Dynamic Balancing Machines - How do they work?

A detailed look at current technology and functions of dynamic balancing machines

Dynamic balancing machines measure vibration (soft-bearing machines) or centrifugal force (hard-bearing machines). This information alone is not very useful for the purpose of dynamic balancing. It is therefor converted to realistic information the operator can use to balance the part. For instance, the information is given in the format of "that much mass to be removed (or added) at this angle". Modern balancing instrumentation systems show this information in a graphical and easy to understand way, making it intuitive and efficient for the operator to perform unbalance corrections.

Soft-bearing machines are so called because the rotor support is able to freely move (soft) in a horizontal plane. Rotor unbalance causes the rotor journals to vibrate. The vibration is transmitted from the rotor journals to the soft-bearing support structure. Vibration sensors connected to the support structure produce an electrical signal, which is amplified and filtered in the balancing Instrumentation.

Hard-bearing machines are so called because the rotor support is rigid (hard). Rotor unbalance causes centrifugal forces. These forces are transmitted from the rotor journals to the hard-bearing support structure. Force sensors connected to the support structure produce an electrical signal, which is amplified and filtered in the balancing Instrumentation.

Phase reference sensor is positioned near the rotor journals so that a one-per-revolution signal is produced. This signal serves two purposes:1. Measure RPM2. Provide a phase reference.

Modern balancing machines can measure 1-plane unbalance, 2-plane unbalance, and static/couple unbalance. However, static/couple unbalance is typically not measured directly but calculated from 2-plane unbalance. 

Hard-bearing balancing machines are calibrated once, and can measure rotor unbalance without the need for trial runs. In simple terms, it is similar to a bathroom scale: After the scale has been calibrated (by the manufacturer), you simply step on the scale and it reads your weight.

Hard-bearing calibration is applicable to vastly different rotor geometries fitting the size, mass and speed envelope of a given machine. Hard-bearing calibration should be checked periodically, and if necessary a re-calibration should be performed.


Soft-bearing balancing machines have to be calibrated for each rotor type. 1-plane measurements require two calibration runs, and 2-plane balancing requires three calibration runs. These runs are called
1. Reference run
2. 1st calibration run (or 1st trial run)
3. 2nd calibration run (or 2nd trail run)

Once calibration runs are complete, influence coefficients will be calculated and applied to the initial measurement (the reference run). Now trial masses are removed from the rotor and the unbalance result is displayed. Modern balancing instrumentation can store these influence coefficients for any particular rotor and can recall these coefficients from rotor memory when the rotor is recalled for a new measurement.


Different types of balancing machines

Dynamic balancing machines are mainly classified as hard-bearing or soft-bearing. What does that mean?

A balancing machine is a measuring device for static or dynamic unbalance. It is designed so that a rotor can be placed on the machine for unbalance measurement. A typical design has 2 pedestals, with some kind of roller bearings on top. A rotor is placed on these roller bearings so that the rotor can rotate easily. A belt is slung around the rotor and driven by an electric motor to make the rotor spin. The terms "soft-bearing" and "hard-bearing" indicate how the rotor is supported on the pedestals.

If the rotor support can move freely (soft), the machine is called a soft-bearing balancing machine.
In soft-bearing machines this will lead the rotor support to move (vibrate). The movement is measured with vibration sensors.

If the rotor is supported in rigid fashion (hard), the machine is called a hard-bearing balancing machine.
In hard-bearing machines the rotor support is rigid. the spinning rotor cannot vibrate; instead the unbalance forces are measured with force sensors (not vibration sensors).

balancing instrument, front panel

A direct effect of dynamic unbalance is Centrifugal Force
F = m r w²

An Indirect effect of dynamic unbalance is Vibration

The amount of vibration depends on several factors, for instance:

Mass of rotating part
same unbalance, smaller part = more vibration
Bearing stiffness
same unbalance, stiffer bearings = less vibration (in this rotor)
same unbalance, stiffer bearings = more vibration (in the rest of the assembly)
Mass of support structure
same unbalance, heavier structure = less vibration


Soft-bearing balancing machines

A spinning rotor will always try to spin around the mass axis. If the geometric axis (the journal axis) does not coincide with the mass axis, the rotor journals will want to move (vibrate) to let the rotor spin around its mass axis.

In soft-bearing machines this will lead the rotor support to move (vibrate). The movement is measured with vibration sensors.

To get a feel for the magnitude of mass eccentricity of a part, we can use this formula:

e = U / m

Example:
Unbalance U = 0.5 oz-inch
mass m = 100 lb (1,600 oz)
eccentricity e = 0.5 oz-inch / 1600 oz = 0.000,312 inch
The vibration amplitude (in soft suspension) will be 0.312 mil


Hard-bearing balancing machines

A spinning rotor will always try to spin around the mass axis. If the geometric axis (the journal axis) does not coincide with the mass axis, the rotor journals will want to move (vibrate) to let the rotor spin around its mass axis.

In hard-bearing machines the rotor support is rigid. Vibration is not present; instead the unbalance forces are measured with force sensors (not vibration sensors).

To get a feel for the magnitude of mass eccentricity of a part, we can use this formula:

e = U / m

Example:
Unbalance U = 0.5 oz-inch
mass m = 100 lb (1,600 oz)
eccentricity e = 0.5 oz-inch / 1600 oz = 0.000,312 inch
The vibration amplitude (in soft suspension) will be 0.312 mil
The vibration amplitude (in hard suspension) will be 0 mil
This unbalance will create a centrifugal force of
12.79 N (2.87 lb)
at a rotor speed of 1,800 RPM