Chapter09 seals

1. Seals
Mechanical Design

2. Aims
• Seals are devices used to prevent or limit
leakage of fluids or particulates.
• The aims of this section are to introduce
the variety of seal configurations, give
guidelines for the selection of seals and
introduce calculation methods for the
quantification of some seal leakage rates.

3. 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.

4. Introduction
• The purpose of a seal is to prevent or limit
flow between components.
• Seals are an important aspect of machine
design where pressurised fluids must be
contained within an area of a machine
such as a hydraulic cylinder, contaminants
excluded or lubricants retained.

5. Static and dynamic seals
Seals fall into two general categories.
1) Static seals, where sealing takes place
between two surfaces that do no not
move relative to each other.
2) Dynamic seals, where sealing takes
place between two surfaces that move
relative to each other by, for example,
rotary or reciprocating motion.

6. Leakage
• Any clearance
between two
components will
permit the passage of
fluid molecules in
either direction, the
direction depending
on the pressures and
momentum
associated with the
fluid.
FLUID 2
BOUNDARY
BOUNDARY
FLUID 1

7. Seal classification
SEALS
STATIC SEALS DYNAMIC SEALS
GASKETS SEALANTS ROTATING
SHAFT SEALS
RECIPROCATING
SHAFT SEALS
FACE SEALS INTERSTITIAL SEALS PACKINGS PISTON RINGS
AXIAL SEALS RADIAL SEALS
LIP RING SEALS PACKINGS
FELT SEALS
CIRCUMFERENTIAL
SPLIT RING SEALS
BUSH SEALS
LABYRINTH
SEALS
RIM SEALS
BRUSH SEALS FERROFLUIDIC
SEALS
MECHANICAL
SEALS

8. Considerations in seal selection
Some of the considerations in selecting the
type of seal include:
• the nature of the fluid to be contained or
excluded,
• 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.

9. Seal selection
RELATIVE MOTION?
Yes
No
Reciprocating
low
Flange
Rotating
Shaft
medium high
USE STATIC
SEAL
USE GASKET
Is accurate
face location
essential?
USE O RINGUSE SEALANT
Is temperature
>100 C
o
USE NON-CONTACTING
SEAL OR CARBON SEAL
USE GLAND
OR LIP SEAL
USE A
NON-CONTACT
BUSHING
USE GLAND,
U OR CHEVRON
SEAL
USE O RING
USE FACE
SEAL
NoYes
NoYes
speedspeed speed
speed
medium
speed
high

10. Static seals
• Static seals aim at providing a complete
physical barrier to leakage flow.
• To achieve this the seal material must be
resilient enough to flow into and fill any
irregularities in the surfaces being sealed
and at the same time remain rigid enough
to resist extrusion into clearances.

11. O rings
• The ‘O’ ring is a simple
and versatile type of seal
with a wide range of
applications for both
static and dynamic
sealing.
• An ‘O’ ring seal is a
moulded elastomeric ring
‘nipped’ in a cavity in
which the seal is located.
ENLARGED
SECTION
SECTION
DIAMETER

12. O ring operation
• The principle of operation for an ‘O’ ring sealing against
a fluid at various pressures is illustrated below.
• Elastomeric seal rings require the seal material to have
an interference fit with one of the mating parts of the
assembly.
200 bar100 bar65 bar30 bar0 bar

13. 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
DIA
MET
ER
(mm
)
INTERNAL
DIA
MET
ER
(mm
)
REFERENCE
NUMBER.

14. Example
• Specify suitable groove dimensions for an
0195-30 ‘O’ ring to seal against a solid
cylinder.

15. Solution
• From Table 9.1 and
with reference to
Figure 9.6, B=3.7
mm, R=1 mm, groove
fillet radius = 0.2 mm.
d D d D
B
R
0.1 to 0.2R
5o
max
(b)(a)

16. 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
DIAMETE
R (mm)
INTERNAL
DIAMETE
R (mm)
REFERENCE
NUMBER.

17. 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.

