The balancing of rotating bodies is important to avoid vibration. in heavy industrial machines such as gas turbines and electric generators, vibration can cause catastrophic failure, as well as noise and discomfort. in the case of a narrow wheel, balancing simply involves moving the center of gravity to the center of rotation. For a system to be in complete balance both force and couple polygons should be closed.
The process of “Balancing” is the removal or addition of weight to the unit such that this effective mass center line approaches the true axis.

The simplest form of, “Static”, balance involves placing the unit on low friction bearings and allowing it to rotate and “settle” with the “heaviest” point falling to the bottom. Material is then removed from this point (or added at the top point) and the unit gently rotated until, when stopping, the new “heavy” point again falls to the bottom. This process is then repeated until no obvious “heavy” point seems to exist.

Advancing this one stage further, the unit can be mounted on a purpose built “Balancing Machine” which has its bearings connected to sensors (displacement or acceleration type depending on the design of the machine) which detect the “heavy” point, in relation to a datum on the unit, whilst it is being rotated. This increases the sensitivity and, hence, the accuracy of the balance. if one considers correction at a single position along the length of the unit, the balance is said to be “Single-Plane”.

Dynamic Unbalance

Unbalance can occur at any lateral (axial) position along the rotating body and with any magnitude. This unbalance is a combination of static unbalance and couple unbalance and is called dynamic unbalance. The couple unbalances component only appears when the object is rotated, so measuring dynamic unbalance requires rotating the object. Dynamic unbalance must be corrected at two locations in the axial direction (two-plane correction for two-plane unbalance). Put simply, unbalance apparent when the object is perfectly still (no rotation) is called static unbalance and unbalance that becomes apparent after an object begins to rotate is called dynamic imbalance.

Static unbalance

When a rotor is placed on smooth rails, the heavy side will turn to face downward. The unbalance experienced when an object is static (not moving) is called static unbalance. Static unbalances can be corrected using only one location (one-plane correction for one-plane unbalance).

  • Causes of dynamic unbalance.
  • There are many factors than can cause dynamic unbalance.
  • For formed or machined parts, potential causes include:
  • Variations in the specific weight of materials
  • Shape is not symmetric with respect to the center axis
  • Variations in forming, machining or other processes
  • For part assemblies, potential causes include:
  • Variations in assembly process
  • Variations in the mass of individual parts
  • Variations in placement

Importance of dynamic balancing

1.One important reason for balancing is that the forces created by unbalancing are detrimental to the life of the machine – the rotor, the bearings, and the supporting structure. The amount of force created by unbalancing depends on the speed of rotation and the amount of unbalance.

2. The goal of rotor balancing is to reduce unbalance enough such that it can operate properly once installed on site. Reducing unbalance reduces vibration and increases efficiency and life of the rotor and bearings. Additionally, during production and repair, it is necessary to balance rotors before full assembly because there may be limited access to the rotor.

3. A balancing machine is used to determine the location and amount of unbalanced masses on a rotor. The rotor is mounted on the machine bearings and the machine spins the rotor. Soft bearing machines measure the displacement of the ends of the rotor and bearings. The machine measures this displacement and the phase angle, then computes the unbalance present.

4. When assessing a rotor, unbalance cannot be visually identified. A hole or added weight on a rotor may be there from a initial balancing of the rotor, not the cause of unbalance. The only way to assess unbalance is via the vibration or the force it generates.

5. A balancing machine is used to balance parts before re-installation, ensuring smooth operation of the machine. A balancing machine can be an extremely valuable asset to any maintenance department that repairs pumps, motors, and other rotating equipment. it can save costs due to delays by eliminating the need to send parts to outside specialists to be balanced. Often the savings on one job alone can justify the expense for a balancing machine.

One of the prime concerns is the balancing quality of the rotating components. Through experience, the manufacturer can established a tolerable limit of unbalance which can be accepted in a particular machine. The manufacturer knows that by exceeding this limit, customer complaints and machinery downtime will reflect in the quality of the his product. To meet this situation, parts will be balanced at the time of the manufacturer, in most instances, prior to assembly and sometimes trim balanced as an assembly.

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