""A Shaft in a Shell""
The pump must be used with many varying liquids, consequently the smallest residual left within the pump is unacceptable. You want to pump adhesive substances like paints, or liquids which tend to crystallize or harden.

The inner tube houses the shaft. It is sealed at the bottom by mechanical seal through which the shaft passes. Consequently the shaft does not come into contact with the liquid except for the small section between the impeller and seal.