Design News for Design and mechanical engineers 22367

March 2, 1998 Design News

DESIGN APPLICATIONS From the regional editors

AT THE NATIONAL DESIGN ENGINEERING SHOW

Hydraulics power a convertible top

A micro-hydraulic system saves space and cost on the Mercedes SLK Roadster

Karen Auguston Field, Managing Editor

Schongau, Germany--Having worked almost exclusively with electric drives in the past, engineers at Mercedes-Benz were initially skeptical about the idea of using hydraulics to actuate the convertible top on the SL Roadster. Thoughts of hydraulic fluid leaking into the passenger compartment of a luxury car were extremely unappealing.

On the other hand, hydraulics--if proven to work--would provide powerful actuation forces and save cost and space by requiring only a single power source. Typically, an electric drive is needed for each axis of movement in a powered roof system.

Believing that the benefits outweighed the risks, Mercedes decided to push ahead in the development of a micro-hydraulic system. Critical design criteria included safety, efficiency, space constraints, and, of course, demanding performance requirements. For example, the complete roof opening/closing cycle could take no longer than 25 seconds.

Working closely with engineers at Hoerbiger Hydraulics, the automaker was able to develop a cleverly thoughtout hydraulic circuit diagram. Operating at a pressure of 200 bar, the compact, yet powerful system consists of five cylinders (two main, two lid, and one locking), a radial piston pump including two complete control valves (which actuate both the trunk and roof top), a pressure relief valve, and a check valve. The combined power supply incorporates the electric motor/pump assembly, reservoir, control valve manifold, and filtration.

Several unique features of the system include:

After extensive testing of the system--early prototypes were operated on "mystery models"--the engineers' early concerns were put to rest. In addition to the compact design and high force capability, the system has even exceeded Mercedes' expectations in terms of reliability. Today, Hoerbiger Hydraulics manufactures up to 200 complete systems a day for the SL Roadster.

Clearly, this development is part of a growing trend: BMW's Z3 roadster, the Porsche 996, the Fiat Punto and the GM Opel Astra also employ special micro-hydraulic systems.

Additional details?Contact Hoerbiger Hydraulics, 100 West Lake Drive, Glendale Heights, IL 60139.