Shafts, rotating, sealing mechanical face seals

Mechanical Face Seals This type of seal forms a running seal between flat precision-finished surfaces. It is an excellent seal against leakages. The seahng surfaces are planes perpendicular to the rotating shaft, and the forces that hold the contact faces are parallel to the shaft axis. For a seal to function properly, there are four sealing points ... 

This dial indicator is fixed to the volute mounting adapter collar of the pump and the needle is on the shaft (Figure 14-21). The shaft should be moved radially by hand (see the arrows) up and down. Note the movement in the indicator. This is a check of the radial tolerance in the bearing. Some people use the word run out . Radial dellection causes misalignment of the rotating and stationary faces of the mechanical seal. This shortens the seal life by eausing drive pins and springs to wear and rub in relative motion. 

In the process industries the conditions at the pump seal are often harsh and more complex seals are needed. Mechanical face seals are used, Figure 5.14. They are generally referred to simply as mechanical seals, and are used only on rotating shafts. 

Mechanical seals offer a more reliable seal than compressed packing seals. Because the spring in a mechanical seal exerts a constant pressure on the seal ring, it automatically adjusts for wear at the faces. Thus, the need for manual adjustment is eliminated. Additionally, because the bearing surface is between the rotating and stationary components of the seal, the shaft or shaft sleeve does not become worn. Although the seal will eventually wear out and need replacing, the shaft will not experience wear. 

A mechanical seal s principal components are two smooth rings carbon and a hard metal alloy. One ring rotates on the shaft while the other is stationary and connected to the pump. The two faces are forced together with a spring. When a seal leaks, it is usually because these faces become separated or are no longer smoothly rotating against each other. 

Another characteristic of mechanical seals is balance. Unbalanced seals have the same shaft diameter for both the rotating and stationary elements, as shown on the left of Figure 21-29. The large area of pressure acting to close the unbalanced seal, shown as arrows at the bottom of the cross-section sketch, results in a large force on the seal faces. A balanced seal can reduce the pressure force acting to close the seal faces, by reducing the area on which the pressure acts, as... 

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