Load sending and twin flow mechanical spool valves – GPA20 and GPA40 rear axles

The proportional auxiliary spool valves, of the Bosch/Rexroth SB23 Load Sending type, are fitted to the high flow rate high pressure circuit. They are fed by the oil coming from the priority block(s). When no functions are being activated, the entire flow rate is directed towards the spool valves. If certain priority functions are being supplied, the remaining flow is then available for the auxiliary spool valves. Each spool valve has a flow regulating system controlled by a black, red, green or yellow button located directly on the each spool valve regulating valve stem. The main spool of the valve is controlled by a cable linked to a lever in the cab. The spool directs oil towards either outlet port. Each outlet port is linked to the LS pilot line of the variable displacement pump regulating valve via the priority block(s). The auxiliary spool valves are made up of elements themselves containing spools and valves. Some elements cannot be identified as spare parts.

Different types of spool valve block components

A spool valve block can consist of three, four or five of the following components. It can combine components of similar type of or different types.

Component b
4-position component with flow rate regulation, double acting and automatic return to neutral.
Component c
4-position component with flow rate regulation, double acting, zero leak and automatic return to neutral.

Quick-disconnect couplings are located on the spool valve support. They are sealed by covers of different colours (red, green and black) corresponding respectively to hydraulic ports A and B of auxiliary spool valves. The dirty oil coming from the separation of hydraulic couplers flows into a transparent plastic tank that is fixed at the back of the left trumpet housing.


When the lever in the cab is pulled (spool (1) pushed), the oil enters via channel P and travels towards the restrictor of the flow regulation control spool (5) and grooves of the main spool (1). The oil is then carried towards a main channel and lifts the ball of valve (2) to enter a chamber. displacement of the spool (1) simultaneously opens the one-way valve (3). The oil flows from the chamber towards port A and returns to the housing through port B via the hydraulic slave device. The operating principle is identical when the lever is pushed (spool (1) pulled). The oil flows from the chamber towards port B and returns to the housing through port A via the hydraulic slave device.

Layout of components and identification of ports

Layout of components 
(1) Main spool (2) Valve (3) Non-return valve (4) Flow regulation control rod (5) Flow regulation control spool (6) Housing and locking system (7) Flow regulation control button

Identification of ports

on the inlet block (14) 
P High pressure supply from the variable displacement pump via the priority block(s) (depending on version).
R Return from auxiliary spool valves (beneath the oil level)
LS Signal to the variable displacement pump regulator via the lift control valve and the priority block(s) (depending on version).

on the auxiliary spool valves 
A Outlet to coupler
B Outlet to coupler
C Restrictor
D Grooves
E Channel
F Chamber
P, R and LS
Marks identical to those of the inlet block (14)

Parts list 
(1) Auxiliary spool valves (5) Intermediary blocks (6) End plate (14) Inlet block (17) Lift control spool valve


Parts list 
(17) Lift control spool valve
BP Priority block(s)
BB Balance block