Hydrostatic steering (80/205cm3 and 80/240cm3)


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 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:
- OSPDF 80/205 cm3,
- OSPDF 80/240 cm3;
- a double acting steering ram (1). The steering unit is supplied via the priority block(s) (depending on version) by the high flow rate high pressure circuit. This circuit also supplies the integrated spool (18) and the two gears (27)(28), (30)(31). 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, only the steering unit gear (27) (28) 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:
- two gears of different capacity, each consisting of a rotor and a stator. These two gears are mechanically coupled to a drive shaft, itself connected to the steering column;
- a spring centred supply sleeve;
- an integrated spool in the body (19).
The steering unit comprises five ports:
- pressure;
- return to the 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) Lip seal (2) Centring springs (3) Bush (4) O’ring (5) Cotter pin (6) Sleeve (7) Spool valve (8) O’ring (9) Link shaft (10) O’ring (11) Plate (12) O’ring (13) Plate (14) O’ring (15) Spacer (16) O’ring (17) O’ring (18) Integrated spool (19) Spool body (20) Seals (21) O’ring (22) Washer (23) Needle bearing (24) Washer (25) Relief valve (26) Orbitrol steering unit (27) Rotor (gear type 80 cm3) (28) Stator (gear type 80 cm3) (29) O’ring (30) Stator (gear type 205 cm3 or 240 cm3) (31) Rotor (gear type 205 cm3 or 240 cm3)
(32) Plate (33) Spring (34) O’ring (35) Closing plate (36) Screw


Parts list 
(1) Lip seal (2) Centring springs (3) Bush (4) O’ring (5) Cotter pin (6) Sleeve (7) Spool valve (8) O’ring (9) Link shaft (10) O’ring (11) Plate (12) O’ring (13) Plate (14) O’ring (15) Spacer (16) O’ring (17) O’ring
(18) Integrated spool (19) Spool body (20) Seals (21) O’ring (22) Washer (23) Needle bearing (24) Washer (25) Relief valve (26) Orbitrol steering unit (27) Rotor (gear type 80 cm3) (28) Stator (gear type 80 cm3)
(29) O’ring (30) Stator (gear type 205 cm3 or 240 cm3) (31) Rotor (gear type 205 cm3 or 240 cm3) (32) Plate (33) Spring (34) O’ring (35) Closing plate (36) Screw (37) Shock valves (38) Non-return valve (39) Spacers (40) Non-return valve (41) Suction valve (42) Cotter pin

Layout of channels and ports


Parts list 
(1) CARRARO fixed 4 WD front axle (2) CARRARO suspended 4 WD front axle

Legend
A Double acting steering ram
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 (7) is centred in relation to the sleeve (6) by the springs (2) (Fig. 7). 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 (37) and two suction valves (41) are located in ports L and R of the spool valve. The shock valves (37) protect the circuit between the steering ram and the spool valve from eventual pressure overloads caused by shocks to the wheels. The suction valves (41) allow the oil released by the shock valves (37) 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.


Legend 
A Double acting steering ram
BP Priority block(s)
L LH hydraulic port of the steering ram
LS Signal
P Pressure
P1 Supply to metering devices (stators (28) (30)
and rotors (27) (31)).
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) (Fig. 8) produces an angular displacement of the spool valve (7) in relation to the sleeve (6) and the opening of the closed Orbitrol circuit. A pilot pressure coming from the Orbitrol arrives in channel (4) and allows the passing into channel (1) of the flow necessary to the integrated spool (18) and the stator metering devices (stators (28) (30) and rotors (27) (31)).
The rotors (27) (31) are driven in rotation and force towards the ram a quantity of oil that is proportional to the degree of rotation. The rotation of rotors (27) (31) is equal to that of the steering wheel.

Example: Let us suppose that the steering wheel is turned by 5°. An angular displacement of 5° of the spool valve (7) is produced in relation to the sleeve (6). The rotors (27) (31) are driven in rotation as long as they are supplied with pressurised oil. They drive with them the link shaft (9) and the sleeve (6). When the latter have turned 5°, the spool valve (7) and the sleeve (6) are once again centred by the springs (2). The rotors cease to be supplied and stop. 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. Depending on whether a left-hand or right-hand lock is applied, the spool valve (7) directs the oil discharged by the metering devices (stator (28) (30) and rotor (27) (31)) towards the L or R port. During rotation, the sleeve (6) 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 rams, on the other hand. A non-return valve (40) 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 (25), located in the steering unit, opens and the excess pressure is released to channel T.