FEATURE

5Sealing Technology January 2004

IntroductionIn many lift augmentation carbon shaft seals, asoft material primary seal, usually carbongraphite, is mated with a hard material.Sometimes in severe industrial environments ahard material primary seal is mated with a hardmaterial. The seal itself is established by thesmall positive clearance between the primary andmating rings at the annulus.

The secondary seal can take a wide varietyof different shapes and mechanisms. It has the essential function of making sure that theprimary and mating ring faces are allowed toself-align and maintain close proximity under all operating conditions while at thesame time sealing between the ring andmounting. There are several options thatinclude O-rings, PTFE, carbon/metallic-seg-mented rings or direct machining on the pri-mary ring. The secondary seal is used for stat-ic sealing and also to guide small movementsof the carbon ring in the seal housing foralignment (Figure 1).

Many different types of springs are used.Usually, once the seal is under pressure, the loadimposed by the spring is small compared to theload that comes about due to lift augmentationand sealed pressure. The drive mechanism has avery important function in allowing the seal pri-mary ring to self-align without causing undueforce on the runner ring.

Design principle and modelling ofaxial seal with lift augmentationThe operating point gap design is at openingFo = closing Fc. The faces of the carbon andrunner seek an equilibrium position to avoidany contact. The gap must be small enough toensure a minimal air leakage, but it must belarge enough to limit friction due to shear inthe gas film and face deformation by shaft dis-placement, misalignment and vibration. Withaxial displacement of the shaft, the seal gapwill be increased or decreased, the lift aug-mentation force will be then decreased or

High-performance liftaugmentation dynamicseals for turbine bearingcompartmentsHung Tran, Techspace Aero, Snecma Group, Belgium and Peter Haselbacher,Burgmann, Germany

Conventional bearing shaft seal systems used in gas turbine engines are often lim-ited to a sliding velocity of about 100 m/s, differential pressure of 3 bar, gas tem-perature of 300°C and a seal life less than 8000 h. Advanced engines will requirebearing shaft seal systems to operate up to sliding velocity of 200 m/s, differentialpressure of 6 bar, gas temperature of 500°C and seal life in excess of 30 000 hours.For seals operating in these advanced conditions, a design with no rubbing contactwill be required to achieve long life and reliability. A good validated approach isthe use of a gas lift augmentation seal. The design objective for a seal of this type isto have the faces of the seal seek an equilibrium position to avoid any contact. Thegap must be small enough to ensure a minimal air leakage, but it must be largeenough to limit power dissipation, due to shear in the gas film, and face deforma-tion by shaft displacement, misalignment and vibration. Dynamic seals for a bear-ing compartment have the following main functions: provide static and dynamicsealing in order to prevent oil leakage from the bearing oil compartment to the aircompartment and consequently no oil smell pollution by the use of bleed air; con-trol air leakage to the bearing oil compartment in order to improve performanceof the engine and to reduce oil consumption; reduce volume of the oil tank andlubrication system and hence provide weight reduction; to operate in extreme con-ditions of temperature and with normal and reverse pressure; and reduce the meantime between overhaul (MTBO) and have a very long life. Techspace Aero andBurgmann have carried out design, development and testing of lift augmentationcarbon seals and demonstrated that high life and performance levels of these sealsare possible in a gas turbine engine environment.

closing force

opening force

operating point: Fo = Fc

1 2 3

720

450

550

axial gap [µm]

gas openingforce [N]

Fo is lift augment. force given by pumping grooves to open gap.

Fc is force given by spring and pressure on air side primary face seal to close gap

Runner

Primary face seal

Secondary seal

Figure 1. Design principle of axial seal.

6Sealing Technology January 2004

increased to adapt the seal gap to the operat-ing point design position as shown in Figure1. This is the dynamic equilibrium position ofthe seal gap.

In gas turbine engine applications, the liftaugmentation effect is created by pumpinggrooves machined on the runner surface, these grooves pump air from the air sidethrough the interface of runner and primaryring to the oil side. This air creates the air filmin the interface and avoids friction during sealoperation. The groove design is shown inFigure 2 and a lift augmentation pressure pro-file in Figure 3.

The lift augmentation pressure is alsodesigned as a compromise between the air leak-age objective, seal life and the other engine para-meters.

The aero thermal design of the seal isachieved by modelling with Reynolds equa-tions for lift augmentation forces, balancingof the primary and secondary seals, and alsoheat generation by shear of the air in the sealgap, Figure 4.

The mechanical design is carried out by usingfinite element modelling for integration of theseal in an engine environment, Figure 5.

