AXIAL DEFLECTION OF THE SHAFT A036
There are many causes of axial deflection of the pump’s rotating pump shaft that include:
- Thermal growth causes shaft axial growth that can be compensated for in the coupling internal clearances. Unfortunately this does not help the mechanical seal face loading.
- Impeller adjustment of open and semi-open impellers can move the shaft towards or away from the volute depending upon the pump design. In the United States the Duriron Pump Company is unique in that their impeller adjustment moves the shaft in the direction of the bearings in most of their designs. Remember that there is an initial impeller setting and “on going” settings that have to be made for casing and impeller wear, as well as thermal growth.
- Sleeve or journal bearings allow a lot of axial movement. Precision bearings limit axial movement but can still allow enough radial movement to open lapped mechanical seal faces.
- Up to 65% of its efficiency a centrifugal pump thrusts towards the thrust bearing. Beyond 65% of its efficiency the shaft thrusts towards the volute. At pump start up there is a lot of axial movement of the shaft. This is one of the reasons why we lose so many mechanical seals at pump start up.
- Attaching a mechanical seal to the shaft adds to the axial thrust of the shaft because the stuffing box pressure works on the seal area attached to the shaft or sleeve. The resultant thrust is normally towards the bearings.
- Any form of vibration will add axial movement. This vibration includes cavitation and water hammer.
- Mounting a pump in a vertical position will increase the axial loading.