SG Iron as a replacement for Steel in Hydraulic Manifolds

Article Brief.

With new and innovative materials being developed every year for a plethora of applications, maybe it is time we look past the ago old practice of machining hydraulic manifolds in Steel. While aluminium is a popular replacement, the metal is not desireable for applications over 210 bar (3000 psi). Nodular or SG Iron, especially continually cast, provides the ideal replacement for steel in hydraulic manifolds. Superior surface finish, lower power requirements and easier machinability make it the perfect choice for hydraulic manifold users and manufacturers.

Cast Iron is a material with a history stretching over two and a half millennia. The earliest recorded use of cast iron was in China in the 5th century BC and has since been used in almost every aspect of human life from cookware to cylinders, banisters to bridges and ovens to overhead gantry cranes.

The last century saw many significant changes, both in the production and the material grades. One of these developments is the manufacture of continuously cast irons. This process has seen the development of cast iron from the original low alloy grey/flake irons material through to today’s family of irons including the alloyed Ductile (Spheroidal Graphite or Nodular iron), developed in 1943 by Keith Millis, as an engineering material to meet the demands of ever more challenging environments, and compete with and improve on some steels.

Physical Structure of Ductile Iron

Ductile iron is not a single material but is part of a group of materials which can be produced to have a wide range of properties through control of the microstructure. The common defining characteristic of this group of materials is the shape of the graphite. In ductile irons, the graphite is in the form of nodules rather than flakes as it is in grey iron. The sharp shape of the flakes of graphite create stress concentration points within the metal matrix and the rounded shape of the nodules less so, thus inhibiting the creation of cracks and providing the enhanced ductility that gives the alloy its name. The formation of nodules is achieved by the addition of nodulizing elements, most commonly magnesium (note magnesium boils at 1100°C and iron melts at 1500°C) along with the less common Cerium, Tellurium and Yttrium.

The Process of Continuous Casting

1. The main furnace supplies the metal to one of many refractory lined receivers.

There can be more than one receiver that is supplied by the main furnace. Each receiver, with a capacity of about 8 tons, has one die.

2. The bars are bottom poured and pulled through a graphite die

A starter plug is used to start off the drawing process through the die. The die is kept cool by circulating coolant throughout the process. Eventually, the die will be destroyed completely.

3. Pulling rollers draw out and align the bar throughout cooling

Once the bar starts to take shape, pulling rollers will continue the process of drawing out the bar. To maintain quality, the first few lengths are discarded, but not before undergoing quality checks to find out the composition.

4. A cut-off saw notches the bar for cutting

To make the eventual break off accurate and easy, a notch is made into the bar.

5. The bar is broken into smaller bars of by a break-off ram and anvil

A hydraulic ram cracks the bar into easily manageable sizes for storage and shipping.

6. The entire process is controlled electronically via the datacenter

The nerve centre of the entire operation takes feedback from multiple locations to ensure only the finest quality iron is produced.

Properties of Continuous Cast Ductile Iron

Homogenous Structure:

The close grained structure of Ductile Iron gives excellent machinability, good wear resistance and ability to withstand hydraulic or pneumatic pressures.

Strength and Ductility:

SG/Nodular iron is comparable to most low alloy/free cutting steels in this regard.

Reduced Defects:

Freedom from usual defects associated with other production methods/materials.

Thermal conductivity:

Continuous cast SG Iron bars are recommended for applications in which heat dissipation is the priority, since graphite is an excellent heat conductor.

Lower residual stress:

The core of Continuous cast SG Iron bars remains liquid while the periphery is solid at the moment the bar exits the die and cooling system. Hence the bar undergoes a heat treatment, intrinsic to the process, from the inside to the outside. This releases the majority stresses in the Continuous cast SG Iron bars.

Improved dimensional stability:

Continuous cast SG Iron bars has high dimensional stability due to the low residual stresses during the slow unrestricted cooling, and a subsequent annealing process. Ideal for applications where machining is required, and subject to high pressure.

Excellent bearing properties:

There is an excellent dampening of both noise and vibration in flake and nodular due to the graphite in the structure.

Improved Corrosion Resistance:

Continuous cast SG Iron bars offers high resistance to corrosion better than steel and as good in many cases as non-ferrous materials.

