Servo Valve Diagnosis
Servo valves used in bend arms are very sensitive to contamination and require a lot of cleanliness to work properly. A 40 micron particle can cause a servo valve to "stick." (1 micron = 0.001 mm. 40 microns = 0.0016 inches) Even valves that are not completely stuck can act strangely if they are not completely clean.
Servo valves require filters in the system. The first time a system is turned on is the most critical. Particles that are in the system from installation can migrate and they must be stopped by the filter before the valve.
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Signs of a sticking valve
- The servo controller cannot be tuned.
- The axis drifts in one direction at power up when it did not drift before.
- The axis oscillates
- The axis overshoots then recovers by pulling back when it did not do so before.
- The valve was pulled off the system then put back on - at which point it stopped working
Fixes
- Throughly clean the valve
- Check the valve for need of repair
- Change out the filter periodically
- Leave the valve on the system if it is working well
Vickers Documentation
Routine Maintenance for Servo and Proportional Valves
This information is from: Servo Controls of India
1. As a general rule, hydraulic components should not be disturbed while they are operating normally.
– This rule particularly applies to Servo or Proportional Valves.
– They should not be removed unless trouble-shooting has shown that they are malfunctioning.
– If the system must be “opened” (for example, adding new pipework, hoses, valves or actuators), then the valve should be removed, replaced by a flushing plate and the new system start-up procedure followed.
2. If the valve must be removed, the area around the subplate should be thoroughly cleansed using non lint producing materials before raising the valve from the surface.
– If the valve function is critical, a spare Servo or Proportional Valve should always be held in stock.The spare should be fitted in place of the removed valve and the shipping plate placed on the removed valve. A newly installed valve may develop a fault soon after fitting if care is not taken with cleanliness.
– Where no spare valve exists the surface should then be covered by a clean plastic sheet so as to limit the possibility of contaminants entering the system.
3. Some valves are fitted with manual override operators. Most have null adjust screws. No other adjustments are possible in the field, apart from the null adjustment.The style of this adjustment varies with different valves.
4. The torque motor is a precision device. Do not attempt to remove or dismantle. Permanent and expensive damage can result. Likewise, do not try to remove the spool in the main stage.
5.Servo and Proportional Valves are used when precision control is required.The Servovalve is a precision instrument and consistency in performance is not possible with “dirty oil.”
6. There are two considerations in filtration for Servo and Proportional Valves.
6.1 Particle Contamination:
Larger particles from approximately 40 microns and upwards can lodge in the Servovalves’ pilot stage filter screen. Particles smaller will generally pass through. This is a last chance filter and is not intended as a system filter.
7 Silt Contamination:
This can, under certain circumstances, lead to seizure of the main spool in the bushing. However, this is rare due to the very high spool positioning forces employed. Silt does affect valve life by eroding the sharp metering edges on the valve spool and bushing.
8. Contamination Control
Contamination enters the fluid at many points:
8.1 New oil supplied from refineries contains noticeable residue in the bottom of empty drums. Particles of 100 micron and larger are quite usual in new oil.
8.2 Filling Methods:
Contamination can enter via dirty funnels or other unhygienic filling methods. It is recommended a transfer pump upstream of the 3 micron low pressure filter. When in doubt, remove the valve, fit a flushing block and flush the system out before refitting the valve.
8.3 Airborne Contaminants:
Many factories have very dusty air in the vicinity of the hydraulic power unit. Sometimes the dust is corrosive. In systems with large air flows in and out of the reservoir,
a 3 micron breather filter is recommended.
8.4 Airborne contaminants are also collected on the rod end of hydraulic cylinders and are drawn into the cylinder in varying amounts, depending on rod wiper efficiency.These are normally collected in the 3 micron low pressure filter.
8.5 Contaminants from rubber hose, teflon tape, metal silt particles from wearing valves, pumps, etc. all contribute to fluid contamination.
9. How often do I change the fluid?
Fluid change frequency depends on whether the filtration quality is high, oil temperature is maintained at reasonable levels, moisture condensation is low, and the oil is not breaking down. Regular observations of oil color in the sight glass are sufficient for monitoring the condition of the fluid. If it remains clear and machine operation is normal, do not change the fluid.There is no firm and fast rule for fluid change. Hydraulic system fluids are not like the oils in engines, as they are not subject to continuous chemical contamination. If the fluid is scheduled to be changed at a time when filters are not showing indication of contamination, then leave the old filter elements in for one or two days before changing them for new elements.Where the hydraulic power unit is dedicated to the valve alone, it may be several years before an oil change is necessary.Where the valve is fed from a larger power unit which services other functions, a more frequent change will be necessary.
10. Adjusting the Null on a Valve.
As stated in section 3, no other field adjustment exists on a valve other than the null adjustment.This is set at the factory and should not normally be disturbed. If a valve is indicating an excessive null drift, it may be indicative of contamination. The null adjustment allows the spool to be centered and thus limit any actuator motion when the valve electrical signal is zero. It is preferable to disconnect the valve connector when carrying out this adjustment.With critical axis cut spools, the null may drift slightly with temperature change and valve age. One to two percent drift is permitted as it will be corrected with the closed loop control in operation. For instructions on how to adjust the null of a valve, see individual valve series data sheets.
