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Vertical Motors VHS vs VSS

The concept of the use of the HOLLOSHAFT® motor with turbine pump envisions a considerable length of pump column, inner tubing and shafting. This gives a considerable amount of weight and represents part of the thrust. The rest of the thrust is from hydraulic force and weight, which puts considerable stress and pull on the motor pump head and particularly the pump shaft. A certain amount of eccentricity of shafting and flange tolerances will not be noticed due to the magnitude of force. Due to combined weight and thrust forces the shaft is in tension.

In short, setting pumps without this weight hanging on the shaft, the eccentricity and tolerance are of considerable importance to the operation and life of the pump. Without the heavy weight of line shaft, there is a tendency for the shaft to gyrate if excessive tolerances are permitted. This could be eliminated if motor and pump tolerances were held to +001 to 0.0 tolerances. But this would drive the cost of the motor up beyond market value.

There are two more practical ways of satisfying this mechanical requirement. One, the use of a vertical solid shaft motor; and two, the use of a steady bushing on a vertical HOLLOSHAFT motor. Various pump manufacturers will solve this problem in different ways be sizing up the limitations of their line and the cost factors between vertical Solid Shaft and vertical HOLLOSHAFT installations.

Vertical Hollow shaft (VHS) vs. Vertical Solid Shaft (VSS)

For many years, pumps for deep wells used vertical HOLLOSHAFT motors and short-coupled booster used vertical Solid Shaft motors. However, it is time for pump OEM’s, engineers and salesmen to evaluate the features and benefits that VHS motors offer. The features and benefits are as follows:

Vertical Holloshaft

Features

1. Less Parts
2. Lower profile required
3. Coupling part of motor
4. Steady bushing available
5. Vibration standard of .008" maximum peak to peak vibration
6. Thrust bearing normally at the top of the motor
7. Distance from steady bushing to seal is normally less on VHS

Advantages

1. Coupling furnished
2. Non-reverse ratchet standard accessory
3. Capable of heavy downthrusts
4. Self-release coupling standard

Benefits

1. Less cost and assembly problems 2. Less chance of vibration problems, space factor less 3. Balanced as part of motor. One part versus three parts for Solid Shaft coupling. Less surfaces to maintain tolerances 4. Supports head shaft with same run-out as Solid Shaft motor. Easily adapted for 1800 RPM motors and .002 TIR maintained 5. No extra field balance required on coupling is advantage for manufacturer and user. Longer seal life from less vibration 6. Easily inspected without disassembly of motor 7. Less total indicator runout at seal since distance is usually less with VHS.

Vertical Solid Shaft

Features

1. Low thrust NEMA ratings, may use normal or inline motors 2. 100 HP, 2 Pole and larger ratings are designed to inherently handle up and down thrust

Advantages

1. Adjustment more accessible
2. Eccentricity less of problem
3. No canopy cap to remove
4. Lower cost driver

Benefits

1. Lower cost than high thrust
2. Limits failures on applications with varying suction and discharge and pressure

Conclusion

HOLLOSHAFT motors with steady bushings offer many benefits for long life on industrial and short-coupled applications.

Solid Shaft, 3600 RPM motors, 440 frame and larger, offer standard self-cooled thrust bearings for use in petro-chemical plants where cooling water is not available.