HYDRAULICS 1.105 STEERING Description The steering system is fully hydrostatic. This means that the steering motion is transmitted hydraulically with no mechanical linkage between the steering wheel and the steered wheels. The hydraulic supply is by a dynamic system working with a load-sensing (LS) pump. Pump supply passes through a priority valve and generates a load-sensing signal. Operation The OSPF steering unit interfaces the steering valve with an adapter. Depending on the steering mode selected in the cab, (two-wheel steer, four-wheel steer or crab steer), the selector valve directs oil to the selected steering cylinder when input is made from the steering wheel.
In the neutral position, oil is allowed to flow freely from the priority valve through the steering unit to return (LS to T). As the steering wheel is turned the spools of the valve rotate against each other and proportionally direct oil to the selected steering cylinder. Not until the rotary valve is fully open is the LS connection to the tank fully closed. C1 To rear cylinder C2 To rear cylinder C4 To front cylinder P Pressure from LS – pump T Tank A pressure relief valve is fitted in the steering unit. If the pressure rises above 175 bar (2537 psi), the PRV will open and return excess pressure to tank.
Shock valves are fitted to in the steering unit to protect the cylinders and hoses and are set at a higher pressure than the PRV. When hydraulic pressure fails, the vehicle can be steered manually. The steering wheel, when turned, drives the outer spool with the cross pin and the rotor is turned by the cardan shaft which acts as a manual pump to supply oil to the steering cylinder. In this case the steering effort required will be increased significantly. Specification Type OSPF250LS PRV 170-175 bar (2465-2537 psi) Shock valve 225-245 bar (3262-3552 psi) Displacement 250 cu.cm/rev C3 To front cylinder O OSPF250 valve (21) A Adapter E Selector valve (22)