notes/uni/mmme/2044_design_manufacture_and_project/seals.md

author	date	title	tags	uuid
Akbar Rahman	\today	MMME2044 // Seals	uni mmme2044 seals	ff74e8ad-090d-47eb-b9cc-41ccdefb9a63

Function

• used to prevent or limit leakage of fluids or particulates
• keep dirt out
• allow motion
• allow disassembly

Classification of Seals

O-Rings

• good for static and low speed applications

• requires interference fit

• application is codified---use a standard where possible (BS 4518:1982, BS ISO 3601-1~5)

• sized by internal diameter (ID) and section diameter (A)

• works by having the rubber o-ring squeezed against the two surfaces

  • pre-compression ensures sealing when there is no pressure

Challenger Space Shuttle Disaster

• Space Shuttle Challenger broke apart 73 seconds into flight
• at the low temperatures, the o-rings lost their resilience
• the booster cases distorted during launch
• insufficient groove width in o-ring design

Common Forms of Failure

• Abrasion---sealing face is worn away. Can be reduced with smoother surfaces, harder materials, better lubricant, and reduced speed

• Compression set---the sealing face is permanently distorted. Can be solved with more temperature resistant o-rings

• Installation damage---can be avoided with chamfers and blending radii

• Extrusion---the o-ring is forced into the sealing clearence. can be avoided with:

  • smaller clearances < 0.13 mm;
  • harder material
  • different cross section
  • backup rings---come in a range of shapes and sizes (BS 5106:1988)

• Spiral damage---o-ring is twisted and leaks. can be avoided with:

  • lower reciprocating speed
  • harder material
  • different cross sections (e.g. quadrant seals, which have a x-shaped section)

Groove Dimensions

• up to 100 bar for static loading
• up to 14 bar & 3.8 m/s for dynamic loading

i don't know when this is the case but it just says this in the [lecture slides](./lecture_slides/Lecture 6 Seals_v1.0.pdf) (and i haven't watched the lecture yet)

• $F$---radial depth
• $E$---groove width
• $G$---total clearance
• $C$---lead-in chamfer
• $R$---radius

Groove in Cylinder

Groove in Piston

Example of a Dimensions Table

O-Ring Properties

Material	Hardness (Shore A)	Temperature Range ($^{\circ}$C)	Uses
Nitrile rubber	70	-30 to 110	general purpose, fuel, oils, water
Silicone	70	-60 to 200	medical, body fluid resistant, low temp, alcohol, oxygen
Viton (fluorocarbon)	85	-20 to 200	high temperature, extreme chemicals
Polyurethane	95	-30 to 110	high temps, oil, gas, hydraulics, good wear

Shore A context:

Item	Hardness (Shore A)
Rubber band	20
Pencil rubber	40
Tire tread	70
Shoe heel	80

Gaskets

• a material or combination of materials clamped between two separable members to form a joint

Gasket Anatomy

1. Base material---a compliant element such as cork, cellulose, or a fibre
2. Binders---temperature and chemical resistant material such as rubber, elastomers, and resins
3. Fillers---fill voids in base material. usually rubbers, elastomers, or resins
4. Reinforcements---prevents distortion. usually a metal or fabric core

Seals in Rotating Shafts

• low pressure and low temperature---o rings, radial lip seals, axial lip seals
• high pressure---high performance lip seals, mechanical seals
• high temperature---labrynth seals

O-Rings

• o-rings can be used in dynamic applications
• friction is high but sealing is good within operating range (14 bar, 3.8 m/s)
• surface roughness must not exceed 0.4 mm Ra, should be around 0.1 mm Ra
• spiral and abrasion are main failure modes, if installed correctly

Radial Lip Seal Anatomy

• low pressure---0.3-0.6 bar, up to 7 bar with backing plate
• moderate speed---up to 16 m/s
• high temperature
• normal environmental conditions
• slight leakage permitted
• normally made of nitrile rubber compounds

Types of Lip Seal

• BS ISO 6149-1:2007 defines 4 basic types of rotary shaft lip seal:

  

  

  • $D_1$---nominal diameter of shaft
  • $D_2$---nominal diameter of housing
  • $b$---nominal seal width
  • $A$---air side
  • $B$---fluid side