by Jim Bryan, Electrical Apparatus Service Association (EASA)
December 17, 2011

Accessories

Other options are available for vertical motors.

Steady Bushing

To prevent pump seal damage from “shaft whip” and excessive vibration, a brass steady bushing can be installed at the lower end of hollow shafts to stabilize and center the headshaft. If the hollow shaft extends beyond the motor's lower end bracket, it will be installed as shown in Figure 7. If there is no  shaft extension (such as with an oil-lubricated lower bearing), the steady bushing will be machined and pressed into the hollow shaft.

Non-Reverse Ratchets

Non-reverse ratchets (See Figure 8) prevent pump backspin when the power is off and the column of water falls through the pump impeller. Depending on the well depth, backspin can reach dangerous speeds if unchecked. As the water level falls, it leaves the water-lubricated pump bearings without lubricant. 

Two common types of non-reverse ratchets are ball ratchets and pin ratchets, both of which work on the same principle.

Figure 5. Guide bearing

Centrifugal force supports the balls or pins until the rotating speed drops enough that the weight of these elements overcomes it. As the rotor comes to rest and begins turning backwards, a ball or pin engages a stationary tooth to stop rotation. An odd number of teeth versus balls/pins ensures that the ratchet will travel only a few degrees before stopping its reverse rotation. The ratchet is robust to withstand full motor torque in reverse. So if the motor can lift the column of water, the ratchet can stop backspin from that same column.

Figure 6. Back-to-back bearings

Lubrication

Table 1 lists some turbine oil lubricants that are suitable for use in vertical motors, as well as the ISO grades and corresponding viscosities. It is important to use the correct viscosity relative to the ambient temperature conditions of the motor application. If the oil is too thin (low viscosity) to maintain the separation of the rolling elements and races, the bearing will be damaged. If it is too thick (high viscosity), fluid shear in the oil will create excessive temperature that will quickly degrade the oil.

Figure 7. Steady bushing

Large, enclosed motors and motors with roller bearings usually require external oil cooling. The extra material and effort necessary to provide this can often be avoided by using synthetic turbine oil. The synthetic will operate successfully at temperatures 30 degrees C higher than mineral-based oils.

Replace the oil according to the motor manufacturer's guidelines. As an alternative, test it regularly for oxidation, contamination (e.g., wear metals or moisture) and viscosity, and then replace it when tests show degradation.

Figure 8. Non-reverse ratchet

Vertical Motor Lubrication Cautions

These hard-learned lessons describe common pitfalls of vertical motor lubrication: 

  • Changing the oil viscosity may cause leaking or overheating.
  • Do not mix oils. They may not be compatible.
  • If water cooling coils are used, check for water leaks.
  • Some thrust bearings must be preloaded beyond the rotor weight. Check for warning plates on the motor.
  • Rotating the rotor shaft by hand may require a strap wrench due to the high downward loading on the thrust bearing -- especially with plate-type or spring-loaded bearings.
  • When servicing the motor, drain and replace lubricating oil.
  • Oil sump temperatures:
  • Normal 80 degrees C mineral/110 degrees C synthetic
  • Alarm 90 degrees C mineral/120 degrees C synthetic
  • Shut down 100 degrees C mineral/130 degrees C synthetic
  • Hollow-shaft motors may use a steady bushing.
  • Motor lifting devices lift only the motor.

Conclusion

Vertical motors are unique in their ability to carry external thrust. The thrust bearings that make this possible require care for optimum service and performance. Oil-bath lubrication is a special consideration for vertical motors that must be understood and maintained correctly.

Table 1. Recommended bearing oils for typical vertical motors

 

Pumps & Systems, September 2011

 

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