Modern-day tractors rely heavily on hydraulics. These are used to ensure a constant plowing depth, compensating for load differences, bundling hay bales and even positioning the vehicle as it works its way up and down the field. Each hydraulic device requires its own processor. In the search for a robust yet compact electronic control unit, manufacturers turn to one of the world’s leading specialists in the field of drive and control technologies: Rexroth.
Developed by global manufacturer Rexroth at its plant in Schwieberdingen, Germany, the EHS2 directional control valve fulfills the need to integrate a number of functions in one central, space-saving electro-hydraulic control unit. Depending on the amount of hydraulic equipment to be installed in a tractor, each EHS2 is composed of three to seven SB33 hydraulic valve slices supplied with compressed oil from a central pump. Each of these slices is responsible for a certain task, whether rotating, tilting, lifting, or compressing. While this is now a best-selling product for Rexroth, the team of developers first had to overcome several hurdles – especially in terms of sealing.
Bosch-Rexroth EHS2 electrohydraulic control unit
No Easy Task
To begin with, agricultural machinery is exposed to extreme conditions 365 days a year and often around the clock. The control unit has to withstand operating temperatures of up to +120°C/+250°F, as well as possibly an ice-cold soaking when the tractor drives through water. In addition, the unit has to remain watertight despite the partial vacuum inside. All this makes heavy demands on seals.
The task is further complicated by the need for the seals to protect the control unit against oil, detergents, manure, high-pressure steam jets and even acidic lubricants. At the same time, the aluminum EHS2 control unit has to be perfectly sealed off from the SB33 hydraulics housing made of cast iron. A simple solution to sealing two metals with different thermal expansion coefficients is the use of a larger sealing cross section. However, this was not possible in this case, since the unit was specifically designed to be slim line.
Bosch-Rexroth hydraulic valve blocks.
Fortunately, the team had expert partners on hand at Trelleborg Sealing Solutions. Norbert Glaser, Key Account Manager, explains, “As the long-term partner for Rexroth we already knew their high demands regarding quality and tolerance levels by heart. Otherwise we might have underestimated the requirements a little bit.”
Tailored to requirements
The initial talks between the two partners made it clear that a special sealing solution would be necessary, designed to meet the demands of the application. Hermann Zollner, Manager Engineered Products at Trelleborg Sealing Solutions, recalls that it was not easy to find the optimal mixture for the molded seal that fulfilled the requirements in terms of chemical and thermal resistance, while keeping the manufacturing process technically and economically viable.
This obstacle was overcome thanks to both partners displaying a willingness to collaborate beyond the factory gates. Dr. Ulrich Muller, who heads Engineering Mobile Electronics Hydraulics at Rexroth explains, “The interaction between our developers and Trelleborg Sealing Solutions was perfect. Everyone worked over and above corporate restrictions. This joint expertise led, for example, to the optimum design of the aluminum casing for the control unit.”
Christoph Jochum, Manager Engineering Mobile Electronics – Sensors, Hydraulics at Rexroth, confirms, “The partnership showed me how important it is to integrate the seal manufacturer at the initial stage of development. This is especially the case as Trelleborg Sealing Solutions is not only an expert in seals and related materials, but also has wide-ranging experience with surface finishes at mechanical interfaces. This was extremely useful in terms of the different thermal properties involved here.”
Thanks to the use of computer simulation and the combined talents of Rexroth and Trelleborg Sealing Solutions, the team was able to reach its target without undue trial and error. This was evident, for example, in the small number of immersion tests that needed repeating, as well as the minimum amount of time spent experimenting on the elastomer formulations.
As a result, clients in the agricultural machinery industry now have a new generation of modular electro-hydraulic control units available, which remain compact while allowing for increasingly complex systems. In turn, farmers benefit from a perfect control system that is virtually maintenance-free and unrivaled in its resistance to internal and external influences. After all, the crops will not wait for the tractor to be repaired.
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