Servohydraulics keeps railway testing on track

Nencki’s new Evolution test stand is designed for the final inspection and quality control of new, overhauled and repaired railway bogies. An innovative hydraulic system applies extremely precise loads to a bogie to simulate different vehicle weights, forces and load distribution.

Accurate load tests rely on compact,quiet and efficient servo-axes.

Andreas Schnurrenberger | Global Sales Manager Industrial SystemsBucher Hydraulics | Neuheim, Switzerland

Using energy more efficiently and reducing total operating costs are worthy and ambitious goals. But substantial improvements can seldom beachieved by simply changing components. Innovative hydraulic systems increasinglyrequire new design approaches, often with software, control systems and networkingto Industry 4.0 levels, to succeed in today’s ever-changing markets.

Conventional hydraulics remains the standard in numerous applications. In particular, throttling control systems are still frequently used despite their high losses. Due to their operating principle—using throttling to control the supply of energy—they offer enormous potential for improving energy efficiency. Often, the better solution is based on the concept, “Pump control—Power on demand,” where only as much pressure and flow is generated to meet a drive’s immediate needs.

Instead of a central power unit that involves long pipe runs, the drives are now arranged in a decentralized fashion, right at the actuators. Pipeline losses, installation space, as well as installation time and costs are therefore significantly reduced.

Both for new projects and for enhancing existing machines, the changeover from inefficient throttling control to a servohydraulic pump-control system can substantially reduce energy consumption, result in sizeable cost savings, and offer a comprehensively intelligent system architecture that is Industry 4.0-compatible.

Implementing these quiet, compact, self-contained axes, however, requires expertise in various disciplines. Based on their extensive experience and combined know-how, Bucher Hydraulics together with controls supplier Jetter AG, Ludwigsburg, Germany, have developed a cross-technology system that offers new and obvious benefits for the user.

A bogie test rig is a typical application for linear-drive technology involving control of force, displacement and speed, and which requires high power density and robustness. Conventional hydraulics in the form of throttling control is still often used for this purpose, but servohydraulic axes offer substantial energy, cost and space savings.

Bogie testing

One example of an innovation of this type is the direct servohydraulic drive for two double-rod cylinders. Engineers from Bucher Hydraulics have incorporated this power-on-demand concept in a bogie test rig for the Railway Engineering Div. of Nencki AG, Langenthal, Switzerland, an important supplier to railway markets worldwide.

A bogie, more commonly called a railroad truck in North America, is the structure underneath a railway vehicle (such as a boxcar or passenger coach) that contains the wheels, axles, suspension and other components. Bogies—typically two per car—support and guide the vehicle body, provide stability, and minimize transmission of impact loads and vibrations due to irregularities in the track.

Nencki’s newest offering, the Evolution bogie test stand, was designed for the final inspection and quality control of new, overhauled and repaired bogies. The test rig operates according to DIN Standard 250437 and is based on long-standing experience developing, together with its customers, more than 100 previous testing machines.

Accurate tests and adjustments of wheel loads and bogie geometry are vital to optimize travel comfort and minimize the wear and operating costs of rail and rolling stock. An exact check is also imperative to avoid derailing accidents. For example, it is critical for wheel loads to be adjusted correctly to withstand lateral forces in curves or under strong side winds.

Key to success is an innovative hydraulic system that applies extremely precise loads to a bogie. The rig’s hydraulic load cylinders simulate different vehicle weights and forces for the fully automatic testing of wheel loads and load distribution. It also ensures high stability for accurate measurements of axle distances and parallelism, wheel concentricity and diameter, deflection and other parameters. The data are logged, are permanently traceable and can be transferred to a higher-level system. The test rig also automatically calculates the shim plates needed to correct for differences in wheel loads, by adjusting the secondary spring height.

According to Hans Borer, technical director at Nencki, cooperation with Bucher Hydraulics was essential. “We needed a reliable partner with experience in hybrid technology, who knew how to combine the implementation of the highly energyefficient servodrives with hydraulics. Bucher Hydraulics identified what we needed to happen after the components were delivered, highlighted opportunities and, in close cooperation with us, developed a professional, state-of-the-art solution. We also consider it important that this new system is capable of being enhanced and optimized to meet future requirements in the areas of efficiency and costs.”

Bucher Hydraulics designed the self-contained servohydraulic drive that mounts directly on the test rig structure. Position and force are controlled by means of displacement and force sensors together with application-optimized software on the subsystem controller, which communicates with a higher level industrial computer via fieldbus.

Servohydraulic axis

The bogie test rig is a typical application of linear drive technology involving control of force, displacement and speed. Previous Nencki designs relied on a conventional throttling-control hydraulic system, consisting of a power unit with an electric-motor/pump assembly, cylinders and proportional valves.

The new, self-contained servohydraulic axis forms one module in a building-block system. It includes an application-specific cylinder, variable speed QXM 4-quadrant internal gear pump/motor, equalizing tank, valve block (with safety functions), and a precharge and filling unit. Other critical components include a servomotor with a standard Jetter servo controller, a subsystem controller with application-optimized motion-control software, pressure and temperature sensors, and a stroke-measurement system.

