Sealing advances in aircraft hydraulics

Sealing advances in aircraft hydraulics

Sealing advances in aircraft hydraulics

Trelleborg Sealing Solutions responds to more challenging sealing environments within aircraft hydraulics

The introduction of Fly-By-Wire and Electro Hydrostatic Actuators has changed the design of hydraulic flight controls. Resulting increases in stroke frequency and compact design envelopes, combined with service-life demands, makes sealing of hydraulics in modern aircraft more challenging. Trelleborg Sealing Solutions has had to respond with enhanced sealing materials and optimized seal designs.

Two technologies recently introduced into aircraft flight control systems are Fly-By-Wire (FBW) and Electro Hydrostatic Actuators (EHAs). These have increased the difficulty of sealing in these systems.

New technologies make sealing more difficult

Due to the artificial stability of Fly-By-Wire (FBW) systems, where mechanical links or wires are replaced by electric wiring, the stroke of the actuator is more rapid and travel of piston rods is extended. Temperatures are also increased by high frequency movements and there is less cooling of hydraulic fluid because of reduced circulation. Exceptional friction characteristics are needed from seals along with outstanding resistance to wear, as well as the ability to operate at elevated temperatures and in contact with high temperature media.

Image shows Turcon® Variseal® SA which gives easy installation into closed grooves.

In Electro Hydrostatic Actuators (EHAs), all the components of a traditional hydraulic system; reservoir, motor, pump, accumulator and manifold, are in an individual unit. Designing a sealing system for the EHA is more challenging than for traditional hydraulic systems. The design envelope of the compact unit is small, temperatures and pressures are high, while dynamic movement is rapid over a short stroke.

Flight hour expectations and pressures are increasing

Other factors are also making sealing within modern aircraft more challenging. The demand for greater efficiency and more reliable systems with less downtime has resulted in a vastly increased service life requirement for hydraulic actuators. This has quadrupled from 12,000 flight hours expected for the Boeing 737 originating in the 1960s to 48,000 hours, representing five and a half years of continuous work, for the recently launched A380.

Operating pressures of the hydraulic systems have increased substantially in order to handle larger loads, at higher speeds; all within a smaller design envelope to reduce system weight. Currently the 5,000 psi/ 35 MPa of the A380 and the new B787, seems to be the accepted limit. This puts very high demands on the sealing system, especially when combined with the increase in movement speed and frequency associated with FBW.

Extended life of seals is a requirement

Another issue is the surface finish of rods. Hard chrome coatings have long been established in the industry. These are being superseded by even harder ceramic coatings due to environmental concerns and the need to extend the lifetime of rods. To meet seal-life expectations seals must give exceptional wear resistance against these hard dynamic counterparts.

On top of this, sealing systems have changed from something that was replaced regularly when a leak was discovered, to more complex sealing configurations that are an integral part of the EHA or FBW. They are expected to operate on the aircraft for many years without the need for service. Generally the initial value of the installed sealing system has increased. However, the need for spare parts and service hours has decreased, reducing the total cost for the operators.

Optimized sealing configurations for flight controls

Trelleborg Sealing Solutions has responded to changing sealing needs by creating optimized sealing configurations for flight controls.

Typical examples:

Application: Military aircraft with full FBW without any mechanical back up

Pressure: Up to 4,060 psi/ 28 MPa

Service life: 6,000 flight hours

Operating temperatures: From –54°C to +135°C/ –65°F to +275°F

Counter surface: Hard chrome rod.

Sealing configuration: Includes Turcon® Plus Seal® used in tandem along with a pressure relieving primary seal. Energizers with Nitrile sealing elements are optimized for use in red oil according to Mil-P-25732 or Mil-P-83461.

Application: Civil aircraft with full FBW without any mechanical back up

Pressure: Up to 4,060 psi/ 28 MPa

Service life: 30,000 flight hours

Operating temperatures: From –54°C to +135°C/ –65°F to +275°F

Counter surface: HVOF ceramic coated rod.

Sealing configuration: Includes Turcon® VL Seal™ in tandem. Energizers in ethylene propylene material are optimized for use in phosphate ester fluids such as Skydrol or Hyjet.

Back-pumping effect: On the forward stroke of the shaft, an oil lubricating film is distributed under the seal. On the return stroke, the oil is back-pumped into the system, preventing leakage, reducing dynamic friction and breakaway force, even after extended periods of rest.

Application: Civil aircraft with full FBW without any mechanical back up

Pressure: Up to 4,060 psi/ 28 MPa

Service life: 38,000 flight hours

Operating temperatures: From –54°C to +135°C/ –65°F to +275°F

Counter surface: HVOF ceramic coated rod.

Sealing configuration: Includes Back-up Rings in high modulus plastic. Within the rod sealing system it incorporates tandem Turcon® VL Seal™ at the same pressure and temperatures as the previous configurations. There is also a piston sealing system with a Turcon® Double Delta® seal.

Advances include Turcon® Variseal® SA

Sealing technology is likely to continue to develop in line with increasing demands from the aerospace industry. One such advance is Turcon® Variseal® SA which gives easy installation into the closed grooves usually preferred by aerospace engineers. Its unique Slantcoil® spring is not susceptible to compression set and can accommodate more deformation than other springs during installation for fitment in even the smallest diameter Mil-G-5514F grooves. Providing all the inherent capabilities of a Turcon® Variseal® such as operation in extremes of temperature and resistance to aggressive aerospace fluids, several projects are now successfully using this design of seal in flight control systems.

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