FLUID POWER
A truly dynamic boost
68 iVT International Off-Highway 2020
DYNAMIC LOADSENSE SOLUTION
OFFERS AGRICULTURAL
EQUIPMENT A BOOST
ere’s a new dynamic in the agricultural
equipment world. Following the trend of
consolidation in agriculture (farm size and acres
tilled), manufacturers are building bigger
implements to perform tilling, seeding, planting,
and spraying over larger elds faster and more
e ciently. Tractors too are being made available
with more auxiliary hydraulic capacity than ever
before. Because of the increase in ow demands
many load-sensing, power-beyond hydraulic
systems su er from poor performance due to low
margin pressure. Hydraulic pressure drops in the
system caused by steering and braking priority
valves, and longer plumbing runs to the actuators
on the implement. is erodes the available
margin needed for load-sensing systems to
perform and meet ow demands.
Load-sensing hydraulics
Higher ow auxiliary hydraulic systems typically
employ a variable displacement, load-sensing
open circuit pump. e basic operation of the
pump is to provide ow on-demand to the
hydraulic system, and it is able to reduce the
displacement when the ow is not required.
When the pump is in standby mode, there is
no ow at the outlet. e pump displacement is
near zero, but in reality it rests at an equilibrium
that displaces enough to overcome internal
leakage and maintain its set margin pressure.
e spring in the pump control sets the pump’s
margin pressure or standby pressure. During
operation, the margin is the di erence between
the pressure at the load and the pressure at
the pump outlet.
Load-sensing hydraulic systems have
a signal input that communicates the ow
demand of the system. e control operates on
the balance of pressure between the pump outlet
and the signal line. At equilibrium the pump
outlet pressure is equal to the load signal pressure
plus the margin pressure. If the pressure in the
signal line rises, the di erence decreases causing
the control valve to command the pump to
increase displacement until the margin is satis ed
and equilibrium returns.
Traditional solution
In addition to the auxiliary hydraulics, the loadsensing
variable displacement pump supplies
ow for steering and braking functions on the
tractor. A priority valve diverts the ow to these
functions. e remaining ow capacity is
available for auxiliary hydraulics. Priority valves
and long auxiliary hydraulic piping runs add
pressure drop to the system which leaves less
margin available for load sensing. is o en
means that the full ow of the hydraulic pump is
not delivered at times of peak demand.
e traditional solution to this issue has
been to improve the margin by boosting the
load-sensing signal at the auxiliary coupling
using a pressure reducing valve. When applied
appropriately, the pressure-reducing valve acts in
reverse to amplify the signal present on the loadsense
line. is approach results in an additive
pressure boost: LSin + Spring Value = LSout. e
increased load-sensing pressure communicates a
higher ow demand. is is a cost-e ective
solution in terms of implementation cost,
HYDRAFORCE