18. Aperture seals
• Aperture seals used, for
example, for doors, windows
and cabriolet bodies are
typically made from
elastomeric extrusions as
production costs are low
relative to fabricated
mechanical seals and as their
assembly can be automated.
• In the case of automobiles the
requirements are demanding
with the need to seal against
differential pressure, exclude
dust, air, water and noise.
GLASS
WEATHER STRIP

19. 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 some
form of surface anti-stick treatment as shown
INNER
SPACER
RING
SPACER
OUTER
RING
INTERNAL
REINFORCEMENT
COMPLIANT
SURFACE LAYER
SEALING ELEMENT
ANTI-STICK
TREATMENT

20. Gasket seal
• The figure shows a
typical application for a
gasket seal.
• When first closed a
gasket seal is subject to
compressive stresses.
• Under working conditions,
however, the
compressive load may be
relieved by the pressures
generated within the
assembly or machine.
GASKET
FLANGES

21. Gasket designs
• Typical gasket
designs are
illustrated.
• The choice of material
depends on the
temperature of
operation, the type of
fluid being contained
and the leakage rate
that can be tolerated.

22. Gasket leakage. (Reproduced from
Muller and Nau (1998)).
U-ring
C-ring
Metal oval-ring
Convex + Ag
Convex + PTFE
PTFE jacketed
Bonded-fibre sheet, 3 mm
Convex + graphite
Serrated + PTFE
Serrated + graphite
Flat metal sheet
Spiral wound + PTFE
Spiral wound + graphite
Corrugated + PTFE
Corrugated + graphite
Metal jacketed
Bonded-fibre sheet, 2 mm
Bonded-fibre sheet, 1 mm
10
-2
10
-1
10
-3
10
-4
10
-5
10 10
-6 -8
10
-7 -10-9
1010 mL/(s m)

23. Sealing of foodstuffs
• The sealing of foodstuffs
amounts for a significant
proportion of seal designs.
• Sealing of foodstuffs
containers involves
consideration of leakage of the
contents, sealing against
contamination and chemical
odours.
• The typical diameter of
bacteria is of the order of one
micrometer and the challenge
in designing containers is to
exclude bacteria for the shelf
life of the product. PLASTIC
WALL
WEDGE
PLASTIC LID
WALL
GLASS
WALL
PLASTIC
PLASTIC LID
PLASTIC LID
SEALING
LIP
WALL
GLASS
WALL
GLASS
WALL
METAL
METAL LID
METAL LID
METAL LID
RUBBER
SEALANT
FIBER
GASKET
THREAD
THREAD
THREAD
THREAD
RUBBER
SEALANT

24. Dynamic seals
• The term ‘dynamic seal’ is used to designate a
device used to limit flow of fluid between
surfaces that move relative to each other.
• The range of dynamic seals is extensive with
devices for both rotary and reciprocating motion.
The requirements of dynamic seals are often
conflicting and require compromise.
• Effective sealing may require high contact
pressure between a stationary component and a
rotating component but minimal wear is also
desired for long seal life.

25. Rotating shafts
• The functions of seals on rotating shafts
include retaining working fluids, retaining
lubricants and excluding contaminants
such as dirt and dust.
• The selection of seal type depends on the
shaft speed, working pressure and desired
sealing effectiveness.

26. 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.

27. Radial lip seals
BONDED METAL
CASE
RUBBER
CASE
ATMOSPHERIC
SIDE
GARTER SPRING
PRESSURE
SIDE
SEALING
CONTACT

28. Mechanical face seals
• A mechanical face
seal consists of two
sealing rings, one
attached to the
rotating member and
one attached to the
stationary component
to form a sealing
surface, usually
perpendicular to the
shaft axis.
RADIAL
SEAL
ANNULAR FACES
FLUID
ROTATING
SHAFT
STATIC
HOUSING
CYLINDRICAL SURFACES
FORM SEAL GAP
SEAL
STATIC
SHAFT
ROTATING
FLUID
HOUSING

29. Mechanical face seal
HOUSING
PRESSURE
SIDE
ATMOSPHERIC
SIDE
SEAL FACES
O RING
SPRING
CLAMPING
PLATE
STATIONARY
SEALING HEAD
ROTATING
DRIVE RING
STATIONARY
SHAFT
ROTATING
SEALING RING
PRIMARY

30. Interstitial seals
• The term interstitial seal is used for seals
that allow unrestricted relative motion
between the stationary and moving
components (i.e. no seal to shaft contact).
• Types include labyrinth, brush and bush
seals.

31. Labyrinth seals
t
p
h
r ori
c

32. Operation
• A labyrinth seal in its simplest
form consists of a series of
radial fins forming a restriction
to an annular flow of fluid.
• In order for the fluid to pass
through the annular restriction
it must accelerate.
• Just after the restriction the
fluid will expand and
decelerate with the formation
of separation eddies
• These turbulent eddies
dissipate some of the energy
of the flow reducing the
pressure.

