Design Considerations for Servo Hydraulic Actuators

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 and are 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. 

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