A balancing machine is a measuring tool used for balancing rotating machine parts such as rotors for electric motors, fans, turbines, disc brakes, disc drives, propellers and pumps. The machine usually consists of two rigid pedestals, with suspension and bearings on top supporting a mounting platform. The unit under test is bolted to the platform 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 or remove 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. Thus the suspension's resulting displacement is caused by the centrifugal force generated by the unbalance of the rotor. In a soft-bearing machine, balancing is done at a frequency higher than the resonance frequency of the suspension. The suspension's displacement is dictated by the distance of the rotor's principal axis of inertia relative to the rotor's rotational axis defined by the rotor bearings. Both types of machines have various advantages and disadvantages. A hard-bearing machine is generally more versatile and can handle pieces with greatly varying weights, because hard-bearing machines are measuring centrifugal forces and require only a one-time calibration. Only five geometric dimensions need 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 versatile with respect to amount of rotor weight to be balanced. The preparation of a soft-bearing machine for individual rotor types is more time consuming, because it needs to be calibrated for different part types which makes the process accuracy dependent on the operator's knowledge and skill. With measuring equipment capable of storing the setup calibration parameters, the need for recalibration becomes unnecessary as long as the original rotor setup on the balancing machine at the time of its calibration is duplicated. Thus use of dedicated fixture for each type of rotor may come at additional cost but offers the advantage of better balancing process accuracy and less dependent on the machine operator's skill level. With the machine setup a little bit more tedious, it is generally more suitable for high-production volume and high-precision balancing tasks. The latter being required when the rotor has a high service speed, or critical application (see ISO 201940 for recommended balancing tolerance.)
Hard- and soft-bearing machines can be automated to remove weight automatically, such as by drilling or milling, but hard-bearing machines are more robust and reliable. Both machine principles can be integrated into a production line and loaded by a robot arm or gantry, requiring very little human control.
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 that 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, a known weight at a known angle is added, and getting the unbalance vector caused by it.
