Hydrostatic steering (125/160 cm3)

The steering system is of the dynamic type (constant 0.5l /min flow rate - 6 bar standby pressure). Its control principle is of the Load Sensing (LS) type. The Orbitrol steering unit (21) type is displayed on an adhesive plate located below the steering unit. The steering unit has no mechanical links, other than the steering column itself, between the steering wheel and steering ram. The system comprises the following main components:
- a pressurised oil feed from the variable displacement pump;
- a steering unit that can be, according to the type of front axle:
- OSPF 125 LS dynamic,
- OSPF 160 LS dynamic;
- a double acting steering ram (1) with a different profile according to the type of tractor or front axle. The steering unit is supplied via the priority block(s) (depending on version) by the high flow rate high pressure circuit. When the steering wheel is turned, the necessary flow of oil is directed to the corresponding side of the steering ram. The other side of the ram is connected to the gearbox selector cover return ports. In the event of a variable displacement pump failure, the steering unit acts as a manual pump, thus allowing the tractor to continue to be steered. This operation, performed without hydraulic assistance, obviously requires significant effort to turn the steering wheel.

Description of the steering unit
The steering unit comprises a selector spool valve, a spring centred supply sleeve and a drive shaft linked to the steering column. The steering unit comprises five ports:
- pressure;
- return to selector cover;
- two supplies (left and right) to the steering ram;
- Load Sensing
The circuit is protected by a safety valve, two shock valves and two suction valves.

The principle of Load Sensing

Steering unit
The LSD steering unit possesses a fifth LS port. This port is connected:
- either to the closed steering unit circuit when steering is in neutral position;
- or with the pressure supply line when the steering wheel is turned.

Parts list 
(1) Screw (2) Screw (3) Seal (4) Closing plate (5) O’ring (6) Stator (7) Cotter pin (8) O’ring (9) Rotor (10) Spacer (11) Link shaft (12) Washer (13) Centring springs (14) Needle bearing (15) Bush (16) Washer (17) O’ring (18) Relief valve (19) Seal (20) Shock valve (21) Orbitrol steering unit (22) Non-return valve (23) Suction valves (24) Bush (25) Spool valve (26) Non-return valve (27) Sleeve (28) O’ring (29) Distributor plate

Layout of channels and ports

Parts list 
(1) 2 WD or 4 WD axle (2 WD on figure shown)

A Double acting steering ram
AV Front axle face
B Location of steering column
L Supply to left-hand union of the steering ram
LS Signal to variable displacement pump
P Pressure
R Supply to right-hand union of the steering ram
T Return to housing via gearbox selector cover

Neutral position (engine running)
In this position the spool valve (25) is centred in relation to the sleeve (27) by the springs (13). The channels P1, L, R are not supplied. The oil coming from the variable displacement pump via channel (2) is directed in priority to the Orbitrol unit via channel (1). With the steering in a neutral position, there is no flow in this channel. The circuit is closed centre. Two shock valves (20) and two suction valves (23) are located in ports L and R of the spool valve. The shock valves (20) protect the circuit between the steering ram and the spool valve from over pressure caused by mechanical shocks to the front wheels. The suction valves (23) allow the oil released by the shock valves (20) to pass from the right-hand channel to the left-hand channel or vice versa depending on the movement of the piston inside the steering ram.

A Double acting steering ram
BP Priority block(s)
L LH hydraulic port of the steering ram
LS Signal
P Pressure
P1 Supply to the metering device
R RH hydraulic port of the steering ram
T Return

Steering on lock position (engine running)

Action on the steering wheel (to the left or right) produces an angular displacement of the spool valve (25) in relation to the sleeve (27) and the opening of the closed Orbitrol circuit. The pilot flow coming from the Orbitrol enters via channel (4) thus allowing for the flow required for the metering device (stator (6) and rotor (9)) to pass into channel (1). The rotor (9) is rotated and directs back into the cylinder a quantity of oil proportional to the angle of rotation. The rotor (9) turns proportionally to the steering wheel.

Example: Let us suppose that the steering wheel is turned by 5°. An angular displacement of 5° of the spool valve (25) is produced in relation to the sleeve (27). The rotor (9) is driven in rotation as long as it is supplied. It drives with it the link shaft (11) and the sleeve (27). When the latter have turned 5°, the spool valve (25) and the sleeve (27) are once again centred by the springs (13). The rotor ceases to be supplied and stops. This same reasoning applies to greater angles. The quantity of oil delivered by the steering unit to the steering ram A is therefore proportional to the rotational angle of the steering wheel. The spool valve (25) allows, whether steering lock is applied to the left or right, to direct oil fed by the metering device (stator (6) and rotor (9)) to port L or R. During rotation, the sleeve (27) ensures the synchronous communication of the cavities in the metering device with the circuit from the pump, on the one-hand, and the circuit to the steering ram, on the other hand. A non-return valve (26) is screwed into the supply port of the steering unit. This valve is one-way. It prevents excessive pressure exerted to the front wheels from being transmitted to the pump when the steering lock is applied. If the pressure in the steering circuit is too high, the relief valve (18), located in the steering unit, opens and the excess pressure is released to channel T.