Pumps & Systems, June 2008
Vibration is one of the most damaging forces in the industrial environment. Vibration not only reduces the life of equipment, but can also adversely affect the quality of the product or the reliability of the production process. In addition to quality and performance issues, vibration costs money. You have to pay for the energy it takes to shake your equipment. Forces generated by unbalance are among the most common sources of vibration encountered in machinery-rich environments. Balancing is one of the most common corrective activities needed to resolve vibration issues.
Seven Pre-Balance Checks to Assure a Smooth Balance Job
1. Look before you Leap-Visual Inspection
A visual inspection of the fan is extremely important. Look for signs of blade rubbing or cracks in hubs. Correcting these issues prior to balancing is not only necessary for the machinery, but can also save your life.
Many years ago, a maintenance crew at the Geysers Geothermal area in Northern California was investigating a five-bladed cooling tower cell which exhibited a high vibration at the fan RPM. The fan was running, so they did not visually inspect the fan (although they could have used a timing strobe light.) The spectrum readings showed not only a large vibration at RPM but also a vibration at five times RPM and a very high 20 times RPM vibration. Not sure what was causing the very high reading, they decided to shut down and inspect the fan.
They found cracks on the fan hub stemming from axial blade loading. There were four structural components directly in the airflow of the fan that supported the gearbox. The compression of air between the four gearbox supports and the five fan blades caused the 20 times running speed vibration. Just a few months before, another cooling tower cell had catastrophically failed due to a fan hub failure.
2. Do a Vibration Analysis Prior to Balancing
This may sound obvious, but it is important to ensure the vibration's source is really imbalance. If a fan has an eccentric pulley or is misaligned, balancing will not work.
A few years ago, a technician from a Seattle balancing shop captured a quick spectrum before a balance job and told the customer that the fan was in balance and the real problem was that it was misaligned. The customer insisted the fan was out of balance as they had just installed the fan and motor. The technician then set a laser alignment system and proceeded to show the customer that the misalignment did indeed exist. After alignment, the fan ran smoothly and required no balance correction.
Causes of Imbalance
•· Difference in fit between the balancing and ventilator shafts
•· Centrifugal force deformation
•· Blade sag
•· Blade cracks
•· Improper paintwork or coating
•· Strain from rotor disk weld
Imbalance from Operation