Seals

Mechanical Design

Aims

• Seals are devices used to prevent or limitleakage of fluids or particulates.

• The aims of this section are to introducethe variety of seal configurations, giveguidelines for the selection of seals andintroduce calculation methods for thequantification of some seal leakage rates.

Learning objectives

At the end of this section you should:• be able to identify a number of the different

types of sealing devices,• be able to select a seal type for rotating,

reciprocating or static conditions,• be able to determine the groove dimensions for

a standard O ring,• be able to estimate the leakage flow through a

labyrinth seal,• be able to calculate the leakage flow through a

bush seal.

Introduction

• The purpose of a seal is to prevent or limitflow between components.

• Seals are an important aspect of machinedesign where pressurised fluids must becontained within an area of a machinesuch as a hydraulic cylinder, contaminantsexcluded or lubricants retained.

Static and dynamic seals

Seals fall into two general categories.1) Static seals, where sealing takes place

between two surfaces that do no notmove relative to each other.

2) Dynamic seals, where sealing takesplace between two surfaces that moverelative to each other by, for example,rotary or reciprocating motion.

Leakage

• Any clearancebetween twocomponents willpermit the passage offluid molecules ineither direction, thedirection dependingon the pressures andmomentumassociated with thefluid.

FLUID 2

BOUNDARY

BOUNDARY

FLUID 1

Seal classification

SEALS

STATIC SEALS DYNAMIC SEALS

GASKETS SEALANTS ROTATINGSHAFT SEALS

RECIPROCATINGSHAFT SEALS

FACE SEALS INTERSTITIAL SEALS PACKINGS PISTON RINGS

AXIAL SEALS RADIAL SEALS

LIP RING SEALS PACKINGS

FELT SEALSCIRCUMFERENTIALSPLIT RING SEALS

BUSH SEALS LABYRINTHSEALS RIM SEALS

BRUSH SEALS FERROFLUIDICSEALS

MECHANICALSEALS

Considerations in seal selection

Some of the considerations in selecting the type of seal include:

• the nature of the fluid to be contained orexcluded,

• pressure levels either side of the seal,• the nature of any relative motion between the

seal and mating components,• the level of sealing required,• operating temperatures,• life expectancy, serviceability,• total cost.

Seal selection

RELATIVE MOTION?

Yes

NoReciprocating

low

Flange

Rotating

Shaft

medium high

USE STATICSEAL

USE GASKETIs accurate

face locationessential?

USE O RINGUSE SEALANT

Is temperature>100 Co

USE NON-CONTACTINGSEAL OR CARBON SEAL

USE GLANDOR LIP SEAL

USE ANON-CONTACT

BUSHING

USE GLAND,U OR CHEVRON

SEAL

USE O RING

USE FACESEAL

NoYes

NoYes

speedspeed speed

speedmedium

speedhigh

Static seals

• Static seals aim at providing a completephysical barrier to leakage flow.

• To achieve this the seal material must beresilient enough to flow into and fill anyirregularities in the surfaces being sealedand at the same time remain rigid enoughto resist extrusion into clearances.

O rings

• The ‘O’ ring is a simpleand versatile type of sealwith a wide range ofapplications for bothstatic and dynamicsealing.

• An ‘O’ ring seal is amoulded elastomeric ring‘nipped’ in a cavity inwhich the seal is located.

ENLARGEDSECTION

SECTIONDIAMETER

O ring operation

• The principle of operation for an ‘O’ ring sealing againsta fluid at various pressures is illustrated below.

• Elastomeric seal rings require the seal material to havean interference fit with one of the mating parts of theassembly.

200 bar100 bar65 bar30 bar0 bar

Some O ring seal dimensions (mm) BS4518

1.06.445.35554.7455.744.30443-57

1.03.720.22524.8203.019.50195-30

0.53.15.398.752.44.60046-24

0.53.14.387.742.43.60036-24

0.52.311.71413.811.51.611.10111-16

0.52.310.71312.810.51.610.10101-16

0.52.39.71211.89.51.69.10091-16

0.52.38.71110.88.51.68.10081-16

0.52.37.7109.87.51.67.10071-16

0.52.36.798.86.51.66.10061-16

0.52.35.787.85.51.65.10051-16

0.52.34.776.84.51.64.10041-16

0.52.33.765.83.51.63.10031-16

R (mm)B (mm)dnominal (mm)(Fig 9.5b)

Dnominal (mm)(Fig 9.5b)

Dnominal (mm)(Fig 9.5a)

dnominal (mm)(Fig 9.5a)

SECTION DIAMETER (mm)

INTERNAL DIAMETER (mm)

REFERENCE NUMBER.

