Main Relief Valve Function - SANY SY195C9 Shop Manual

SY195/205/215/225C9 Crawler Hydraulic Excavator

4.5.2.10 Main relief valve function

The main relief valve is installed on the valve
block A (101). Its function is described below:
● Working oil flows into chamber (a) from
passage (P) via the hole of valve seat (541)
and hole on valve rod (512). Valve rod (512)
is pressed against valve seat (541), form-
ing a tight seal.
● When pressure in passage (P) exceeds
the elastic force of spring (621), cone valve
(611) is pushed open. Therefore, working
oil fl ows through the hole on valve sleeve
(103) via the periphery of cone valve (611)
Shop Manual - June 2013
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Structure and Function
and enters low-pressure path (R).
● Opening of cone valve (611) causes pres-
sure in chamber (a) to drop and valve rod
(512) is open. In this way, working oil in
passage (P) flows into low-pressure path
(R) directly.
● When pressure oil high than 3MPa is sup-
plied to port Pz, this pressure oil pushes
plunger (614) to the left. The set load of
spring (621) is changed and pressure of
the main relief valve rises.
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