Copyright Goodheart-Willcox Co., Inc.
444 Hydraulic Systems for Mobile Equipment
Torque-Limiting Control
Some variable-displacement LSPC pump systems are designed to provide
high amounts of fl ow or high amounts of pressure, but use an engine that
does not have the capacity to deliver enough horsepower to provide both max-
imum system fl ow and maximum system pressure simultaneously. In these
applications, a torque-limiting spool is incorporated into the pump compen-
sator assembly. It destrokes the pump when the operator requests too much
hydraulic horsepower.
Corner Horsepower
Corner horsepower is the point on a PQ curve that shows the total amount
of horsepower required to deliver both maximum system hydraulic pressure
and maximum system hydraulic fl ow simultaneously. See Figure 18-20. The
corner horsepower is located in the upper right-hand corner of the PQ curve.
A pump with torque-limiting control shaves off the corner horsepower
requirement by preventing the operator from requesting both maximum fl ow
and maximum pressure simultaneously, Figure 18-21. Manufacturers save on
production costs by designing a machine equipped with torque-limiting con-
trols. The system allows a smaller engine with reduced fuel consumption com-
pared to unequipped machines.
In addition to housing the pressure compensator and fl ow control spools,
the variable-displacement LSPC pump in Figure 18-22 uses two components
as torque-limiting controls: a piston that senses the angles of the pump’s swash
plate and a torque-limiting spool valve. The piston uses an orifi ce that drops oil
pressure based on the angle of the pump’s swash plate. When the pump swash
plate is at a large angle, delivering high amounts of hydraulic fl ow, the swash
plate sensing piston is pulled out of its bore resulting in little pressure drop.
As a result, high pump outlet pressure causes the torque-limiting spool valve
Figure 18-20. A hydraulic system that is producing maximum flow and maximum pressure simultaneously is depicted
in the upper right-hand corner of a PQ curve.
Pump
displacement
Corner
horsepower
Metering point
Usable
power
Operating Pressure (psi)
Pump
Flow
Rate
(gpm)
Conserved power
Wasted power
Pressure
compensator
stall pressure