MACHINERY LUBRICATION- INDIA MARCH-APRIL 2020 | Page 20

HYDRAULICS
viscosity will be. Thicker oil cannot move as rapidly through an
orifice as thinner oil can.
Fixed Orifice
Figure 2. A fixed-orifice flow control symbol
The fixed-orifice flow control (Figure 2) may or may not be a proper
component in a machine. By definition, it is nothing more than a
hole of a specific size that cannot be adjusted. It may be simply a
drop in line size. Sometimes when a 50 mm hydraulic line is teed off
to a 6 mm line, a draftsman may represent this as a fixed orifice. If
you remove the valves from a manifold, you will find large holes and
small holes drilled through it. The fixed-orifice flow control symbol
may represent one of the smaller holes. It may also represent an
orifice that can be removed from the manifold with an Allen wrench.
Whatever form it takes, it should never be removed and replaced by
a variable-orifice flow control. The designer had something specific
in mind when calling for a fixed-orifice flow control. For whatever
reason, the designer did not want it to be adjusted. Its purpose may
be to synchronize flow to more than one actuator, or it may be for
safety purposes to keep an actuator from moving too rapidly.
Variable Orifice
Figure 3. A variable-orifice flow
control symbol
Figure 4. A flow control symbol with a bypass check valve
The variable-orifice flow control symbol in Figure 3 has a diagonal
arrow to indicate that it can be adjusted. It often is called a needle
valve because a common construction uses a conical needle that
seats to close off the valve. The conical needle is called a vernier. The
purpose of the vernier is to make the adjustment proportional to the
number of turns made on the adjustment. A common number of
turns between fully open and fully closed is five, so each full turn of
the knob will change the orifice size by 20 percent.
Manual valves such as ball valves, gate valves and butterfly valves
should never be used to control speed in a hydraulic system. These
types of valves are meant to be open or closed. Keeping them
partially open in a high-pressure hydraulic system will cause them
to be unable to seat properly, and they will not close all the way. The
vernier is specifically designed to adjust flow.
There are also cartridge-type flow controls that mount in a manifold
or valve stack. While these are not true needle valves, they are
engineered with a spool that is cut to limit flow.
Figure 5. A pressure-compensating flow control symbol
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It is not uncommon for either fixed- or variable-orifice flow controls
to have a built-in bypass check valve (Figure 4). The flow control
with a bypass will limit flow in one direction but will allow free flow
in the opposite direction. The purpose of these controls is usually to
enable independent forward and reverse speed control.