Improved wear resistance:

Due to a self-lubricating network of graphite, tool wear resistance is improved.

High Fatigue strength:

The absence of defects, as well the cohesion of the structure compared to sand cast products makes it the ideal material for applications where higher levels of fatigue strength are required.

Machining Ductile Iron

The nodular structure of Ductile Iron makes it extremely consummate for machining providing a more economical solution to regular steel. The machinability of Continuous cast SG Iron bars are improved over other materials due to the presence of microscopic particles of graphite in the structure which act as a lubricant. In addition to this, the homogeneity of its structure and the absence of the abrasive inclusions of sand which are typical in sand casting, improve machinability and therefore prolong the life of tools, drastically cutting costs.

Chip Control

Chip control is another important benefit of Ductile Iron over Steel. While Steel chips are typically stringy, uncontrollable, inconsistent and tangled Ductile and Flake iron chips are fine, compact, consistent and controllable

S355 Steel chips (left) Typical stringy, uncontrollable chip formation. Flake iron chips (middle) fine, compact, consistent and controllable chip formation. Nodular iron chips (right) Consistent and controllable chip formation

Machining Productivity and Production Economy

Metal removal rate (cm³/min) Improved feeds and speeds by 35-50%

Tool Life Improved tool (insert) life up to 60%

Power Consumption (kWh) Torque/Power required reduced by 50%

Lubricant Costs Machining without lubricants and coolants

Excellent Surface Finish

Avoidance of abortive machining costs

Purchasing Cost Economy

SG Iron has a lower density and hence is 12% lighter than comparative steel products. Continuous cast SG Iron bars eliminates need for costly patterns and dies and with no restriction on design or volume changes. This results in lower overall costs and quicker deliveries when compared with alternative production processes. Unlike steel which needs to be rolled (‘bright bars’) to achieve intermediate sizes, there is just one process to produce SG Iron continuous cast bars in any standard size or even shape (eg. semi circular, lobed) needed.

Case Studies

Along with Sandvik Coromant, tests were carried out by United Cast Bar, UK, to check the various parameters under which various grades of SG Iron can outperform steel (S355 and AISI1212). SG Iron grades used were Unibar 400 or GGG40, Unibar 500 or GGG50, and Unibar 600 or GGG60. The results were tabulated as follows.

Required Power

The net power in kilowatts required to machine the same dimensions of SG Iron and Steel. While Unibar 500-7 with ceramic additives needed far more power to machine than steel, the standard grades of 500-7 and 400-15 fared much better than steel using almost half the power.

Specific Cutting force

SG Iron fared much better than steel (1800 Mpa) with almost 21% less force generation for Unibar 500-7 (1350 MPa). Unibar 400-15 required almost half the force of steel with a reduction of 47% in the cutting force at 900 MPa. The annealed nature of the grade has a much lower requirement in cutting forces.

Machinability Rating

Machinability rating of the 1212 Steel used was about 80%. Most SG Iron grades fared much better with Unibar 500 touching the baseline at 100% and the annealed Unibar 400 going up to 160% machinability The rating for Unibar 600 was a lot lower at 30%.

Conclusion

In conclusion we see that not only does continuous cast SG Iron provide all the features of steel, it surpasses most steels in the same areas. Careful selection of the correct grade for the application will result in cost savings not only in terms of material used, but also in overall reduction of overheads and tooling costs.

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

bC India 2013 Exhibition

With the conclusion of bC India on the 8th of February 2013 at the BKC Grounds in Mumbai, it is our pleasure to thank all those who took the time to come and visit our stall from all over India and abroad.

Despite the challenging situation in the hydraulics industry, we have been able to hold an exemplary exhibition showcasing all our hard work and dedication that goes into our products. Besides the high-quality of visitors, the show also proved to be an excellent platform for doing business, with several deals closed and a lot of fresh business prospects generated. We were kept very busy by the sheer volume of visitors to our stall which occupied 36 square meters of exhibition space. 

Below are some of the highlights from the exhibition for those of you who could not attend. Please feel free to contact us for specific pictures.

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. 

