WHAT IS A POOL PUMP SHAFT SEAL?

The shaft seal prevents water from leaking along the motor shaft, just behind where it connects to the impeller. The motor shaft goes through the seal plate, or the bracket that connects to the pump. The design of a shaft seal allows the shaft to spin without any direct contact (which would immediately cause a seal failure with the shaft rotating at 3250 RPM).
Shaft seals  are composed of two halves, a round ceramic ring encased in rubber (what I call the Donut), and the other half with the spring, or the Spring Half. The donut usually sits in the seal plate, and the spring half slips over the impeller. As the impeller is tightened down, the spring half mates perfectly with the donut half, sealing up the shaft from water leakage, without actually touching the shaft.