Example

• Specify suitable groove dimensions for an0195-30 ‘O’ ring to seal against a solidcylinder.

Solution

• From Table 9.1 andwith reference toFigure 9.6, B=3.7mm, R=1 mm, groovefillet radius = 0.2 mm.

d D d D

B

R

0.1 to 0.2R5omax

(b)(a)

Solution cont.

1.06.445.35554.7455.744.30443-57

1.03.720.22524.8203.019.50195-30

0.53.15.398.752.44.60046-24

0.53.14.387.742.43.60036-24

0.52.311.71413.811.51.611.10111-16

0.52.310.71312.810.51.610.10101-16

0.52.39.71211.89.51.69.10091-16

0.52.38.71110.88.51.68.10081-16

0.52.37.7109.87.51.67.10071-16

0.52.36.798.86.51.66.10061-16

0.52.35.787.85.51.65.10051-16

0.52.34.776.84.51.64.10041-16

0.52.33.765.83.51.63.10031-16

R (mm)B (mm)dnominal (mm)(Fig 9.5b)

Dnominal (mm)(Fig 9.5b)

Dnominal (mm)(Fig 9.5a)

dnominal (mm)(Fig 9.5a)

SECTION DIAMETER (mm)

INTERNAL DIAMETER (mm)

REFERENCE NUMBER.

Movement• A particular problem associated with O rings is ability to cope with

small movements of the housings and sealing faces.• A range of solutions have been developed to produce seals that are

resistant to, for instance rotation, within the seal groove.• An X ring, also known as a quadring, and a rectangular seal are

illustrated opposite.

Aperture seals• Aperture seals used, for

example, for doors, windowsand cabriolet bodies aretypically made fromelastomeric extrusions asproduction costs are lowrelative to fabricatedmechanical seals and as theirassembly can be automated.

• In the case of automobiles therequirements are demandingwith the need to seal againstdifferential pressure, excludedust, air, water and noise.

GLASS

WEATHER STRIP

Gaskets• A gasket is a material or composite of materials clamped between

two components with the purpose of preventing fluid flow.• Gaskets are typically made up of spacer rings, a sealing element,

internal reinforcement, a compliant surface layer and possibly someform of surface anti-stick treatment as shown

INNERSPACER

RINGSPACEROUTER

RINGINTERNAL

REINFORCEMENTCOMPLIANT

SURFACE LAYER

SEALING ELEMENT

ANTI-STICKTREATMENT

Gasket seal• The figure shows a

typical application for agasket seal.

• When first closed agasket seal is subject tocompressive stresses.

• Under working conditions,however, thecompressive load may berelieved by the pressuresgenerated within theassembly or machine.

GASKET

FLANGES

Gasket designs

• Typical gasketdesigns areillustrated.

• The choice of materialdepends on thetemperature ofoperation, the type offluid being containedand the leakage ratethat can be tolerated.

Gasket leakage. (Reproduced from Muller and Nau (1998)).

U-ringC-ring

Metal oval-ringConvex + Ag

Convex + PTFE

PTFE jacketed

Bonded-fibre sheet, 3 mm

Convex + graphiteSerrated + PTFE

Serrated + graphiteFlat metal sheet

Spiral wound + PTFESpiral wound + graphite

Corrugated + PTFECorrugated + graphite

Metal jacketed

Bonded-fibre sheet, 2 mmBonded-fibre sheet, 1 mm

10-210-1 10-3 10-4 10-510 10-6 -810-7 -10-9 1010 mL/(s m)

Sealing of foodstuffs• The sealing of foodstuffs

amounts for a significantproportion of seal designs.

• Sealing of foodstuffscontainers involvesconsideration of leakage of thecontents, sealing againstcontamination and chemicalodours.

