18
|
July - August 2019
|
www
.
machinerylubrication.com
e speed of a hydraulic
system is determined
by the amount of f low
delivered. Normally, fl ow
controls are used to accomplish this.
While many people are aware that
a fl ow control or orifi ce will limit
the hydraulic fl ow in a system, they
may not realize that orifi ce size isn't
the only variable that will aff ect the
fl ow and therefore the speed of a
hydraulic actuator, such as a cylinder
or hydraulic motor. ere actually
are three variables that aff ect fl ow:
the orifi ce size, the pressure diff er-
ence between the inlet and outlet of
the orifi ce, and the oil temperature.
Orifi ce Size
e size of the orifi ce is fairly
straightforward. e bigger the hole,
the more fl ow will pass through it.
Many fl ow controls have a variable
orifi ce size, so turning the adjust-
ment counterclockwise will increase
fl ow, while turning it clockwise will
close the valve, limiting the fl ow and
slowing down the actuator.
Pressure Diff erence
Whenever a f low control is
adjusted so that it limits fl ow, there
will always be a pressure drop
across the orifi ce. Any restriction of
fl ow causes back pressure to build
upstream of the valve. e greater
the pressure drop, the more fl ow 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. is is to be expected,
because the higher the oil tempera-
ture, the lower the oil viscosity
will be. icker oil cannot move as
rapidly through an orifi ce as thinner
oil can.
Fixed Orifi ce
The fixed-orifice f low control
(Figure 2) may or may not be a
proper component in a machine. By
defi nition, it is nothing more than a
hole of a specifi c size that cannot be
adjusted. It may be simply a drop in
Controlling the Speed
of a Hydraulic System
HYDRAULICS
Jack Weeks
|
GPM Hydraulic Consulting
"If speed
control is
important to
your operation,
switching to
one of these
flow controls
may help."
Figure 1. The higher the upstream pressure reading on
gauge A (as compared to gauge B), the harder the fl ow is
pushed through the orifi ce.
A B
