Testing for unbalance

Unbalance is the most common malfunction in turbomachines. In an unbalanced condition, the machine’s rotor mass centerline does not coincide with the axis of rotation (condition of unequal mass distribution at each section of the rotating assembly). Unbalance represents the first, fundamental mechanism to transfer the rotational energy into the vibration.

The rotor response depends on both the rotor combined dynamic stiffness (the combined effect of stiffness, damping, others) and the unbalance excitation. Any change in the rotor response could result from either a change in the unbalance or a change in the restraining rotordynamics stiffness components (for example, a shaft crack would reduce the rotor stiffness).

The synchronous vibration response provided by the lateral displacement transducer(s) are usually presented in the ‘Bode’ (‘vibration amplitude vs. rotating speed’ and ‘phase lag vs. rotating speed’) and the ‘Polar’ plot formats. These formats are widely used for unbalance diagnosis and corrections.

The damped unbalance dynamic response test (known as ‘synchronous test’) consists of inputting a controlled unbalance to the rotor and measuring its synchronous response. It is a very important dynamic test. However, only a more sophisticated non-synchronous testing yields comprehensive dynamic data of a turbomachine.

In a non-synchronous test, a rotor assembly is excited by a non-synchronous external excitation, generated by an auxiliary shaker-like device. The sweep frequency response data for a sequence of rotating speeds provides the necessary information for identification of speed dependent characteristics. Non-synchronous testing should become a routine procedure during testing of turbomachines (particularly for critical machines, new models, innovative designs, high-pressure machines, new seal designs, and similar).

(Amin Almasi is a rotating equipment consultant in Australia)