Principle of radial seal with lift augmentation by Rayleigh stepRayleigh steps between the carbon and runnersurfaces create the lift augmentation pressure,Figure 6. The Reynolds equation is used to calculate this lift augmentation pressure inthe radial direction in order to create equilib-rium between the opening and closing forces. The principle is described in Figure 1:The design operating point gap is at openingforce, Fo = closing force, Fc. The faces of the carbon and runner will seek an equilibri-um position to avoid any contact. The gap must be small enough to ensure a mini-mal air leakage, but it must be large enough to limit friction as described in the sectionabove. When radial displacement of the shaft occurs, the seal gap will be increased or decreased, the lift augmentation force willthen decrease or increase to adapt the seal gapto the operating point design position. This is the dynamic equilibrium position of seal gap.

Limits of operation in aeronauticaland industrial applicationsThe lift augmentation carbon shaft seal has thepotential to be an extremely versatile device. Itcan tolerate wide ranges of pressure and velocityas well as misalignment and has the followingcapabilities:

• Seals are designed for pressure differentialsfrom 0 to 5 bar for aeronautical and much higher for industrial compressor applications.

• Sliding speeds may range from 0 to 200m/s and higher.

• Seals can support a large axial or angularvibration without leakage because the netclosing load is relatively large.

• Relative radial motion is tolerated by eccen-tric operation of the two faces. Axialmotion is accommodated by an axial free-dom of motion provided by the secondaryseal and springs. The secondary seal alsotolerates angular motion.

• Air/air, air/oil or gas/liquid may be sealed.

• The seal is a non-contacting. The pressuretimes velocity value (PV) is less dependent ontribological factors such as friction, wear andheat transfer during the variable conditionsexperienced in flight and can accommodatestarting and reverse wind milling conditions.

• Because the heat generation is very low, theseal can be used in cold and hot sections ofan engine. The air temperatures can befrom –60°C to 500°C.

FEATURE

Figure 2. V pumping groove.Figure 3. Axial lift augmentation pressure profile on runner and carbon surface.

Lift augmentation pressure in function of radius

0

0,5

1

1,5

2

2,5

3

3,5

4

0,155 0,16 0,165 0,17 0,175 0,18 0,185

Radius of rubbing surface (m)

Ope

ning

pre

ssur

e P

o (b

ar)

Max lift off pressure

Outlet pressure

Limit of V groove pressure

Inlet Air pressure

Figure 4. Aero thermal modelling of axial lift augmentation pressure in V grooves of aero gas turbine engine in take-off mission. Heat generation less than 60 watt and air leakage less than 0.5 gr/s.

Figure 5. Finite element modelling of axial lift augmentation seal.

7Sealing Technology January 2004

FEATURE

• Seal life of more than 30 000 h.• No oil leakage during transient conditions

such as reverse pressure.

Design principlesThe design objective for an axial/radial liftaugmentation shaft seal is to have the faces ofthe seal seek an equilibrium position to avoidany contact. In the case of high-viscosity fluidsthe gap should be adjusted to limit frictionheating by shear in the film and to avoid wearin varying upset conditions such as shaft mis-alignment and vibration.

The design objectives to be met for an enginewill include, cost, weight, life, leakage and inte-gration. Table 1 shows the design parameterchoices and the technology advantages that liftaugmentation can provide.

Aero-engine and industrial applicationsThe high-performance lift augmentation carbonseal has been designed, developed, tested andproduced based on the design principles shownin Table 1. An example of the application ofthese seals to an aircraft engine is discussedbelow.

Requirements and input parametersfor seal design

Client requirements:

• No need for air side seal pressurization.• No oil loss → cabin bleed air quality.• No cooling oil for runner → weight

reduction.• Life and MTBO engine overhaul interval

30 000 hours.• Low air leakage.• Low oil consumption by air/oil separator →

oil pollution reduction.

Client input parameters:

• Duty cycle (idle, takeoff, climb, cruise anddescent).

• Shaft speed (carbon/plate rubbing speed)and direction of rotation.

• Seal air/oil pressure drop.• Air/oil compartments temperature.• Axial/radial displacement and unbalance.• Spring load to overcome drag and inertia

forces.• Wear rate, carbon lip height, rubbing speed,

face loading, friction.• Out of balance and vibration.• Seal plate, hard-face coating durability.

CFM56 engine rig applicationIn the CFM56 engine, there is a requirement toreduce the oil consumption caused by high airleakage of the bearing labyrinth seals. For test

Figure 6. Left/right Rayleigh step following direction of rotation of shaft in order to compress air ingroove to create lift augmentation pressure.

