Check / High Pressure Relief Valve
LDT21/24/32 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 or into the transmission case via charge pressure relief valve (CPRV).
C
Caution
The High Pressure Relief Valve (HRPV) function is intended for short duration over-pressure protection /
regulation only.
Check/High Pressure Relief Valve with Orifice
Check/High pressure relief valve with orifice
As an option, LDT21/24/32 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 creep. 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
Valve
LDT21
LDT24
LDT32
Check / HPRV without
orifice
6 l/min [1.6 US gal/min]
4.7 l/min [1.3 US gal/min]
4.7 l/min [1.3 US gal/min]
check / HPRV with orifice
18 l/min [4.8 US gal/min]
N/A
N/A
P400005
C
Caution
HPRVs 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.
C
Caution
The High Pressure Relief Valve (HRPV) function is intended for short duration over-pressure protection /
regulation only.
Service Manual
LDT 21/24/32 Closed Circuit Axial Piston Transmission
Operations
©
Danfoss | June 2022
AX417167456877en-000101 | 9
