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 upand 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.

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.

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.

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

Modern machines and work equipment are subject to steadily increasing requirements. Ever more functions are needed. On the one hand, machines can be deployed in a greater variety of duties, but on the other hand their development is also more complex. More than ever, solutions for the simplification of complexity are in demand. These have to support a modern control structure and enable flexible, adaptable use. Therewith, the development of a machine should become clearer, more modular and hence simpler.

 The decentralisation of control tasks is one of the most important approaches in finding solutions that comply with the demand for simplification and modularity. It allows the “outplacement” of technology-specific knowledge – such as hydraulic control tasks – to third parties. Thus the strain on the developer’s and designer’s task is relieved, and they can concentrate on the core tasks of the machine – which also has a positive effect on the time-to-market factor.

 The SD7 control for stationary systems with control cabinets, and the MD2 control for mobile hydraulics were developed particularly from this point of view. Sub-processes can run decentrally. For example, SD7 or MD2 can take over the task of controlling the position of an axis “on site”. Thus the central control system is freed of specific control tasks, less computer capacity is needed and less attention has to be paid to control cycle times. The communication with the main control system can take place via analogue or digital signals, or via a field bus.

 The digital MD2 amplification and control module was specifically developed for the control of hydraulic valves under extreme environmental conditions. The use of this module is particularly appropriate for wet conditions, where there are vibrations, extreme temperatures or fluctuating supply voltages. By the robust and compact design, it is excellently suited to mobile applications.

The operation of control electronics has to comply with a range of different demands. It must be easy to adjust and allow tools for quick and easy commissioning. These requirements can be fully met through the digitalisation of the control, using the options of graphic display of the process data flow and the analysis and diagnostics tools.

PASO Operating Software

The units are available purely as amplifiers, for the control of proportional and switching valves or also in combination with control functions. In this way it is possible to simply and rapidly build up a range of controllers such as pressure, volume flow or position controllers. A standard-conforming CANopen and Profibus DPinterface is available for communication in a modern field bus system.

 The modules have digital inputs and outputs as well as powerful PWM outputs for the operation of the solenoid valves. In addition, the digital inputs can process frequency or PWM signals without any problems. Up to four analogue inputs with a maximum resolution of 16 bits are available for processing the analogue process signals.  The allocation of inputs and outputs to each other is variable. This allows optimum utilisation of the existing hardware and guarantees flexible adaptation of the application without any programming knowledge.

Signal Recording

The parameterisation is carried out via a software called PASO, which is very easy and intuitive to use. The connection between computer and SD7, or computer and MD2, takes via well-known and proven USB interfaces. The amplification and control modules have a new concept for illustrating the entire signal process path – from uploading the command value via various additional functions through to the valve flow calculated from these. The user will see at a glance the condition of his control system at any point in time. If needed, the individual process parameters can be displayed on the screen in real time. If the module is used as controller, it is practical in many cases to view for individual or groups of process data at exactly the same time. This is the only way to optimise efficiently deviations from the rule, and hence the overall control behaviour of the system. For this purpose, the SD7 and MD2 are capable of recording and graphically displaying up to four freely selectable process data simultaneously. This oscilloscope function provides the commissioning engineer with the tool to quickly find potential improvements for the controller settings and to visualise the result of the correction in a simple manner. In this way, the parameters can be easily changed and saved without losing the overview. Simply a brilliant matter!

A Hose Burst Valve (also called a Velocity Fuse)With the ascent of the involvement of safety regulating bodies in equipment manufacturers coupled with end users becoming more conscious about the safety features in their equipment in the last few decades, the design departments of OEM companies were in a tizzy to develop the safest and most reliable operating equipment. The International Standards Organization concluded that all excavators would have to function as cranes. This implied that the earth moving equipment would now have to have a provision to hold the load in place as well as have a system in place to prevent failure in case of a hose burst situation.

One out of the many safety concerns were, what happens if the hose connecting the cylinder to the valve was to rupture? Without any failsafe in place, ruptured hoses would allow for the oil pressurized in the cylinder to have a free path to atmosphere. This would bring the load down catastrophically causing massive damage to man and machinery. Since there were not many options to rely on, some Italian companies developed what was called a “Hose Rupture” or a “Hose Burst” valve. It is, in fact, a “Velocity Fuse”.

How does it work?

The simplicity in the design of the HBV is reflected in the crudeness of its function. The metering of the flow through the valve cannot be effectively controlled or even set

The valve functions on one of the fundamental principles of hydraulics, the pressure drop created across an orifice. The term “drop” is used to imply that the pressure is always lesser downstream. More the flow across the orifice, greater is the pressure differential between the inlet and outlet.

The velocity fuse is composed of a body with a central stem that retains a disk (Figure 1). The stem is secured on one side by a locknut while the disk is held in place by a spring. The thickness of the gap between the body and the disk is essential as it is this distance that regulates the flow of oil out of the actuator and prevents failure in case of a hose burst. Radial holes around the circumference provide a path for the oil from one port to another.

