Danfoss LDU21C Service Manual page 10

Service Manual
LDU21C Closed Circuit Axial Piston Transmission
Operation
Check / High Pressure Relief Valve
CPRV (Charge Pressure Relief Valve)
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10 | Danfoss | April 2025
LDU21C 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 pump 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 pumps with
only HPRV is a reflection of the HPRV setting.
The system pressure order code for pumps configured with pressure limiter and HPRV is a reflection of
the pressure limiter setting.
Check/High pressure relief valve with orifice
As an option, LDU21C offers Check / HPRV with an orifice produce a larger neutral deadband.
In some applications, it is desirable to use Check / HPRV with an orifice to expand null dead band, which
would help provide a larger margin of safety for vehicle movement in neutral and provide easier
adjustment of the vehicle linkage for machine neutral. 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. Check / HPRV with an orifice has possibility to
increase downhill creap. It is recommended to install the orifice in a specific working loop, which is
pressurized when the vehicle moves in reverse.
The HPRV are set at the following flow rates
Check/HPRV without orifice
Check/HPRV with orifice
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.
The charge pressure relief valve maintains charge pressure at a designated level above case pressure. The
charge pressure relief valve is a direct acting poppet valve which opens and discharges fluid to the HST
case when pressure exceeds a designated level. For external charge flow the CPRV is set according to
below table. The charge pressure relief valve setting is specified on the model code of the pump.
6 l/min [1.6 US gal/min]
18 l/min [4.8 US gal/min]
P400005
AX342138485422en-000102