Rotor Unbalance
Rotor unbalance causes premature damage to bearings and seals and greatly reduces the mean time between failures (MTBF) of machines. The vibration from rotor unbalance can cause false brinelling in standby machines and can reduce product quality. Vibration analysis can be used to detect rotor unbalance (or imbalance if you prefer) and most vibration data collectors come with a balance software option so you can resolve the problem by doing in-situ balancing.
If you don’t believe that unbalance can cause damage, just take a look at the video below. (don’t try this at home)
Unbalance Forces
Unbalance forces (F) are proportional to the unbalance mass (m) times the radius (r) from the axis of rotation times the rotor rotational speed (ω) squared.
F=mrω²
An increase in force results in an increase in vibration, but the two are not the same. Therefore, some rotor balance standards are based on vibration amplitude while others are based on residual unbalance. Residual unbalance is the unbalance left in the rotor after it has been balanced. It is given as mr from the equation above and limits on mr are determined based on rotor weight and rotation speed (ω).
It is important to adopt rotor balance standards in your plant, to collect vibration readings on a routine basis to detect rotor unbalance and to resolve unbalance problems before they cause damage.
Click here to purchase a copy of: Audit it. Improve it! Getting the Most from Your Vibration Monitoring Program by Alan Friedman from the MRO-ZONE and make sure that you are getting the most from your vibration monitoring program!
