Carraro suspended front axle control unit


Technical characteristics

Description Values
Nominal pressure                                                                      250 bar
Hydraulic oil type                                                                    DIN 51524
Ambient temperature                                                      Min -30 °C / Max +90 °C
Hydraulic fluid temperature                                              Min -30 °C / Max +70 °C
Type of hydraulic unions to ports P, T, A, B                                 M18 x 1.5
Type of hydraulic unions to LS port                                            M14 x 1.5
Filtration: maximum pollution level allowed in service fluid            NAS 1638
Solenoid valve                                                                           25 Nm
Accumulator tightening torque                                                    200 Nm
Nominal voltage                                                                        12 VDC

Parts list 
(1) LS pilot flow (2) KSP1 accumulator (3) KSP2 accumulator (4) RSP1 accumulator (5) Bleed screw (6) Solenoid valve WV1 (7) Solenoid valve WV2 (8) Return (9) Supply from pump (10) Top ram supply (11) Bottom ram supply





Hydraulic description of the control unit with Open Centre hydraulics

Parts list 
(1) Front axle suspension ram (2) KSP1 accumulator (3) KSP2 accumulator (4) RSP1 accumulator (5) Bleed screw (6) Solenoid valve WV1 (7) Solenoid valve WV2 (8) Control unit (9) Filter (10) Proportional valve (11) Controlled non-return valve (12) 250 + 20 bar pressure relief valve (13) 170 bar pressure relief valve (14) High pressure inlet (15) To auxiliary spool valves

Legend
A Upper ram chamber
B Lower ram chamber
C Hydraulic channel
D Hydraulic channel
E Hydraulic channel
F LS pilot line
G Hydraulic channel
LS Load Sensing pilot flow
R Return to housing

Neutral position

In low position, chamber A of the ram (1) is empty. The suspension is in contact with two mechanical central beam stops. In high position, the ram is balanced by accumulators (2), (3) and (4). If the pressure in the ram is too high (shocks received by the front axle), the pressure relief valve (12) opens and the excess pressure is directed to the return. If the ram position has changed, the position sensor value has also changed. If after 3 seconds the sensor value does not match to the calibration value, the calculator supplies the solenoid relevant valve(s) (WV1 (6), WV2 (7)) to make the two values correspond.

Lifting position 


When lifting, the two solenoid valves WV1 (6) and WV2 (7) are activated. The WV1 solenoid valve (6) allows oil to reach the accumulators (2) and (3) and chamber A of the ram (1). When the pressure in chamber B reaches 250 bar, the oil returns to the tank via channel C. Channel D also supplies accumulator (4). Solenoid valve WV2 (7) allows priority for the suspended front axle in pilot flow line F.
The proportional valve (10) stops the supply to the auxiliary spool valve and linkage. When the pressure in the circuit reaches 180 bar, the circuit resumes its course via the proportional valve (10) and supplies the linkage and auxiliary valves.

Lowering position 


When lowering, only solenoid valve WV2 (7) is supplied. The oil supplies chamber B of the ram (1) via channel E. The pressure created in the pilot flow line F allows priority for the suspended front axle. The proportional valve (10) stops the supply to the auxiliary spool valve and linkage. When the pressure in the circuit reaches 180 bar, the circuit resumes its course via the proportional valve (10) and supplies the linkage and auxiliary valves (15). The pressure in pilot flow line F also allows the non-return valve (11) to open so that the oil in chamber A returns to the tank via channel G.

Hydraulic description of the control unit with Closed Centre hydraulics

Parts list 
1) Front axle suspension ram (2) KSP1 accumulator (3) KSP2 accumulator (4) RSP1 accumulator (5) Bleed screw (6) Solenoid valve WV1 (7) Solenoid valve WV2 (8) Control unit (11) Controlled non-return valve (12) 250 + 20 bar pressure relief valve  (14) High pressure inlet

Legend
A Upper ram chamber
B Lower ram chamber
C Hydraulic channel
D Hydraulic channel
E Hydraulic channel
F LS pilot line
G Hydraulic channel
LS Load Sensing pilot flow
R Return to housing

Neutral position

In low position, chamber A of the ram (1) is empty. The suspension is in contact with two mechanical central beam stops. In high position, the ram is balanced by accumulators (2), (3) and (4). If the pressure in the ram is too high (shocks received by the front axle), the pressure relief valve (12) opens and the excess pressure is directed to the return. If the ram position has changed, the position sensor value has also changed. If after 3 seconds the sensor value does not match to the calibration value, the calculator supplies the solenoid relevant valve(s) (WV1 (6), WV2 (7)) to make the two values correspond.

Lifting position 


When lifting, the two solenoid valves WV1 (6) and WV2 (7) are activated. The WV1 solenoid valve (6) allows oil to reach the accumulators (2) and (3) and chamber A of the ram (1). When the pressure in chamber B reaches 250 bar, the oil returns to the tank via channel C. Channel D also supplies accumulator (4). Solenoid valve WV2 (7) allows priority for the suspended front axle in pilot flow line F.

Lowering position 


When lowering, only solenoid valve WV2 (7) is supplied. The oil supplies chamber B of the ram (1) via channel E. The pressure created in the pilot flow line F allows priority for the suspended front axle. The pressure in pilot flow line F also allows the non-return valve (11) to open so that the oil in chamber A returns to the tank via channel G.

Hydraulic description of the control unit with Twin Flow Load Sensing hydraulics

The operation of the different positions (neutral, lifting and lowering) of the hydraulic unit with Twin Flow hydraulics is similar to that described. The only difference is that the hoses (pressure and LS) are connected at different places.

Identification of ports

on the control unit 

A Supply (hydraulic chamber with a large section)
B Supply (hydraulic chamber with a small section)
LS Load Sensing pilot flow
P High pressure
T Return

on the tractor 

LS Load Sensing pilot flow
P High pressure
T Return

Disassembling – reassembling the control unit

Some of the following pictures may not show your exact hydraulic control unit, but the procedure is the same.

Preliminary operations
Remove any components that could hinder control unit removal.
If necessary, remove the tractor complete control unit.

Disassembly
Mark the position of accumulators (2) (3) (4) on the control unit. Remove them. The accumulators may be of different capacity depending on their position and on the front axle model.


Loosen the A screw of each solenoid valveSplit the B solenoids from C spoolsUnscrew the C spools from the control unit. Gather the components.



Reassembly
Check the components. Replace any defective parts.
Screw the C spools on the control unit. Tighten to a torque of 25 Nm.
Assemble the B solenoids on C spoolsTighten the A nuts moderately.
If necessary, fit new seals on each accumulator.
Install the accumulators using the marks made during removal. Tighten to a torque of 200 Nm.

If removed, Install the control unit on the tractor.
Install any removed components around the control unit.
Screw the control unit AV bleed screw.
If the control unit and/or one or more solenoid valves needed to be replaced:
- check P and LS pressure at the control unit ports;
- calibrate the front axle.