On the Knife’s Edge

‘Static balancing’ dates back to the days when rotors were placed on knife edges (on a mandrel if required) and the unbalance would cause the heavy spot of a rotor (unbalance) to fall to the bottom through the force of gravity. The unbalance could therefore be identified without the need to drive the rotor, or in other words ‘static’.

When the heavy spot had fallen to the bottom, weight would be added at the top or removed at the bottom and the rotor would be rotated, and where required, the process would be repeated until the rotor remained stationary at any angle it was positioned at (balanced).

Balancing on knife edges still takes place, although typically when there are very low production volumes and where the rotor requires only a rough balance. Advances in technology allow for the same theory of balancing taking place through non-rotating balancing machines where the rotor is mounted in a vertical axis on what is effectively a set of polar scales.

The benefit of this over using knife edges is, as a result of stored calibration procedures on the machine, the exact amount and angle of unbalance can be established rapidly meaning parts can be balanced faster. Not all static balancing machines measure without rotation – this is also a common misconception. If greater degrees of accuracy are required, and/or if production levels dictate that on-station unbalance correction facilities are required, then a rotating vertical balancing machine will be the better choice.

There are three types of unbalance, static, couple and dynamic. Static unbalance can also exist in rotors that are typically balanced dynamically. Static unbalance is also referred to as single or one plane balancing, meaning unbalance is removed or countered in a single plane of correction.

In summary, if you refer to the term ‘static balancing’ as correcting unbalance in a single plane of correction and pay less attention to whether the unbalance is measured while the rotor is driven or not.

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