An unload conveyor and a sweep conveyor can be positioned below and above a floor of the grain bin, respectively. A below floor gearbox and an above floor gearbox can be positioned below and above a sloped wall, respectively, of a sump. The sloped wall can be designed to shed grain toward a sump basin. A coupling shaft can extend through a non-circular sump shaft aperture in the sloped wall to couple the below floor gearbox to the above-floor gearbox. The coupling shaft can extend vertically. A pair of seal cover plates can form a horizontal wall extending perpendicular to the coupling shaft. The horizontal wall can have a circular sealing aperture therein and through which the coupling shaft extends. A washer can be provided adjacent the circular sealing aperture that can be free to move laterally with wobbling of the coupling shaft.