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.
