System Design Parameters; Case Drain; Charge Flow Requirements; Loop Flushing - Danfoss LPV Technical Information

Technical Information
LPV Axial Piston Pumps

System design parameters

Case drain

Charge flow requirements

Loop flushing

Bearing loads and life

10 | © Danfoss | September 2017
Connect the case drain line to one of the case outlets to return internal leakage to the system reservoir.
Use the higher of the outlets to promote complete filling of the case. Case drain fluid is typically the
hottest fluid in the system. Return case drain flow through the heat exchanger to the reservoir.
All LPV pumps applied in closed circuit installations require charge flow. The charge pump provides flow
to make up internal leakage, maintain a positive pressure in the main circuit, provide flow for cooling and
filtration, replace any leakage losses from external valving or auxiliary systems, and to provide flow and
pressure for the control system.
Many factors influence the charge flow requirements and charge pump size selection. These factors
include system pressure, pump speed, pump swashplate angle, type of fluid, temperature, size of heat
exchanger, length and size of hydraulic lines, control response characteristics, auxiliary flow
requirements, hydrostatic motor type, etc. When sizing and selecting hydrostatic units for an application,
it is frequently not possible to have all the information necessary to accurately evaluate all aspects of
charge pump size selection.
Maintain charge pressure at the level specified in the table
operating conditions to prevent damage to the transmission. Danfoss recommends testing under actual
operating conditions to verify this.
Charge pump displacement should be at least 10% of the total displacement of all axial piston
components in the system. However, unusual application conditions may require a more detailed review
of charge pump sizing. Refer to Selection of Drive line Components, BLN-9985, for a more detailed
selection procedure, or contact your Danfoss representative for assistance.
Closed circuit systems may require loop flushing to meet temperature and cleanliness requirements. A
loop flushing valve removes hot fluid from the low pressure side of the system loop for additional cooling
and filtering. Ensure the charge pump provides adequate flow for loop flushing and the loop flushing
valve does not cause charge pressure to drop below recommended limits.
LPV utilizes a special loop flushing spool design. On dual path systems, take special care to verify
acceptable performance.
Bearing life is a function of speed, system pressure, charge pressure, and swashplate angle, plus any
external side or thrust loads. The influence of swashplate angle includes displacement as well as
direction. External loads are found in applications where the pump is driven with a side/thrust load (belt
or gear) as well as in installations with misalignment and improper concentricity between the pump and
drive coupling. All external side loads will act to reduce the normal bearing life of a pump. Other life
factors include oil type and viscosity.
In vehicle propel drives with no external shaft loads and where the system pressure and swashplate angle
are changing direction and magnitude regularly, the normal L20 bearing life (80 % survival) will exceed
the hydraulic load-life of the unit.
In non propel drives such as vibratory drives, conveyor drives, or fan drives, the operating speed and
pressure are often nearly constant and the swashplate angle is predominantly at maximum. These drives
have a distinctive duty cycle compared to a propulsion drive. In these types of applications a bearing life
review is recommended.

Applications with external shaft loads

LPV pumps have bearings that can accept some external radial and thrust loads. When external loads are
present, the allowable radial shaft loads are a function of the load position relative to the mounting
Technical specifications on page 20 under all
520L0954 | BC00000044en-US0102