While the detailed design is driven by the particular requirements of this application, it is based on proven Bucher Hydraulics and Jetter components. This makes it possible to achieve high-performance yet economical solutions, even for moderatesize production quantities. In addition to use in test rigs, other suitable applications include forming machines, simulators, oscillating axes, packing machines and conveyor systems, lifting devices, and cutting machines for the food sector.

Benchmark technical data for the modular servo-axis include nominal force output up to 1,000 kN; stroke length up to 1,500 mm; working speed up to 100 mm/sec and rapid-traverse speed up to 400 mm/sec; and drive power up to 30 kW. Positioning accuracy depends on the measuring system and the required working speed. Ambient operating temperature is from 0° to 40° C.

Notable benefits

Servohydraulic axes combine the advantages of an electrical servodrive with those of a robust, high power density and efficient hydraulic drive. At the same time, the system is overload-proof and precise. By switching to servohydraulic axes, users can reduce energy consumption by up to 70%. Conventional throttling control systems have two significant drawbacks. The system’s intrinsic throttling losses have a negative impact in themselves, and the need to remove the resulting heat can increase system complexity and cost. Typical power units, together with cooling equipment, can therefore reach a considerable size.

Hans Borer noted that bogie test rig’s previous power unit had a 250 liter volume. “Thanks to the new axis, just 30 liters of oil is required for the hydraulic components today. In addition, the unit had to be freely accessible from all sides, and its footprint has now been eliminated,” he said.

For large installations in other markets, benefits can be even greater. For example, in six-axis hexapod flight simulators, the power unit for a conventional hydraulic system can easily fill a separate room. In applications of this type, Bucher Hydraulics has substantially lowered energy demand and reduced oil requirements by about 90%, because only the actual pendulum volume must be supplied with oil.

Reducing energy demand, oil volume, and floor-space requirements are by no means the only advantages over a conventional hydraulic system. Rather, it is an overall consideration of the system, including noise levels, installation and commissioning work, operating costs, user know-how required, as well as operating safety and the system’s degree of future-proofing.

Summarizing the benefits of the Bucher Hydraulics smart system architecture, Borer said, “We are seeing better energy efficiency and optimized life-cycle cost. Acoustically, the new system is in a different league from the unpleasant sound of the old hydraulic or ball-screw drives.” There are additional technical improvements, he continued. “We can run up to the specified loads with greater accuracy and keep them within the resolution range of the load cells. The quality of the control is similar to the position or force control of an electromechanical servodrive, but without the frictional losses.”

Servohydraulic axes are an integral part of the bogie test rig. They eliminate the central hydraulic power unit and long hydraulic lines to the cylinders, and reduce overall space requirements.

These features make servohydraulic axes suitable not only for test rigs, such as in the Nencki installation, but also for presses where long life is a very important factor; for injectionmolding machines, where the bulk of the product manufacturing cost consists of energy costs, which means that Total Cost of Ownership plays a prominent role; and in lifting systems which need precision movement under large loads and with the lowest possible level of jerks and oscillations.

From fieldbus to Industry 4.0

Merging the technology worlds of hydraulics and automation gives rise to intelligent systems that, thanks to their system design and simplified integration into the overall plant concept, provides new and significant advantages. Unlike electromechanical servodrives, servohydraulic actuators offer the advantages of a hydraulic drive. For example, while a classic worm-gear transmission with inherent frictional losses has a maximum efficiency of 50% in the bogie test rig, the servohydraulic transmission usually achieves an efficiency of around 80%.

Another plus is that the servohydraulic axis acts essentially like an electromechanical drive, even though a hydraulic transmission behaves fundamentally differently from a mechanical one. The plug-and-play system from Bucher Hydraulics includes a subsystem controller and software (firmware) developed specifically for the application, and with hydraulic characteristics already programmed in.

The subsystem controller communicates via fieldbus with the industrial computer operated by the customer, who can retain the familiar user interface and database. The firmware ensures that target values for the cylinder (position, speed and force) are controlled by the servo pump unit. Installation and operation therefore require no special hydraulic expertise. “For our part, we input an acceleration ramp and a set point for the bogie test rig. The Bucher system takes care of everything else,” said Borer.

Thanks to microcontrollers and associated sensors, the system’s distributed intelligence offers the opportunity to exercise complete control of all characteristics via fieldbus. This is the precondition for preventive maintenance planning. The fact that the entire system now speaks the same language via fieldbus also enables successful integration into Industry 4.0 concepts, so that machines or even entire factories can be networked globally.

The clever combination and respective advantages of different technologies thus makes the difference that allows systems designed by Bucher Hydraulics and implemented using their proven components to be referred to as “smart.” Hans Borer summarized, “The important thing is know-how in the fields of electromechanical drives, industrial hydraulics, automation technology, and the combination of these disciplines. Bucher Hydraulics proved this knowhow in our project, just as they demonstrated their flexibility regarding new ideas.”

Bucher Hydraulics | www.bucherhydraulics.com

Jetter Automation | www.jetter.de