Balancing equipment falls into two major classes: that spins the workpiece and that doesn’t.
These are known as dynamic and static balancers respectively.
A dynamic balancer is also known as a centrifugal balancer. Dynamic balancers are further separated into two distinct classes – soft bearing and hard bearing balancers. This distinction is made according to the relative stiffness of the measuring system. Each is discussed further below. Static balancers depend totally upon the force of gravity to detect unbalance. Consequently, they are only sensitive to static unbalance and are completely unable to detect couple unbalance. A dynamic balancer with 2 sensing elements is required to sense couple unbalance.
STATIC BALANCERS
Static balancing equipment does not spin the part and do not depend on centrifugal force to measure unbalance. Their operation is based on gravity generating a downward force at the mass center. The downward force causes the part to gently rotate or roll until the mass center is down and at its lowest point. In this way the location of the heavy spot is identified and corrections can be made. This type of balancing is often done on level ways or rollers. Typically, with level way balancing, the unbalance amount is not known with precision and the part is corrected by trial and error until the part no longer rotates. Although extremely time consuming, this method is effective at minimizing static unbalance. It is possible to measure unbalance amount on a level way balancer by rotating the heavy spot up 90° and measuring the moment or torque required to keep the heavy spot at 90°. The torque measured is equivalent to unbalance.
DYNAMIC BALANCERS
Dynamic balancers rely on the effects of centrifugal force to detect unbalance. They are capable of detecting all forms of unbalance – static, couple, dynamic or quasi-static. The distinction between soft and hard bearing is made based on the natural frequency of the suspension and the relative speed of operation. Those balancers operating at speeds below the natural frequency of the suspension (usually less than half) are classified as hard and those operating at speeds above the natural frequency are classified as soft (usually more than double).
DYNAMIC SOFT SUSPENSION BALANCERS
Soft suspension balancers are also referred to as soft bearing balancers. The soft suspension balancer operates above the resonant frequency of the balance suspension and measures the displacement associated with unbalance. With this type of balancer the part is force free in the horizontal plane and rotates about the central principal axis. The amplitude of vibration is measured at the bearing points to determine the amount of unbalance.
The most significant drawback to the soft suspension is the requirement to recalibrate for each unique part. Left and right bearing outputs are heavily influenced by the total weight of the workpiece and its mass distribution. Calibration requires that weights be alternately placed in the right and left correction planes. Each weight normally causes vibration at both supports. The ratio of amplitudes can be used to quantify the crosstalk between planes or their independence. This is known as the correction plane interference ratio or plane separation. Plane separations of 100:1 can be achieved with some difficulty. Each calibration is speed dependent and unique to the part used for calibration.
Dynamic suspension balancers are also referred to as hard bearing balancers. The hard suspension balancer operates at speeds below the suspension resonant frequency and measures the force generated by the spinning rotor. The amplitude of vibration is very small, and the centrifugal forces potentially very large.
Hard suspension balancers employ rigid work supports and are typically easier and safer to use. Tooling can be configured to hold almost any type of part and there is no restriction that the mass center lie between cradles as there often is with soft suspensions.
In between hard and soft suspensions is a class of balancing equipment known as Quasi-Hard or Quasi-Soft. These balancers use natural resonance to amplify output and take advantage of a mechanical gain to boost sensitivity. Performance in this region can be non-linear and unpredictable. Precise speed control is required to preserve amount and angle accuracy as both change rapidly at resonance. With more modern electronics, transducer outputs can be processed with adequate gain and this region is typically avoided for the benefit of a more stable operating range.