MEC2020W Design I

Seals

Dr Reuben Govender

April 2013

Department of Mechanical EngineeringMEC2020W Design I

Function of seals

● Keep lubricant or process fluid inside● Lubrication retention – essential for bearings● Process fluid retention – cost or safety concerns

● Keep contaminants out● Water / dirt ingress into oil supply of bearings● General contamination of product

Department of Mechanical EngineeringMEC2020W Design I

Classes of seals

Static Dynamic

Rotary Translating

Contact Y Y Y

Non-contact (gap)

N Y ?

Department of Mechanical EngineeringMEC2020W Design I

Static seals

● Why ? ● Surface roughness – leaves leak path

● Deformable element between two components which “fills” leak paths

●

● Easily replaced● Examples: Gaskets, O-Rings, Packing

Department of Mechanical EngineeringMEC2020W Design I

O-Rings

● Cheap, easily installed●

Department of Mechanical EngineeringMEC2020W Design I

O-rings

● Fit inside groove● Groove sized to leave 25~30% of O-ring

thickness for compression● NB rubber is nearly incompressible!

Department of Mechanical EngineeringMEC2020W Design I

O-rings contd

Department of Mechanical EngineeringMEC2020W Design I

Gaskets

● Material – to suit process fluid and environment ● Plastics, rubber, metals (soft)

Department of Mechanical EngineeringMEC2020W Design I

Dynamic contact seals

●Spring component to ensure sealing faces are in contact●Frictional losses higher

SKF manual

Department of Mechanical EngineeringMEC2020W Design I

Lobed seals

● X-rings, U-rings● Fluid pressure forces lobes against sealing

surfaces, preventing leakage

SKF.com

Department of Mechanical EngineeringMEC2020W Design I

X-Rings

Seal in both directions

Department of Mechanical EngineeringMEC2020W Design I

Piston rings

●Operate under extreme pressures (Combustion inside cylinder)●Large Temperature differences (expansion)●Multiple rings – sealing gas in chamber, preventing burn up of oil●Friction high relative to other seals●Other seals provide insufficient life

Department of Mechanical EngineeringMEC2020W Design I

Mechanical Seals

Pump Internals:

Department of Mechanical EngineeringMEC2020W Design I

Mechanical Seals

●Multiple components●Static seals (O-rings) between :

● shaft and rotating seal face● Housing and stationary seal face

●Thin fluid film between seal faces●Does not stop fluid flow ('leakage')●Limits / controls flow to acceptable levels and direction

Department of Mechanical EngineeringMEC2020W Design I

Non-contact seals

● Fluid flow rate :● Pressure differential● Geometry of flow path

● Changes in direction limit flow rate● “Tortorous path”

Department of Mechanical EngineeringMEC2020W Design I

Different orientations