By: Thomas Yoon

A centrifugal water pump contains a few basic components as follows:

  1. The housing for containing the water.
  2. The impeller attached to a rotating shaft that moves the water.
  3. The coupling that transmits the rotating action of a motor (or other prime movers) to the pump shaft.
  4. The seal between the housing and the rotating shaft.
  5. The bearings for ensuring that the shaft turns smoothly.
  6. The inlet and the outlet ports for channeling the water flow through the pump.

In order for a pump to work, the pump impeller must generate pressures inside the pump housing. The seal between the housing and the rotating shaft plays a critical role in making the pump work.

One of the most common ways of sealing a rotating pump shaft is by using mechanical seals.

The 3 components of a mechanical seal are:

  1. The rotating component that moves together with the rotating shaft.
  2. The stationary component that is fixed to the pump housing.
  3. The spring that presses the rotating component onto the stationary component axially.


Rotating Component

This is a ring, usually consisting of 2 types of materials – a soft carbon piece with a highly polished axial surface, and a rubber material or o-ring that is able to grip the shaft for sealing against water leak through the shaft surface.

Stationary Component

The stationary ring is usually made of stainless steel, although there are some made of ceramic or similar hard materials. The axial surface is also highly polished. At the circumference of the stationary piece, a rubber o-ring will be installed to seal against water leak through the pump housing.


The spring(s) is installed around the shaft and rotates with the rotating component. The purpose of the spring is to press the rotating component of the mechanical seal against the stationary component so that the matching surfaces are in contact with each other axially to the shaft.

Because the axial rubbing surfaces in contact are very highly polished, there is almost no leak there. Water does not leak through the pump housing opening because of the o-ring pressed between the stationary component and the pump housing. Water will also not leak through the shaft because of the o-ring in the rotating component gripping against the shaft.

So, if there is a leak in a mechanical seal of a pump, look for the sources of leak in those 3 places. Surely one of them is not sealing properly.

For more information on mechanical seals and pump troubleshooting, download the free software