Q. Another alternative in determining vertical turbine pump design is solid versus hollow shaft motors. What are the considerations for making that selection?
A. Vertical pump drivers of solid shaft or hollow shaft construction are shown in Figures 184.108.40.206.4a and 220.127.116.11.4b.
The hollow shaft configuration has no external shaft extension. The top pump shaft, called the head shaft, extends through the driver shaft (also known as the quill), which is hollow and coupled at the top with a key and adjusting nut arrangement (see Figure 18.104.22.168.4c). The coupling is located within the motor, under the motor’s drip cover. This coupling permits the shaft to be adjusted to compensate for the tolerance stack-up of the pump rotor and casing components and provide the desired axial running clearance for the impellers. The manufacturer normally specifies this clearance, which is determined by mechanical and efficiency considerations, as well as thermal and pressure elongation expectations of the column and shafting.
This hollow shaft arrangement provides optimum access to the head shaft-adjusting nut where impeller lift is sensitive, such as on deepwell pumps. A single-piece head shaft is typically used on outside deepwell pump installations where headroom clearance for disassembly is not limited.
Hollow shaft motors are coupled to the pump head shaft with either a self-release or bolted coupling.
A self-release coupling is a device that can be used to keep a pump’s lineshaft from unscrewing, due to torque reversal, and protect the driver from damage. Should a torque reversal occur, for example if motor leads were wired incorrectly, the driver coupling will lift and disengage the pump shaft. Self-release couplings should not be used on motors subject to upthrust.
Bolted couplings rigidly connect the pump lineshaft to the motor. Bolted couplings will handle upthrust but will not protect a motor or lineshaft in case of torque reversal. Lock screws usually secure the threaded adjusting nut to the driver rotor flange (clutch). The lock screws usually provide limited pump shaft upthrust protection.
A bottom steady (quill) bushing option is normally offered with hollow shaft drivers to provide added shaft support. This bottom bushing is often recommended with two-piece head shafts, long one-piece head shafts or to solve head shaft vibration problems.
Solid shaft drivers are connected to the pump through a rigid or flexible coupling. Rigid couplings must transmit torsional and axial loads, maintain shaft alignment and permit adjustment of the shaft as required to set the desired axial running clearance for the impellers. Pumps with tight rotor end float clearances may require motors specified with limited rotor movement from the pump axial thrust.
The driver lower bearing (usually rolling element type), in conjunction with the rigid coupling, provides pump shaft support for mechanical seals. When precision manufactured, this coupling is not normally dynamically balanced. The rigid flanged-type coupling is capable of transmitting both axial up and down thrust. This style coupling is preferred when pump shaft axial upthrust occurs. Flexible couplings are normally only used with solid shaft drivers when a separate thrust bearing is provided in or above the pump discharge head and below the flexible coupling. The flexible coupling then only transmits the torque, with the separate thrust bearing providing the axial and radial shaft support and allowing for the required axial adjustment of the rotating element.