by Jim Bryan, Electrical Apparatus Service Association (EASA)


Construction features are key to vertical motor application and maintenance.

The definite-purpose application of vertical motors dictates a number of construction features that set them apart from horizontal motors. Some differences are obvious—such as a vertical motor typically drives a pump and has a P-base mount without feet and a horizontal motor may have a footed or footless mount with a C or D flange or no flange). Less apparent is the unique capacity of vertical motors to carry external thrust. For optimum service and performance, understanding how construction features affect application and maintenance requirements  is important.

Figure 1. Typical vertical motor

Vertical Motor Classifications

Vertical motors can be broadly classified based on shaft type—solid or hollow. Solid-shaft models couple to the pump shaft at the lower end of the motor. The shaft extension on these motors normally has an annular (ring-shaped) keyway to carry the thrust of the pump and a radial keyway to transmit torque. This type coupling is more common on shallow pumps and tanks than on deep-well applications.

On hollow-shaft motors, the pump headshaft passes through the motor shaft and couples at the top of the motor. An adjusting nut at the top of the headshaft greatly simplifies the adjustment of the pump impeller depth. Although commonly used on deep-well pumps, hollow-shaft motors may be applied whenever easy adjustability is an advantage.

Vertical motors are also classified by their thrust load: normal, high or in-line. Normal thrust loads can be determined from tables based on motor speed and horsepower rating. High thrust is a multiple of normal thrust—for example, 175 percent, 250 percent or 300 percent. In-line pump motors are a little different because the pump is “in line” with the piping, no external thrust is applied to the shaft.

Thrust Bearings

The thrust bearing, which is usually at the top of the vertical motor (Figure 1), may consist of one or more angular contact bearings; a spherical roller bearing;

or a hydrodynamic, plate-type bearing. The thrust applied by the pump—manifesting in upward, downward or balanced axial loading—determines the type and number of bearings used. The applied load and speed of the thrust bearing may require oil lubrication. To achieve the best service life and performance, it is important to correctly match the thrust bearing configuration and the pump thrust.

Angular Contact Ball Bearings

Angular contact ball bearings are the most common thrust bearings in vertical motors. A single bearing is used for normal thrust loads. Two bearings can be used in tandem for 175 percent of normal thrust (see Figure 2), and three bearings can be stacked for 250 percent of normal thrust. Stacks of up to five bearings have been reported. (Note that using two bearings in tandem does not double the thrust capacity.)

Figure 2. Angular contact ball bearings

Spherical Roller Bearings

Spherical roller bearings are used when even higher thrust loads are applied (Figure 3). Such high-thrust loads are common on very deep wells, where the column of water, the pump shaft, the impeller and the force applied to move the water can total several thousand pounds. Spherical roller bearings can handle 300 percent or even 500 percent of normal thrust load.

Hydrodynamic Bearings

Hydrodynamic—tilting pad, plate or Kingsbury—bearings (Figure 4)can handle extremely high thrust loads. Like sleeve bearings in horizontal motors, they can last a long time. The load is carried on an oil film that separates the moving parts, so theoretically, no wear on the plates occurs.

Guide Bearing

The bearing opposite the thrust bearing (usually the lower or “bottom” bearing) is called the guide bearing (Figure 5). Its function is to locate the lower end of the motor shaft and withstand momentary upthrust (i.e., 30 percent of standard high thrust in the direction opposite the thrust bearing for no more than ten seconds). The guide bearing is typically a Conrad deep-groove ball bearing and is usually grease lubricated. In normal operation, it is subject to little or no load.

Figure 3. Spherical roller bearing

Special Thrust Case

Occasionally, a pump design will exceed the momentary upthrust limits (more common with 3,600-rpm designs). If the guide bearing cannot accommodate this thrust for the duration necessary, the upper bearings are arranged back-to-back (Figure 6) or face-to-face. (Note that arrows indicating thrusts in the bearings are in opposite directions.)

Such configurations require additional parts. For example, a hold-down ring must be mounted on the outer race of the upper bearing and fastened to the upper bracket to prevent the upthrust from lifting the bearings from the bearing bore. A nut must also be threaded onto the bearing carrier below the lower bearing inner race to keep the upthrust from pushing the bearings off the mount. The motor vendor should be informed if the application experiences upthrust, so that these steps can be applied.

Figure 4. Tilting pad bearing