MECHANICAL SEAL CLASSIFICATIONS

Over the years, mechanical seals have undergone continuous improvement. Today, numerous classifications and design features are available to end users. Each mechanical seal design offers specific strengths that make its use advantageous for certain situations, and tradeoffs that may make its use impractical or ill-advised for particular applications. As the technological sophistication associated with mechanical seal design has increased, users often have a difficult time deciding which seals to use, and where to apply them most appropriately.

Mechanical seals can be classified by design, or by application.

Classification by design:

1. Component or cartridge seals

2. Spring-type seals

3. Stationary or rotary seals

4. Balanced or unbalanced seals

5. Pusher or bellows seals

6. Split vs. non-split seals

Classification by application

1. Pump or mixer seals

2. Metallic versus nonmetallic seals

3. High-temperature seals

4. Single versus dual seals (such a tandem, back-back or face-face de signs)

5. Wet lubricated seals or gas seals

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How an oil seal is designed

In an oil seal environment there is a shaft that is rotating inside a housing or bore. Because of friction, there must be some clearance between the shaft and the bore. The function of the oil seal is to stop whatever fluid is inside from leaking out the clearance between the shaft and housing. The seal may also be used to prevent outside materials, such as dirt, from moving in through the clearance.

"Dynamic sealing" is the relationship between the rotating shaft and the seal and is handled by the sealing element. A garter spring may used in the oil seal to increase the radial interference between the seal lip and contact point on the shaft. In order to achieve this interference, the oil seal's ID must be slightly smaller than the diameter of the shaft.

"Static sealing" is the relationship between the housing and the seal. In order to achieve this interference, the oil seal's OD must be slightly larger than the diameter of the housing or bore.

An oil seal normally consists of three basic components: the sealing element, the metal case and the spring. The purpose of the sealing element is to stop the fluid from leaking between the shaft and housing. The metal case will give rigidity and strength to the seal while it is being held in the bore or recessed groove. The spring will help make the sealing element more effective. All materials must be selected depending on the environment in which the oil seal will function.

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Introduction of Oil Seals

Oil seals have the main function of sealing off oil, greases and other types of fluids that must be contained within a machine, also avoiding the entry of any foreign matter or impurities from without, such as dirt, sand particles, dust etc. Employed in a varied array of uses, they are present in our daily lives. From automobiles to trucks, buses, motorcycles, farming machines, even home appliances, airplanes, and industrial machines.



Oil Seal Composition

Main Seal:Fluid sealing function, whether the shaft is in a dynamic or static condition.
Spring: Radial load exerted over the shaft compensation function.
Auxiliary sealing: Main seal protection from dust and other elements, function.
External diameter: Offers an interference between the housing and the oil seal.
Types in existence:
- With a smooth rubber lining
- With an ondulated rubber lining
- Metallic- Half Rubber / Half Body
- With rubberized painting
Body: Performs the function of a structure for the oil seal, supporting its profile while also enabling the mounting of the necessary housing.

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Introduction of Mechanical Seals

Mechanical seals are being used increasingly on fluid pumps to replace packed glands and lip seals. Pumps with mechanical seals perform more efficiently and generally perform more reliably for extended periods of time

Mechanical seals are provided to prevent pumped fluids from leaking out along the drive shafts. The controlled leakage path is between two flat surfaces associated with the rotating shaft and the housing respectively. The leakage path gap varies as the faces are subject to varying external loads which tend to move the faces relative to each other.

The mechanical seal requires a different shaft housing design arrangement compared to that for the other type of seals because the seal is a more complicated arrangement and the mechanical seal does not provide any support to the shaft.

In order for the mechanical seal to perform over an extended time period with low friction the faces are generally hydrodynamically lubricated. The fluid film will need to carry substantial load. If the load becomes too high for the film surface contact will take place with consequent bearing failure. This lubricating film is generally of the order of 3 micrometres thick , or less. This thickness is critical to the required sealing function. Mechanical seals often have one face of a suitable solid lubricant such that the seal can still operate for a period without the fluid film.

The mechanical seal generally includes three static seals.

1.The sleeve seal - this is usually an O-Ring
2.The seals between the moving seal member and the shaft or sleeve.- This is often an o-ring but can be a wedge or vee seal. This seal may not be used for bellows type mechanical seals
3.The housing seal is generally an o-ring of a gasket.

All of these seal must be compatible with the fluid being contained and the associated environment. These seals may limit the design for high temperature applications. In this case the bellows type alternative may be the best option.

The use of mechanical seals generally involve the use of additional equipment primarily for the flushing /coolant systems. This includes pumps, coolers, strainers, filters etc.

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