HYDRAULIC SYSTEM THEORY OF OPERATION

TM 5-2410-237-23

HYDRAULIC SYSTEM THEORY OF OPERATION

SYSTEM DESCRIPTION

The hydraulic system supplies a controlled flow of filtered hydraulic oil for operation of the bulldozer blade and ripper.

Principle components of the hydraulic system are: pilot valves (1 and 2), pumps (3 and 4), pressure control valve (5),

ripper control valve (9), quick drop valves (13), blade lift cylinders (14), blade control valve (lift and tilt) (17), blade tilt

cylinder (18), ripper lift cylinders (22), filter (25) and hydraulic tank (26).

A two-section vane pump (3 and 4) draws oil from the hydraulic tank (26) whenever the engine is running. The small

section of pump (4) supplies oil for the pilot system and operation of the blade tilt cylinders (18). The large section of

pump (3) supplies oil to operate the blade lift cylinders (14) and the ripper lift cylinders (22). If the cylinder is not being

operated, oil flow from both sections of the pump is combined within the blade control valve (17) to supply the blade lift

cylinders (14).

Pilot oil flows from the small section of the pump to the pressure control valve (5). This oil flow moves a valve spool

against concentric springs. The force of these springs produces a back pressure of 115 psi (758 kPa) on the small section

of the pump, and also provides operating pressure for pilot valves (1 and 2).

The blade tilt and ripper lift functions are controlled by pilot valves (1 and 2). The respective control levers connect to

the spools of the pilot valves. The blade lift control lever directly controls the spool (36) of the blade lift control valve;

no pilot valve is involved.

Return oil from all circuits is passed through a filter (25) before entering the tank. The filter has a bypass feature, which

operates a visual indicator that shows when bypassing is occurring.

BULLDOZER BLADE LIFT SYSTEM

The blade control valve (17) has four operating positions: RAISE, HOLD, LOWER and FLOAT. The lift valve spool is

moved to the desired operating position by fore and aft movement of the blade control lever.

The RAISE and LOWER positions raise or lower the blade. In the HOLD position, the spool is centered and oil flow to

and from the lift cylinders is blocked. The FLOAT position (lever fully forward) has detents to hold the valve spool in

position. In the FLOAT position, both ends of the blade lift cylinders are at "tank" pressure; thus, the blade can be

moved up or down by outside forces.

With the blade control lever in the HOLD position and the engine running, oil flow from both pump sections combines

in passage (28) of the control valve. When pressure in passage (28) exceeds the 80 psi (552 kPa) pressure exerted by

spring (29), dump valve (30) will unseat and the oil flow from both pump sections will return to the tank via oil return

line (24).

With the blade control lever in the RAISE position, blade lift valve spool (36) is moved into the valve body. Pressure

buildup in the valve unseats load check valve (37). Oil then flows under pressure through quick drop valves (13) to the

rod ends of the blade lift cylinders (14). Cylinder movement raises the bulldozer blade. Exhaust oil from the head ends

of the cylinders returns to the tank through passages in the control valve (17).

Moving the blade lift control lever to the LOWER position directs pressurized flow to the head ends of the blade lift cyl-

inders (14). Return oil from the rod ends of the blade lift cylinders can either pass to the tank through the control valve

(17) or it can be added to the flow going to the head ends through the makeup valve (38) in the control valve.

If working pressure of the lift cylinders is greater than the setting of relief valve (32), the relief valve will unseat. Dump

valve (30) will then open, allowing overflow to momentarily return to the tank. The relief valve (32) will reseat when

cylinder working pressure drops below relief setting. The relief setting is adjusted by adding or removing shims.

BULLDOZER BLADE TILT SYSTEM

The blade tilt control spool and related valves are located in the upper portion of the blade control valve (17).

Pilot oil to operate the blade tilt control spool and supply oil for the tilt cylinder (18) is provided by the small section of

the pump (4). All flow from the small section of the pump goes into pressure control valve (5). As mentioned previously,

spring force within the pressure control valve sets a minimum pump operating pressure of 115 psi (758 kPa).