1. Field of the Invention
The present invention relates to a rotating object dynamic balancing system and method having particular utility in connection with dynamically balancing an out of balance condition of an object or hub mounted on a rotating shaft.
2. Description of the Prior Art
Rotating machinery is never perfectly balanced, and inevitably with the passage of time and use, increasing imperfections of balance cause increasing levels of vibration. Increasing vibration leads to undesirable effects, such as power loss, increased noise, increased wear and early failure of components. Decreasing the vibration by partially or fully restoring balance, reduces these undesirable effects, and so providing a device that is easily attachable to an existing shaft or hub provides a means by which a fan, impeller, mixer, propeller or other parts of the rotating assembly likely to degrade in balance can gain from these desirable benefits.
It is known in the art of millwrighting to apply static weights to balance rotating systems. It is also known that rotating objects generate great vibrations when the speed of the rotating object reaches a resonant speed (i.e., when the rotating object speed coincides with a natural frequency of the rotating system). When the rotating object speed is less than the resonant speed, the center of gravity of the rotating system is offset from the center of the rotating object. When the rotating object speed exceeds the resonant speed, the phase of vibration of the rotating system is shifted 180°, so that the center of gravity of the rotating system is shifted to the center of rotation from the center of the rotating object.
Such prior art balancers generally include a counterweight having a weight of fixed value which is located at a determined position from the axis of rotation to help oppose an imbalance in the rotating body. The magnitude of the imbalance can generally be measured and, accordingly, the necessary weight and position of the counterweight can be calculated so that the weight is positioned where it will act to help counter the newly determined position and magnitude of imbalance. This strategy ceases to perform satisfactorily where imbalances are either slowly or rapidly changing, or where the imbalance is finer than is practical to balance statically.
Marine propellers are sometimes constructed by fastening individually fabricated blades to a hub. This is always the case with controllable pitch propellers (CP propellers) and can also be the case with propellers having fixed blades. Like any rotating device, it is desirable that a marine propeller be balanced to minimize vibration and thereby prevent undesirable loadings of the propeller components, the ship's shafting, the shaft seals, bearings and mountings, and to minimize noise and power loss. The blades of built-up marine propellers are usually hand-finished and are balanced against a standard blade weight by removing material from the blade surfaces. The resulting blades often deviate somewhat from the desired profile, which results in small, but nonetheless undesirable, differences in hydrodynamic performance among the blades.
The effect on the balance of the assembly by installing or removing balance masses on the hub or blades is, of course, a function of the masses of the balance members and the distances of the balance members from the axis of rotation of the shaft. When balance members are installed in the blade flanges, the distance from the propeller axis is relatively small, and balance members with relatively large masses (high weights) are required. To minimize the masses of the balance members, it has been conventional to balance the blades in sets and to install them as sets on the hub. For both balancing and installation, the blades are arranged in a sequence, based on their weights, that will make the propeller most closely balanced before balance members are added and minimize the sizes and the number of balance members required to achieve balance. An important disadvantage of statically balancing the blades in sets is that they have to be installed and used in matched sets. If one or more, but less than all, of the blades of a propeller have to be replaced for some reason, it is a practical necessity to change the whole set, as the static balancing is typically done at an offsite location with specialized equipment.
Furthermore, balanced blades can be damaged during use by impacting an object by wear, by corrosion, marine growth, repainting or repairing, and therefore place the blades in an unbalanced state.
Conventionally, correction for ship propeller shafting systems vibrations due to such causes as damage to the propeller or shaft or misalignment of bearings supporting the shaft require dry-docking of the ship and balancing of the propeller and dunce cap, together with replacement of the seals and bearings, aligning of bearings or straightening of the shaft if required. Such operations are costly and time consuming and require inactivation of the ship for some period of time.
While the above-described devices and repair procedures fulfill their respective objectives to partially or substantially correct imbalances due to damage or wear, the repairs are static and therefore do nothing to further counteract subsequent sources of imbalances, they do not describe a rotating object dynamic balancing system and method that allows for unattended and automatic dynamic balancing of any hub mounted blade, and in particular the unattended and automatic dynamic balancing of a marine propeller.
Therefore, a need exists for a new and improved rotating object dynamic balancing system and method that can be used for dynamically balancing a marine propeller. In this regard, the present invention substantially fulfills this need. In this respect, the rotating object dynamic balancing system and method according to the present invention substantially departs from the conventional concepts and designs of the prior art, and in doing so provide an apparatus initially developed for the purpose of dynamically balancing a marine propeller, but also useful and desirable for balancing various shaft mounted devices.