MACHINERY LUBRICATION- INDIA MARCH-APRIL 2020 | Page 19

Jack Weeks | GPM Hydraulic Consulting
HYDRAULICS
Controlling the Speed
of a Hydraulic System
“If speed
control is
important
to your
operation,
switching to
one of these
flow controls
may help.”
A
The speed of a hydraulic
system is determined
by the amount of flow
delivered. Normally, flow
controls are used to accomplish
this. While many people are
aware that a flow control or orifice
will limit the hydraulic flow in a
system, they may not realize that
orifice size isn’t the only variable
that will affect the f low and
therefore the speed of a hydraulic
actuator, such as a cylinder or
hydraulic motor. There actually
are three variables that affect
flow: the orifice size, the pressure
difference between the inlet and
outlet of the orifice, and the oil
temperature.
B
Figure 1. The higher the upstream pressure reading on
gauge A (as compared to gauge B), the harder the flow is
pushed through the orifice.
Orifice Size
The size of the orifice is fairly
straightforward. The bigger
the hole, the more f low will
pass through it. Many f low
controls have a variable orifice
size, so turning the adjustment
counterclockwise will increase
flow, while turning it clockwise
will close the valve, limiting
the flow and slowing down the
actuator.
Pressure Difference
Whenever a f low control is
adjusted so that it limits flow,
there will always be a pressure
drop across the orifice. Any
restriction of flow causes back
pressure to build upstream of the
valve. The greater the pressure
drop, the more flow will pass
through it. Figure 1 provides a
good example of this.
Oil Temperature
You may notice some machines
move more slowly at startup
than they do once the oil gets
up to temperature. This is to be
expected, because the higher the
oil temperature, the lower the oil
www.machinerylubricationindia.com
| March - April 2020 | 17