notes/uni/mmme/2xxx/2044_design_manufacture_and_project/seals.md

---
author: Akbar Rahman
date: \today
title: MMME2044 // Seals
tags: [ uni, mmme2044, seals ]
uuid: 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

![](./images/vimscrot-2022-11-07T15:17:10,365197721+00:00.png)

## 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

![](./images/seals-019.png)

### 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

![](./images/seals-045.png)

## Groove in Piston

![](./images/seals-044.png)

## Example of a Dimensions Table

![](./images/seals-046.png)

# 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

![](./images/seals-047.svg)

# 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

![](./images/seals-056.png)

![](./images/seals-057.svg)

## Types of Lip Seal

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

  ![](./images/seals-059.png)

  ![a secondary lip can keep out dirt but cause temperature to ride](./images/seals-060.png)

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