Amount of unbalance

Obtained by multiplying unbalance mass with its radial distance of its center of gravity from the shaft axis without referring to its angular position.

Angle of unbalance

Angle at which an unbalance mass is located (referring to a polar coordinate system, which is fixed to the shaft axis and rotating with the rotor).

Angle reference generator

Component of a balancing machine which generates a reference to determine the angular position of the rotor.

Angle reference mark

Mark at the rotor in order to create a fixed angular reference system.

Auto-balancing device or active balancing system

System, which automatically compensates changes of the unbalance during operation.

Axis of rotation

Straight line, about which a rotor actually rotates.

Balance quality grade

Possibility to classify rigid rotors; product from mass eccentricity and angular velocity at service speed or trajectory speed of the center of mass each in mm/s (refer also to ISO 21940-11).

Balance tolerance, maximum permissible residual unbalance

The amount of unbalance (with respect to a measuring or correction plane) that is specified as the maximum below which the state of unbalance is considered acceptable.

Balancing

Procedure during which the mass distribution of a rotor is checked and typically is corrected to ensure that the residual unbalances, the vibration of the journals, the forces on the bearings and/or the bearing vibrations at service speed or for a speed range are within specified limits.

Balancing bearings

For example, rolling element bearings with reduced clearance to support rotors from aero-engines in a low-speed balancing machine.

Balancing machine

A machine that provides a measure of unbalance in a rotor, and that can be used for adjusting the mass distribution of that rotor to reduce the unbalance.

Balancing machine sensitivity

Resolution of the unbalance display of a balancing machine.

Balancing putty

Putty normally used to add a test mass to a rotor during balancing.

Balancing run

Cycle consisting of one measuring run and the successive unbalance correction.

Balancing speed

Speed at which the rotor is being balanced.

Bearing support or work support

Component of a balancing machine containing the rotor support, which attaches to the machine bed.

Belt drive

In a balancing machine, a mechanical drive consisting of a motor, belt, pulleys and belt tensioner. Diameter of motor pulley and driven rotor diameter should not be close in order to avoid the generation of phantom unbalances.

Bob weight

Mass attached to the crank pins of a crankshaft to simulate a portion of the rotating and oscillating mass of piston and rod during balancing.

Bode plot

With balancing the amount and angle of vibration at shaft speed or twice the shaft speed plotted against running speed.

Calibration

In balancing, the process of adjusting a machine or device so that it converts the measuring information into unbalance units referred to bearing and/or correction planes of a rotor.

Calibration mass

Defined mass in order to calibrate a balancing machine using a special rotor or to calibrate a balancing machine for a certain rotor type.

Center of mass or center of gravity

Point in a rotor with an axial and radial expansion where its inertia effect against linear distortions can be represented by a single point mass.

Centering

Design of the interface between rotor components to maintain a certain concentricity.

Centrifugal (rotational) balancing machine

Machine which generates measuring values for the unbalance caused by the centrifugal forces of a revolving rotor supported by it.

Compensate

To offset an error or undesired effect.

Component correction

Unbalance correction at least at two angular positions, when in a balancing plane the correction is restricted to given angular positions.

Contract balancing

Industrial balancing services provided by a company on an as-needed basis, either production balancing or rotor unbalance evaluation.

Controlled initial unbalance

Limitation of the initial unbalance by balancing of individual components, smart design and specific machining and assembly.

Correction mass

Mass changed in a balancing plane, in order to reduce unbalance.

Correction or balancing plane

Plane perpendicular to the shaft axis, where unbalance can be corrected.

Correction plane interference or cross-effect

When change of the unbalance in one plane causes a change in the other plane. This cross-effect is expressed in percentage. The better the plane separation, the smaller the cross-effect.

Couple unbalance

Type of unbalance that displaces the main principal axis from the shaft axis and intersects the shaft axis at the center of gravity. This type of unbalance may be caused by two unbalances that are equal in amount and are exactly 180 degrees out of phase.

Critical speed

A characteristic speed, at which a resonance occurs with a rotor-bearing-system.

Dummy rotor

Balancing tooling that represents a sub-assembly of a rotor. Therefor the dummy rotor should have the same main geometrical, mass and stiffness properties as the original part.

Dynamic (two-plane) balancing machine

Balancing machine which utilizes centrifugal forces in order to balance a rotor for two planes.

Dynamic unbalance

Condition of unbalance in which the main principal axis is neither parallel to the shaft axis nor does it intersect.

Electrical runout

Systematic error of eddy current sensors with runout or shaft vibration measurements, which may be caused by residual magnetism or electrical irregularities of the measured part.

End drive

In a balancing machine, a mechanical drive consisting of a motor, an optional gearbox and a drive shaft. The drive shaft will finally connect to one end of the rotor to be balanced.

