New Holland WorkMaster 50 Service Manual page 481

The control valve incorporates a metering unit which regulates the volume of oil supplied to the cylinder so that it is
proportional to the angular movement of the steering wheel. The metering unit in combination with the check value
also allows the steering to be operated manually without pressurised oil being supplied from the pump.
The system is fully hydrostatic and as such there is no mechanical connection between the steering column and the
steering wheels.
Straight - ahead driving ( A, a, d)
With steering wheel (V) stationary, rotary valve (5) takes neutral position relative to sleeve (6). This position is main-
tained through the action of springs (2), (see figure 2 section A-A) and the following conditions exist:
• Pin (1), (see figure 2 section B-B) is central in the valve (5) aperture.
• Ports (13) and (14) are in alignment (see figure 2 section C-C) and the oil pressure from pump (P) is returned to
tank.
• Passages (15), (17) and (19) on the valve (see figure 2 sections D-D and E-E) are off register relative to ports (16),
(18) and (20) on the sleeve (i.e. all ports in communication with the power cylinder remain closed).
Right-hand side steer ( B, Sd, b, e, f)
Upon turning steering wheel (V) clockwise, springs (2), (see figure 2 section A-A) deflect allowing valve (5) to rotate
relative to sleeve (6) until gap (G), (see figure 2 section B-B) is taken up.
Thus:
• Ports (13) and (14), (see figure 2 section C-C) go out of alignment to discontinue oil return.
• Six passages (15), (see figure 2 section D-D) line up with an equal number of ports (16) connected instant by instant
with the rotor recesses during the inlet phase.
• Six pressure passages (17), (see figure 2 section E-E) line up with ports (18) communicating with the power cylin-
der. Moreover, the oil pressure passages communicate with the remaining ports (16), (see figure 2 section D-D)
connected instant by instant with the rotor recesses during the actuation phase.
• Six exhaust passages (19) line up with ports (20), (see figure 2 section E-E) communicating with the power cylinder.
Once the gap (G) is eliminated valve (5) positively transmits steering wheel input to both sleeve (6) and rotor (9)
through pin (1) and shaft (7). Diagrams (e and f) show the principle of operation at start of right-hand side steer and
after a certain amount of wheel rotation.
The flow of oil pressure from pump to rotor during inlet, and from rotor to power cylinder line during the power actuation
phase, is provided instant by instant.
Left-hand side steer ( B, Ss)
Upon turning the steering wheel anti-clockwise, a reversal of the above sequence is obtained and delivery passages
(17), (see figure 2 section E-E) supply ports (20) to bring about left-hand side steering.
Emergency hydraulic steer ( C, Sed, Ses)
Steering is possible even in cases of loss of hydraulic pressure. Upon turning the steering wheel, valve (5) takes up
the normal operating position, whilst the rotor functions as a hand pump directing oil pressure to the power cylinder.
Check valve (4) opens, thereby permitting the flow of oil from tank to rotor by-passing the pump. Valve (23) remains
closed preventing leakage in connecting line between pump and control unit.
Power cylinder safety and make-up valves (N2)
As of safety valves open (33), pressure created by piston (H) is exhausted in one cylinder chamber under the action
of strong external stress on wheels. Simultaneously, vacuum in the opposite chamber is compensated by oil flow
through the opening of the associated make up valve (34). Pressure in one cylinder chamber deriving from small
external stress which are not sufficient to open cylinder safety valve, is exhausted through normal leakage past control
valve (D), while vacuum in the opposite chamber is compensated through make-up (34), as shown in detail N2, for
DANFOSS OSPC 100 control valve.
Steering - Steering control
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