http://en.wikipedia.org/wiki/Rotating_unbalance
http://www.nrc.gov/reading-rm/doc-collections/cfr/part050/part050-appb.html
http://www.api.org/publications-standards-and-statistics/
http://pumpturbo.tamu.edu/exhibit
http://www.universal-balancing.com/en/balancing-information/iso-quality-grades