Equivalent n-th modal unbalance

The smallest single unbalance which creates the same effect on the n-th flexural principal mode as the real unbalance distribution of a flexible rotor.

Field balancing equipment

Measuring equipment used outside of a balancing machine with balancing under operation.

Flexible rotor

Rotor with an unbalance distribution that shows deflections depending on speed, which cannot be neglected.

Flexural critical speed

Speed at which a maximum bending deflection of a rotor occurs. This deflection should then be bigger than the vibration displacement of the journals.

Flexural principal mode or eigen mode

Characteristic deflection curve that a rotor shows at a critical speed.

Foundation

Structure which supports a balancing machine.

Gravitational (non-rotational) balancing machine

Machine which generates a measuring value for the static unbalance caused by the gravitation of a non-revolving rotor supported by it.

Half key

Special key used with balancing which creates the unbalance of the part of the real full key with regard to the shaft or hub (also refer to ISO 21940-32).

Hard bearing (force-measuring) balancing machine

Machine used for balancing of rotors at speeds below the critical speed of the system rotor and balancing machine support.

High-speed balancing

Balancing of flexible rotors at speeds, when the rotor to be balanced cannot be treated as rigid any more.

Horizontal balancer

A balancing machine that measures the unbalance for a horizontal orientation of a rotor’s axis.

Inboard rotor

Rotor where the center of gravity is located between the bearings.

Index balancing or tooling compensation

Compensation for tooling unbalance and for unbalance caused by clearance and runout in the mounting surfaces between tooling and a rotor to be balanced to identify the true rotor unbalance. Therefor the rotor will be turned against the tooling between consecutive measurements.

Indexing

Turning the rotor into a required angular position.

Initial unbalance

Unbalance of any kind that exists in the rotor before balancing.

Isotropic bearing

Bearing system with identical dynamic properties in each radial direction.

Journal

Part of a rotor that is in contact with or supported by a bearing in which it rotates.

Journal center

Axial rotor plane, where a bearing force acts.

Local mass eccentricity

Distance between the center of mass of (imaginary) thin rotor disks and the shaft axis.

Low-speed balancing

Balancing of a flexible rotor when it is rigid still.

Low-speed or slow roll runout

Distortion of the runout of a rotor surface measured at low speed, where there are no substantial unbalance induced vibrations yet. Value will usually be compensated for measurements at variable speed.

Mandrel (balancing arbor)

Precisely machined shaft to receive a rotor with no own journals for balancing.

Mass centering

Evaluation of the principal axis of a rotor. Information will be used during the successive machining of journals, recesses and pilots or reference planes in order to closely align the later rotational axis with the principal axis.

Mass eccentricity

Distance between mass center and shaft axis with a rigid rotor.

Master rotor

A derivative of a real rotor with defined accurate positions to attach calibration masses. It will be regularely used to check a balancing machine.

Measuring plane

Radial rotor plane for which an unbalance vector will be determined.

Mechanical adjustment

Actions in order to setup the mechanical part of a balancing machine to balance a rotor.

Method of correction

Change of the mass distribution of a rotor to reduce unbalance, or vibration caused by unbalance, to an acceptable value. Corrections are usually made by adding material to, removing material from or relocating material at the rotor.

Minimum achievable residual unbalance

Smallest value for the residual unbalance, which can be achieved with a balancing machine.

Modal balance tolerance

Limit for the equivalent modal unbalance of a flexural principal mode of a flexible rotor.

Modal balancing

Method that uses information about the flexural principal modes obtained in the vicinity of the respective critical speeds to balance a flexible rotor.

Mode function

Describes the course of a flexural principal mode depending on the rotor’s axial coordinate. Function is limited to a max. value of one.

Multiplane balancing

Methods which use more than two planes when balancing flexible rotors.

N-th modal unbalance

Unbalance distribution which only excites the n-th flexural principal mode of bending deflection of a flexible rotor.

Nilling

Measure to compensate the current unbalance values and by software create a perfectly balanced rotor. For example used to check cross-effect.

Non-dimensional speed

Ratio between the rotor speed and one of its critical speeds.

On-site or field balancing

Balancing a rotor not on a balancing machine, but in its operational bearings.

Outboard rotor

Rotor where the center of gravity is located outside the bearings.

Outboard rotor

Rotor where the center of gravity is located outside the bearings.

Parasitic mass

Components with mass of a balancing machine, which are excited to vibrate by the unbalance forces of the rotor to be balanced.

Perfectly balanced rotor

Ideal rotor with no unbalance.

Permanent calibration

Calibration performed once, which then is valid for all rotors within the weight- and speed-range of a hard-bearing balancing machine.

