Mounting Servo Valves

At this time, two different mounting brackets are in used for mounting servo valves on the WF-30: PN 53-3634 and its replacement, PN 53-4378. In situations involving mounting bracket PN 53-3634, a number of instances of servo breakdown have been reported. These breakdowns are caused by the incorrect orientation of the mounting bracket.

Within the servo is a spool (figure 3) which must have a total clearance of 15/1000” in order to move freely within the servo valve body. If the valve is mounted to an incorrectly oriented bracket, the plastic valve head is placed under a lateral stress. This stress results in either a bending force applied to plastic head in relation to the aluminum body (figure 1) or an offsetting force applied to the head (figure 2). Either of these forces restricts the movement of the servo spool and causes premature or immediate breakdown of the valve. (Note: the distortion of the servo valve heads in figures 1 and 2 are exaggerated for photo clarity.)

These stresses can be prevented by mounting PN 53-3634 in an orientation in which the entire mounting bracket is clear of the back of the plastic servo head. No stress can be exerted on the servo valve head if the mounting bracket is in this orientation (making contact with the aluminum valve body only).

The second cause of servo damage results from the use of a cord set of inadequate length. A cord set which is too short for the connection to the electrical enclosure can cause a bending pressure on the servo head. This pressure can result in a compromise in the movement of the servo spool or even an eventual splitting of the servo head (figure 4).

To avoid cord-related stress to the servo head, always use a cord set which is so long as to cause no sideways stress to the unit.