Copyright Goodheart-Willcox Co., Inc.
Chapter 18 Load-Sensing Pressure-Compensating (LSPC) Hydraulic Systems 445
to shift against its spring, opening a passage to allow oil to destroke the pump.
A torque limiter acts like an infi nitely variable relief valve that destrokes the
pump anytime too much hydraulic horsepower is requested.
Torque limiting is a common descriptor used by many manufacturers.
However, horsepower limiting is a more accurate descriptor because the con-
trol is limiting the total amount of hydraulic horsepower in an effort to prevent
the engine from lugging, stalling, or even dying.
Figure 18-21. Torque-limiting control reduces the required amount of engine horsepower by preventing an operator
from requesting maximum flow and maximum pressure simultaneously. Note the movement of the corner horsepower
point on the PQ curve for a hydraulic system equipped with torque-limiting controls.
Figure 18-22. A hydraulic pump with torque-limiting control allows the operator to request maximum flow and
maximum pressure individually but not simultaneously. A piston senses the angle of the swash plate, and when too
much hydraulic horsepower is requested from the system, it causes a torque-limiting spool to destroke the pump.
Pump
displacement
Torque limitation =
smaller power source
Metering point
Usable
power
Operating Pressure (psi)
Pump
Flow
Rate
(gpm)
Conserved power
Wasted power
Pressure
compensator
stall pressure
Bias spring
Swash plate
Axial
pistons
Pressure
compensator
spool (2500 psi)
Flow control
spool (300 psi)
Control
piston
Sensing
piston
Torque-limiting spool
DCV