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.

Available in different speed ranges,
covering 50 RPM up to 800,000 RPM
Standard speed range: 100 to 10,000 RPM
High speed range: 500 to 50,000 RPM
Ultra-high speed range: 1,000 to 100,000 RPM


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 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