Seals

Question 1

Seals:

Answer 1

The purpose of a seal is to prevent or limit the flow of fluids (or passage of particles) between two components.

Important aspect of machine design to contain pressurised fluids within an area of a machine, exclude contaminants , retain lubricants.

Question 2

Categories of seals:

Answer 2

Static Seals

Dynamic Seals

Question 3

Static seals:

Answer 3

Sealing takes place between two surfaces that do not move relative to each other.

Static seals aim to provide a complete physical barrier to the 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 to resist their slipping/extrusion into clearances.

Question 4

Dynamic Seals:

Answer 4

Sealing takes place between two surfaces that move relative to each other (e.g. rotation or reciprocating motion).

Question 5

Applications of seals:

Answer 5

• Automotive
• Healthcare and medical applications
• Agriculture
• Chemical processing/petrochemical
• Construction/Engineering
• Energy
• Food/Beverage/Dairy
• Manufacturing
• Marine
• Mine and exploration
• Oil and Gas
• Paper, pulp & board
• Semiconductor/electronics
• Wate treatment and waste

Question 6

Considerations in seal type selection:

Answer 6

• The nature of the fluid to be contained or excluded
• Static pressure levels in either side of the seal
• The nature of any relative motion (e.g. rotating, reciprocating, contacting or non
  contacting ) between the seal and mating components.
• The level of sealing effectiveness required
• Operating temperatures
• Life expectancy
• Serviceability
• Total cost

Question 7

Elastomeric seals: O-Rings:

Answer 7

• Simple, cheap and versatile type of seal with a wide range of applications
• Both static and dynamic sealing
• Large standard range of sizes and materials
• In dynamic applications where speed and pressures are not very high.

Question 8

Elastomeric seals: O-Rings fit and applications:

Answer 8

• Elastomeric seal rings require the seal material to have an interference fit with one of the mating parts of the assembly.
• Diverse applications: sealing casings,
  reciprocating, oscillating and rotating components, seats in solenoid valves and plumbing, etc.

Question 9

Variation on O-Ring seals:

Answer 9

• A particular problem is the ability to cope with small movements of the housing and sealing faces.
• X- ring and square-ring seals have been developed as alternative configurations.
• Broader sealing area
• Resists twisting in sliding applications
• Potentially higher friction

Question 10

Aperture seals:

Answer 10

• Typically, an elastomeric extrusion
• Low cost and automated assembly
• Seal against differential pressure, dust, water, noise
• Typical applications:
• Automotive doors
• Windows
• Cabriolet bodies

Question 11

What are gaskets:

Answer 11

• Gasket is a material (or composite
  material) clamped in a (typically bolted)
  joint between two components with the
  purpose of preventing fluid flow.
• Typically made up of spacer rings, a
  sealing element, internal reinforcement, a compliant surface layer and possibly some form of surface antistick treatment.
• Simple gaskets use a single material
  while composite gaskets use two or more materials for better performance.

Question 12

Gaskets uses:

Answer 12

• Gasket seals are widely used in flanges, to fill
  the spaces and the irregularities between two
  mating flanges to prevent the leakage of the
  fluid as shown in the figure.
• When first closed the gasket is subject to
  compressive stresses.
• Under working conditions, however, the
  compressive load may be relieved by the
  pressures generated within the assembly.

Question 13

Gasket types:

Answer 13

Flat

Reinforced

Flat with bead

Flat with grommet

Corrugated

Profile

Spiral Wound

Question 14

Gaskets in an internal combustion engine:

Answer 14

Various types of seals used in an internal combustion engine:

Cylinder head cover gasket, head gasket, oil pan gasket, distributor O-rings.

Question 15

Seals for foodstuffs:

Answer 15

• Particularly demanding requirements to
  seal against biological contaminants such
  as bacteria.
• Seal material also subject to constraints.
• The typical diameter of bacteria is of the
  order of one micrometre and the challenge in designing containers is to exclude bacteria for the shelf-life of the product.

Question 16

Dynamic seals:

Answer 16

• Dynamic seals are used to limit flow of fluid between surfaces that move relative to each other.
• Extensive range of dynamic seals for both rotary and reciprocating motion.
• Conflicting requirements of high seal pressure with low friction and wear, requiring
  compromise.
• Dynamic seal performance may be substantially affected by a number of
  operating environmental factors e.g. seal swell in fluids, lubrication, system
  pressure, thermal cycling and friction

Question 17

Seals for rotary motion:

Answer 17

• They retain working fluids (such as lubricants) and prevent contamination from
  dirt and dust.
• The selection of seal type depends on the shaft speed, working pressure and desired sealing effectiveness.
• 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.

