European safety laws — governed by TUV — require that all tractors sold in Europe must be able to steer manually during dead-engine situations. In other words, the operator must have steering ability if either the engine or hydraulic pump fails while the tractor is moving. Case Corp., Racine, Wis., developed the latest generation of their Magnum tractor line for both the approxidomestic and European market, so they had to meet this requirement.
An average-size operator can exert a maximum torque of about 1060 in.-lb on a steering wheel. A typical hydraulic steering unit with a displacement that is greater than 160 cc/rev requires more torque than this when the engine is dead — and therefore cannot be turned manually. For these larger steering units, tractor manufacturers usually install a separate DC-motor/hydraulicpump circuit in parallel with the standard hydraulic system. If the engine stops or the main pump fails, this system provides enough hydraulic flow to the steering unit to keep it operable.
Danfoss Fluid Power showed Case a new steering unit developed for this type of application. Called the OSPD LS (in which D signifies dual gerotor), this 3000-psi steering unit has two smaller gear sets instead of the single large gear set found in many steering units for large vehicles. The Danfoss design limits the smaller of the two displacements to 160 cc or less, and incorporates a 2-position, NC spool valve.
With inlet pressure available, the spool pilot-shifts to combine the two gear sets for normal steering displacement. If no pressure is available, the valve spring-returns to its closed position and blocks flow to the larger gear set, disabling it so only the smaller gear set can function. The smaller set draws flow from the tank line and allows the operator to steer the tractor manually.
Although the OSPD steering unit is more costly than a standard large steering unit, Case found that it decreased their overall cost and simplified their hydraulic system by eliminating other components. Case became the initial North American customer for the OSPD steering unit and plans to use it on other European tractors in the future.
Mike McCoy, OEM account manager, USMH, Danfoss Fluid Power, Racine, Wis., described this application.
