Balancing machine

A balancing machine is a measuring tool used for balancing rotating machine parts such as rotors for electric motors, fans, turbines, disc drives, 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, air, 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.

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 resonance frequency of the suspension. In a soft bearing machine, balancing is done at a frequency higher than the resonance frequency of the suspension. Both types of machines have various advantages and disadvantages. A hard bearing machine is generally more flexible and can handle pieces with greatly varying weights, because hard bearing machines are measuring centrifugal forces and require only a one-time calibration. Only 5 geometric dimensions have to be fed into the measuring unit and the machine is ready for use. Therefore it works very well for low and middle size volume production and in repair workshops. A soft bearing machine is not so flexible in respect of amount of rotor weight to be balanced. The preparation of a soft bearing machine for individual rotor types is more time consuming, because it has to be calibrated for every individual part. It is very suitable for high production volume and high precision balancing tasks. Hard- and soft bearing machines can be automated to remove weight automatically e.g. by drilling or milling, but hard bearing machines are more robust. Both machine principles can be integrated in a production line and loaded by a robot arm or gantry, requiring very little human control. Soft bearing machines are also generally more expensive because of the higher complexity in the design and manufacturing.

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.

Other Types of Balancing Machines

Static balancing machines differ from hard and soft bearing machines in that the part is not rotated to take a measurement, and rather than resting on its bearings, the part rests vertically on its geometric center. Once at rest, any movement by the part away from its geometric center is detected by two perpendicular sensors beneath the table and returned as unbalance. Static balancers are often used to balance parts who's diameter is much larger than their length, such as fans. The advantages of using a static balancer are speed and price, however a static balancer can only correct in one plane, so its accuracy is limited.

A blade balancing machine attempts to balance a part in assembly, so minimal correction is required later on. Blade balancers are used on parts such as fans, propellers, and turbines. On a blade balancer, each blade to be assembled is weighed and its weight entered into a balancing software package. The software then sorts the blades and attempts to find the blade arrangement with the least amount of unbalance.

Portable balancing machines are used to balance parts that cannot be taken apart and put on a balancing machine, usually parts that are currently in operation such as turbines, pumps, and motors. Portable balancers come with displacement sensors, such as accelerometers, and a photocell, which are then mounted to the pedestals or enclosure of the running part, and based on the vibrations detected, calculate the part's unbalance. Many times these devices contain a spectrum analyzer so the part's condition can be monitored without the use of a photocell and non-rotational vibration can be analyzed.

Literature

* Adolf Lingener: "Auswuchten. Theorie und Praxis". Verlag Technik, Berlin und München 1992, ISBN 3-341-00927-2
* Hatto Schneider: "Auswuchttechnik". 6. Auflage. Springer, Berlin u. a. 2003, ISBN 3-540-00596-X