Many vertical electric motors are used to drive water pumps. The selection of electric motors requires attention to many details including the power supply, environment and load. This article is an introduction to the effects of the load on the bearing system. Here, the bearing system refers to the core, housing, seals, method of lubrication and cooling.
Vertical motors are like horizontal ones with respect to electrical characteristics, but differ mechanically.
Since they are assembled in a vertical position, the upper bearings alone support the weight of the rotors and, in some applications, the weight of the pump’s impeller, shaft and the hydrodynamic thrust.
Image 1 represents a vertical motor assembled directly on a pump’s head. Large and/or heavy motors can require a separate structure or foundation.
The upper bearing is expected to support the rotor and the external thrusts and, when specified, maintain radial stability, last a reasonable amount of time, require reasonable maintenance intervals and operate with reasonable losses.
To design such a bearing system, the following parameters* are necessary:
- normal, continuous downthrust—the basis for calculation of bearing life
- maximum, momentary downthrust for tilted pad bearings
- minimum downthrust—needed for spherical roller bearings
- momentary upthrust—if continuous upthrust is predicted, the motor manufacturer should be informed
*[pound force (Lbf)] or [kilogram force (kGf)] or [kilonewton (kN)]
Many types of bearings are used on electric motors, according to the manufacturer’s standard. Large pump systems are likely to require custom bearing systems according to specifications. The most used types of bearing systems are the deep groove ball bearing, the angular contact ball, the spherical roller and the tilted pad sleeve bearing.
Types of Bearings & Use
Guide bearings—The guide bearing is at the lower part of the vertical motor, toward the shaft end. It provides stability to the rotor and can be used, when properly dimensioned, to sustain momentary axial upthrust.
Normally, a deep groove bearing is used with grease for lubrication. However, when the motor is fitted with a tilted pad type, the guide bearing is also a sleeve type.
Thrust bearings—A thrust bearing is installed on the upper part of the motor, opposite to the shaft end. Image 3 depicts an angular contact ball bearing type, which is typically used in smaller motor frames. The angular contact bearing can withstand forces in both radial and on axial directions due to the inner and outer raceways being shifted from each other with reference to the bearing axis.
The motor designer can select angular contact bearings with double rows of rolling elements to increase the downthrust capacity. The increase of the angular contact represented by the Greek letter α, between the rolling elements and the raceway, increases the load carrying capacity.
Grease can be used to efficiently lubricate this type of bearing in a wide range of speeds and downthrust values. When grease is no longer efficient, the bearing operates submerged in an oil reservoir. In larger frames, a serpentine with running water is installed inside the oil reservoir to maintain the temperatures as specified. The production facility must supply fresh water.
The spherical roller bearing (Image 4) supports large downthrusts. Its capacity to self-align compensates minor misalignment between the motor and the driven equipment without detriment to service life. The spherical roller bearings do not withstand thrust in the upward direction and must be loaded at all times to avoid losing radial stability. When momentary upthrusts are predicted, they are handled by a special configuration of the guide bearing.
Operation at this condition must be limited to avoid fatigue of the guide bearing.
To ensure that the roller bearing is always loaded, a set of springs is installed to push the lower race upward. When an upthrust occurs, the springs lift the lower race to maintain its contact with the rollers. In general, the load (hydraulic thrust) applied by a water pump decreases when the water flow increases, and increases when the water flow decreases.
When a variable frequency drive (VFD) powers the electric motor, it is possible to vary the speed of the pump to adjust the volume of pumped water while operating near the maximum efficiency of the pump system. The reduction of water flow unloads the bearing, and the springs will act to maintain the lower race in contact with the spherical rollers.