R975 Radial engines

[Malcolm Towrie]

Originally Posted by Jesse Browning

I learned from a pump manufacturer that the holes are to relieve the vacuume pressure, not to protect the bearings, but so that sewage (or oil in engine application) doesn’t get sucked past the seal.

Gotta disagree, Jesse. As far as I know, those holes are to reduce axial load on the pump, or in this case supercharger, bearings.
What can cause oil getting sucked in past the shaft seal on an overhung impeller design like this is having the impeller back shroud too close to the supercharger rear housing. The tight gap between the shroud and casing causes a pumping effect which can reduce the pressure down at the shaft below atmospheric.

Malcolm