Designers of industrial equipment are continually confronted with demands for lower up-front and operating costs while maintaining or improving overall performance. That’s especially true for hydraulic power units (HPUs) as a core element for supplying flow and pressure for hydraulic actuation systems, according to engineers Bastian Beckmann and Martin Laube of Bosch Rexroth AG, Lohr am Main, Germany.
In a paper “Downsizing of hydraulic power units by revolutionizing the design,” presented at the 11th IFK fluid-power conference in Aachen, Germany, they discussed an innovative engineering package called “myCro” (short for my Customized reduction option), that aids in building small, customer-specific and cost-efficient power units via optional add-on equipment. The goal is to help users reduce size, weight and cost of the fabrications, minimize the required oil quantity, and optimize fluid conditions.
According to the authors, initial costs of an HPU are, in addition to component costs, largely influenced by the hydraulic oil tank and oil volume, while maintenance and repair costs make up a large part of the operating costs. HPUs are generally characterized by a large variety of sizes and shapes that arise from application demands such as available footprint and hydraulic power requirements. Regardless, the use of hydraulic power units in confined installation spaces is on the rise, and efforts to increase in the power density of HPUs (downsizing) has become more frequent.
Because the size, shape and cost of HPUs are much influenced by the tank, it is advantageous to size this as small as possible, said the authors. In addition to reduced up-front costs due to the smaller tank, there are also lower costs for the initial oil fill, as well as replacement oil when a change-out is needed.
One constraint to downsizing is regeneration of the fluid (such as degassing), a necessary resting time in the tank that lets air bubbles in the oil rise out of the fluid and outgas. This process can be relatively slow depending on bubble size as well on physical and chemical properties of the fluid. The recommended tank size is approximately 3 to 5 times the circulated oil volume flow.
If the relationship between the circulated flow, tank size and corresponding oil conditioning are not taken into account, damage—and increased maintenance and repair costs—will result. In addition to contamination with solid particles, oil contamination with air and water must be considered. Major problems are caused by undissolved air that, depending on the type and viscosity of the oil, forms bubbles. That leads to foaming, adversely affects the heat dissipation from the oil and worsens lubrication of friction surfaces. Other negative characteristics due to undissolved air are accelerated oil aging due to diesel effects or cavitation phenomena in hydraulic components. The negative effects of water in hydraulic oil pose similar problems.
However, reducing the required amount of oil represents an extraordinary challenge, because hydraulic oil is a decisive factor for meeting demanding tasks in addition to power transmission, such as lubrication and corrosion protection of moving parts. Water and air deteriorate the required properties of the fluid by actions such as oxidation, corrosion, and dieseling which reduce the life of oil and components and lead to undesired effects such as noise, erosion damage and a lack of system stiffness during operation.
From this it can be concluded that HPU´s can be dimensioned much smaller, in principle, if the above described phenomena can be treated by suitable fluid conditioning. This is possible by controlling the air and water content, cooling and filtration, or by a targeted management of the oil flow or tank design.
Because these measures can vary greatly depending on the application, Bosch Rexroth has developed the complete engineering package myCro—consisting of active and passive measures—to counteract the complex and multi-layered problems and reliably ensure appropriate oil conditioning.
For suitable oil conditioning by means of active measures, the desired parameters to be conditioned in the oil must be measured, interpreted and then possibly improved. Key components to aid in that task include sensors, standalone degassing modules, water separation modules, and hydraulic components suitable for vacuum pressures. In the same way, filtration can be addressed via a secondary circuit.
In passive measures, the HPU is designed such that entrainment of air or water in the hydraulic oil is prevented or at least slowed down. For an optimized flow and the resulting tank design, modern simulation methods such as 3D CFD simulation are available.
The myCro package comes in three levels. The first, myCro, downsizes the oil tank with the least possible effort. It includes initial evaluation of an application for oil tank downsizing by trained sales staff and supporting tools, and passive measures (intelligent tank design) to improve natural degassing to the desired level.
The next level of the package is myCro+, in which options for reducing the size of the tank are further exploited with additional active measures. These include, for example, a degassing module that actively improves oil conditioning and thereby enables even smaller power unit design. It also includes regulation and monitoring of the oxygen content in the operating medium.
The highest level package, myCro++, uses 3D-CFD simulation of flow and geometry to extensively explore possibilities for tank size reduction.
The effort for the additional hardware or the intelligent tank design is often not justified, so trained and experienced sales staff carries out an initial evaluation and plausibility check. Thus, in the beginning technical discussions with the customer, decisions in favor of smaller and less-expensive HPUs can be made. Because all elements in myCro are modular and flexible, it opens up further potential for miniaturization of the unit. The best technical/economic design of the HPU can be determined individually with the customer and result in the optimal hydraulic unit for an application with significantly less restriction from the above relationship between tank size and the desired oil condition.
Considering the technological change in industrial hydraulics towards electrification and electronification, Bosch Rexroth’s modular engineering package myCro option offers a completely new approach for the efficient design of optimized systems using the latest technologies in terms of numerical methods, computational system assessment as well as newly developed fluid technologies and controls. Projects where myCro is implemented could achieve a reduction in tank size of up to 70% and the required floor space by up to 50% with an immediate return on investment (ROI), which underlines the efficiency of this approach.