Balancing Machine

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A balancing machine is a measuring tool used for balancing rotating machine parts such as rotors for electric motors, fans, turbines, propellers and pumps. The machine usually consists of two rigid pedestals, with suspension and bearings on top. The unit under test is placed on the bearings and is rotated either with a belt or end-drive. As the part is rotated, the vibration in the suspension is detected with sensors and that information is used to determine the amount of unbalance in the part. Along with phase information, the machine can determine how much and where to add weights to balance the part.

[edit] Hard Bearing vs. Soft Bearing

There are two main types of balancing machines, hard bearing and soft bearing. The difference between them, however, is in the suspension and not the bearings. In a hard bearing machine, balancing is done at a frequency lower than the resonant frequency of the suspension. In a soft bearing machine, balancing is done at a frequency higher than the resonant frequency of the suspension. Both types of machines have various advantages and disadvantages. A soft bearing machine is generally more flexible and can handle pieces with greatly varying weights, however it takes longer to use as it has to be calibrated for every part that is put in. It works well in a repair shop or in low volume production where flexibility is required. A hard bearing machine is not as flexible with the amount of weight it can balance. However, because of the rigidity of machine, it only requires a one-time calibration. Because of this, it is much more suited towards high-volume applications where a large amount of similar parts can be balanced quickly. A hard bearing machine can be automated to remove weight automatically by drilling or milling. It can be placed in an assembly line and loaded with a robotic arm, requiring very little human control. Hard bearing machines are also generally more expensive because of the precision required in their manufacturing.

[edit] How it works

With the rotating part resting on the bearings a vibration sensor is attached to the suspension. In most soft bearing machines, a velocity sensor is used. This sensor works by moving a magnet in relation to a fixed coil which generates voltage proportional to the velocity of the vibration. Accelerometers, which measure acceleration of the vibration, can also be used.

A photocell(sometimes called a phaser), proximity sensor or encoder is used to determine the rotational speed, as well as the relative phase, of the rotating part. This phase information is then used to filter the vibration information to determine the amount of movement, or force, in one rotation of the part. Also, the time difference between the phase and the vibration peak gives the angle at which the unbalance exists. Amount of unbalance and angle of unbalance give an unbalance vector.

Calibration is performed by adding a known weight at a known angle. In a soft bearing machine, trial weights must be added in correction planes for each part. This is because the location of the correction planes along the rotational axis is unknown, and therefore it is unknown how much a given amount of weight will affect the balance. By using trial weights, you are adding a known weight at a known angle and getting the unbalance vector caused by it. This vector is then compared to the original unbalance vector to find the resultant vector, which gives the weight and angles needed to bring the part into balance. In a hard bearing machine, the location of the correction plane must be given in advance so that the machine always knows how much a given amount of weight will affect the balance.