What is a servo-valve?

A servo valve is an electro-hydraulic operated valve that electrically controls how hydraulic fluid is supplied to an actuator. Positional movements by an hydraulic cylinder can be set by a varying analogue or digital control signal providing precise pressure, position, velocity and force for the valve. Signal can be also applied to ensure any movement is conditioned by a dampening effect.

What happens if I fit a proportional valve or servo valve with the wrong flow rate on a blow moulding machine?

Moog proportional and servo valves come in a number of different flow rates.

Generally speaking, low flow is used on very small machines while high flow are fitted on larger machines. It is not always obvious if you have fitted one with the incorrect flow rate. Often the error may be indicated by a slight reduction in repeatability.

What are the typical tests carried out on Moog proportional and Moog servo valves?

All our proportional and servo valve tests are carried out to the original manufacturers published specifications. The Typical tests carried out on a Moog proportional or a Moog servo valve are:

  1. Input Signal/Flow
  2. Flow Linearity and Symmetry
  3. Null bias
  4. Input Signal/ Pressure
  5. Pressure Gain
  6. Internal and External Leakage Stability
  7. Coil resistance and Coil Insulation

Moog mould movement servo-valve faults

When these servo-valves go faulty it is normally only the pilot stage that is blocked with dirt.

If you suspect the servo-valve is faulty remember to check that the safety solenoid (if fitted) is energised and that the +15V and –15V supply to the servo-valves electronics are present and correct.

If you find that the servo-valve you are fitting (either new or repaired) does not work look to see that the pilot pressure and pilot return are correctly ported. This may involve moving grub screws from one port to another as these are not always factory set to the machines porting.

Many machine have a separate filter in the pilot pressure line and if this is partially blocked the Moog servo-valve will not work properly.

How is a servo-valve used in the blow moulding process?

A conventional extrusion blow moulding machine incorporates an annular die gap through which the moulding material is extruded to produce the tubular parison which is subsequently subjected to blow moulding to produce a product which is outputted from the machine.

The die gap can be controlled by a movable member which is displaceable by supply of hydraulic fluid to a cylinder under the control of an electrohydraulic servovalve. The servovalve is under the control of a parison programmer which is set by the machine operator to the desired wall thickness and weight settings and which receives a feedback signal from a transducer indicative of the actual position of the movable member, and which supplies an output signal to the servovalve to effect movement of the movable member to a desired position corresponding to the desired width of the die gap.

Furthermore the parison programmer may be programmed to vary the wall thickness during extrusion of the parison so that the wall thickness of the generally tubular parison varies along its length in the manner required by the form of the final moulded product, that is to take account of features such as the shaped bottom and the necked outlet of a container, for example.

How does an electro-hydraulic servo-valve work?

Electrical signals are used to control hydraulic fluid flow to and from a hydraulic actuator. The hydraulic actuator itself is used to manipulate a devices such as a steam control valve or a gas turbine inlet guide.

These devices often have a requirement to be positioned anywhere between fully open and fully closed. It is thus the electrical signals controlling the flow of hydraulic fluid supplying actuator that dictates how these devices position themselves. The electrical signals can of course be controlled by other devices such as micro processors or computers.

How could better control of a servo valve be used to improve blow moulding production?

An extrusion blow mounding system for controlling parison wall thickness could contain a closed loop control system monitoring the actual position of a movable die member and feedback signal from an automatic weighing system for the product.

This information can be used to set a desired position signal from the wall thickness controller to an electro-hydraulic servo-valve controlling the position of the movable die member that is providing the desired width of the die gap. The process ensures the weight of the final product is within a narrow weight range resulting in improved product consistency and quality. It also enables the total quantity of moulding material used in moulding of the product to be decreased.

Does the best tool change-over procedure involve servo-valve removal?

  1. Firstly, keep the servo-valve on the parison programming cylinder. If the tool change also involves a head change (Single to Twin, PE to PVC for example) then you do need to take some precautions. The only justification in removing the servo-valves is to avoid inadvertently damaging the valve.
  2. Disconnect the pressure and tank hoses.
  3. Plug up the open ports on the valve block.
  4. Plug up the open hose ends.
  5. Do the head change.

The goal here is not to introduce dirt into the hydraulic system. Some servo-valves are more dirt sensitive than others, do not believe the valve manufacturers claims. Moog 76 or 760 series are more sensitive than Moog 62 or 631 series.

If carefully done, you will not have problems.