213 TRACK-TYPE EXCAVATOR How the Tank Pressurization Valve Functions Caterpillar


How the Tank Pressurization Valve Functions
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Fig. WH 152

The tank pressurization valve is integrated in the tank return line of the left valve bank in order to prevent damage to the hydraulic running gear motor as a result of excessive thrust during downhill travel (avoidance of cavitation or insufficient fluid). The pressurization valve guarantees a back pressure of approximately 30 bar (3000 kPa) in the return line system of the hydraulic running gear motor and thus also guarantees the oil supply to the inlet of the running gear motor.

When the pilot control valve for the running gear motor is switched on, the pressurization valve at connection "X" is simultaneously subjected to a maximum of 30 bar (3000 kPa) pilot pressure (when pilot control valve is fully on). This control pressure passes through a jet, item 11, (section A-A), and acts on the front of the piston, item 7. Together with the spring, item 3, this pressure pushes the piston, item 7, out into the position shown on the T4/T2 side. Return oil arriving at T5 and T5 is prevented from flowing to T2 and T1, i.e. the pressure control function of the pressurization valve creates a pressure differential of approximately 30 bar (3000 kPa) between T4 and T2 and between T5 and T1. This 30 bar (3000 kPa) back pressure makes for a definite improvement in the function of the makeup valve on the valve bank, left (3rd section), so that gradients (slopes or inclines) can be negotiated without oil deficiencies in the running gear motor.

The pressurization valve is actuated only when the machine is traveling. When excavating, i.e. when the pilot control valve for the running gear motor is not actuated, there is no back pressure function (unwanted here) and the return oil can flow from T4 to T2 or from T5 to T1 with little resistance.

The pump pressure for the running gear motor reaches connection P of both covers, item 5, behind the piston, item 8, by means of the jet, item 11. The surface ratio of the two pistons, items 7 and 8, is selected in such a way that at a pump pressure of approximately 70 bar (7000 kPa), the back pressure function of the pressurization valve is eliminated, piston 8 pushing piston 7 in (shown on side T5/T1). This automatic control on the high pressure side is important because it prevents a loss of traction in uphill travel, as would be the case with a torque from 30 bar (3000 kPa).

The protection function of the pressurization valve is provided by the oblique channel fitted with a jet (item 1). Even if there is no pump pressure at connection P of the covers, item 5, during (downhill) travel this does not cause the back pressure at T4 and T5 to exceed 30 bar (3000 kPa), since this pressure acts on the back of the piston, item 7, via the jet, item 1, and thus creates a pressure balance.

T1 Discharge to hydraulic oil tank

T2 Discharge to hydraulic oil tank

T3 Discharge to hydraulic oil cooler

T4 Inflow from left valve bank, discharge in T2

T5 Inflow from right valve bank, discharge in T1