in the body due to contamination particles in the land areas, it receives too much of this first wave of maximum current and it can permanently damage the device by allowing the coil to burn. By contrast, DC solenoids experience no alteration in currents and do not run the risk of being damaged by the current. Off late, all solenoids are made to function on DC voltage. An AC Solenoid would have a DC Coil rated to a voltage close to the required input AC voltage (195 VDC coil for a 220 VAC input) and a rectifier plug that would convert the incoming AC current into DC (Figure 17). Although DC circuits can be utilized with AC solenoids without a problem, DC solenoids cannot be used on other circuits without becoming noisy and overheated and possibly burning out due to excess current. 

Due to the inherent problems with AC Solenoids, a lot of Original Equipment Manufacturers are rethinking their designs and opting for DC Solenoid Valves. Using the same voltages that are predominantly used in their equipment’s PLCs, it is less of a hassle giving longer service lives free from maintenance issues. 

Conclusion

In conclusion, the selection of a proper directional control valve is as important as its use in the circuit. The above article will help new initiates in hydraulics to select a proper valve for their circuit while refreshing the fundamentals of people already well versed in hydraulics. It’s best to check requirements of the system and decide the best valve for the job. Some of the circuits are highly traditional and outdated and need a major revamp on spool and solenoid selection. Some applications become a lot easier with the help of poppet valves instead of sliding spool valves.

Click Here for Directional Control Valves - Part 1
Click Here for Directional Control Valves - Part 2

Directional Control Valves - Part 2

Spool Selection:

Spools in Directional Control Valves come in various configurations depending on the applications they are required for. The extreme positions usually have the same configuration where P is connected to A and B is connected to T at one extreme and vice versa for the opposite side. Some of the more common spool types are listed below with examples of some of the applications they serve.

Spools for 2-Position Valves:

2-Position spool valves have only one actuator and the spools are usually spring offset. Although they have all the four ports ( P, T, A and B) on their interface, they may or may not necessarily use all of these ports. All 2-position spools can either be normally-open or normally-closed.

This is a misnomer, however. It is not necessary for the valve to be “closed” or “open” it may just be redirecting flow to a different direction. It simply means that the solenoid and the spring switch positions so the neutral position changes.

On-Off Spool: 

 On-Off spools are the most basic of all spools. They allow flow to pass in one position and restrict flow path in the second position. If the neutral position allows for flow, the valve is called “normally-open” and if flow is restricted in the neutral position, the valve is “normally closed”. The valve is most commonly used for switching on or switching off flows to different sub circuits or as a main on-off valve. It can also be used as a manual drain valve to unload pumps or accumulators. It should be noted that since the valve is of the spool type, it may have certain leakages. For simple on-off valves, it is more plausible to use Solenoid Poppet valves.

Selector Spools (Exhaust Spool): 

Selector Spools are used when one inlet has to feed two outlets depending upon the actuation. Depending upon the connections made to the various ports of the valves, they are also called Exhaust Spools. Exhaust spools are used with Pressure Gauges or Hydraulic Clutches or Parking Brakes or any other accessory that needs to be referenced to tank when not in use. For a normally closed valve, when actuated, the pressure line is connected to the accessory at the outlet; let us say it is a pressure gauge, which gives us the pressure reading. On releasing the actuator, the pressure gauge is reference back to tank, which is connected to the third port, and hence shows zero pressure. For a normally open valve, in the neutral position, the valve is referenced to tank, which, in the case of Parking Brakes, keeps them engaged. Using a hydraulic pressure line as an actuator, the pressure in the two lines of the motor is sensed and opens the valve allowing the pressure line to be connected to the parking brake cylinder, releasing it.

Parallel and Cross Spool: 

Parallel and Cross spools are spools that connect P to A and B to T in the parallel position and vice versa in the cross position. Parallel and cross- positions are more commonly the extreme positions in 3 position valves. These are versatile spools that can be used for any of the applications above simply by blocking the port not required on the subplate.

Spools for 3-Position Valves:

3-Position spool valves have two actuators and the spools are usually spring centered (although they can be detented). They usually use all the four ports ( P, T, A and B) on their interface.

