Tractors rely heavily on hydraulics to ensure a constant plowing depth, compensate for load differences, bundle hay bales, position the vehicle as it works its way up and down the field, etc. Each hydraulic device requires its own processor. In the search for robust yet compact electronic control units, manufacturers often turn to Bosch Rexroth, a leading specialist in the drive and control technologies.
One such directional control valve, the EHS2, developed by Rexroth at its plant in Schwieberdingen, Germany, fulfills the need to integrate a number of functions in one central, space-saving electrohydraulic 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.
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 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.
It became clear during initial talks between Rexroth and Trelleborg Sealing Solutions (TSS) that a special sealing solution would be necessary to meet the demands of the application. According to Hermann Zollner, Manager of Engineered Products at TSS, 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 due to both partners displaying a willingness to collaborate "beyond the factory gates," according to Ulrich Muller, Head of Engineering Mobile Electronics Hydraulics at Rexroth. “Everyone worked over and above corporate restrictions,” he said. “This joint expertise led, for example, to the optimum design of the aluminum casing for the control unit.”
Another thing that led to "the optimum design" was integrating TSS at the initial stage of development, according to Christoph Jochum, Manager of Engineering Mobile Electronics—Sensors, Hydraulics at Rexroth. “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.”
Through the use of computer simulation, 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 of the partnership, clients in the agricultural machinery industry now have a new generation of modular electrohydraulic control units, which remain compact while allowing for increasingly complex systems. In turn, farmers benefit from a control system that is virtually maintenance-free and resistant to internal and external influences.
Donna Guinivan, Editor, PR and Media, Trelleborg Sealing Solutions, wrote this article for SAE Off-Highway Engineering.