Chuck Yung
EASA Technical Support Specialist
The problem: We recently rebuilt a 2-pole motor and the centrifugal blower it drives. When the customer reinstalled them, he reported high vibration levels. Everything runs smoothly for 10-15 minutes after a cold startup. Then the vibration starts to climb. We balanced the rotor and blower to G 1.0 tolerances. We even balanced each of the 7 blower impellers separately using a balancing mandrel. Shaft runout was less than 0.0002" on the motor and blower when we finished the job. The customer uses laser alignment. He is convinced that us to rebuild the blower again. What did we do wrong?
The solution: First, you probably did nothing wrong. The precision balance was a smart move, because the relatively small diameter shaft on this type of blower tends to be flexible. With several impellers stacked on a common shaft, multiple planes of imbalance exert radial force on the shaft in several directions at once.
With the impellers shouldered against each other, it is possible to deflect the shaft when tightening the clamping nuts. "Stacked tolerances" — when several mating surfaces have slight imperfections — can add up to unacceptable total deviations. Your final shaft runout indicator readings make it unlikely that this is the case. The cause of the vibration is probably misalignment.
The clue is the absence of vibration until the unit warms up. That indicates that something is changing as the temperature increases. It could be mechanical looseness, but there is another, more likely possibility. This style of blower does not operate at a uniform temperature. Air enters one end at ambient temperature, and is com- pressed further by each stage. Work creates heat, so the air gets hotter at each step. Air leaving the outlet end of the unit may be over 100°F hotter than the inlet end. With the feet bolted to a rigid base, the outlet end of the blower ÒgrowsÓ further vertically than the inlet end. The result is angular misalignment between motor and blower shafts.
Laser alignment is a great tool, but it is only as good as the parameters input by the user. The equipment service manual should have specific alignment instructions that include a final adjustment made at the conclusion of the alignment process. If not, calculate the correction as follows:
- Measure the shaft height of the motor and blower.
- Record the operating temperature of the motor, as well as the inlet and outlet ends of the blower.
- Use the coefficient of expansion for the housing material.
For cast iron, the coefficient of expansion is 0.0000067" per 1°F per inch. This means that a cast-iron frame blower with a 20" shaft height, and a 100° temperature difference between inlet and outlet will lift 0.013" higher on the hot end:
20" x 0.0000067" x 100° = 0.0134"
That can cause a lot of vibration, especially when a flexible shaft is involved.
Because the outlet end is so much hotter than the inlet end, the ambient alignment must include some deliberate misalignment. In our example, lowering the outlet end of the blower by 0.013" will correct the problem. (If you know this ahead of time start with the appropriate shim — in this case, 0.013" — in place, and remove it as the last step.) The shafts will be properly aligned when the motor and blower reach operating temperature. Some vibration may be evident while the machinery warms up, but it will decrease as the machinery approaches operating temperature.
Points to ponder
Most manufacturers of blowers, pumps and gearboxes are familiar with the thermal growth quirks of their equipment, and are willing to share information when asked. Gearboxes and piping add a horizontal element, which may require horizontal angular corrections as well. Again, machinery manufacturers willingly share infor- mation about this! Given the model number and operating temperature, most will provide the alignment guidelines. (Detailed alignment information is often included in the original equipment manual.)
For steel, the coefficient of expansion ranges from 0.000008" to 0.000015", so choosing the correct value makes a lot of difference. Assume the wrong value, and the correction could be off by a factor of two.
Some coupling designs withstand misalignment better than others, but that does not mean the equipment can take more misalignment just because of the coupling. The blower in question is a good example of this. (Rule-of-thumb: 3600-rpm machinery / anything with a relatively small diameter shaft / equipment with a long shaft extension: All are sensitive to misalignment.)
When aligning unfamiliar machinery, always consider the effects of temperature. Some gas compressors operate at extremely cold temperatures — the effect of temperature changes holds true for cooling as well as heating.
One added value of precision alignment is the perception it creates. Lower vibration levels will convince the customer of a quality job. Even if alignment is not part of the service your com- pany provides, your knowledge about alignment can prevent harmful misperceptions about the quality of your workmanship. More than one service center has been blamed for a motor failure that was actually caused by misalignment. Once that blame has been placed, the damage to your reputation is difficult to undo.
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