Flow Regulators Part 2

Priority Flow Regulators

Bypass valves work very well for single systems where all the flow of the pump is to be fed into one system. But what happens when there is more than one actuator in action? Prioritizing flow for different actuator is a part of its regulation as well. Priority type flow regulators supply oil to systems where it is absolutely essential and then divert excess flow to secondary systems which fall lower on the importance list. The excess oil is supplied regardless of the working pressure of the secondary system.

HIC Style Priority Flow Regulator

The logic element used in Priority type Flow Regulators is a 4 Port element where Port 1 is connected to the inlet of the needle valve. The needle valve outlet is connected to Port 4 whose pressure is sensed in the spring chamber harbouring an 80 psi spring. Port 3 goes off the Priority System while Port 2 is connected to the secondary circuits.

The flow to the priority circuit is set on the needle valve. Oil will flow from the Inlet port across the needle valve to Port 4 of the logic element. From here it goes to Port 3 of the element and out to the priority system.  When the flow from the Inlet to the Priority port creates enough pressure drop to match the spring in the logic element (80 psi) the spool moves backward diverting excess flow to the secondary system.

In this way, the priority system is always fed with the required amount of oil before any supply reaches the secondary system. This is possible provided the basic flow rate of the pump is higher than that which is required by the priority system.

If more flow tries to get past the needle valve to the Priority system, there will be a pressure imbalance across the spool. This pressure imbalance will cause it to shift backwards closing the priority line and further opening the Inlet to the Secondary system. This exercise will continue till the flow rate to the Priority system creates just enough pressure drop to counteract the effect of the compensator spring.

Excavators use priority system to ensure oil is always available for steering.

In case the flow into the Inlet port decreases to a level below what is required in the priority, the pressure drop across the spool will fall below that of the compensator spring. This would cause the spring to push the spool forward completely closing off the Secondary system from receiving any oil and diverting all of the incoming oil to the priority port. This would ensure that regardless of the input flow, the secondary system gets oil only after the priority system is COMPLETELY satisfied.

An increase in load in any of the two systems would eventually reflect in the Inlet pressure. Upon increasing the load in the Priority line, the pressure here rises decreasing the flow rate. The pressure in the line is sensed through the orifice plug in the spring chamber causing a force imbalance. This pushes forward the spool and reduces the flow of oil to the secondary system diverting more oil to the priority line.  This occurs till the pressure drop matches the spring force which brings the spool to a state of equilibrium. Thus the flow is brought back up to normal by the metering effect of the spool. Conversely, a surge in flow from the pump will cause the pressure at the inlet to rise. This will create a pressure imbalance across the spool which will push it backwards against the spring force opening up more flow to the secondary line and reducing the flow to the priority bringing it back down to normal. Thus the priority line will be fed with only as much oil as it needs regardless of the working pressure of either the secondary circuit.

Cartridge Style Priority Flow Regulator

Similarly, in cartridge constructions, a compensating spool senses pressure imbalance caused due to increased or reduced flow to the priority line and meters the radial holes at the secondary line. If the flow falls below the requirement of the priority line, the radial holes at the secondary line close completely directing the entire flow to the priority line.

The pump will work at a pressure which is 80 psi more than the higher of the two system lines (secondary or priority) the bypass line is higher than that in the regulated line then there would be a tendency for the flow to   increase through the regulated line, increasing the flow would increase the pressure difference across the needle valve and cause the compensating spool to meter the regulated line.

Circuit schematic for Priority Flow
Regulators used in Fast-Slow Systems

Priority valves can be used for Rapid Speed-Slow Feed systems where the Priority Line is connected back to the Secondary line via a check valve. During the Rapid Traverse, both lines contribute to the flow providing the actuator with increased speed. A solenoid valve at the Priority Port is opened when Slow Feed is required venting the line to tank and cutting off the flow. The system works on the logic of “bleed-off” flow control where the flow to tank is accurately metered rather than the flow to the system. This system works when the flow rate from the pump is constant and not prone to surges.

Electronic and Proportional Systems

As in the case of any hydraulic system, there is always that extra dimension of electronics that can be added to the valves. Electronic systems coupled with flow regulators open up quite a few avenues for customization of flow patterns depending on feedback from the environment.

