Skytrak 6042 Service Manual User Manual download pdf (Page 284)

Hydraulic System

8.52

Model 6042 Legacy Origin 7/02

8. Controlling the force applied to the service brake

pedal determines the amount of braking power.

9. If the first application of force applied to the service

brake pedal was not enough to deliver the braking

required, the one-way orifices at the work ports help

give the brake valve “pump up” ability. The one-way

orifices allow unrestricted flow to the brake lines but

also restrict the rate of decompression.

8.8.20 Boom Extend/Retract Circuit

Description

Hydraulic flow is applied in the boom extend/retract circuit

(Fig. 8-5 and Fig. 8-6) by the hydraulic gear pump

through the unloader valve. System pressure is directed

to either side of the extend/retract cylinder by the shifting

of the spool valve in the main control valve. The spool

valve is shifted by the operator joystick and its associated

control cable.

a. Boom Extend Position (Fig. 8-5)

When the joystick is in the boom EXTEND

position, the spool valve is shifted by the cable

so flow is directed through extend/retract spool

valve ports, through the counterbalance valve to

the base end of the extend/retract cylinder. Return oil

from the rod end of the cylinder exits through the check

valve. The oil will pass from the cylinder to the spool

valve, to the unloader valve, to the oil cooler, to the return

filter and then to the reservoir.

If system pressure exceeds 3,250 psi (224 bar), the

extend/retract port relief will open and allow hydraulic oil

to return to the reservoir. If the return filter is plugged, the

oil returning to the reservoir bypasses the filter when the

internal pressure of the filter reaches 25 psi (1,7 bar).

b. Boom Retract Position (Fig. 8-6)

When the joystick is in the boom RETRACT

position, the extend/retract spool valve is shifted

by the cable so that system pressure is directed

through the spool valve, to the rod end of the

extend/retract cylinder. The counterbalance valve is

piloted open by high pressure, allowing return oil from the

base end of the cylinder to flow through the spool valve,

to the unloader valve, to the oil cooler, to the return filter

and then to the reservoir.

If system pressure reaches 3,250 psi (224 bar), the main

relief valve opens, allowing hydraulic oil to return to the

reservoir. If the return filter becomes plugged, return

hydraulic oil will bypass the filter when pressure reaches

25 psi (1,7 bar) and return to the reservoir unfiltered.

The extend/retract cylinder includes a counterbalance

valve and a pilot-operated check valve. The valves serve

two main functions. One of the functions is to help prevent

component damage. The other valve function can be

described as follows:

• If the vehicle is travelling and the boom runs into

a solid object, pressure will build up on the base

end of the cylinder. Without a counterbalance

valve, the cylinder rod will bend, damaging the

cylinder. With the counterbalance valve installed,

the pressure will build up until it reaches

3,250 psi (224 bar) and then the counterbalance

valve will be internally piloted open, allowing the

pressure to return to the reservoir. This condition

will only exist if the cylinder has been extended. If

the cylinder is already fully retracted, the

counterbalance valve will have no effect and

damage can result.

• Should any of the hydraulic lines going to the

extend/retract cylinder fail, there will be a loss of

hydraulic system pressure to the cylinder. For

example, if there was a hydraulic line failure in the

pressurized retract line during retracting, the

counterbalance valve would lose pilot pressure,

closing off flow returning to the reservoir. The oil in

the base end of the cylinder would then be trapped,

which would immediately stop boom retraction,

preventing an elevated load from uncontrolled

retraction. The load can be lowered safely to the

ground by following the procedures in the appropri-

ate Owners/Operators Manual.

IMPORTANT: DO NOT attempt to reset a counterbalance

valve cartridge. In the event that a counterbalance valve

cartridge was disabled for emergency boom lowering or

any unauthorized adjustments are ever made to this

cartridge, remove and replace the cartridge with a new

part. Failure to replace this cartridge with a new part may

alter the holding characteristics of the counterbalance

valve, creating an unsafe condition for vehicle operation.

8.8.21 Boom Lift/Lower Circuit Description

Hydraulic flow is applied in the lift/lower circuit

(Fig. 8-7 and Fig. 8-8) by the hydraulic gear pump

through the unloader valve. System pressure is directed

to either side of the left and right lift/lower cylinders by the

shifting of the spool valve in the main control valve. The

spool valve is shifted by the operator joystick and its as-

sociated control cable.

MA7220

MA7230

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Skytrak 6042 Service Manual User Manual Page 284

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Skytrak 6042 Service Manual User Manual Page 284

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