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Design Considerations for Servo Hydraulic Actuators

Supplier: Hylec Controls By: Hylec Controls
29 November, 2018

Recent developments in elastomers and the ability to make a two part seal, has overcome most of the limitations of former seals.

Recent developments in elastomers and the ability to make a two part seal, one part being carbon fibre reinforced plastic and the sealing part being a special low friction, high modulus plastic has overcome most of the limitations of former seals. The CFRP component and the special plastic stabilises the seal geometry, giving a previously unachievable, narrow contact surface between seal and rod or barrel.This prevents the energised back up O ring from increasing the seal contact pressure, which increases friction and wear to seal, rod or barrel. Hence the development of long life actuators with low friction, independent of load, and suitable for low amplitude high frequency operation is now possible. However to prevent loss of seal lubrication the cyclic amplitude of movement must still be greater than seal contact width.  Because this type of seal cannot be stretched or compressed the seal must be slid into its groove then a sleeve installed after it to hold it in its groove. However this type of actuator is still much cheaper than a hydrostatic bearing actuator without the leakage problems.

Actuators with no seals and hydrostatic self-centring rod bearings have almost no friction, infinite life, high side load capability and no limitation on cyclic amplitude of movement andare well suited for precise control and low amplitude high frequency operation. However they suffer from two draw backs.Firstly they are expensive and secondly they have high internal piston leakage and high rod leakage. Techniques are available to prevent external leakage but the unavoidable internal leakage reduces positioning accuracy and energy efficiency.

Very recently another limitation of hydraulic actuators, the weight of the piston assembly, and also of the barrel, has been greatly reduced by the use of carbon fibre reinforced plastic for these components.  Reducing the piston mass reduces the effect of oil compressibility and allows for higher frequency operation. Low barrel mass is also required for high frequency multi axis operation.

Hylec Controls offers European actuators with the design features mentioned above.