The shaft frets under the shaft seal

If the mechanical seal is cocked or misaligned onto the shaft (Figure 14-10), or the seal chamber face is not perpendicular to the shaft, the shaft o-ring will have to flex to maintain face contact. The flexing causes the shaft seal to rub on the shaft under the seal, which can eat or erode a groove into the shaft. The pumped fluid can leak under the o-ring, or the o-ring can hang into the groove and drag the seal faces as the shaft moves within the bearing tolerance. This is considered a seal problem, but is actually an alignment problem. There is more information on this later in this chapter regarding pump reconstruction. Figure 14-11 shows how an o-ring in a misaligned seal can fret the pump shaft.

Aligned

Face

Misalignment.

Face

Misalignment. (\

1/2 Revolution.

The seal rotary face moves to maintain alignment with the stationary face. The o-ring slides and frets the shaft.

Aligned

Face

Misalignment.

Face

Misalignment. (\

1/2 Revolution.

The seal rotary face moves to maintain alignment with the stationary face. The o-ring slides and frets the shaft.

The o-ring seal gripping the shaft moves

■ 1,800 rpm x 2 movements/revolution = 3,600 movements/ minute.

■ 3,600 movements/min x 60 minutes/hour = 216,000 movements/hour

■ 216,000 movements/hour x 24 hours/day = 5,184,000 movements per day

At 5,184,000 movements (rubs) per day on a shaft spinning at 1,800 rpm the constant friction will eat a fret mark (groove) into the shaft or sleeve in just a few days (Figure 14-12). The next o-ring seal, installed onto this pump, will ride in the groove (cut by the previous seal) and never give good service. Again, this is an alignment problem and not a seal problem. Many seal companies have addressed fretting corrosion with a product called self-aligning faces. Both the rotary and stationary faces are spring loaded as the faces push against each other. The opposing springs tend to cancel themselves and the union between the seal faces will always be perpendicular to the shaft axis. This prevents the flexibly mounted shaft seal from dancing and rubbing on the shaft if the pump parts should be out of alignment. Another way to resolve fretting corrosion is to align the pump parts upon rebuilding the pump.

Figure 14-12

0 0

Post a comment