Fasteners Lubrication Alignment Balance


Imbalance increases the force on contacting surfaces, thus increasing the likelihood of surface-to-surface contact and wear.

Imbalance occurs when there is a mass distribution inequity in a rotating machine that produces relative centrifugal force (RCF).  The amount of RCF is a function of the weight of the mass imbalance (e.g. in grams), its distance from the rotating centerline and the rotational speed.  The increase in RCF is linearly related to the weight of the mass imbalance and its distance from the rotating centerline and geometrically related to the rotational speed.  In other words, high speed equipment is particularly sensitive to imbalance.  As with fastener induced looseness and misalignment, imbalance increases the force on contacting surfaces, thus increasing the likelihood of surface-to-surface contact and wear by overwhelming the protective film provided by the lubricant. Imbalance is typically observed at one-times running speed in the vibration spectra.  Here are some common problems seen on the plant floor.

  1. Failure to shop balance equipment during rebuild.  
    1. Unless you’re pumping abrasive slurry, where impeller wear is the dominant failure mode, all pumps should be shop balanced during rebuild.  The ISO 1940 standard calls for balancing to G6.3 precision for pumps, fans and other balance of plant equipment. Balancing is preferred to G2.5 and preferably G1.0 or better for this equipment. It is surprising how rarely this simple process of shop balancing is employed in plant shops.  In most instances, low speed dynamic balancing works fine for process pumps and other balance of plant equipment.  High-speed or at-speed balancing is only required for high value critical turbo-machinery.  Low speed dynamic balancing is relatively easy and inexpensive to conduct.  It requires some skill, but it’s not that hard and very few organizations do it.
  2. Failure to incorporate balance standards into your contracts.  
    1. We routinely send electric motors and other rotating equipment out for rewind or rebuild.  Do our contracts require the shops to dynamically balance the equipment?  Typically not.  There are reasons why rebuilt equipment doesn’t typically last as long as new equipment – lack of precision balance is one of the big ones.  And, as is the case for pumps that are rebuilt on-site, low speed dynamic balancing is typically sufficient for most electric motors, so the process is pretty easy and inexpensive.  Again, the preference is for standards set to G2.5, G1.0 or better.
  3. Failure to manage dynamic balancing in fans.  
    1. Whether it’s a rebuild in the shop or routine maintenance in the plant, we must pay special attention to fans.  Fan blades commonly extend well out from the rotating centerline. Because RCF increases geometrically the further the mass imbalance is from the centerline, we really have to pay attention to fans.  Moreover, fans are susceptible to wear and corrosion, especially at the tips, and accumulation of debris, which introduces unbalance forces.  Fortunately, fans are easy to balance in the field by adding (or occasionally removing) mass. 
  4. The unbalance cure creates other problems. 
    1. In some applications, we must periodically wash down fans to clear away accumulated debris and restore balance.  This is commonly done with water spray directed at the fan blades.  Exercise caution when cleaning fans, as the water spray has been known to migrate past labyrinth seals and into bearings.  It’s not a good bargain to trade an unbalanced fan for water contaminated grease or oil.  If the risk is high, take precautions by using deflectors, greasing before and after cleaning, putting a bead of grease around the exterior of the labyrinth or, in the case of oil lubricated fans, change the oil or, better yet, use a filter cart with water removing elements to decontaminate the oil during and after the wash down.


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