Tri State Hydraulics

7 Validation of "Ball-..." Reliability

Despite the advantages of the "ball-in-hole" solution, there are legacy products and other valve manufacturer products that continue to use the slotted spool configuration.  A second series of life expectancy tests demonstrates the superior performance of the "ball-in-hole" technology over "ball-in-slot" mechanical configuration resulted in visible wear marks only in the spool slot in both the carbide and sapphire ball test cases after 1 billion cycles.  Both the carbide and sapphire ball did not exhibit any signs of the wear under magnification.

In this next analysis two different models of Moog's servo valves were evaluated.  Both servo valve models were assembled with steel balls with the major difference being a "ball-in-slot" versus "ball-in-hole" configuration.  The valves depicted in this test data were removed from installations in the field that had a up to 50,000 service hours.  A post mortum was performed on each of the valves in order to measure the wear on the ball of each servo valve.  The data depicted in Figure 7, "Distribution of Ball Spool Wear of Slotted vs. Ball-in-Hole", plots the distribution of ball wear of the ball-in-slot design and ball-in-hole design.  The wear on the ball is distributed in a range of 0.0 to 8 mil (thousands of inch).  While the wear specifically in the slot of the "ball-in-slot" valve and the hole of the "ball-in-hole" were not measured directly in this analysis, the wear on the steel ball is used as a proxy. Furthermore, visual inspection was performed on the slot and hole to validate these assumptions.  

Figure 7 - Distribution of Ball Spool Wear of "Slotted" vs. "Ball-in-Hole"

The wear on the ball-in-hole remains well below 1 mil for the duration of the test with a statistical variance near zero.  In contrast, the ball-in-slot wear pattern is distinctly different in that the wear pattern has a wide statistical variance independent of the service hours.  The fact is that the ball-in-slot configuration demonstrates wear patterns that are almost independent of service life.  Specifically, the "ball-in-slot" has a number of instances of high wear in under 1000 hours in service.  This is a result of early infant mortality in the servo valve suggesting the difficulty in maintaining manufacturing tolerances.  Further investigation concluded that the wear in the ball-in-slot is the result of free rotation of the spool causing "adhesive wear."  Slow spool rotations between 1 to 4 RPM cause the most significant damage.  These test results confirm that the ball-in-hole solution has far superior performance characteristics over the ball-in-slot.

PUMPS, VALVES & MOTORS MOOG Valves Design of Long Life... 7 Validation of "Ball-...