How wear affects Servo Valves in Hydraulics/Fluidpower accuracy and efficiency

In the realm of high-performance hydraulics, the servo valve is the central heart of the process. It can be used to control the flight surface on an aircraft or the speedy injection process of a molding machine; these valves transform small electrical signals into large, precise physical forces.

However, like all precision instruments, they're not invincible to the passing of time. Wear and tear, often invisible to the naked eye, can silently reduce the efficiency and accuracy of your entire power fluid system.

Here's a rundown of the wear that manifests in servo valves and how it's important to the bottom line.

1. The Crucial "Sharp Edge": Spool and Sleeve Erosion

The core of a servo device is the spool and sleeve combination. To ensure precise control, all edges on the spool (the "lands") as well as the ports inside the sleeve have to be extremely clear.

• Its Mechanism: In time, the high-velocity liquid contaminated with microscopic particles behaves as a sandblaster in liquid form. This process, referred to by the names of silt erosion and sandblasting, smooths out those sharp edges.

• The Effect: Loss of "metering" precision. When edges are rounded, the valve is unable to shut off flow in a flash or regulate small movements with precision. You'll see "hunting" or oscillations when the controller tries to pinpoint the precise position.

2. The Internal Leakage Problem (Efficiency Loss)

The efficiency of hydraulics is mostly about controlling flow. A brand new servo valve has been made with extremely tight tolerances - often measured in microns between the sleeves and spool.

• Increased Null flow. When the spool and sleeves wear down, the distance between them gets larger. This allows fluid to escape through the port that is used for pressure to the tank, even if it is not in a "null" (neutral) state.

• Energy Waste: Internal leakage basically wastes energy. The pump must perform harder to keep the pressure of the system constant, and this results in increased electric bills and a rise in energy consumption. The heat, in turn, also degrades fluids used to make hydraulics, resulting in an unending loop of wear.

3. Degradation of Response Time (Accuracy Loss)

The Servo valves are admired because of their responses to frequency--their ability to respond with signals hundreds of times a second.

• Stiction, Friction Wear, and dirt can result in rough surfaces across the spool. This causes "stiction" (static friction), which means that the valve needs a bigger electrical strike to begin moving.

• Hysteresis Wear can increase hysteresis, which is the variation in the valve's response when it is instructed to shift in one direction and then the other. In a precise process, this results in "overshooting" targets and low reproducibility.

4. Feedback Mechanism Fatigue

A majority of servo valves utilize the mechanical feedback cable (or Torsion bars) to relay the spool's current position and direction back to the pilot stage.

• Metal Fatigue: High-frequency vibrations that are constant could trigger the wiring to reduce its spring rate or even cause micro-fractures.

• The Effect: If the feedback is not accurate, it means that this valve "lies" to the controller. The controller assumes that the valve is open at 10%, yet it may be at 12percent. The disconnect causes the closed-loop accuracy needed to ensure high-end manufacturing.

How to Combat Servo Valve Wear

It's impossible to stop physics; however, you can prolong the life of your valves using three methods:

1. Extreme Filtration Servo valves generally need ISO 4406 cleanliness levels of 14/12/9 or higher. Standard filters don't suffice; you require high-efficiency non-bypass filters.

2. Oil Analysis: Always check for acidity and varnish. Degraded oil is chemically abrasive and speeds up the degradation of the internal component.

3. Watch "Null" Change of direction: If your machine requires a different electric bias than it did a year ago Your valve is probably getting worn out.