Closed Spool (Cylinder Spool): 

Closed spools have their ports isolated from each other in the central position. The lands are fairly large which keeps leakages down to a minimum. This spool is also the easiest to manufacture. These valves are not to be used as load holding valves in the puritan sense since a certain amount of leakage can expected in this configuration (5 to 10 cc/ min from A and B to T). The transition positions can be either closed as well or completely open.

In the central position, pump flow has no recourse to the tank line due to the blocked position. Hence the pump either needs to be pressure-compensated where the system pressure is at the pump compensator setting until all pump flow is going to the actuators at their working pressures or unloaded to tank either with a relief valve which will relieve it at the set pressure or an unloading valve which unloads the valve at a minimum pressure and conserves power and electricity. The spool is used in cylinders which cannot be drained to tank and need to be kept pressurized (but only for a short while) and the P port is needed in a secondary system to perform other operations.

Float Spool (Motor Spool): 

Float spool valves have their A and B ports connected to T in the central position. Again in this spool, the lands are sufficiently long to prevent massive leakages between ports. The valves are used to relieve pressures at the A and B ports while keeping the P port isolated in order to service a secondary system or be separately unloaded to tank. The valve is commonly used with hydraulic motors (hence the colloquial term “motor spool”). This is because when the oil supply to motor is cut off, the momentum of the motor keeps it rotating. Oil is picked up from the inlet port due to rotational inertia and is deposited to the outlet port. Hence, instead of the system driving the motor, the motor drives the system. 

Connecting A and B to T serve two purposes; the first is any spike at the outlet due to excess oil being deposited is prevented since it connects to tank (unlike in blocked centre where the oil would have nowhere to go). The second is that the inlet has a direct line to tank in the case oil is required in the prevention of cavitation which may permanently damage the pump.

Another common use of this spool is with single or double Pilot Operated Check Valves or Overcentre Valves. These valves, which serve as means to lock cylinders in place, need their downstream ports connected to tank. For Pilot Check Valves this is essential since any pressure entrained in the line may serve as a pilot pressure and open the pilot check valve in the opposite line. 

For Overcentre Valves, they act as load holding valves as well as thermal relief valves. Not only can a wrong pilot signal be given to the opposite line as in the case of Pilot Check Valves, but the quick draining of the Pilot port as well as the Valve port makes it easy for the poppet to sit quickly and give a leak free performance.Also in the event of a pressure spike caused by an external load or temperature increase, the relieved oil needs to be given a direct pathway to tank.system. Connecting A and B to T serve two purposes; the first is any spike at the outlet due to excess oil being deposited is prevented since it connects to tank (unlike in blocked centre where the oil would have nowhere to go). The second is that the inlet has a direct line to tank in the case oil is required in the prevention of cavitation which may permanently damage the pump.

Open Centre Spool: 

Open Centre spools have all ports open to each other in the central position. However, due to the smaller land width that needs to be maintained in order to allow flow, in the actuated positions, cross port leakages are highly probable.

One of the advantage of this spool is the pump directly unloads to tank. A circuit normally using a fixed-volume pump is used in conjunction with this type of valve. Ideally, the open center allows all of the pump’s flow return to tank with little or no back-pressure. This saves energy and reduces heat to the point that a heat exchanger is not necessary on most circuits. But the advantages end there.

 These spools find their use in applications where the valves are not taken up to the maximum flow and the cylinder is horizontal. Since the pump is constantly unloads to tank in the central position, clogging of the filter and restrictions in the line are possible which may lead to higher back pressures. Using this spool for vertical cylinder is impossible without holding valves as the ports are connected to tank and would bring the cylinder down. Even the use of Pilot Operated Check Valves or Overcentre Valves is debatable as the Pilot port may end up being pressurized due to restrictions in the tank line. 

Furthermore, even in horizontal cylinders, the restrictions in the tank line would put pressure on both the sides of cylinder (i.e. the head end as well as the rod end). Ultimately, in the neutral position the cylinder would gradually extend; a funny but true situation! This is because, the head end, having a greater area, would create a greater force forward for the same pressure as would the rod end. With this force imbalance, the cylinder would end up extending even in the central position!

