SkyTrak 8042 Service Manual - Manual (Page 474)

8.59 Model 8042, 10042, 10054 Legacy Rev. 10/03
Hydraulic System
4. Pressure in both the front and rear axle brake lines is
equal. A groove in each of the two spools opens to a
cross-hole between the two spool bores, preventing
uneven brake pressure. This feature is called the
“equalizer” function.
5. Power-mode braking begins when the metering
notches (6) in the spool (7) meter oil from the inlet
port to the spool chamber. Fluid flows from the spool
chamber, through the one-way orifice (8) and into
the brake lines. Brake pressure is controlled by a
force balance between the force applied at the pedal
and the resulting force from pressure in the spool
area.
6. The load sense bleed orifice helps provide stability
and aids in purging air from the system.
Manual Mode
In the event that hydraulic pressure at the inlet port is too
low to provide the braking force required, the service
brake valve automatically transitions to the manual mode
to provide braking power.
Flow paths out of the valve that would rob efficiency are
automatically blocked in the manual mode. The inlet port
check valve seals off the inlet port, and the load sense
shutoff valve closes. The service brake valve then acts as
a two-stage pump.
1. Brake pedal displacement is transmitted to the
service brake valve (1) at the plunger (2).
2. The series spring (3) transmits plunger displacement
to the piston (4).
3. The piston (4) and spool (7), functioning as pumps,
send oil to the brake lines. Initially, the brake lines
are vented to the reservoir by an annular gap
between a stepped-down piston diameter and the
piston bore. Flow to the reservoir is restricted when
the piston contacts the piston sealing ring (5).
4. Volume generated by piston displacement causes
the spool make-up check device (9) to unseat. Fluid
flows into the spool chamber and out to the brake
lines.
5. Pressure in both the front and rear axle brake lines is
equal. A groove in each of the two spools opens to a
cross-hole between the two spool bores, preventing
uneven brake pressure. This feature is called the
“equalizer” function.
6. At a set piston chamber pressure, a relief valve (10)
begins to meter oil and limits the pressure within the
piston chamber. The pressure in the piston chamber
becomes isolated from the pressure in the spool
chamber when the make-up check device (9)
reseats itself.
7. Further displacement of the service brake pedal
causes higher pressure in the spool chamber. The
load sense pin in the make-up check device (9) is
sensitive to the increased pressure and transmits a
force to the relief ball. The increasing force on the
relief ball effectively lowers the relief setting within
the piston chamber. Eventually, piston chamber
pressure reaches zero, and the valve has moved
completely from the first stage to the second stage.
The smaller area of the second stage permits the
brake valve to develop higher braking pressure.
MS1880
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