Hydraulic steering unit

The steering unit (Orbitrol) is attached to the pedal console which is housed at the centre of the front firewall in the cab. Its characteristics are written on a plate riveted to the lower part of the spool valve.
The hydraulic steering system has no mechanical connection between the steering wheel and steering ram.
The system comprises the following main components:
- a pressurised oil supply from the low flow rate stage of the hydraulic pump,
- a hydrostatic (Orbitrol) steering unit fitted in parallel,
- a central double acting ram.

Hydraulic steering operation
The spool valve receives a priority supply from the low flow rate circuit. When the steering wheel is turned, the necessary flow of oil is directed to the corresponding side of the steering ram. Excess flow rate not required by the ram is directed via return ports to the 17 bar valve located on the left-hand hydraulic cover.
In case of an engine breakdown or hydraulic failure, the spool valve acts as a hand-operated pump so that the steering can be controlled. Operating in this way requires greater effort to be applied to the steering wheel.

Description of the hydraulic steering unit (Orbitrol)
The Orbitrol comprises a selector spool valve, a spring centred supply sleeve and a drive shaft linked to the steering column. It has four hydraulic ports:
- pressure;
- return to the 17 bar valve;
- two supplies to the steering ram.
The circuit is protected by a safety valve, two shock valves and two suction valves.

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) Ring (16) Washer
(17) O’ring (18) Relief valve (19) Seal (20) Shock valve (21) Orbitol steering unit (22) Non-return valve (23) Suction valves (24) Ring (25) Spool valve (26) Non-return valve (27) Sleeve (28) O’ring (29) Distributor plate (30) O’rings (31) Manifold (32) Screw

Layout of the main components of the front axle and hydraulic ports

Front axle system 
(1) 2RM axle beam (2) 4WD fixed front axle (3) 4WD suspended front axle
A Steering ram
B Location of steering column
LM Mechanical links 

Hydraulic ports 
L Supply to left-hand union of the steering ram
P Low flow rate - low pressure supply from the hydraulic pump (right-hand cover)
R Supply to right-hand union of the steering ram
T Supply to the 17 bar valve housed in the left-hand hydraulic 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) and (R) are not supplied. The oil coming from port (P) passes directly via hose (T) to supply the 17 bar valve. The circuit is open 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 overpressure 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.

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). The flow coming from the pump is directed to the metering device (stator (6) and rotor (9)). The rotor (9) is rotated and directs back into the cylinder a quantity of oil proportional to the rotational angle. The rotor (9) turns proportionally to the steering wheel.

For 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 sleeve (27). When these have turned 5°, the spool valve (25) and sleeve (27) are centred again, by 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 cylinder (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 metering device cavities with the circuit from the pump, on the one-hand, and the circuit to the cylinder (A), on the other hand. A non-return valve (26) is screwed into the supply port of the spool valve. This one-way valve stops excessive pressure or blows received by the front wheels from being transmitted to the pump whenever steering lock is applied. If the pressure in the circuit is too high, the relief valve (18) located in the spool valve is activated: the excess pressure is then directed to the channel (T).

Manual steering (engine stopped)

When the pump is no longer operating or the available pressure is too low, the metering device is no longer hydraulically driven. It is no longer power assisted. In this case, action on the steering wheel compresses the centring springs (13). 

The angular clearance between the pin (7) and the sleeve (27) is reduced to zero resulting in mechanical rotation of the metering device (stator (6) and rotor (9)). The steering unit then operates in the same way as a hand pump. The oil returning from steering ram (A) passes through the non-return valve (22) and supplies the metering device. The pressure generated is proportional to the torque applied to the steering wheel. The effort therefore required to turn the wheel in order to steer the tractor is much greater.