Hydraulic Interlocks: Hydraulic interlocks use direction control valves to ensure that conflicting motions are
prevented.
[a] Single actuator can move only in one direction. The cylinder ram can either move in
an outstroke, or an instroke. A manually-operated valve automatically ensures this. The
operating lever [or pedal] can be moved towards the right or left, moving the ram in
outstroke or instroke. Similarly, a pedal can be pressed downward, or allowed to return
to a spring-actuated, up position. Manual operation also ensures that a hydraulic motor
moves either clockwise or anti-clockwise.
When the Direction Control [D.C.] valve is operated by solenoid [electromagnets], the
interlocking is effected by a contactor, which switches off the other coil, when one solenoid
is energized.
[b] Two or more actuators can also be interlocked, by using a common direction control
valve, before using individual direction control valves. The interlocking
D.C. valve will allow the pressurized oil to flow to only one of the actuators, thereby,
preventing the operation of the other. This can ensure that the horizontal actuator will
not function when the vertical actuator is in motion, and vice-versa.
In an automated drilling machine, drill-feeding ram will not function when the clamping
[or workpiece feeding] ram is moving. Each interlock will require an additional direction
control valve.
McGraw-Hill, Machine Tools Handbook : Design and Operation, P.H. Joshi, 2007, P. 561
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