How does the tall fuel pump seal?

[rgafitanu@gmail.com]

Hello,
I am a mechanical engineer and there is an element of the tall fuel pump that baffles me. So far we know:
- the bellows seal is sealed at the bottom by the o-ring
- fuel enters from the top of the seal (red arrows) inside the seal (first picture) and through centrifugal force during rotation pushes the bellows outwards (blue arrows) which in turn extend themselves upwards (green arrows) sealing against the black piece of plastic (second picture).
I get all this.
However, what stops the bellows from relaxing when the shaft is not turning and there is no more centrifugal force? The bellows would drop (compress) breaking the seal formed with the black piece and allowing fuel to go up through the gap between the shaft and the black piece into the compartment above.
Maybe I am not getting the principle of this seal correctly, however it must work, hundreds of thousands of vehicles were equipped with this pump and I am sure they weren't meant to even "sweat". Even after 50 years and a rough rebuilt (by myself) my fuel pump is not leaking one gasoline molecule (my wife would sniff it from a mile).

Radu

[dirkbalter]

Hello,
Isn’t the bellow when installed, slightly pre-compressed and acts like a spring, always generating a certain seal force against the black plastic washer?
Dirk

[Tyler S]

Dirk is correct. The bellows is merely a closed spring that provides the resistance needed to seal. It allows up and down movement without putting the shaft into a bind. Compensates for temperature changes or bearing wear.

[rgafitanu@gmail.com]

Makes sense but it's not entirely following the technical manual explanation "fluid enters from the top creating a fluid seal". If the bellows were used for the spring aspect I would question the material choice, brass has a very low module of elasticity, you don't see brass springs. I managed to restore 2 pumps out of 3 so far and when I extend the bellows there is very little spring back, I am more in favor for the bellows being used for the reverse "chinese finger trap" capacity.
Not a big deal, after all, it works.