Tri State Hydraulics



4 Ball-in-Slot Design...

...Marked First Generation

As with all servo systems, the performance of the overall system is highly dependent upon the performance of the feedback mechanism.  In a servo valve the elements of the system which have undergone considerable innovations are the feedback mechanism and spool, which are in direct mechanical contact, resulting in mechanical wear in each or both.  Failure of the ball-in-slot technology can occur in as few as 100 million cycles when a life expectancy is a billion cycles.  The wear in either ball or slot, prior to catastrophic failure, directly affects the dynamic performance of the servo valve.  Potential symptoms of wear in the spool or feedback mechanism are exhibited as jitter, chatter, uncommanded motion, limit-cycling in control, dithering, oscillations, bang-bang control, and infinite gain response in the null area.


Figure 4 - Carbide Ball and Spool Wear in Slot

PUMPS, VALVES & MOTORS MOOG Valves Design of Long Life... 4 Ball-in-Slot Design...