Service Information
Document Title: Function Group: Information Type: Date:
Pressure build up, steering 640 Service Information 10/17/2011
Profile:
ART, A40F (37535) [GB]
Pressure build up, steering
Introduction
The pumps are numbered according to the connections to the steering and dumping valve, and the numbers in the text below
refer to Hydraulic diagram, complete.
Function description
Engine is running and the machine is parked. The engine-dependent hydraulic pumps suck hydraulic oil from the hydraulic tank
and build up a flow to the steering and dumping valve 11 (neutral position). The pumps are connected to each other via
channels in the steering and dumping valve. Pump 2 has connection to the dumping valve slide through the steering valve
slide's "open centre", but when the steering slide is moved from its neutral position, then BV2 prevents the oil flow from pump 2
from going to the dump function. In this way, steering is prioritized. The remaining engine-dependent pumps are directly
connected (past the steering valve slide) to the dumping valve slide and pressure reduction valve R35 for servo pressure to the
dump lever 13.
The pressure is built up to the steering and dumping valve and its non-return valves. The pump's compensator valve is affected
through the pump's internal channels, and the pump angles down. Now there is stand-by pressure at the valve. At this point, the
pressure is only determined by the spring force in the compensator valve since there is still no pressure obtained from the
steering system to the compensator's LS–connection.
In neutral position the steering and dumping valve permits a small, controlled oil flow between its connection P2 and
connections A and B to the steering cylinders. An equal oil flow is also permitted between connections A – B and connection T4
for the tank line. The pressure is built up to the steering cylinders 15, the damping cylinder 16, the shock valves CHV1 and CHV2
with anti-cavitation valves and the cross-over valves VV1 and VV2. Steering is stabilized by pressurizing the steering cylinders.
The pressure is also built up via non-return valves BVLS1, BVLS2, and BVLS5 to outlet C2 and the compensator's LS–connection
on the engine-dependent pumps. The pressure in the pumps increases accordingly. At the same time, oil is forced out from the
steering and dumping valve's connections A and B via back-pressure valve (MTRV1) and its spring force to its connection T4 and
is returned to the tank.
Pressure is also built up via BVLS3 and BVLS4 when oil is forced out from the steering and dumping valve connections B1- and
A1+.
Neutral position pressure at the outlet for the engine-dependent pumps is the sum of the LS-pressure (MTRV1's spring force)
and the spring force in the compensator valve.
P=C+F P = pressure at pump outlet
C = pressure at LS–line
F = spring pressure in the compensator valve
Figure 1
Pos. Description Pos. Description
A Connection to the steering cylinders P Pressure line
A1+ Connection to the hoist cylinders P1 Pressure line
B Connection to the steering cylinders P2 Pressure line
B1- Connection to the hoist cylinders T Connection to the tank
Components
Pressure build-up, ground-dependent pump 8
The engine-dependent pumps and the ground-dependent pump use common LS–pressure. The operation of the ground-
dependent pump 8 is directly connected to the wheels via the drivetrain to the dropbox power take-off. When pump 8 rotates in
the correct direction (the machine is driven forward) pressure is built up to non-return valve BV4 in the steering and dumping
valve. The pump's compensator valve is affected by via internal channels and the pump angles down as there is no oil flow. The
same neutral position pressure now exists at the outlet for all pumps.
Steering function, engine-dependent pumps
The steering valve spool is displaced inwards and the connections P1– P5 from the pumps (also pump 8) are now in connection
with the steering cylinders via connection A. Pressure is built up in the cylinders' A–connection and the pistons are acted upon
on the plus side and minus side respectively. Pressure is built up simultaneously via the cross-over valves VV1 and VV2 to
damping cylinder 16 and to the valve spool end face. The valve spool is stabilised. The same pressure is now also in the LS–line
to the compensator's LS–connection. The compensator valve is acted upon and adapts the flow of the pumps to existing
requirements.
Restrictions C2 and C3 in load signal block 12
If there is a leak on the LS-signal line between C2 and the engine-dependent hydraulic pumps (Red central warning is activated),
the LS-pressure to the secondary steering pump (ground-dependent) can be maintained to a certain extent thanks to restriction
C2. Thus, the secondary steering pump can supply required flow to the steering valve.
The opposite applies for C3 if leakage arises on the LS–signal line between C3 and the secondary steering pump.
Load signal block 12
The purpose of this valve block is to deliver the highest LS-pressure from the load & dump system to the hydraulic pumps' LS-
connection, so that the pumps can angle out and provide required flow. The block also contains two restrictions that drain the
system's LS-pressure to tank.
Anti–cavitation and pressure–limiting valves (shock valves) CHV1 and CHV2
In order to prevent negative pressure (vacuum) or excessively high pressure peaks from occurring in the steering cylinders, the
steering valve contains two anti-cavitation and pressure limiting valves.
Pressure-limiting valve (shock valve) CHV3
Reduces max. pressure during active lowering. Prevents the load body from lowering if, e.g., body lock is not removed or
prevents undesired pressure build-up in the cylinders if the dump lever is held in the warm-up position active lowering after the
cylinders have been pushed in.
Non-return valve with back-pressure function, MTRV1
To ensure refilling of oil in the steering valve as well as steering cylinders, there is a non-return valve with back-pressure function
installed in the steering and dumping valve. MTRV1 also has the function to build up the LS–pressure and the servo pressure.
Non-return valve with back-pressure function, MTRV2
To ensure refilling of oil in the dumping valve as well as hoist cylinders, there is a non-return valve with back-pressure function
installed in the steering and dumping valve.
Non-return valve function in the steering and dumping valve
The function of the non-return valves BV1, BV2, BV3 and BV4 in the steering and dumping valve is to distribute the oil from the
hydraulic pumps to the steering valve spool.
Software description
The hydraulic pressures are monitored by the electronics using pressure sensors SE9102 for engine-dependent pumps and
SE9103 for ground-dependent pump. A warning is generated if the pressure in the system drops below a certain level, or if the
difference between the pressure values from the sensors becomes too big.
Supplementary information
For more information on sensors, see 3021 SE9102/SE9103, description and measuring
For more information on the steering system, see 600 Steering system, description.
For more information on hydraulic diagram, see 990 Hydraulic diagram, complete.
For information on pressure values for the functions described below, see 990 Hydraulic pressure, specifications
