Service Manifold Manual

drops, downstream pressure and a control spring fully close the main control valve. The HSM returns to
the pressure-off mode.
About HSM Pilot Pressure
The HSM is equipped with a hydraulic circuit which provides separately filtered fluid at HPU pressure to
operate the pilot stage of multistage servovalves. This portion of the HSM operates automatically and
requires only routine filter maintenance.
Note:
Turning pressure on and off, or changing between low and high output pressure, does not affect
the pilot pressure output. Pilot pressure is always present, as long as pressure is available from the
HPU. The pilot circuit takes time to bleed down after high pressure is removed from the HSM.
HSM Accumulators
The pressure, return, and pilot pressure accumulators reduce pressure and flow fluctuations caused by
changing system demands. When the HPS is located some distance from the actuator, the flow and
pressure of the hydraulic fluid may be inadequate at certain frequencies.
The type and frequency of the servovalve command signal affects the efficiency of the accumulator. Square
waves cause a greater demand than sine or ramp signals. At low frequencies, fluid flow in the lines can
stop completely and fluid inertia can become an operational factor.
The pressure accumulator reduces the inertia and line restriction considerations. When the servovalve
opens and line pressure begins to drop, the accumulator supplies part of the fluid volume to help maintain
line pressure. Then, when the servovalve closes, the accumulator recharges, causing fluid in the lines to
remain in motion.
An accumulator in the return line reduces pulsing caused by the varying amounts of fluid being discharged
into the lines as the actuator moves. Movement of hoses or "hammering" of hard lines is thus reduced.
HSM Slow Turn-On
The control manifold contains the slow turn-on accumulator. The fixed control circuit inlet orifice, and the
variable low-pressure adjustment orifice affects the rate at which the control circuit operates. When the
low-pressure solenoid is energized, hydraulic fluid is allowed to flow through the control circuit inlet orifice.
As it passes through the orifice into the control circuit, the initial pressure is decreased substantially. As
the control circuit fills, the slow turn-on accumulator begins to fill, extending the pressure rise time and
slowing the rate at which the main control valve operates. Control circuit bleed-off through the low-pressure
control also increases the time required for the main control valve to balance. A controlled, gradual increase
(or decrease) occurs during the shift from low to high pressure (or from high to low pressure).
20 | Model 293.32 Hydraulic Service Manifold
Introduction