Design parameters Technology advantages

Face width. Face materials and Low wear and friction on starting of engine. Constant gap grade of carbon and low air leakage during whole engine life. Reduction of

oil consumption through air oil separator

Balance ratio and lift augmentation Constant gap. No rubbing contact

Relative circumferential stiffness Constant seal performance

Heat transfer environment Low heat generation. No cooling of carbon ring and runner,size reduction on lubrication system (pump, oil tank,piping, etc.)

Pressure, temperature, velocity Damper effect by air film and lift augmentation forces willcaused axial-radial displacement reduce risk of contactand vibration

Reverse pressure No oil lost in reverse pressure. Oil consumption and oil pollution reduction

High life Reduction of MTBO (mean time between overhaul). Lowoperating cost

Table 1. Design parameters and technology advantage.

Figure 7. Front bearing compartment with axial lift augmentation carbon seal.

purposes we have replaced the front labyrinthseal with an axial lift augmentation carbon seal,Figure 7, with the objective that the two sealingsystems must have the same weight and cost.

Rig for seal validationThe test rig replicates the engine bearing config-uration, Figure 8, with respect to size of enginebearing compartment, bearing load, shaft speed,pressures and temperatures at the air and oil sideof the engine. The specification of the test rigincludes:

• Maximum oil temperature: 200°C (400°F)• Maximum oil flow: 1200 l/h (320 gal)• Maximum air temperature: 400°C (750°F)• Air/oil pressure differential: 5 bar (75 psi)

FEATURE

8Sealing Technology January 2004

Performance tests Unbalance 6 g tests Reverse pressure tests Endurance testsParameters of test: Parameters of test: Parameters of test: Parameters of test:Seal diameter: 250 mm (10 inches) Seal diameter: 250 mm (10 inches) Seal diameter: 250 mm (10 inches) 16 000 cycles, each cycle is 120 sShaft speed, pressure and Shaft speed, pressure and Shaft speed, difference air–oil Shaft speed cycle from 0 to temperature varied in function of temperature varied in function of pressure = 0.2 bar take-off speed engine mission engine mission Air pressure cycle from 1 bar to

take-off pressure Max take-off air T° Max take-off oil T°

Result: Result: Result: Result:Seal air leakage < 0.5 gr/s Seal air leakage < 0.5 gr/s No oil leakage Seal air leakage < 0.5 gr/s No oil leakage No oil leakage No heat and no wear No seal oil leakageNo oil consumption at No heat and no wear Seal air leakage < 0.5 gr/s No heat and no wearair–oil separator No oil consumptionNo heat and no wear

Table 2. CFM56 No. 1 axial lift augmentation carbon seal rig results.

Figure 8. Bearing compartment rig for seal validation.

Figure 9. Seal components.

Figure 10. Wear control after each test.

FEATURE

9Sealing Technology January 2004

• Shaft speed: 20.000 rpm• Instrumentation: 64 lines

Seal validation

Axial lift augmentation sealThe seal is formed by two parts, the first partis the runner fixed to the shaft and the secondis the carbon in the bearing housing. The run-ner has grooves on the rubbing surface, Figure9, in order to pump air from the air side to oilside through the rubbing surface and createlift augmentation pressure in the seal gap. Thecarbon is guided by springs and a secondaryseal in order to follow the lift augmentationpressure and misalignment of the runner. It isrestrained by an antirotation device so thatthe carbon can not rotate with the runner.The test programme parameters are shown indetail in Table 2.

Wear measurements after each test showthat the wear is less than 2 microns until the11 700 cycles test and is then unchanged untilthe end of testing (Figure 10). The typicalcycle for cycle endurance testing is shown inFigure 11.

Radial lift augmentation seal withRayleigh step on the carbonA comparison was carried out with a circum-ferential segmented seal. The seal consists of a seal housing containing a carbon segmented ring, Figure 12, held in dynamicsealing contact with the runner by a garter spring and in static sealing contact withthe inboard face of the housing by axialsprings.

Dynamic sealing is effected in the radialdirection by the carbon segments mating with the runner, primary seal. The radial lift

augmentation is generated in the gap toreduce wear and friction and increase life.

Semi-static sealing, the secondary seal, is by means of mating the axial face of the car-bon segmented ring with the seal housing.

The carbon segmented ring is preventedfrom rotating with the runner by a series

0

1000

2000

3000

4000

5000

6000

0 20 40 60 80 100 120 140

Sha

ft s

peed

in r

pm

Endurance Cycle

Times in second

16000 cycles for endurance test

Figure 11. Typical cycle for endurance test.

Figure 12. Radial lift augmentation seal components.