These spools are also extensively used in vehicles where any external load, such as a pull or a push, can be adjusted without any cavitation. Equipment such as front-end loaders when they need to crawl without load or road paving machines where asphalt has to be laid in an even fashion find use of Open Centre Spools. In Front End Loaders or Side Dump Loaders needing to push material on the ground, the cylinder has to be biased to extend without there being any real pressure from the pump. At the same time, an obstacle such as a bump on the road causing the implement to get stuck may cause severe damage to the system. With Open Centre Spools, the cylinder is allowed to follow the contour of the path without being too rigid with minimal effort on the part of the pump thus reducing the power needed as well as protecting the implement and cylinders. 

Tandem Spool:

Tandem Spools connect the P port to the T port in the neutral position while blocking the A and B ports. Flow is directed from the P to the T port, ideally at a low pressure. Since there is no Use of the pump in the neutral position, two or more Tandem Centre valves can be connected in series to be sequentially operated. This may incite a lot of designers to opt for series connected Tandem Centre Valves, but beware! A circuit may look good on paper, but can run hot because of wasted energy. The spool is hollow, and ports P and T have cross drilled holes. This substantially increases the pressure drop of the valve for oil flowing from P to T. Because of the higher pressure drop most manufacturers’ catalogs show a lower nominal flow or higher pressure drop curve for tandem-center valves. Connecting these valves in series will only amplify the pressure drop since it is additive. Two or three valves connected in series could have pressure drops as high as 20-30 bar for large flows!

Tandem Centre Spools can beconnected in series to actuate systems one after the other using the same pump. The back pressure at T will be the cumulative back pressures of Valves 1, 2, 3 and 4

Another cause for concern is when the second spool is in operation, the load on the second P port is felt on the tank line of the first T port. In many Directional Control Valves, the tank line pressure is limited to 140 or 210 bar while the load on the P, A and B ports could be as high as 350 bar. This may cause damage to the valve if it is not used in a sensibly designed circuit.

Pressure Spool:

Pressure Spools have the P line connected to A and B to allow the actuator ports to remain pressurized in the neutral position. Equal pressures on both the actuator ports would not affect rotary actuators such as hydraulic motors. Liner actuators, however, would extend in the neutral
position due to unequal forces unless double rod cylinders are used. 

Although not very commonly seen in regular practice, they are used in machine tools with dual clamping systems where single acting cylinders are used. Here either one of the clamps has to be engaged with the work piece when it is being fed into the machine. While the operation is being performed, both clamps need to be engaged with the work piece for better stability.

Regenerative Spool:

Regenerative Spools have the P line connected to T in the center or neutral position to allow the pump to unload into the tank. In one extreme position, the A and B ports are connected to the P Line. This allows the flow coming out of the rod end of the cylinder (the ‘B’ line in this case) to feed oil into the cap end of the cylinder. This type of a system is quote common in hydraulics and is referred to as a “Regenerative Circuit”. 

(Left) Regeneration using two check valves. (Middle) Regeneration using a sequence valve and a check valve 

(Right) Regeneration using regenerative directional control valves.

Regenerative Circuits are used to save on the cost of the pump since oil is reused and a smaller capacity pump can be used for the same circuit (In cases of cylinders with 1:2 area ratios, the pump capacity will be slighter greater than half the original size required). Although these spools are slightly more expensive than the other spools mentioned earlier, there are huge savings on external valves to create the same regenerative circuit.

In the case shown in the spool allows the actuator ports to connect in the retract position. Cylinders would extend quicker than in other valves in this position due to outgoing flow being put back into the cap side. Although not very commonly seen in regular practice due to unawareness, they are used in many applications where the force of the cylinder is not in question but simply the speed. This is because pressurizing both ends of the cylinder would  mean a resultant extension, albeit reduced, force due to the pressure on the rod end acting against the pressure on the cap end. As in all cases, the regenerative spool also has its pro’s and cons!

For a view at our full range of Directional Control Valves, click here.

Click Here for Directional Control Valves - Part 1 
Click Here for Directional Control Valves - Part 3

Directional Control Valves - Part 1

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.

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).

Detented Valves: Some Directional Control Valves with manual actuation hold the valve in a particular position using a detent mechanism. These valves have notches on their spools. Using spring loaded pins the spool can be held in place by pushing the pin into a notch. Upon releasing the actuation mechanism, the spool does not shift back until the actuator is made to change the position. Shifting in detented valves is slightly jerky since the pin has to be forced out of the notch to change positions.

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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