THRUST LOADS

"Thrust" is an external mechanical load — a force — imposed on a motor shaft, directed parallel to that shaft. That load must be sustained by one of the motor bearings (not both bearings; assemblies allowing an endwise force to be divided between bearings at both ends of the motor are not practical). Hydraulic pumps are the most common source of thrust.

A few special types of horizontal-shaft drives involve thrust loadings. But such external forces are normally encountered only by vertical-shaft pump motors, or simply "vertical motors" as they're usually called. Actually, almost any integral horsepower of "medium size" induction motor could be operated with its shaft either horizontal or vertical. But in the world of pumps, those imposing significant thrust force on the driving motor are confined to a particular group of vertical-shaft designs. These have three characteristics in common, besides that shaft orientation:

1. They are used on deep wells involving high lifts of water to the surface. Irrigation service is typical.
2. They are "high speed" — either 1800 or 3600 rpm.
3. The thrust loads are high but span a wide range of values depending upon pump design, system hydraulics, and well configuration.

The industry standard pump construction includes a flat upper surface (on the pump itself or on an attached adapter called a "pump head") for mounting the motor. The mating surface on the motor's lower bearing bracket is called a "P-flange" or "P-base," and its dimensions are standardized by NEMA over the range of industrial motor frame sizes. This construction is denoted on motor nameplates by the letter "P" following the frame size; e.g., "444TP."

Not all suppliers offer standard P-base motors. Those who do may restrict their availability to 75-100 hp and below. Common practice is to offer three different values of maximum thrust load capability: "normal," "high" and "extra high" is one example. Each requires a different thrust bearing assembly inside the motor (normally at the top or upper end of the shaft). All thrust loading adds motor friction loss and reduces efficiency.

For most pump drives, the thrust load is a continuous force directed downward ("downthrust"), plus a much lower value (such as 30%) appearing "momentarily” in the upward direction ("upthrust") when the pump is shut down. Momentary upthrust is usually carried by the motor's lower bearing. Some units, however, are built to carry equal up-and down thrust continuously, which requires a different type of thrust bearing. In the "high" or "extra high" thrust categories, oil lubrication is necessary. Some smaller motors, and the lower bearings of some larger units, can still get by with grease.

These motors typically use the same electrical designs and parts as their standard horizontal counterparts, but are considered "definite purpose" by NEMA standards. Proper application involves many choices of options too numerous to outline here, and often dependent upon pump design, such as a non-reverse backstop, self release coupling, and hollow-shaft vs. solidshaft design. Choosing the right motor is no job for an amateur.

Richard L. Nailen, P. E.