mech_eng_dsn2_5

8/11/2019 mech_eng_dsn2_5

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Engineering Design II

Ng Poh Kiat


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Is this a Seal?


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Is this a Seal?


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Is this a Seal?


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Seals

Definition

A Seal is a device for separating twofluids.

resting

-resting-moved

sealSpace 1

With

Fluid 1

Space 2

With

Fluid 2

avoid

exchange


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Seals

Seals applied in a standard pump

1.1-1.10:

Gaskets

2.1-2.4: radial,

rotary seals

3.1-3.5: form seal

6: centrifugal seal


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Seals

Seals

Static Dynamic

Contact Clearance

Rotary Reciprocal ReciprocalRotary

Sealant

Gaskets

O-Rings

Shrink Fits

Pipe Couplings

Gland packing

O-Rings

Felt seals

Lip/Gartner sealsMech. Face seals

O-Rings

Packing Ring

Piston ring

BellowsDiaphragm

Rolling diaphr.

Fixed & floating

Bush

Labyrinth

Viscoseal orWindback

Centrifugal

Ferromagnetic

hydrostatic

Fixed &

floating Bush

Labyrinth


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Seals

What is impermeability?

Has to be defined (molecules, humidity, dropletsetc.)

!Technical impermeability

Thermo- and fluid-

dynamic effects

Fluid to

be sealed

leakage

Aux. material

Part 1

Part 2

leakage

Part 2

Part 1Fluid to be

sealed

Principle of a static seal Principle of a dynamic,

clearance seal


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Seals

What is impermeability?

Sym. Axis of a piston seal

Sym. Axis of a rod or shaft seal

Movements

SealTappet

Force Housing

Support

Sealing surfaces

Sealing gap

static dynamic

Principle of a dynamic, contact seal

Sealing gap


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Seals Static

Sealant

Gaskets

Compliance with rigidsurface contours

Many different materialsused: e.g. Paper: light duty

Cardboard: larger size light duty

Asbestos based materials: hightemperature and pressures

Etc.

Sealant

Gasket


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Seals Static

Gaskets

Different mounting possible

Be careful about the line of force of screw


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Seals Static

Metal seal ring:

high temperatures (!300C) and pressures (!70MPa)


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Seals Static

O-Rings

Various cross sections: ring, lobed,rectangular

Seal function: preload + system pressure

Allows metal-to-metal contact fordimensional accuracy

Housed in grooves

Many materials for a variety of applications


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Seals Static

O-Rings

Assembly & Types

Undercut & the use of extrusion rings

Undercuts Slitting

caused

by extrusion

Extrusion ring


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Seals Static

Shrink fit

Sealing due to dimensional interference Permanent sealing

Pipe couplings

Taper threads

Compression fittings: Olive for smaller pipes

Flange and Gasket for bigger pipes

Cone seal

Ductile deformation

Detail ZDetail Z


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Seals Dynamic Rotary

Felt seals

For grease lubrication Prevents dirt ingression

Often used with other types of seals

Bearing Lubricant (grease)

Flange

Felt Seal

Dirt

Housing

Shaft


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Lip/Gartner seal

Most commonly used cheap seal Lip faces fluid to be sealed

Seal surface mustbe scratch free

Cheap long life and easily replaced

Dont useat reciprocal, oscillating conditions;

p > 100 kPa; gas/gas situations


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Mechanical or Face seals

Extends desirablefeatures of lip seal to

wide range of pressures

Dry and lubricated

Very susceptible to dirt Difficult to replace in-line

Thermal distortion

causes problems

Materials: carbon, PTFE,ceramics, brass alloys

Cooling fluidSealed fluid

Twist lock

Shaft

BushingSpring

Rotating

ring

Static

ring

Add. Sealing

(O-Rings)


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Seals Dynamic Reciprocating

O-Rings

Packing rings

Pressure increases

sealing effect Dry and lubricated

Single seal: unidirectional

Double seal: bidirectional

Smoothsurface

Seal ring

Support ring

Pressure


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Seals Dynamic Reciprocating

Piston rings

Metallic, musthave lubrication

Cascaded

Wiper ring

Wipes off dirt and

water

Let oil film pass

Guide ring

Support ring

Seal ring Seal ring

Tension ring

Dirt, ice

water

Let pass

Oil film Wipe off dirt

Pressure slope not

too steep!


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Seals Dynamic Contact Reciprocating

Bellow

Diaphragm

Short stroke

High pressure

Rolling diaphragm

Very flexible rubberdiaphragm

Long strokes Low speed

Positive sealing betweentwo fluids

Connection ConnectionBellow


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Seals Dynamic Clearance Rotary

Fixed and floating bush

Leakage approx 100 ml/h, Not often used

Labyrinth

Commonest type of clearance seal

Leakage is half of bush type seal

!V =C3!p

3l


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Visco- or Windback seal

Helical grooves pump leakage back intosystem

Viscous fluids or at high speeds, moderate

pressure


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Seals Dynamic Clearance RotaryCentrifugal seal

Disc rotating in annular space Used to seal gas at high rotational speed

Attention

Quite expensive because of accurate

manufacturing

No sealing effect when not rotation

Sometimes need backup seals


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Ferro-Magnetic seal

Based on fluid which increases viscosity in amagnetic field

Only for gas/gas applications

Seal pressure: 0.2 MPa per cm of seal length

Hydrostatic seal

Like thin film hydrostatic bearing

Product contamination or leakage inevitable

Designed for specific applications


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Seals Dynamic Clearance Reciproc.

Fixed and floating bush

Labyrinth

High leakage

Provide no static seal properties


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Seals O-Ring Design Recommendations

Sufficient preload

pre-pressure 10-20%

Groove width

Surface roughness

Avoid extrusion! Extrusion rings

Mounting damage

Chamfering

Undercut

Surface roughness


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What might happen if you disobey design

recommendation!Challenger disaster 1986

Whats happened?!Totally destruction of space shuttle

Challenger 75 s after lift-off

Technical reason:

!Poor fuel tank design and sealing (O-Rings)


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Build-up of

the spaceshuttleboosterrockets


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Leak check


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Joint rotation

O-Ring resiliency

Due to low temperatures the resiliencywas on 1/5 of expected value.


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Failure is the best way to learn

!New booster hull design

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