Low Flow Systems

Needle valves can be replaced by Proportional Flow Control Valves which give the added advantage of in-process flow control. This application is quite popular amongst agricultural equipment manufacturers who need the flow of pesticides and such controlled depending on the speed of the vehicle. This provides for an even spray. The proportional valve directly controls flow to the Outlet or Priority ports in Bypass and Priority type systems respectively. These systems are apt for low flow applications since proportional valves for higher flows turn out to be more expensive.

High Flow Systems

For systems involving the proportional regulation of higher flows, an ingenious concept of a pilot valve is used. Coupling a proportional pressure reducing valve to a fully balanced Overcentre valve (OSIBD Series) we can regulate flows upto 350 lpm!! The OSIBD Series of valves have a fully balanced piston which senses the load pressure on both sides of the poppet. The poppet is now completely under the influence of the Pilot Pressure which is regulated proportionally by the pressure reducing valve from 0-25 bar. A higher electronic signal to this valve would mean an increase in Pilot Pressure which would push back the poppet further and allow more flow to pass. Since the flow required for the pilot port is minimal, cost reduction can be obtained on the system due to the small size of its proportional components.

HIC Style High-Flow Flow-Regulators using Fully Balanced Overcentre Valves, Proportional Valves and Logic Elements. (Click to enlarge)

Accurate regulation of flow is vital to the safe and efficient operation of hydraulic systems and therefore the machines on which they are used. Each application is different and will demand different control solutions and pressure compensated flow control devises, either mechanically or electronically operated, are designed to offer that accurate, cost effective, reliable and repeatable control.

Link to Flow Regulators Part 1

Flow Regulators Part 1

Processes when allowed to run free unhindered rarely do any good to the designer as well as the user. A lawless system, not unlike a lawless society, can rarely function up to its potential. Checks and balances always have to be maintained to ensure the maximum possible extraction of work from any process. Policemen maintain order in public to ensure that the majority is safe and secure while weeding out the minority that cause problems, traffic lights ensure smooth running of traffic, predators control the population of their prey, sea breezes give costal towns and cities a maritime and pleasant weather while our own moral balance ensure that we don’t veer off course! Each process, in order to be fruitful and productive is gently regulated by employing these checks and balances.

Thankfully, a hydraulic system is more conducive to being regulated and intelligent applications of control points can give an exceedingly precise result as the level of control increases. We have temperature controls, filtration controls, electrical controls, mechanical controls and the most common, pressure and flow controls. While most of the former are either already discussed or will be at a later stage, today we concentrate on the flow control, more specifically, its regulation.

The Control and Regulation of Flow

Flow control boils down to only one particular and very vital concept, the orifice. The orifice controls any and everything about flow in a hydraulic circuit. How much back pressure will be generated in the system due to the flow, what is the speed of the oil flow and what is the basic rate of flow?

Flow control and flow regulation are two separate entities which should not be confused. Where flow control is a predominantly static and crude concept mainly limited to switching flow on and off or setting the flow where parameters do not change by much, flow regulation is a more dynamic exercise with varying entities and effects updated by constantly changing causes. Hence flow can be controlled or regulated by valves as simple as a humble shut off valve or as complicated as a proportional pressure and temperature compensated priority flow regulating valve.

The main reason for using flow regulators is the behavior of fluids passing through the orifice. Fluids flow from an area of higher energy to one of lower energy (height or pressure). Orifices create back pressure upstream due to the fact that all the fluid is not allowed to flow freely. This gives rise to a pressure differential or a pressure drop across the orifice through the restriction creates. Increase in pressure upstream will reduce the pressure drop across the orifice and ultimately the rate of flow passing through it. Increase in pressure downstream will increase the pressure drop and the rate of flow through the orifice. Flow regulators take feedback from the upstream and downstream pressures to regulate orifice size. The size is regulated in such a way that there is always a constant pressure drop across the valve and hence the flow rate remains the same.

Flow regulators come mainly in three basic constructions depending on the system used. There is the simple Restrictive type of flow regulation, the slightly more complicated Bypass type and the Priority type of flow regulators.

Restrictive Flow Regulators

Restrictive type of regulator valves in a hydraulic integrated circuit (HIC) format are made of a needle valve element and a 3 ported pressure compensated orifice regulating logic element. The logic element has compensating spools that shifts backwards and forwards depending on the pressure drop to regulate the flow set on the needle valve element.

