Sealing Solutions–Longer Lives, Less Leaks

Aircraft seals face tough challenges as they bear more extremes to contain fluids.

Printed headline: Sealing the Deal


Whether for a super-jumbo commercial jet or a single-engine piston model, fluids, ranging from highly refined propulsion fuels to viscose lubricating greases and oils, are an aircraft’s life blood. But fluids are at risk of loss due to leakage. For aerospace seal manufacturers, that presents a dual engineering challenge—more robust containment and longer on-wing life.

New technology applications are moving that along. Among them is carbon graphite, a material that has been manufactured by Metallized Carbon Corp. (Metcar) for nearly 75 years. Specifically, the company is targeting the seal face, which is the primary sealing surface—in a mechanical assembly—that Metcar supplies to the seal OEMs.

Keith Hoge, an application engineer for the Ossining, New York-based Metcar, says the company “has significantly advanced the properties of carbon graphite material.” It has also developed a proprietary process to impregnate carbon graphite with a variety of materials, which has improved its temperature resistance, among many other properties.

“At higher temperatures, oil can carbonize—or coke—which tends to increase wear and can ultimately lead to seal failure,” he points out. “To combat this, we have been developing materials with higher thermal conductivity and lower coefficients of friction. By reducing the coefficient of friction, we have been able to improve wear resistance by minimizing the amount of thermal buildup at the sealing surface.”

In addition, he says, the company’s proprietary impregnation process has enabled Metcar to develop pressure-tight materials to stem fluid leakage.

Hoge confirms that with implementation of the new carbon graphite technology, in tandem with new seal designs, durability has improved. “Our customers have reported that seal service intervals have increased from 5,000 to 20,000 hr.,” he says. The seals are used in a variety of systems, including engines, gear boxes, auxiliary power units (APU) and hydraulics. “Carbon graphite material has been replacing old labyrinth seals, which had a higher acceptable leakage rate than seals using impregnated carbon graphite,” he adds.

Looking ahead, Hoge says that a challenge is to design seal faces that provide a longer life with even higher temperature resistance, at higher rotating speeds.

Temperature Extremes

In that regard, Vinay Nilkanth, vice president of the Global Mobility Sector for Freudenberg Sealing Technologies, reports that the Weinheim, Germany-based company has focused its research and development on technology impacting high- and low-temperature performance, friction reduction and safety. That has been driven, in part, by the necessity for seals to respond to temperature extremes.

“Jet aircraft are flying at higher altitudes to save fuel,” Nilkanth says. “For short-haul regional aircraft, this means a severe cold soak at altitude followed by a rapid descent for landing. This puts a major thermal shock on all components, including seals.”

In addition, fuel and hydraulic systems “have been driven to increasingly higher pressures” to provide the required performance, Nilkanth notes. “When you couple thermal shock with higher system pressures, this puts a premium on seal resiliency and the seals’ abilities to cope with these pressures and thermal changes.”

The resiliency of a seal to temperature fluctuations varies, depending on the material selection and design. “If material and design variables are not considered by application, the sealing function may not be optimal and could potentially be compromised,” he says.

Freudenberg Sealing Technologies unveiled several high- and low-temperature-resistant products at the 2019 Paris Air Show. They include a new high-temperature, fireproof material; a developmental low-temperature ethylene propylene diene monomer (EPDM) material for commercial aircraft hydraulic systems; and a developmental low-temperature fluoroelastomer (FKM) material for engine fuel and lubrication systems. Among other developments announced by the company in Paris is its EPDM LM426288 material for application to low-pressure static sealing. According to Nilkanth, the material offers temperature resistance down to -77C (-107F), and short-term resistance to +150C, for high-temperature hydraulic systems—such as braking.

Another material he cites, the FKM LM426776, is used for low-pressure static sealing in such aerospace applications as O-rings and small, homogeneous rubber-molded shapes. “It is engineered to withstand temperatures as low as -67C and high-temperature resistance—short-term—to +270C,” he explains.

Extending Seal Life

Jared Manry, an engineering specialist at Swedish seal OEM SKF, reports that innovations are aimed at increasing the functional life of the seal on-wing. “For most engine manufacturers, 20,000 hr. of seal life is now the minimum requirement our designs must meet, with many engine programs demanding even longer life,” he notes.

Citing a few examples of recent technology trends, Manry includes “hydrodynamic liftoff,” which enables seals to operate on a thin film of gas rather than making full contact between the static and rotating components. Others include hard coatings applied to the sealing surfaces. “That has focused on improved tribology and wearing characteristics between those sealing surfaces, which must remain in contact, and are often exposed to fretting wear due to engine vibration,” he explains.

Increasing shaft speeds, along with higher pressure differentials across the seals, contribute to increased seal heat generation and wear.

“New engine designs are running with faster shaft speeds, hotter air and oil temperatures, and higher sealing pressures,” he says. “To address this, we have utilized a patented hydrodynamic lift augmentation to the sealing bore of our circumferential seals along with a hardened coating applied to the secondary axial sealing surfaces to reduce both heat generation and wear. Testing has demonstrated a 30% reduction in heat generation, which reduced measured seal interface temperatures by 10-15% and yielded a three-times improvement in measured seal wear when compared to a conventional circumferential seal,” Manry notes.

SKF has investigated the mating and wear characteristics of various carbon grades against a variety of aerospace materials and coatings. “We have evaluated the relative performance of these mated materials for both ‘sliding’ wear and fretting wear,” he adds. “These evaluations will be ongoing as new materials emerge on the market.”

Asked about self-lubrication/polishing features being applied to some sealing applications, Manry states that “carbon grades advertising self-lubricating properties” have been shown to reduce friction—and therefore heat generation—in laboratory testing. However, he cautions that full-scale testing to quantify the advantages of those properties is warranted.

He adds that new hydrodynamic technologies developed by SKF have been shown to extend seal life “so that the seal is no longer a limiting component” in engine maintenance and overhaul—and may either be replaced or reused during engine removals for scheduled maintenance events.

Changing Materials

Torben Anderson, director of the global aerospace segment for Trelleborg Sealing Solutions—headquartered in Sweden—points out that the exclusion of chromed rods in hydraulic systems, due to the European REACH (Registration, Evaluation, Authorization and Restriction of Chemical Substances), and emergence of High-Velocity Oxygen Fuel (HVOF) coatings, have prevented the use of elastomer contact seals in dynamic aerospace sealing applications.

“This has necessitated their replacement with tougher seals in polytetrafluoroethylene-based (PTFE) materials,” he explains. “Coatings made from HVOF are more corrosion-resistant and therefore far less likely to sustain scratches, extending the life of the seal and the system as a whole. Also, polymer seal materials have become more wear- and fatigue-resistant [to the] millions of cycles they are exposed to, especially in fly-by-wire systems,” he says.

Seal manufacturers, Anderson says, have responded well to the challenges presented to them by primary flight-control applications. “During the past 25 years, the working pressure of aircraft hydraulic systems has increased from 1,500 to 5,000 psi, and the expected service life of the sealing system in the hydraulic cylinder has gone from 1,500 to more than 100,000 flight hours,” he reports. “At the same time, leakage criteria has been lowered from one drop in 25 cycles to ‘dry rod.’”

Anderson specifically attributes that to the use of PTFE materials, developed to withstand the “many thousands of miles” a seal has to work in the tough environment of an electro-hydrostatic actuator or fly-by-wire system.

Asked if life-extending seal technology could mean increased time between engine and hydraulic system overhauls, “for hydraulics, we are already there,” Anderson says. “For gearboxes and engines, substantial amounts of R&D resources are being spent on developing sealed-for-life solutions.”

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