HPRV (High Pressure Relief Valve)
LDU20/24 is equipped with a combination high pressure relief and charge check valve. The high-pressure
relief function is a dissipative (with heat generation) pressure control valve for the purpose of limiting
excessive system pressures. The charge check function acts to replenish the low-pressure side of the
working loop with charge oil. Each side of the transmission loop has a dedicated HPRV valve that is non-
adjustable with a factory set pressure. When system pressure exceeds the factory setting of the valve, oil
is passed from the high pressure system loop, into the charge gallery, and into the low pressure system
loop via the charge check.
The HST order code allows for different pressure settings to be used at each system port. HPRV valve with
orifice is available to gain wider neutral dead-band. When HPRV valves with orifice are used, it is only for
High pressure ports when vehicle goes in reverse. The system pressure order code for HST with only
HPRV is a reflection of the HPRV setting.
The system pressure order code for HST configured with pressure limiter and HPRV is a reflection of the
pressure limiter setting.
HPRV settings
HPRV valve without orifice
5 l/min [1.3 US gal/min]
HPRV valve with orifice
17 l/min [4.5 US gal/min]
HPRV (High Pressure Relief Valve)
P400005
C
Caution
HPRV´s are factory set at a low flow condition. Any application or operating condition which leads to
elevated HPRV flow will cause a pressure rise with flow above a valve setting. Consult factory for
application review.
Charge Check Relief Valve with Orifice
As an option, LDU20/24 offers a charge check relief valve with an orifice in order to enlarge the neutral
dead-band.
In some applications, it is desirable to use charge check valve with orifice for expanding null dead band,
giving both the safety measure to prevent the vehicle movement in the neutral position of the control
shaft and easy adjustment of neutral position when connected to vehicle linkage. The orifice connects
the working loop, which is a main hydraulic circuit, to a charge circuit. It always allows some internal
leakage to ensure the expanding null dead band around neutral position of control shaft. However, it
decreases the volumetric efficiency, particularly at high system pressure in the working loop. It is
recommended to install the orifice in a specific working loop, which is pressurized when the vehicle
moves in reverse. A cross section and characteristics are shown above. The charge check valves with
orifice are available with 0.85mm orifice today. This option is allowable for only high pressure port when
vehicle goes in reverse.
Service Manual
LDU20/24 Closed Circuit Axial Piston Transmission
Operation
©
Danfoss | June 2021
AX152886481311en-000304 | 9
