Electrohydraulic servo valves are typically used to control how hydraulic fluid is ported to an actuator, allowing for precise control of the position, velocity, pressure and force of the actuator. For example, many aircraft flight controls and engine controls are actuated using servo valves. A typical electrohydraulic servo valve comprises an electrical torque motor (a first stage) which controls the flow of hydraulic fluid driving a spool valve or servo (a second stage), which can in turn control flow to an actuator.
The hydraulic second stage often comprises a spool disposed within a sleeve. The position of the spool within the sleeve is controlled by the first stage, which establishes a differential pressure across the servo, causing the spool to move within the sleeve. The position of the spool within the sleeve controls the flow of hydraulic fluid to the actuator. A hydraulic amplifier acts between the first and second stages to vary the piloting flow. The hydraulic amplifier can have various different designs, for example a jet pipe and receivers, a jet deflector and receivers, or a nozzle flapper.
Many servo valve designs incorporate a mechanical feedback member to equilibrate the torque applied by the first stage. This feedback member is fixed at one end to the spool and at the other end to the hydraulic amplifier. Connection between the feedback member and the spool is often achieved with a ball-in-slot joint. The feedback member is terminated by a ball that engages a slot or hole at the centre of the spool. Advantages of a ball-in-slot design include that it alleviates binding problems. However, premature wear of the ball in the feedback mechanism can degrade the valve's performance and reduce its lifetime. To provide the ball with extended wear protection, the ball can be made from materials such as tungsten carbide or sapphire, but these can be expensive to produce.
In addition, the use of a ball-in-socket joint does not allow for the adjustment of the null-bias position of the feedback member relative to the spool, thereby accommodating any tolerances established on assembly or movement during use. It is thus desirable to provide an adjustable joint that can secure a feedback member to a spool of a servo valve.
US 2015/0176720 A1 discloses an arrangement whereby a feedback spring is attached to the spool by at least one fixing member, such as one or more screws. For instance, an end of the spring member may be clamped between a pair of screws passing axially through the spool. One or more spacers can be positioned between the screws and the feedback spring. The one or more spacers may be used to accommodate any tolerances from the screws.
There remains a need for an improved design of an adjustable joint that can secure a feedback member to a spool of a servo valve.