 Flow-control
or speed-control valves are used to control the volume of oil that
passes a given point. A flow-control valve is a metering valve and
a check valve built into one housing. The volume may be controlled
as it approaches a component or as it leaves a component. The design
of the metering valve may vary from a needle to a cam-shaped device.
Cam-operated,
flow-control valves are of value where flow control is only required
for a portion of the cylinder stroke. They can be used as a cushioning
device to provide extra long cushioning for a cylinder or for slip
feeding arrangements by placing multiple cams on a trip bar. The
one disadvantage of this type of valve is that the orifice must
be designed for each size of pump as there is no adjustment means
available.
Flow-control
valves are normally built in sizes from ¼ -inch up to 2-inch
pipe size. The majority are built for 1500 lb per sq in. maximum
pressure but a number are available for much higher pressures.
Flow-control
valve gives trouble due to one of the following reasons:
- If
they are placed incorrectly or backwards in the circuit- This
occurs inspite of the fact that they are usually marked with a
large arrow showing the direction of the controlled flow.
- Dirt
lodging in the orifice causes it to clog up and produce erratic
feed. Dirt under the check valve seat does not allow the check
to seal off thus producing leakage.
-
A considerable change in oil viscosity has a definite effect on
the flow-setting, especially on a needle-type metering device.
-
Excess internal leakage produces poor flow control. In this case,
if the leakage becomes too much, it is possible that even changing
the setting on the needle from one extreme to the other will not
change the flow rate appreciably.
Types of Flow Control Valves
The flow control valves could be either:
a) throttle valves or flow restrictors that are pressure dependent
b) flow control valves that are pressure independent.
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