The pressure drop relationship to the flow and gap spacing is given in Graph 1 for various sizes of the hose burst valve. The graph is an approximation of the setting to be achieved as the entire system is quite crude and cannot guarantee that the valve will close at the required flow rates.

Graph 1: The relationship between the flow and orifice thickness is shown for Velocity Fuses of various sizes

When the differential force caused by the flow across the gap “T” reaches that exerted by spring, the disk closes and the flow stops. The pressure on Port 2 then holds the disk closed to lock the load in place. This is very similar to blow out of an electrical fuse that melts and closes the connection when there is a spike in the current.

Valve is generally set by adjusting the gap ‘T’ from the graph above. This is a standard procedure setting and although quite easy, is not accurate since variations in valve manufacture govern it mechanics more than standardized graphs. The alternative is to set approximately to the required flow and then iterate the gap using a flow meter to check the flow. The latter method is time consuming although it gives a more accurate result. Moreover, the setting may not be the same on the field as on the test stand as the viscosity of the oil can alter the flow rate. The pressure drop in the opposite direction (PoRt 1 to Port 2) will increase as the setting, and consequently gap T, is reduced. It is best to locate the valve in the cylinder port by using its cartridge form. Else, line mounted (male-male or male-female) bodies can be used.

How safe is the Velocity Fuse?

The Velocity Fuse may not be the best way to do the job, but it is cheap. It has carved out a niche so strong that customers do not want to look past the valve for an alternative. It retains its popularity in Italy although in India it can still be found on some archaic cranes.

The main issue with the Velocity Fuse is that what is needed is the exact definition of the term “Hose Burst”. If it is set for very high flow, small cuts in the hose and small opening in the fittings will not be detected by the fuse and can be dangerous. If set for a very low flow so as to be able to detect these problems, passing heavier flows through the valve will be difficult. Hence judicious methods of flow setting are required.

Another problem with the hose burst is that the load, although held in place, now has no simple means of being lowered back down to safety.

Once the fuse is shut, the only way to open it would be to apply pressure at Port 1 to reduce the pressure differential across the seal.

In case of the hose being ruptured, this would entail that the load can be lowered only after fitting a new hose and completing the hydraulic circuit. There is no way to bring the load down simply by draining the oil to atmosphere.

What are the other options?

In the Overcentre valve the flow is free from Port V to Port C over the check valve, but reverse flow is blocked and can only be opened by the pilot pressure acting on Port P along with the load pressure on Port C.Instead of Velocity fuses to control the load in case of a hose rupture, the safer option is to install an Overcentre valves. These load controlling valves are typically mounted on the cap of a linear actuator or they can be line mounted with rigid piping between the actuator and the valve.

Figure 3: Rigid Piping is used between the valve and the cylinder to prevent accidental hose burst in these lines. The cartridge can even be mounted in a cavity machined in the cylinder.

The cylinder port of the valve is connected to one end of the cylinder while the valve port is connected to the directional control valve (see figure). The pilot port is connected to the oppositeend of the actuator and serves as a pressure feedback. For retraction, the Overcentre valve restricts the flow of oil out of the cap end which results in the pressure rising at the opposite end.

This pressure is sensed at the pilot port of the Overcentre valve which, assisted by the load induced pressure, compresses the spring giving an outlet to the oil from the cap end.

In this way, hose burst protection can be offered safely and securely to almost any hydraulic cylinder, without the use of archaic technology.Since the piping is rigid between the cylinder and the overcentre valve, hose burst can only occur in Line A or between the overcentre valve and the directional control in Line B. In this case, the flow out of the cap end would not change since the load induced pressure would still act in conjunction with the pilot pressure on the spring to allow the load to lower. The only change would be the leakage from the burst hose.

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

The Casting Process

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.

Continuous cast bars

Properties of Ductile Iron produced via Continuous Casting

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

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 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.

The exchangeable coil significantly simplifies the logistics, because the solenoid coil can also be retrofitted. The various alternatives make the proportional screw-in cartridges a very flexible system. Different plug – voltage alternatives are available ex stock and are complemented with respect to individual adaptations – with the customary Wandfluh flexibility. In addition, the performance of the valves has been increased by the improved solenoid coil. Therefore also ambient temperatures of up to 70°C can be accepted without any performance loss. With the improvement of the corrosion protection of the solenoid coil, depending on the version, the valves achieve a salt-spray resistance of over 500h.

Available are valves with the standard cavities M22 and M33 in accordance with ISO 7789. Pressure relief valves are available as direct operated and pilot-operated versions up to volume flows of over 230 l/min and pressures of up to 350 bar. Apart from the optimised proportional throttle for volume flows of up to 63 l/min, the All-In-One valve QSPPM33 (tight seating volume flow controller) with mechanically preset Imin is in the Wandfluh product range. By the preset Imin the series spread of the valves is reduced to a minimum, in order to guarantee a simplification of the valve commissioning.

Typical applications for the qualitatively high-grade proportional screw-in cartridges are:
– Sensitive adjustment of pressure, resp., volume flow
– Speed control of hydro-motors
– Cylinder control – and positioning
– Individual adjustment of clamping – and tensioning force

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