• The typical diameter ofbacteria is of the order of onemicrometer and the challengein designing containers is toexclude bacteria for the shelflife of the product. PLASTIC

WALL

WEDGE

PLASTIC LID

WALLGLASS

WALLPLASTIC

PLASTIC LID

PLASTIC LID

SEALINGLIP

WALLGLASS

WALLGLASS

WALLMETAL

METAL LID

METAL LID

METAL LID

RUBBERSEALANT

FIBERGASKET

THREAD

THREAD

THREAD

THREAD

RUBBERSEALANT

Dynamic seals

• The term ‘dynamic seal’ is used to designate adevice used to limit flow of fluid betweensurfaces that move relative to each other.

• The range of dynamic seals is extensive withdevices for both rotary and reciprocating motion.The requirements of dynamic seals are oftenconflicting and require compromise.

• Effective sealing may require high contactpressure between a stationary component and arotating component but minimal wear is alsodesired for long seal life.

Rotating shafts

• The functions of seals on rotating shaftsinclude retaining working fluids, retaininglubricants and excluding contaminantssuch as dirt and dust.

• The selection of seal type depends on theshaft speed, working pressure and desiredsealing effectiveness.

Seals for rotary motion

Seals for rotary motion include • ‘O’ rings,• lip seals,• face seals,• sealing rings,• compression packings and• non-contacting seals such as bush and

labyrinth seals.

Radial lip seals

BONDED METALCASE

RUBBERCASE

ATMOSPHERICSIDE

GARTER SPRING

PRESSURESIDE

SEALINGCONTACT

Mechanical face seals

• A mechanical faceseal consists of twosealing rings, oneattached to therotating member andone attached to thestationary componentto form a sealingsurface, usuallyperpendicular to theshaft axis.

RADIALSEAL

ANNULAR FACES

FLUID

ROTATINGSHAFT

STATICHOUSING

CYLINDRICAL SURFACESFORM SEAL GAP

SEAL

STATIC

SHAFTROTATING

FLUID

HOUSING

Mechanical face seal

HOUSING

PRESSURESIDE

ATMOSPHERICSIDE

SEAL FACES

O RING

SPRING

CLAMPINGPLATE

STATIONARYSEALING HEAD

ROTATINGDRIVE RING

STATIONARYSHAFT

ROTATING

SEALING RINGPRIMARY

Interstitial seals

• The term interstitial seal is used for sealsthat allow unrestricted relative motionbetween the stationary and movingcomponents (i.e. no seal to shaft contact).

• Types include labyrinth, brush and bushseals.

Labyrinth seals

tp

h r ori

c

Operation• A labyrinth seal in its simplest

form consists of a series ofradial fins forming a restrictionto an annular flow of fluid.

• In order for the fluid to passthrough the annular restrictionit must accelerate.

• Just after the restriction thefluid will expand anddecelerate with the formationof separation eddies

• These turbulent eddiesdissipate some of the energyof the flow reducing thepressure.

Labyrinth design features

Labyrinth flow

• Flow through a labyrinth can be estimated using:

• = mass flow rate (kg/s),• A = area of the annular gap (m2),• α = flow coefficient,• γ = carry over correction factor,• ϕ = expansion ratio,• po = upstream pressure (Pa),• ρo = density at the upstream conditions (kg/m3).

oopAm ραγϕ=�

m�

Flow coefficient

• The flow coefficient, α, is a function of theclearance to tip width ratio but an averagevalue of 0.71 can be used for

1.3<c/t<2.3• where• c = radial clearance (m),• t = thickness of fin (m).

Carry over correction factor

12γ=1+11.1(c/p)8γ=1+10.2(c/p)

6γ=1+8.82(c/p)4γ=1+6.73(c/p)3γ=1+5(c/p)2γ=1+3.27(c/p)

NUMBER OF FINS

CARRY OVER CORRECTION FACTOR

Expansion ratio

• The expansion ratio, ϕ, is given by

• pn = downstream pressure following the nth

labyrinth (Pa),• n = number of fins.

( )( )no

on

p/plnnp/p1

+−

=ϕ

Guidelines for the selection of fin thickness, pitch and height

1.8 - 2.21.8 - 2.20.18 - 0.22

3 - 3.54 - 50.28 - 0.32

4 - 56 - 80.3 - 0.4

h (mm)p (mm)t (mm)

Example

• Determine the mass flow rate through alabyrinth seal on a 100 mm diameter shaft.

• The labyrinth consists of 6 fins, height 3.2mm, pitch 4.5 mm, radial clearance 0.4mm and tip width 0.3 mm.

