Tri State Hydraulics

5 Carbide Replaces...

...Stainless Steel

Designers of feedback mechanism and spools sought to overcome numerous sources of early wear in these critical components.  The manufacturing processes used to produce the ball and spool are relying upon machining tolerances held in the submicron level for both the ball and spool because matching is critical.  Despite the precision machining processes used, over the years servo valve performance degraded because of premature ball wear and circular rings along the edge of the slot in the spool.  Early systems used stainless steel for the ball on the feedback mechanism, however by the mid-1990s, the carbide ball was introduced and remains the material with the highest performance characteristics in the field.  A sapphire ball has also been used in the servo valve technology resulting in endurance characteristics that are comparable with carbide.

Test conducted in Moog's R&D laboratory evaluated the wear characteristics of a steel, carbide, and sapphire ball by subjecting them to 1 billion test cycles.  It also compared ball-in-hole and ball-in-slot configurations (See Section 6).  In this controlled environment, hydraulic fluids were kept clean with temperatures held at a steady state throughout the testing.

Figure 5 - Wear Test Results after 1 Billion cycles of Mechanical Feedback Balls; Carbide and Sapphire are Comparable while Steel has noticeable Wear

The test results confirmed that the carbide and sapphire ball do not show any signs of wear in this controlled environment, however the steel has noticeable wear in all three cases.  Researchers concluded that the reliability of the slotted spool design early wear characteristics are independent of the material used for the feedback mechanism ball.  Test results reaffirm that the slotted spool configuration compromises the reliability of the servo valve.

PUMPS, VALVES & MOTORS MOOG Valves Design of Long Life... 5 Carbide Replaces...