My experience is not with the Eheim pumps, but their canister filters.

The ceramic rod through the impellor assembly should be one piece. it fits
(on my filters anyway) into a cylindrical rubber "bearing" at each end -
this holds the shaft steady and centered. The shaft does not turn.

The coil in the pump head makes the magnet spin on the shaft. As the
impellor can only turn about 180 degrees on the magnet it turns with it and
hence pumps the water.

problems that tend to happen is the shaft wears and breaks, causing the
magnet to not stay centered in its "well" and rattle about, not pumping
well, if at all. The other main problem is build up of "bio film" in the
well and on the magnet causing drag which lowers the pumps performance and
in bad cases can cause the pump not to restart after a power failure. After
a long time both the shaft and the inside of the impellor assembly can wear
causing excessive play. They would normally be replaced as a set as if one
is worn the other is likely to also be worn.


wrote in message
ups.com...
Sometime ago, we purchased a tank which came with two Ehiem Model
2015(?) pumps and a third for spare parts. I've taken the head apart
and am examining the impeller, but don't understand how it works. This
is what I am seeing:There is a cylindrical magnet into which the
impeller shaft fits. The impeller snaps on to the top of this shaft and
can rotate maybe 180^ in either direction. Two ceramic rods run thru
the center of this assembly and serve to keep the assembly vertical.
I assume the external coil causes the cylindrical magnet to rotate.
However, there does not appear to be an direct connection between the
cyhlindrical magnet and the impeller shaft, and there does not appear
to be any magnetic interaction between the magnet and the shaft, hence
the impeller shaft only vibrates.
Can someone describe how the impeller is supposed to operate? - Mike