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Balancing equipment

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Categories: Rotor balancing basics

Balancing machines fall into two major classes – those that spin the workpiece and those that don’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 balancers do 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 HARD SUSPENSION BALANCERS

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.

QUASI-HARD or QUASI-SOFT SUSPENSION BALANCERS

In between hard and soft suspensions is a class of balancers 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.