MTS Series 252 Servovalves - Manual (Page 35)

Adjust the Mechanical Null
Series 252 Servovalve Product Information Maintenance 35
B. Remove the filter plug by threading one of the socket head screws,
removed in Step A, into the filter plug and pulling it out of the filter
housing. Remove the filter cover plate.
C. Remove the filter plug O-rings from the filter plug.
D. Remove the filter O-ring from the filter.
E. Remove the filter.
F. Lightly lubricate the filter O-ring with clean hydraulic fluid, install it
on the replacement filter, and insert the filter into the housing.
G. Lightly lubricate the filter plug O-rings with clean hydraulic fluid,
install them on the filter plug, and install the filter plug.
H. Secure the filter cover plate to the housing using the four socket head
screws and washers removed in Step A. Tighten each socket head
screw until it is firmly seated against the filter cover plate. Using the
appropriate sequence, tighten the socket head screws to 4.5 N·m (40
lbf·in). Continue using the sequence and tighten the socket head screws
to a final torque of 9.60 N·m (85 lbf·in).
2. Turn on electrical and hydraulic system power.
3. Apply low hydraulic pressure to the servovalve so that hydraulic fluid
gradually fills the filter cavity.
4. Apply high hydraulic pressure and check for leaks.
Adjust the Mechanical Null
This procedure describes how to adjust the mechanical null for the Series 252
Servovalve. The mechanical null adjustment aligns the servovalve spool to a
position that results in little or no actuator movement when there is no control
signal.
Prerequisites MTS Systems Corporation recommends that you read this procedure before
attempting to adjust the mechanical null. The mechanical null adjustment is quite
sensitive, and you should be familiar with the hazards that can be encountered
when performing the procedure.
Perform the servovalve mechanical null adjustment after the valve balancing
procedure (electrical compensation) has been completed and the results are
judged unsatisfactory.
During the servovalve mechanical null adjustment procedure, the actuator should
be free to move with minimal load.
Valve balance
adjustments
MTS controllers have an electronic mechanical null adjustment called valve
balance. The valve balance adjustment is a convenient way to compensate for a
servovalve that needs a mechanical null adjustment. The adjustment introduces
an electrical offset signal that causes the servovalve to hold the position of the
actuator when a zero command is issued.