Question 18

Seals for rotary motion: V-ring seals:

Answer 18

• V- Ring (also known as axial shaft seal ) is a rubber ring for shafts requiring a dynamic seal in the axial direction.
• It seals axially against a counter face and normally rotates with the shaft.
• The body of V seals holds itself in position by its elastic fit on the shaft.
• Sealing is produced by the contact between the lip and the counter face.
• The purpose of V rings is to exclude contaminants whilst retaining lubricants in the application.

Question 19

Seals for rotary motion: Lip seals:

Answer 19

• Often referred to as an oil seal (or rotary shaft seal), as preventing oil from leaking is its typical use. It also excludes contaminants such as dirt and dust.
• Moderate speeds and low-pressure applications.
• Also used for reciprocating motion in some applications.
• The outer case should be retained in the housing by an interference fit.
• The garter spring maintains a uniform radial force on the shaft ensuring contact between the sealing ring and the shaft.

Question 20

Seals for rotary motion: Sealed bearings:

Answer 20

• Rotary shaft seals or lip seals are also used to protect bearings.
• Incorporate single or dual seal.
• Can be ‘lubricated for life’ bearings.

Question 21

Seals for rotary motion: Mechanical face seals:

Answer 21

• For applications where low leakage, high reliability, low wear are important.
• Consists of two sealing rings, one attached to the rotating member and one attached to stationary component to form a sealing surface, usually perpendicular to the shaft axis. Frictional rubbing contact between the sealing faces is maintained mechanically, for example, through a spring.
• Rubbing action between the surfaces produces heat and wear. Lubrication is used to minimise this.
• Applications: pumps, compressors, gearboxes and domestic washing machines.

Question 22

Seals for rotary motion: Sealing glands
(Compression packings):

Answer 22

• Uses compressible seal packing material.
• Compression packing consists of a number of rings and is usually made from relatively soft, pliant materials.
• Compression packings are inserted into the annular space (stuffing box) between the rotating or reciprocating member, and the
  body of the pump or valve.
• Low leakage at moderate speeds, but require maintenance.

Question 23

Seals for rotary motion: Labyrinth seals:

Answer 23

• Interstitial seals allow unrestricted relative motion between the stationary and moving components (i.e. no seal to shaft contact).

A type of interstitial seal is labyrinth seal.

• A labyrinth seal consists of a series of fins and corresponding chambers causing a restriction to the flow and an expansion of
  the volume, respectively.
• Radial fins restrict axial flow through close control of clearance.
• As it passes through the annular restriction, the fluid accelerates.
• After passing through, the fluid expands and decelerates with the formation of separation eddies.
• These turbulent eddies dissipate some of the energy of the flow reducing the pressure.
• Some leakage inevitable.

Question 24

Seals for rotary motion: Labyrinth seals
applications:

Answer 24

• Turbo machines to reduce leakage flow.
• Bearings to help prevent the leakage of
  the oil or contamination.

Question 25

Reciprocating pressure seals:

Answer 25

They include commonly packing seals and piston rings.

Question 26

Reciprocating pressure seals: packing seals:

Answer 26

* Packing seals consists of a cup, V, U or X section of leather, solid rubber or fabric reinforced rubber. * Sealing principle: direct contact with the reciprocating component.

Question 27

Reciprocating pressure seals: packing seals:

Answer 27

* Packing seals consists of a cup, V, U or X section of leather, solid rubber or fabric reinforced rubber. * Sealing principle: direct contact with the reciprocating component. * Piston rings seal cylinders (e.g. automotive) at operating temperatures above the limit of elastomeric, fabric or polymeric materials. * They seal the combustion chamber/cylinder head, transfer heat from the piston to the cylinder walls, and control the oil flow. * Typically machined from a fine grain alloy cast iron. * Conventionally they are split into three piston rings to allow for assembly over the piston, with two compression rings sealing the high pressure and one oil control ring to control the flow of oil.

Question 28

Types of piston rings:

Answer 28

Endless Butt-cut Bevel-cut Step-cup

Question 29

Dykes ring:

Answer 29

* Uses low spring pressure than other piston rings, allowing cylinder pressure to provide sealing force. * Now rare in engines, but still appropriate in some compressor applications.

Question 30

Seal failure root causes:

Answer 30

* Out of specification temperature may damage sealing material or alter lubricant properties. * Contamination * Misalignment * Poor installation may cause non uniform loading which will lead to premature failure.

Question 31

Conclusions:

Answer 31

* Seals provide a means of preventing or limiting the flow of fluid from one region of a machine to another. * Seals generally represent a trade-off between quality of seal and other factors such as speed, friction and maintenance. * Some applications can take advantage of standard solutions (oil seals, o rings, etc.), but others require application specific design.