SUBJECT : The classification of mechanical seals. 2-11

Talk to a seal manufacturer's sales representative, send for all of the brochures, and you'll learn that the subject is very confusing. What do we really know about mechanical seals? A few things for sure:

For all practical purposes seals should not leak visibly. Look at the seal in the water pump of your automobile, or the seal in the air conditioning unit in your car and ask yourself if you think either of them is leaking. Fugitive emission regulations have changed the definition of leakage to less than a few parts per million, depending upon the chemical involved.

If you like the brand of seal you're using ,have the manufacturer repair it at his facility, or purchase the spare parts from him to insure you'll be getting the correct materials and tolerances. It doesn't make sense to do anything else.

We'll now look at the various methods of classifying mechanical seals and in the process learn which to specify for our applications. I'll give a brief description of each type and list the most obvious advantages and disadvantages of each. Needless to say the advantage of one is almost always the disadvantage of the other

The rotating seal. The springs/ bellows rotates with the shaft

The stationary seal The springs/bellows do not rotate with the shaft.

The inside mounted seal. All components are in the pumping fluid.

The outside mounted seal. None of the metal components are in contact with the pumping fluid. Most designs clamp to the shaft rather than using set screws that damage ceramic or glass coated shafts.

The single seal. It has only one set of sealing faces.

Dual seals. More than one set of faces are installed in a variety of configurations including:

In all of these configurations two-way balance should be specified for safe operation.

The unbalanced seal. The seal faces are subjected to full system hydraulic and surge pressures in addition to the spring pressure..

The balanced mechanical seal. The design allows the seal faces to see only a small portion of the system hydraulic pressure.

The elastomer type of seal , utilizing an o-ring, wedge, chevrons or a U-cup, with the o-ring type having the most advantages.

The metal bellows seal. All elastomers have been eliminated from the design.

The cartridge seal. All components of the seal are mounted on a sleeve that can be secured to the shaft from out side the seal chamber.

The non-cartridge seal. The seal attaches directly to the pump shaft or sleeve, or in some cases, against a shaft shoulder.

The split seal. In a true split seal all the seal components are split in half so that the seal can be installed without taking the equipment apart. Rubber components are not allowed to be glued together as this would cause a "hard spot", interfering with the free movement of the dynamic elastomer.

The solid seal. The equipment must be disassembled to install the seal.

Motion seals. They have larger internal clearances along with different spring arrangements and wider hard faces to compensate for excessive radial and axial motion. A radial movement capability of plus or minus 1/8" (3 mm) would be typical.

Pump Seals. Manufactured for 3/8 inch (10 mm) packing space. Some designs will go into 5/16" (8 mm).

Original equipment seal designs. The type you get if you don't specify a specific brand and model number.

Off the shelf, universal seal designs. Designed to fit into the thinnest, shortest space and still meet the necessary operating conditions. Most are non-fretting designs with universal materials installed as standard.

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