In your past couple of hydraulic maintenance emails,
I explained how to accurately assess the condition
of a hydrostatic transmission using case drain flow.
Today I want to switch gears and talk about
a type of hydraulic control in widespread use,
but not well understood: load sensing.
Load sensing is a term used to describe a type of
variable pump control used in open circuits. It is
so called because the load-induced pressure downstream
of an orifice is sensed and pump flow is adjusted to
maintain a constant pressure drop (and therefore flow)
across the orifice.
The 'orifice' is usually a directional control valve
with proportional flow characteristics. But a needle
valve or even a fixed orifice can be employed,
depending on the application.
In hydraulic systems that are subject to wide fluctuations
in flow and pressure, load-sensing circuits can save
substantial amounts of input power.
A load sensing circuit typically comprises a
variable displacement pump, usually axial-piston design,
fitted with a load sensing controller, and a directional
control valve with an integral load-signal gallery.
The load-signal gallery is connected to the load-signal port
on the pump controller. The load-signal gallery in the
directional control valve connects the A and B ports of
each of the control valve sections through a series of
shuttle valves.
This ensures that the actuator with the highest load pressure
is sensed and fed back to the pump control.
To read the rest of this story - with diagrams,
point your browser to
http://www.insidersecretstohydraulics.com/load-sensing.html
Yours for better hydraulics knowledge,
Brendan Casey
Author of 'Insider Secrets to Hydraulics'
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