How does a HOB motor work?

[Oxboy]

(I would've but this in the Equipment forum but that doesn't seem too populated)

So I've googled how a typical magnetic HOB motor (say Aquaclear 70) works, but I'm still unclear:

- so the magnets on the impeller adhere to the metal shaft, which in turn rotates?
- the only thing the motor inside the housing does is turn the shaft?
- can water get down in through the hole that the shaft sits in?
- why is the plastic impeller vane piece all loose on top of the magnetic impeller piece? How come the aquaclear model has so much sloppy give? Doesn't this contribute to rattle?

thanks for clarity.

[AquaticNerd]

Most modern impeller designs don't actually have a motor that turns the impeller, but instead uses a fluctuating magnetic field to cause the impeller to spin. The impeller and motor housing never actually make contact with each other and the motor housing is completely water-tight. Since there really isn't anything except for magnetism holding the impeller in place, you sometimes notice that the impeller doesn't have to be securely stuck in place. In fact, there should be a little bit of play around the impeller.

It is for this reason that you have to "prime" most HOB filters by adding water to their chamber until they are full - they can't move air. They need a liquid in order to perform their function (suction) to bring water from your tank into the filter box and then out the "waterfall".

HTH

[Oxboy]

So does the shaft turn or just serve to hold the impeller in place for it to spin around?

[bill6148]

To my understanding the shaft does not spin, it is held in place by the rubber grommets.