Skytrak 6042 Service Manual User Manual download pdf (Page 283)

8.51

Model 6042 Legacy Origin 7/02

Hydraulic System

Figure 8-22 Cross-Section of Service Brake Valve

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 (Fig. 8-22, 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 brake

valve (Fig. 8-22, 1) at the plunger (2).

2. The series spring (Fig. 8-22, 3) transmits plunger

displacement to the piston (4).

3. The piston (Fig. 8-22, 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 (Fig. 8-22, 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

(Fig. 8-22, 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

(Fig. 8-22, 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, the 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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