33. Labyrinth design features

34. 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

35. Flow coefficient
• The flow coefficient, α, is a function of the
clearance to tip width ratio but an average
value of 0.71 can be used for
1.3<c/t<2.3
• where
• c = radial clearance (m),
• t = thickness of fin (m).

36. 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

37. Expansion ratio
• The expansion ratio, ϕ, is given by
• pn = downstream pressure following the nth
labyrinth (Pa),
• n = number of fins.
( )
( )no
on
p/plnn
p/p1
+
−
=ϕ

38. 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)

39. Example
• Determine the mass flow rate through a
labyrinth seal on a 100 mm diameter shaft.
• The labyrinth consists of 6 fins, height 3.2
mm, pitch 4.5 mm, radial clearance 0.4
mm and tip width 0.3 mm.
• The pressure is being dropped from 4 bar
absolute, 353 K, to atmospheric conditions
(1.01 bar).

40. 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
23232
i
2
o
m101.342
1053.21053.6rrA
−
−−
×
=×−×=−=

41. 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
10/1.01104ln6
10/4101.011
/pplnn
/pp1
55
55
no
on
=
××+
××−
=
+
−
=ϕ

42. 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
1.7840.71101.342m
5
4
=××
××××= −

43. Axial and radial bush seals
• Simple axial and
radial bush seals are
illustrated and can be
used for sealing both
liquids and gases.
Q
L
cØ
Q
a
b
c

44. Axial bush seal
• The leakage flow though an axial bush
seal for incompressible flow can be
estimated by
• For compressible flow
( )
L12
ppc
Q ao
3
µ
−πφ
=
( )
a
2
a
2
o
3
Lp24
ppc
Q
µ
−πφ
=

45. Radial bush seal
• The leakage flow though a radial bush
seal for incompressible flow can be
estimated by
• For compressible flow
( )
)b/aln(6
ppc
Q ao
3
µ
−π
=
( )
a
2
a
2
o
3
p12
ppc
Q
µ
−π
=

46. Example
• An axial bush seal consists of an annular
gap with inner and outer radii of 50 mm
and 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 and
downstream of the seal are 7 bar and 5.5
bar respectively.
• The viscosity can be taken as 0.025 Pa s.

47. 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:
( ) ( )
/sm104.909
0.040.02512
105.5107100.50.1
Q
34
5533
−
−
×
=
××
×−××××
=

48. Seals for Reciprocating
Components
• The seals principally used for reciprocating
motion are packings and piston rings.

49. Packing seals
• Packing seals essentially
consists of a cup, V, U or
X section of leather, solid
rubber or fabric reinforced
rubber.
• The sealing principle is by
direct contact with the
reciprocating component.
• The contact pressure can
be increased in the case
of V packings by axial
compression of the seals
although this increases
friction and wear.
HIGH
PRESSURE
LOW
PRESSURE
HIGH
PRESSURE
LOW
PRESSURE
HIGH
PRESSURE
LOW
PRESSURE
PACKING
PACKING
PACKING

50. Piston rings
• Piston rings are used to seal cylinders where
the operating temperature is above the limit
of elastomeric, fabric or polymeric materials.
• Piston rings are used in automotive cylinders
for three purposes:
1) to seal the combustion chamber/cylinder
head,
2) to transfer heat from the piston to the cylinder
walls,
3) to control the flow of oil.

51. Piston rings
• Piston rings are usually machined from a
fine grain alloy cast iron and must be split
to allow for assembly over the piston.
• Conventional practice is to use three
piston rings with two compression rings
sealing the high pressure and one to
control the flow of oil.

52. 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)

53. Piston rings
ENDLESS BUTT-CUT
STEP-CUTBEVEL-CUT

54. Piston Ring sections
PLAIN
INTERNALLY
BEVELLED
TAPER
PERIPHERY
WEDGE
SECTION
INTERNAL L
SECTION
BEVELLEDSTEPPED DRILLED SLOTTED

55. Conclusions
• It is frequently necessary in machine design to
provide some means of containing or limiting the
flow of fluid from one region to another.
• Because of the very small nature of fluid
molecules this is a challenging task.
• This section has reviewed a range of static and
dynamic seals.
• Because of the wide range of applications seals
tend not to be available as stock items and
instead must be designed fit for purpose.