HYDRAULIC AND AIR SYSTEMS 2250 SERVICE/MAINTENANCE MANUAL
2-36
Published 07-19-16, Control # 249-01
AIR PRESSURE SAFETY SWITCHES
MAINTENANCE
General
See Figure 2-41 for the following.
The air pressure safety switch consists of an electric limit
switch and a diaphragm-type valve. The pressure switch is
used to control operation of auxiliary electrical devices or
circuits in response to air pressure.
Air pressure safety switches are used for the following crane
functions:
• To monitor the load drum clutch/brake maximum air
pressure to ensure the appropriate clutch/brake is
applied before allowing operation of load drums—This
switch also alerts the programmable crane controller if
the clutch/brake has not applied for a duration of 4
seconds or more. The AIR DRUM VALVES fault will
appear on the crane display. Each drum clutch/brake
safety switch is normally closed (N/C) with a trip point of
4,14 bar (60 psi).
• To monitor the free-fall interlock treadle valve minimum
air pressure—When the selected drum’s service brake
treadle valve is locked down, the safety pressure switch
sends a signal to the programmable crane controller
allowing the operator to confirm the selected drum into
Free-Fall mode. Each free-fall interlock safety switch is
normally open (N/O) with a trip point of 4,14 bar (60 psi).
• To monitor system air pressure—The safety pressure
switch sends a signal to the programmable crane
controller to alert the operator with a LOW AIR
PRESSURE system fault when activated. The air
pressure safety switch is N/O with a trip point of 6,21 bar
(90 psi).
NOTE: Always confirm by crane serial number the specific
pressure at which the limit switch is set and the
operation for which the limit switch is wired by
referring to the corresponding assembly drawing
and air/electrical schematics.
Operation
As pressure increases, the diaphragm moves up causing the
adjusting screw to move up. When the pressure reaches the
specified point, the adjusting screw pushes the limit switch
plunger in, and the switch contacts either open or close.
If the limit switch is wired N/O, the contacts close to turn on
the corresponding circuit when the specified pressure is
reached. If the limit switch is wired N/C, the contacts open to
turn off the corresponding circuit when the specified pressure
is reached.
Adjustment Requirements
Adjustments can be performed when either installed on the
crane or when removed from the crane. Adjustments will be
easier and more accurate when done with the pressure
switch removed from the crane. The following items will be
required to perform the adjustment:
• Air supply capable of being regulated up to 8,27 bar
(120 psi)
• Calibrated 0 to 10,34 bar (0 to 150 psi) air gauge
• DC voltage power supply
• Bulb-type continuity tester
• 1/4” open-end wrench
NOTE: Air pressure and electric current from the crane
source can be used for this adjustment.
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