Page 230 - Root Cause Failure Analysis

218 Root Cause Failure Analysis

Manual control devices (e.g., levers, cams, or palm buttons) can be used as the pri-
mary actuator on most fluid power control valves. Normally, these actuators are used
in conjunction with a secondary actuator, such as a spring return or detent, to ensure
proper operation of the control valve and its circuit.

Spring returns are used in applications where the valve is designed to stay open or
shut only when the operator holds the manual actuator in a particular position. When
the operator releases the manual control, the spring returns the valve to the neutral
position.

Valves with a detented secondary actuator are designed to remain in the last position
selected by the operator until manually moved to another position. A detent actuator is
simply a notched device that locks the valve in one of several preselected positions.
When the operator applies force to the primary actuator, the valve shifts out of the
detent and moves freely until the next detent is reached.

Pilot Although a variety of pilot actuators is used to control fluid-power valves, they
all work on the same basic principle. A secondary source of fluid or gas pressure is
applied to one side of a sealing device, such as a piston or diaphragm. As long as this
secondary pressure remains within preselected limits, the sealing device prevents the
control valve’s flow-control mechanism (i.e., spool or poppet) from moving. How-
ever, if the pressure falls outside the preselected window, the actuator shifts and forces
the valve’s primary mechanism to move to another position.

This type of actuator can be used to sequence the operation of several control valves
or operations performed by the fluid-power circuit. For example, a pilot-operated
valve is used to sequence the retraction of an airplane’s landing gear. The doors that
conceal the landing gear when retracted cannot close until the gear is fully retracted.
A pilot-operated valve senses the hydraulic pressure in the gear-retraction circuit.
When the hydraulic pressure reaches a preselected point that indicates the gear is fully
retracted, the pilot-actuated valve triggers the closure circuit for the wheel-well doors.

Solenoid Solenoid valves are widely used as actuators for fluid-power systems. This
type of actuator consists of a coil that generates an electric field when energized. The
magnetic forces generated by this field force a plunger attached to the main valve’s
control mechanism to move within the coil. This movement changes the position of
the main valve.

In some applications, the mechanical force generated by the solenoid coil is not suffi-
cient to move the main valve’s control mechanism. When this occurs, the solenoid
actuator is used in conjunction with a pilot actuator. The solenoid coil opens the pilot
port, which uses system pressure to shift the main valve.

Solenoid actuators always are used with a secondary actuator to provide positive con-
trol of the main valve. Because of heat buildup, solenoid actuators must be limited to
short-duration activation. A brief burst of electrical energy is transmitted to the sole-