Table 3. Seal technology comparisons.Figure 13. Radial lift augmentation seal testresults.

of anti-rotation devices linked to the sealhousing.

The static sealing between the side of eachcarbon segment and the seal housing allows sufficient radial movement to accommodateshaft radial excursions.

The endurance cycle and wear results areshown in Figure 13. The air leakage of approxi-mately 3.5 g/s is higher than on the axial carbonseal, about 0.5 g/s, but lower than the labyrinthseal where it is approximately 50 g/s. The wearon the radial carbon is 25 microns, comparedwith about 2 microns on the axial carbon.

Seal technology comparisonsTechspace Aero and Burgmann have used theseal technology comparisons shown in Table 3to justify the selection of the lift augmentationcarbon seal.

Contact:Peter Haselbacher, Head of Department,Aerospace/HF,Burgmann Dictungswerke GmbH & Co KG, D-82502Wolfratshausen, Germany.Tel: +49 8171 23 1301, Fax:+49 8171 23 1053, Email: haselbacher@burgmann. com

Hung Tran, Engineering Manager, Techspace Aero,Snecma Group, Aeronautic Engines Components, Routede Liers, 121, B-4041 Herstal (Milmort) Belgium. Tel:+32 4 278 84 46, Fax: +32 4 278 80 29 or +32 4 27050 89, Email: htran@techspace-aero.be

FEATURE

10Sealing Technology January 2004

This feature is based on a paper presented bythe authors at the workshop ‘High performancerotary shaft seals: experiment and modelisation’held at LMS Futuroscope, University ofPoitiers, France on 2 October 2003 and is pub-lished by kind permission of the organizers.

System specificationThe hydraulic pumps to be sealed are used onmany different kinds of equipment in the con-struction and mining industries and also in mili-tary equipment. Therefore, the external environ-ment that they experience can be quite severe.The combination of harsh external environmentand high internal fluid pressure and flow creates achallenge for the sealing system.

The control cover in this particular type ofhydraulic pump is a small square cast iron plate,machined to mount to the pump housing, alsoof cast iron, with four flange-head bolts. Thefunction of the cover is to control the pumpstroke, which in turn controls the flow and pres-sure of the pump output.

Currently, a rubber-edged metal (REM) gas-ket is used to seal this joint, which has two aper-tures. The larger aperture contains low-pressurepump casing oil, which intermittently spikes to14 bar, typically at cold start-up. The other is a

small port, which contains high-pressure chargeoil that can spike to 70 bar. The oil flowsthrough this port at rates up to 11 litres perminute (l/m). As part of the control function,this high-pressure fluid can be subjected to bothpositive and negative pressure pulses.

Rubber-edged compositesREC technology is a relatively new engineeringsolution that uses a compressible composite base(or carrier) in combination with a rubber (elas-tomer) sealing edge (Figure 1). As such, it is a newcategory of sealing technology, and can provideimproved durability, robustness and compression-stress retention in joints that may otherwise provedifficult to seal.

The improved compression-stress retentionin a REC joint results from the use of thecompressible composite carrier (Figure 2).

As in any joint using a rubber polymer as a bar-rier seal, the rubber under compression relaxes

over time. This loss of sealing stress from relax-ation can lead to fluid leaks, especially when thefluid is pressurized. However, when the rubberrelaxes in a REC joint, the base material (compos-ite carrier) also compresses a specific amount,thus further compressing the rubber edge. Thistransfers energy stored in the carrier and the boltsto the rubber sealing edge.

The net effect is that the compression-stressretention of the rubber edge is higher than itwould have been without the energy transferfrom the carrier. This results in improveddurability of the rubber seal.

REC gaskets are manufactured using apatented, QS-registered production process.This does not rely on moulding or the costlyand time-consuming creation of dies or tooling.REC gaskets appeared especially appropriate toreplace the REM seals for the hydraulic control-cover. Not only are the REC gaskets less costly,they are also especially appropriate for multi-port applications. If necessary, different types of

Rubber-edged compositeseals for high-pressurehydraulic applicationsAndrew D. Baum, Interface Solutions Inc

Rubber-edge composite (REC) technology is a recently developed technique thathas been developed by Interface Solutions Inc (ISI) and is marketed as Select-a-Seal. It has been applied to a number of components in the automotive industryand other relatively low-pressure applications. More recently it has been tested athigher pressures for use in hydraulic applications. One such application is sealingcontrol covers on hydraulic piston pumps used on open-circuit hydraulic systemsby a worldwide manufacturer of industrial and mobile hydraulic products.

Figure 1. Example of the construction of atypical gasket.