Jul 19, 2023
Why do hydraulic servo valves have such a long service life?

Analyze the performance of the hydraulic servo valve and the reasons for its life.

The practice has proved that the design of the servo valve feedback mechanism is the key to prolonging its service life. Hydraulic servo valves can operate for up to 25 years. Why do they have such good performance, even in applications of metallurgical machinery and industrial production machinery? Hydraulic servo valves are part of a closed-loop control system that includes electromechanical mechanisms and embedded software algorithms. The design, materials of construction, and manufacturing processes ensure consistent performance over its expected service life.

Three keys of the long life of servo valves:

Three design aspects within hydraulic servo valves increase their service life, reduce downtime, and provide reliable performance. These include:

  • Carbide balls on the feedback mechanism,
  • The design of the hole on the spool,
  • And brazing to bond the ball to the feedback rod.

MOOG introduced hydraulic servo valves with carbide balls to replace the traditional steel material used for decades. While most manufacturers define long life as millions of cycles, Moog's engineers recently offered an even higher bar: over a billion cycles.


Structure of carbide balls

Despite years of precision machining, servo valve designers at Moog found that premature wear of the ball in the feedback mechanism reduced servo valve performance. Most early designs had a stainless steel ball on the feedback mechanism, which would wear out over time. Since the 1990s, carbide and sapphire materials have been used to replace stainless steel and provide more protection for the ball.

Although sapphire is more expensive than cemented carbide, it does not necessarily provide higher performance. In fact, Moog engineers have evaluated steel, carbide, and sapphire made by performing a billion test cycles on each steel ball in clean hydraulic fluid and a controlled environment where the temperature remains stable. Although the stainless steel balls showed significant wear, the results confirmed that the carbide and sapphire balls did not show any signs of wear. In addition to being comparable to sapphire and being less expensive, the carbide material can also be brazed to the feedback mechanism. For cost control, carbide is the clear choice for ball design.

Ball in hole design

Ball groove designs have been the industry standard for over 40 years, but Moog engineers developed a carbide ball bore design in 1998 to improve the life and reliability of their servo valves. This design reduces concentrated contact between the ball and the spool anywhere on the surface, a process that radically increases the overall life expectancy of the servo valve by eliminating wear on the spool. Engineers found that after one billion cycles in a controlled environment, the ball-and-groove design showed visible signs of wear in the spool slot, while the ball-and-hole design showed no signs of wear. In fact, when the life expectancy should be 1 billion cycles, failure of the ball and groove design can occur in as little as 100 million cycles. Additionally, further research concluded that adhesive wear (i.e., slow rotation of the spool between 1 and 4 rpm) caused the most damage to the ball groove design, but had the least impact on the ball hole design. Today, many mechanical feedback servo valves have been converted to ball and hole technology due to their superior performance and extended life in industrial applications.

Cutaway view of Moog G761 Series Servo valve, showing ball bore design

Brazing is a specialized brazing process that joins carbide balls and stainless steel wires at temperatures above 450 oF (232 oC). It involves heating a filler metal above its melting point and distributing it by capillary action between two or more closely-fitting parts to join the parts together. This critical manufacturing process is only possible with cemented carbide (rather than sapphire) and is critical to enabling the ball to withstand high temperatures and deterioration from chemicals in hydraulic fluids. Engineers often use epoxy as an alternative to soldering when connecting the ball and stem of a feedback mechanism. This method is often used when connecting sapphire to stainless steel rods, as sapphire cannot be brazed. Unfortunately, there are other factors in servo valve applications that can cause unexpected failures in epoxy/sapphire technology. In fact, tests have shown that even at normal operating temperatures of  0 oF (-17.7 o C) to 160 o F (71 o C) the epoxy used to connect the feedback mechanism to the ball and stem of the sapphire spherical mechanism Resin may also break down.

The design of the servo valve feedback mechanism is the main reason for prolonging the service life, as well as the professional maintenance and maintenance of the servo valve, the regular cleaning of the valve, and the cleanliness of the system oil circuit all have an impact on the service life of the servo valve.


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