• The pressure is being dropped from 4 barabsolute, 353 K, to atmospheric conditions(1.01 bar).

Solution

• The outer radius of the annular gap is(100/2)+3.2+0.4=53.6 mm.

• The inner radius of the annular gap is(100/2)+3.2=53.2 mm.

• The annulus gap area is

( ) ( ) ( )( )24

23232i

2o

m101.342

1053.21053.6�rr�A−

−−

×

=×−×=−=

Solution cont.

• α=0.71. γ=1+8.82(c/p) for n=6.• c=0.4 mm, p=4.5 mm.• γ=1+8.82(0.4/4.5)=1.784.

( )( )

( )( ) 0.3183

10/1.01104ln610/4101.011

/pplnn/pp1

55

55

no

on

=××+

××−

=+−=ϕ

Solution cont.

• p=ρRT,• so the upstream density is given by,• ρo=po/RTo=4×105/(287×353)=3.948 kg/m3.

kg/s0.06801043.9480.3183

1.7840.71101.342m5

4

=××

××××= −�

Axial and radial bush seals

• Simple axial andradial bush seals areillustrated and can beused for sealing bothliquids and gases.

Q

LcØ

Q

a

b

c

Axial bush seal

• The leakage flow though an axial bushseal for incompressible flow can beestimated by

• For compressible flow

( )L12

ppcQ ao

3

µ−πφ

=

( )a

2a

2o

3

Lp24ppc

Qµ

−πφ=

Radial bush seal

• The leakage flow though a radial bushseal for incompressible flow can beestimated by

• For compressible flow

( ))b/aln(6

ppcQ ao

3

µ−π

=

( )a

2a

2o

3

p12ppc

Qµ

−π=

Example

• An axial bush seal consists of an annulargap with inner and outer radii of 50 mmand 50.5 mm respectively.

• The length of the seal is 40 mm.• Determine the flow rate of oil through the

seal if the pressures upstream anddownstream of the seal are 7 bar and 5.5bar respectively.

• The viscosity can be taken as 0.025 Pa s.

Solution

• The radial clearance is• 0.0505-0.05=0.0005 m.• φ=0.1 m, L=0.04 m.• The volumetric flow rate is given by:

( ) ( )

/sm104.9090.040.02512

105.5107100.50.1�Q

34

5533

−

−

×

=××

×−××××=

Seals for Reciprocating Components

• The seals principally used for reciprocatingmotion are packings and piston rings.

Packing seals• Packing seals essentially

consists of a cup, V, U orX section of leather, solidrubber or fabric reinforcedrubber.

• The sealing principle is bydirect contact with thereciprocating component.

• The contact pressure canbe increased in the caseof V packings by axialcompression of the sealsalthough this increasesfriction and wear.

HIGHPRESSURE

LOWPRESSURE

HIGHPRESSURE

LOWPRESSURE

HIGHPRESSURE

LOWPRESSURE

PACKING

PACKING

PACKING

Piston rings

• Piston rings are used to seal cylinders wherethe operating temperature is above the limitof elastomeric, fabric or polymeric materials.

• Piston rings are used in automotive cylindersfor three purposes:

1) to seal the combustion chamber/cylinderhead,

2) to transfer heat from the piston to the cylinderwalls,

3) to control the flow of oil.

Piston rings

• Piston rings are usually machined from afine grain alloy cast iron and must be splitto allow for assembly over the piston.

• Conventional practice is to use threepiston rings with two compression ringssealing the high pressure and one tocontrol the flow of oil.

Number of piston rings required to seal a given pressure

6+>200

5100 < po < 200

460 < po < 100

320 < po < 60

2<20

NUMBER OF RINGSPo (bar)

Piston rings

ENDLESS BUTT-CUT

STEP-CUTBEVEL-CUT

Piston Ring sections

PLAIN INTERNALLYBEVELLED

TAPERPERIPHERY

WEDGESECTION

INTERNAL LSECTION

BEVELLEDSTEPPED DRILLED SLOTTED

Conclusions

• It is frequently necessary in machine design toprovide some means of containing or limiting theflow of fluid from one region to another.

• Because of the very small nature of fluidmolecules this is a challenging task.

• This section has reviewed a range of static anddynamic seals.

• Because of the wide range of applications sealstend not to be available as stock items andinstead must be designed fit for purpose.