Servo Valve Sleuthing – Tips to Quickly Identifying the Failure Modes of a Servo Valve
Where precise control of a hydraulic system is needed, there are usually one or more electro-hydraulic servo valves in use. There are many on the market these days with countless varieties engineered for many different applications. Servo valves are typically used to control hydraulic actuators and motors. Due to their relatively high complexity, many small moving parts and the presence of electronics in contaminated environments, servo valves are one of the common units we see in the service center.
One of the questions we receive quite often is how does one identify the cause of servo valve failure based on symptoms observed machine side? To answer this it is first important to understand more about the valve itself.
First let us talk about the basic anatomy of a servo valve. Of course in modern servo valves made today there can be dozens and dozens of interconnected components, but for simplicity’s sake we will just go over the major working parts.
OBE (On Board Electronics)
For a totally integrated valve, electrical current is applied to a piece of on board electronics (OBE) which is typically some form of amplifier circuit board. This OBE then applies DC current to a coil. This in essence acts as a transducer, converting electrical energy into mechanical work.
This is a coil of wire set within a pair of permanent magnets that produces torque through a magnetic field, this whole assembly is usually just referred to as “the coil”.
The coil operates the position of a flapper to change fluid pressure between two nozzles which in turn moves the spool. The position of this spool will then determine the direction of flow thus controlling how the hydraulic actuator will perform the desired work.
The center shaft of the valve, this piece is machined to, depending on its position; allow flow to travel in a desired direction within the valve.
Now that we have a basic sense of the different parts that make up the servo valve we will go over different symptoms the machine operator or maintenance technician may see on the machine.
Symptom: full system failure, or the valve has no flow or switching
- Coil breakage. This symptom could mean that an overcurrent coming from the amplifier board or an overload on the output has caused the coil windings to overheat. At this point the valve would definitely need service as the coil would need replacement. This symptom is often seen as weak or slow response or the valve not shifting all the way.
- Jam of the spool. Within the valve body the spool can become mechanically jammed. This can be due to quality of fluid, wear or even a manufacturing defect. In some cases thermal expansion due to extreme temperatures can cause an interference in position of the spool as the clearance tolerance inside the spool housing is usually very tight. The number one cause of this symptom is trash in the systems hydraulic fluid such as grit, metal shavings, plastic or any other contaminant that may exist in your facility specific to where the machine is placed and what type of product it produces. To avoid this most servo valves have filter screens on the ports that can be cleaned or replaced.
- Failure of on board electronics (OBE). One of the most common failures, the electronics portion of the servo valve mechanism can become compromised. These failure modes mirror those of any piece of electronics, but can be exasperated by a high presence of contaminant., most often hydraulic fluid due to leaking or poor maintenance practices. Electronics can also suffer from heat damage from in the valves heat exchangers.
Symptom: Degradation of performance
- Valve core wear. Servo valves contain many sliding and shifting parts. Anywhere where metal can touch metal you have the potential for wear which can lead to failure. If the operator witnesses sluggishness or drag in the way the valve is operating, it could mean that the core of the valve is experiencing some wear and needs to be rebuilt.
- Centering spring wear. Although less common in modern valves, some servo valves make use of centering springs to determine spool position. Like most springs, the centering spring within the valve body will degrade over time depending on how often it is compressed. This is due to the mechanical stress risers that the spring metal is subjected to over the course of its lifetime. This of course is exasperated if the valve is subjected to more movement or excessive load conditions outside of its intended design.
- Leakage, sealing failure. Seals, usually made of rubber or plastic, are prone to failure due to heat, age or contamination. Seal failure can lead to losses of pressure, contamination of electronics, or wear of internal mechanical parts. This will cause valve performance issues or outright failure.
- Failure over time of the OBE. As is the case with most electronics, OBE assemblies on servo valves are subject to the same failure modes as any engineered circuit boards. Common failure components such as capacitors, diodes, resistors are prone to failure due to heat, electrical noise, load issues or contamination.
Symptom: chattering movement of the actuator
- Electrical issues . Chattering can be caused by weak coils, or intermittent electrical connections. It also can come from incorrect feedback signals coming from the controls section of the application if they are not properly set up or have lost parameters specific to your valve.
- Contaminants. Chattering can also be caused by dirty or clogged filter screens, contaminant of the spool housing or nozzle or orifice clogging. This can be caused by a number of conditions mostly related to hydraulic fluid integrity. If the fluid is too cold, it becomes gummy and loses its viscosity. If too hot it can become thin and not properly disperse and coat internal parts leading to mechanical wear. In addition sometimes if you do not use the proper fluid for your application, the valve can suffer. It is best to make sure the type of fluid is the type recommended by the manufacturer of the servo valve.
Using these tips as a guide and communicating to your maintenance personal and the operators of the machinery, you can quickly identify failures in your servo valve systems. You can not only get the problem repaired quickly, but also prevent any further issues down the road. Remember finding the correct failure and eliminating its cause is the key to preventing machine down time.
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