Gene Vogel
EASA Pump & Vibration Specialist
Vertical motors are a common item in most EASA service centers. There are various applications for vertical motors, but most of them are mounted on vertical turbine pumps (VTPs). Vertical motors may be hollow shaft or solid shaft; the hollow shaft motors are used exclusively on VTPs and vibration is a common problem with these installations. A much longer article could address the broader topic of vertical motor vibration, but the focus of this article will be on imbalance and 1x RPM vibration.
Imbalance generates a 1x RPM force that results in vibration at that frequency. But vibration and imbalance are not the same thing. The vibration amplitude may vary depending on numerous factors: the rotating speed, mounting stiffness, the presence of natural frequencies (resonance) and others. Theoretically, the imbalance of a rigid rotor does not change. The units of imbalance are oz-in or gram-mm. The amount of imbalance does not change just because the rotor runs at a different speed or is mounted on a soft base. So why are vertical motors so susceptible to imbalance?
Several of the reasons vertical motors are susceptible to imbalance are related to the design and construction of the motors. Another factor is the vertical mounting that may allow excessive vibration even when imbalance is minor; what is perceived as imbalance may require only a small amount of correction weight.
One factor of vertical motor design and construction is that the top bearing (sometimes the bottom bearing), is mounted on a quill which is a clearance fit to the motor shaft (Figure 1). The clearance fit accommodates adjustment of the motor endplay after assembly. That clearance fit also allows the motor shaft location within the quill bore to vary. Adjust the endplay nut and the shaft may relocate within the clearance fit. Remember that the units of imbalance are oz-in or gram-mm, mass and distance. If the shaft moves within the quill bore clearance, the mass of the rotor is moved some distance creating imbalance. Eccentricity times mass equals imbalance. Example: a 500-pound rotor moved 0.001” equals 8 oz-in of imbalance. A metric example: a 200 kg rotor moved 0.025 mm equals 5000 gram-mm imbalance. It is easy to see how even a very slight movement of the shaft in the quill can result in significant imbalance.
Another factor related to quill bearing mount design is the difficulty it presents in balancing the rotor in the balancing machine. When the quill is removed, the journal where the quill is mounted usually has a keyway and cannot be used to support the rotor in the balancing machine. There are several ways to avoid this dilemma. The rotor can be supported on a section of shaft below the quill mounting journal. Unfortunately, that surface may not be perfectly true to the quill mounting journal. Thus, runout results in imbalance. Some service centers will machine a section of the shaft true to the quill mounting journal, and that works well if tight tolerances are achieved. Additionally, It is an extra step in the repair process. Some service centers have had success mounting the quill to the shaft and using the bearing journal to support the rotor in the balancing machine. This method may work, but the clearance fit of the quill to the shaft does introduce looseness into the balance process. The clearance acts differently horizontal in the balance machine than it does when mounted vertical. The problems introduced by the quill design may translate to less than perfect rotor balance from the balance machine.
Another issue with hollow shaft motors on VTPs is the head shaft which is mounted through the motor shaft bore. The head shaft fits in the bore of the coupling at the top of the motor (Figure 2). The fit of the coupling to the top of the quill and the concentricity of the coupling bore can cause runout of the head shaft relative to the motor rotor axis (Figure 3). Again, eccentricity times mass equals imbalance. The trueness of the head shaft is also a potential variable. Pump Shaft Quality (PSQ) shafting is recommended for all VTP shafting, but the head shaft may not have been considered part of the pump. A piece of cold rolled steel may be way out from the tolerances of PSQ shafting.
Solid shaft motors do not have the potential imbalance from the head shaft, but they do have much more complex couplings that are susceptible to imbalance. Although the imbalance is at the bottom shaft extension, the resulting vibration will be greatest at the top of the motor and balance corrections will likely be made there.
With the combination of the several common causes of imbalance addressed in this article, there is about a 50-50 chance that any vertical motor installed on a VTP will require field balancing. Often the amount of correction weight is small, especially when there is resonance or other mounting issues. For any of these causes of imbalance, the simple solution is to trim balance the motor with correction weight at the top, usually on the coupling hub or on an external cooling fan. It helps when customers are advised ahead of time that trim balancing on site is commonly required – and an expected cost instead of a “warranty” service call.
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