205 AND 211 TRACK-TYPE EXCAVATORS Check and Relief Valve (For Boom Cylinder Protection) Caterpillar

Boom Relief And Check Valve - Fig. WH 214

Function of Boom Relief and Check Valve

The valve is a pilot controlled sliding seat valve controlled through a separate control line. If control line pressure is 0, the valve acts as a normal check valve, i.e., the applied load closes the valve cone (the higher the load, the higher the closing force) on the check valve. The special design allows sensitive opening of the cross section by applying a control pressure. In addition to the secondary protection, a direct controlled pressure relief valve is installed. The balance pipe required for two cylinder operation is provided with a small check valve so that, in case of tube rupture between the balance lines, this small check valve will close tightly. For emergency lowering, an appropriate emergency drain (boom vent) is provided on each valve, which must always be operated manually with wrenches.

Circuit Arrangement

The valve unit is installed on the cylinder immediately with a SAE master gauge. The return lines of the pressure valves and of the emergency drain come directly to the oil reservoir. In case of two cylinder operation, the balance pipes are interconnected. The control line is parallel connected with the control line of the corresponding directional control valve. This offers the advantage that only small ID control lines must be used.

Function on the Machine

Lifting: The oil flowing into the cylinders flows through the check valves integrated into the unit with a low precharge. If a pipe rupture occurs between the unit and control valve bank during lifting, the load is held by the above described boom check valve.

Lowering: The control pressure for lowering is applied to the check valve in parallel with the directional control valve bank. The opening threshold and the opening cross section being determined such that, there is no pressure between the valve unit and the valve bank. Thus, the valve unit and valve bank will not result in a speed increase. In case of line rupture, the lowering speed can be made to be zero by reducing the control pressure.

The design has been made such that the small inner diameters (ID) of the control lines required for sensitive lowering are flushed in each lifting process. This results in particularly high insensitivity to dirt contamination.

The layout of the unit is such that, in case of double cylinders (boom), it may be flange mounted indifferently either to the right hand or to the left hand cylinder. There is no special left and right hand configuration.

Show/hide table

Make reference to WARNING on first page of the TESTING AND ADJUSTING Section.

NOTE: Make these tests when the hydraulic oil is at normal temperature of operation. (1 Bar is equal to 100 kPa; 1 Bar is equal to 14.5 psi; 1 psi is equal to 6.9 kPa)

Procedures To Be Taken After Line Rupture

1. Line rupture between valve bank and check valve in the lift line: put the load down, replace the damaged section.

2. Line rupture in the balance pipe: in case of rupture in the balance pipe, there is no external leakage due to integrated seat valves. Lift operation is possible without limitation. Lowering is possible only for putting down the load, since there are high pressure differences in the two boom cylinders.

Immediate Repair of the Balance Pipe

For this purpose, put down the load, replace the pipe and, prior to putting the machine into operation, apply pressure to the rod side of the boom cylinders, i.e. initiate lowering movement, thereby lifting the machine. In this way, the boom cylinders are sure to be relieved on the bottom side. The pressure in the lift lines being relieved, the check and relief valves in the balance pipe will open.

3. General hydraulic failures:

- No pilot pressure (pilot pump etc.)

- Failure of double axial piston pump

- Engine failure

If it is necessary to lower the implements, use emergency actuating elements integrated into the check valves (see the schematic in Fig. WH 214).