HIC Style Restrictive Flow Regulator

The required flow is set across the needle valve. The pressure upstream is sensed at Port 3 of the logic element which pressurizes the spool along with an 80 psi spring. An increased load on the actuator will increase the pressure downstream. This reduces the pressure differential across the valve and lowers the flow rate. The increased downstream pressure is sensed at Port 1 of the logic element which then pushes back the spool closing the radial holes in Port 2. This increases the pressure drop across the valve due to an increased restriction in flow which once again brings the flow rate back up to normal. In this way, the compensating spool always makes sure that there is a pressure drop of 80 psi across the valve which gives us an even flow rate.

Cartridge Style Restrictive Flow Regulator

In the fully cartridge version, the logic element is converted into a pressure sensing spool. An orifice allows for the flow to pass from Port 1 to Port 2. The orifice size is dependent on the adjustable flow range required of the valve. By varying the pressure on the spring by means of an adjuster, the flow required is selected by matching the pressure drop created by the flow to the pressure on the spring. Hence, if an orifice of Dia ‘X’ allows for 3 liters of flow at a pressure drop of 5 bar, a pressure of 5 bar is set on the spring. An increase in pressure at Port 2 will cause an imbalance in the spool pushing it forward towards Port 1 and opening the radial holes further to allow more flow to pass. In this way, the spool meters the flow constantly to provide an even flow rate regardless of pressure.

In addition to the above variations in valve types, we can also add a return check valve for free flow. In the HIC style, it is done by adding a check valve connecting the outlet to the inlet, while a check seat is added in cartridges to permit reverse free flow.

HIC Style Restrictive Flow Regulator with Reverse Check

Cartridge Style Restrictive Flow Regulator with Reverse Check

If most the flow sent out by the pump is consumed in the system, it is well and good. There is not much excess flow that has to be taken care of. But if the flow through the valve is much lower than that of the pump output the inlet pressure will rise to a point where a relief valve upstream cracks open and dumps excess flow to the tank or in the case of a pressure compensating pump, the compensator reduces the output of the pump to match the flow setting of the flow regulator.

Restrictive Flow Regulators in A or B lines need reverse checks. Restrictive Flow Regulators in P lines don't need reverse checks

Bypass Flow Regulators

To overcome the problem of excessive power usage, we use the Bypass type Flow Regulators. In these valves, we have a similar system of a needle valve being used in conjunction with a compensator logic element. This time, however, the logic element is normally closed by the compensator spring and it “tees off” excessive flow to the pump at a minimal pressure drop. Port 1 is connected to the inlet of the needle valve while Port 3 senses the pressure at its outlet. Port 2 bypasses the oil to tank. 

HIC Style Bypass Flow Regulators

During normal working, oil is fed to the system while the rest is bypassed to the tank line at working pressure plus the logic element spring setting (80-100 psi). When the actuator comes on load, the pressure at the actuator end (downstream) starts to increase. The pressure differential across the needle valve is hence reduced. The reduced pressure drop in turn decreases the flow rate through the valve causing the actuator speed to slow down. The increased pressure is sensed at Port 3 of the logic element causing the spool to slowly close the bypass line. This diverts more flow across the needle valve bringing up the flow rate back to normal along with the actuator speed. In this way, the logic element constantly moves forward and back to meter the flow into the cylinder maintaining a constant flow rate regardless of the pressure. A point of caution is that the tank line should have minimum back pressure as any resistance in the bypass line may cause excessive flow being channeled to the actuators.

Inbuilt relief valves save the additional
costs of piping and line bodies

Bypass valves are superior to restrictive type valves because the pump operates at the working pressure of the system (or actuator) plus a nominal 80 psi rather than increase upstream pressure to the one set on the system relief and consume much more power.

As a supplementary system to Bypass type flow regulators, Relief Valves can also be incorporated to allow for pressure relieving. The pressure is sensed in the Outlet line relieving incoming flow to the Bypass line. Popularly used in the case of unidirectional motors who encounter obstructions, the relief valve opens at a pre-set pressure by sensing the load on the outlet. This reduces the cost of additional relief valves downstream providing a compact package while controlling excessive pressure buildup in the system.

Flow Regulators - Part 2