Gearboxes, when included on vertical pump units, must be of the coaxial type with the pump, motor and gearbox centrelines positioned vertically above each other to ensure an equal load distribution on the support ring.
Coaxial gearboxes may be either of the multi-layshaft or epicyclic designs. With recent epicyclic designs, it has been possible to combine the gearbox and the pump by mounting the impeller on the low speed output shaft of the gear unit (Fig 4.50). This arrangement eliminates the need for one journal bearing, which in turn reduces the overall height of the pumpset leading to a more compact arrangement.
A further reduction in pumpset length is achieved by combining the thrust collar into the coupling hub, as shown in Fig 4.50.
The gearbox is equipped with a complete forced-lubricating oil system to provide a continuous flow of oil to the gear internals and the thrust and journal bearings throughout the pumpset. The oil system includes both gear-driven and standby motor-driven oil pumps, and the piping and non-return valve layout allows delivery of oil to the bearings and gears, whichever direction the pump is rotating.