New servo hydraulic actuators overcoming traditional limitations

Typical limitations in seals have been overcome thanks to recent
developments in elastomers and the ability to make a two-part seal combining carbon
fibre reinforced plastic (CFRP) with a special low friction, high modulus
plastic as the sealing part.

The CFRP component and the special plastic stabilise the seal geometry,
giving a previously unachievable, narrow contact surface between seal and rod
or barrel. This prevents the energised backup O-ring from increasing the seal
contact pressure, which can otherwise worsen friction and wear to seal, rod or

Thanks to these advancements, 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. Since this type of seal
cannot be stretched or compressed, the seal must be slid into its groove and a
sleeve installed after to hold it in its groove. This type of actuator is still
cheaper than a hydrostatic bearing actuator without the leakage problems.

Actuators with no seals and hydrostatic self-centring rod bearings have
almost zero friction, infinite life, high side load capability and no
limitation on cyclic amplitude of movement, and are, therefore, well suited for
precise control and low amplitude high frequency operation. However, there are two
primary drawbacks: they are expensive and they have high internal piston and rod
leakage. While external leakage can be prevented, the unavoidable internal
leakage reduces positioning accuracy and energy efficiency.

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.

All of these design features are included in the European actuators available
from Hylec Controls.

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