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History: US Electrical Motors, working with pump manufacturers, developed the P-Base vertical motor in 1922. The motor was designed for deep well, turbine pumps. The significant design innovations were putting the pump thrust bearing and the motor to pump coupling at the top of the motor. The pump head shaft extended thru the motor Hollowshaft to the top of the motor. The principal benefits included:
- easy access to the thrust bearing for replacement without dismounting the motor,
- easy access to the coupling,
- the bearing size could be increased,
- the bearing lubrication could be improved,
- and easy axial pump adjustment.
The Vertical Hollow Shaft (VHS) motor is typical to the United States pump manufacturers. Most other pump manufacturers still use the thrust bearing in the pump with a zero thrust, vertical motor.
P-Base Design Overview: The defining characteristic for P-base motors is thrust. Unfortunately there is no defining standard for thrust. Each manufacturer defines the Thrust capability for each rating, individually. Of course, the thrust capacity is determined by the thrust bearing and is calculated by Anti Friction Bearing Manufacturers Assoc. (AFBMA) standards, once the bearing is selected.
Although the original P-base was the VHS motor, Vertical Solid Shafts (VSS) designs have also been developed. While all VHS motor are high thrust, VSS motors come in High, Medium and Normal Thrust.
VHS vs. VSS Thrust: VHS: NEMA MG 1, Part 18, Definite Purpose Motors defines the shaft dimensions and tolerances for High Thrust (HT) VHS. The frame designation has a TP suffix, i.e. 444TP. NEMA defines a flange for 143TP thru 445TP. Flanges are identified by their “BD” dimension which is the diameter of the flange outside diameter. Additional P-bases are also available for each frame and identified by a manufacturer unique, additional letter, i.e. 444TPA. VHS HT motors are generally used for shallow well turbine pumps and/or pumping up hill.
The standard flange BD’s are:
- 10”
- 16-1/2”
- 20”
- 24-1/2”
- 30-1/2”
Typically a 12” BD are also available on smaller frames.
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