Phantom unbalance indication

Display of unbalance, which is not caused by unbalance.

Piezoelectric material

Certain class of rigid ceramic material that produces electric charge when mechanical pressure is applied to the material. Used with force sensors in balancing machines to sense the effect of rotor unbalance.

Plane separation

Capacity of a balancing machine to exactly distribute the unbalance for the correction planes and to minimize the cross-effect.

Plane transposition

Computation of the unbalance with reference to other radial rotor planes.

Polar correction

Correction of the amount of unbalance in a balancing plane at one single angular position.

Polar plot

Amount and angle of vibration or unbalance values obtained at variable speed presented with their X- and Y-values on a circular grid.

Practical correction unit

Display of the amount of unbalance for a given radius directly for example in mm drilling depth, number of washers etc..

Principal axis

Depending on the unbalance condition of a rigid rotor the principal axis is offset and inclined with respect to the design axis.

 Progressive balancing

Method to balance a rotor, which consists of several components that can be dismounted. Unbalance is corrected at the component which has been mounted at last until the rotor is complete.

Quasi-static unbalance

Type of unbalance in which the main principle axis intersects the shaft axis at a point other than the center of gravity. This may be caused by a single unbalance in a plane other than the center of gravity, or by a combination of static and couple unbalances where the static mass is in the same plane as one of the couple masses.

Reference plane

Radial shaft plane to which an unbalance is referenced.

Repeatability

With a balancing machine measure for the quality of the unbalance measurement with repeated measurements under the same conditions. Normally evaluated applying statistical methods.

Reproducibility

With a balancing machine measure for the quality of the unbalance measurement with repeated measurements under different conditions. Normally evaluated applying statistical methods.

Residual unbalance

Unbalance of any kind that remains after balancing.

Rigid rotor

With a rigid rotor the deflections caused by a given unbalance distribution can be neglected up to its max. service speed.

Rotor

A body, capable of rotation, generally with journals that are supported by bearings.

Sensitivity to unbalance (influence coefficient

Measure of how much the vibration of a rotor changes due to a change in unbalance. It is represented as ratio between the change of the vibration vector and the change of the unbalance vector.

Service speed

Speed at which a rotor revolves under operation.

Setting

Actions in order to enter rotor- and balancing-specific information into the measuring device of a balancing machine.

Shaft or rotor axis

The straight line joining the journal centers of a rotor.

Side-spindle balancer

Balancing machine configuration in which the rotor is mounted horizontally and overhung to a balancing spindle.

Single-plane (static) balancing

Procedure by which the mass distribution of a rigid rotor is adjusted in one plane to ensure that the residual static unbalance is within specified limits.

Single-plane (static) balancing machine

Balancing machine which utilizes the gravity or centrifugal forces in order to balance a rotor for one single plane.

Soft bearing (displacement-measurement) balancing machine

Machine used for balancing of rotors at speeds beyond the critical speed of the system rotor and balancing machine support.

Specific unbalance

Amount of the static unbalance divided by the rotor’s mass in gmm/kg or µm.

Static unbalance

Unbalance that displaces the main principal axis parallel to the shaft axis. This may be caused by a single unbalance in the same plane as the center of gravity.

Susceptibility to unbalance

Probability for a machine to show a substantial change of unbalance within a certain time of operation.

Swing diameter

The biggest rotor diameter which can be used with a balancing machine.

Test (proving) rotor

Balanced rigid rotor with a suitable mass used to calibrate and check balancing machines. Test or calibration masses can be exactly and reproducibly attached.

Thermally induced unbalance

Change of the unbalance condition of a rotor caused by temperature.

Total indicated runout

Difference between max. and min. value of the radii of a shaft’s cross section with reference to a stationary rotational axis.

Trial mass

Mass with reasonable size, to evaluate the response of a rotor caused by an unbalance in a plane.

Trim balancing

Final correction of small residual unbalances – typically with the rotor in its operational bearings.

Two-plane (dynamic) balancing

A procedure by which the mass distribution of a rigid rotor is adjusted in two planes to ensure that the residual dynamic unbalance is within specified limits.

Unbalance

Condition of a rotor, when unbalance causes forces or vibration at its bearings revolving with the rotor. The unbalance unit is gmm (or similar units).

Unbalance force

Force acting in a radial plane of a rotor, which is caused by an unbalance at a certain speed.

Unbalance mass

Mass with a center of gravity distant from the shaft axis.

Unbalance reduction ratio (URR)

Ratio between the reduction of the unbalance caused by a correction and the unbalance before that.

Unbalance vector

Vector described by amount and angle of unbalance.

Vertical balancer

A balancing machine that measures the unbalance for a vertical orientation of a rotor’s axis.