ROTOR BALANCING

Abstract

A procedure defining a balancing strategy includes: providing a computer model which predicts the vibration amplitude at a given axial position along the spool when the spool is rotated at a given rotational speed; using the model to predict respective vibration amplitudes at the given axial position for different axial positions of a unit unbalance applied to the spool; plotting the predicted vibration amplitudes as data points on a graph of vibration amplitude against axial position of the applied unit unbalance; using the graph to identify axial positions which are more or less likely to contribute to flexing of the spool at the given rotational speed when mass is added or removed from the first rotor module to reduce imbalances at the axial positions; and defining a balancing strategy based on the identification.

Claims

1. A procedure for defining a balancing strategy to reduce mass eccentricity imbalances produced by a first rotor module which in use is attached to one or more further rotor modules to form a spool, the procedure including the steps of: providing a computer model which predicts the vibration amplitude at a given axial position along the spool when the spool is rotated at a given rotational speed; using the model to predict respective vibration amplitudes at the given axial position for different axial positions of a unit unbalance applied to the spool; plotting the predicted vibration amplitudes as data points on a graph of vibration amplitude against axial position of the applied unit unbalance; using the graph to identify axial positions which are more or less likely to contribute to flexing of the spool at the given rotational speed when mass is added or removed from the first rotor module to reduce imbalances at the axial positions; and defining a balancing strategy based on the identification.

2. A procedure for defining a balancing strategy to reduce mass eccentricity imbalances produced by a first rotor module which in use is attached to one or more further rotor modules to form a spool, the procedure including the steps of: rotating the spool at a given rotational speed; measuring respective vibration amplitudes at a given axial position along the spool for different axial positions of a unit unbalance applied to the spool; plotting the predicted vibration amplitudes as data points on a graph of vibration amplitude against axial position of the applied unit unbalance; using the graph to identify axial positions which are more or less likely to contribute to flexing of the spool at the given rotational speed when mass is added or removed from the first rotor module to reduce imbalances at the axial positions; and defining a balancing strategy based on the identification.

3. A procedure according to claim 1, wherein the given rotational speed is in a speed range in which the spool is susceptible to resonant vibration.

4. A procedure according to claim 1 further including the step of: including indicators on the graph, at different axial positions, of likely average unbalance due to module component mass eccentricity at the respective axial position; wherein the defined balancing strategy is also based on the likely average unbalances.

5. A procedure according to claim 1, wherein: the first rotor module has two axially spaced apart balancing planes; the data points include a pair of data points for the predicted/measured vibration amplitudes when the unit unbalance is applied to the spool at the balancing planes; and the graph includes a straight line which extends through the data points at the balancing planes.

6. A procedure according to claim 5, wherein: the graph has a range of axial positions (a “rigid response range”) in which the data points lie on the straight line; and when the first rotor module has an imbalance (a “rigid response imbalance”) which is identified to lie within the rigid response range, a balancing strategy to address the rigid response imbalance is defined in which, to the maximum extent possible, mass is added or removed from the first rotor module at the balancing planes to correct the rigid response imbalance.

7. A procedure according to claim 5, wherein: the graph has one or more ranges of axial position (“flexible response ranges”) in which the data points do not lie on the straight line; and when the first rotor module has an imbalance (a “flexible response imbalance”) which is identified to lie within such a flexible response range, a balancing strategy to address the flexible response imbalance is defined in which, to the minimum extent possible, mass is added or removed from the first rotor module at the balancing planes to correct the flexible response imbalance.

8. A procedure according to claim 7, wherein the balancing strategy includes component balancing, straight building and/or blade distribution procedures to correct the flexible response imbalance.

9. A procedure according to claim 5, wherein: the graph has one or more ranges of axial position (“flexible response ranges”) in which the data points within the flexible response range(s) do not lie on the straight line; and when the first rotor module has an imbalance (a “flexible response imbalance”) which is identified to lie within such a flexible response range, a balancing strategy to address the flexible response imbalance is defined in which the first rotor module is reconfigured to move the straight line closer to a data point at the axial location of the flexible response imbalance.

10. A procedure according to claim 9 including repeating the steps of: providing a computer model; using the model to predict vibration amplitudes; plotting the predicted amplitudes; using the graph to identify axial positions; and defining a balancing strategy, for the reconfigured first rotor module.

11. A method of reducing dynamic imbalance produced by a first rotor module, which in use is attached to one or more further rotor modules to form a spool, the method including: performing the procedure according to claim 1; and balancing the first rotor module according to the defined balancing strategy.

12. A method according to claim 11, wherein the first rotor module is a module of a spool of a gas turbine engine.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Embodiments of the invention will now be described by way of example with reference to the accompanying drawing in which:

[0027] FIG. 1 shows an example of an unbalance response graph for a high pressure compressor and a high pressure turbine of a gas turbine engine.

DETAILED DESCRIPTION AND FURTHER OPTIONAL FEATURES

[0028] FIG. 1 is a graph of dynamic response (i.e. vibration amplitude) to unit unbalance measured at a single fixed sensor (e.g. a bearing location) on a gas turbine engine spool comprising a first module (a high-pressure compressor (HPC)) and a further module (a high-pressure turbine (HPT)) against axial location along the spool. Plotted on the graph are plurality of data points (crosses) at different axial positions. Each point represents the respective vibration amplitude determined at the single sensor when a unit unbalance is applied to spool at the axial position of the data point for a given rotational speed of the spool (e.g. a critical speed for maximum amplitude of resonant vibration). As explained below, the graph is used for defining a balancing strategy to reduce imbalances caused by mass eccentricities in the HPC and HPT.

[0029] Conveniently, the data points can be predicted from a computer numerical model of the spool, suitable models being known to the skilled person. The unit unbalance can then simply be applied as a rotating force. However, this does not exclude that the data points can be measured experimentally (e.g. by an accelerometer, a displacement probe, or a bearing force measurement) from a real spool being rotated in a balancing machine or in situ in an engine, with the unit unbalance being applied by balancing weights.

[0030] In addition, circles are shown on the graph, the relative diameters of the circles representing the likely average unbalance due to mass eccentricity at the respective axial positions. In particular, the axial position of each circle centre corresponds to the axial position of the centre of mass of a respective module component, and each circle diameter is calculated from the mass of that component multiplied by the average (and therefore most likely) eccentricity of the component. Conveniently, the average eccentricity can be obtained from statistical analysis of rotor samples. Each circle is aligned in the vertical direction, purely for convenience, so that its centre is at the vibration amplitude of the data point which shares the same axial position, but the circle diameters themselves are merely scaled relative to each other to show the relative importance of their likely unbalance. Circles relating to components of the HPC module are drawn with solid perimeters, and circles relating to components of the HPT module are drawn with dashed perimeters.

[0031] The HPC module has front and rear balancing planes, the axial positions of which are indicated on the graph. These planes correspond to the axial locations of lands at which balancing weights are added or removed from the module when balancing correction is performed. A straight line X is drawn on graph between the data points at these two axial positions.

[0032] If the data points within an axial range of a module lie on straight line X, then in that range the spool is acting rigidly at the given rotational speed when the unit unbalance is applied. That is, movement of the unbalance within the length of that range has a linear effect, and therefore dynamic two plane low-speed balancing of the part of the module in that range should be possible without exciting a mode shape producing vibration. Such balancing is advantageous on grounds of cost and ease of vibration-reduction.

[0033] However, if the data points within an axial range of a module do not lie on straight line X, then in that range the spool is acting flexibly, and if a bending moment is applied in the part of the module in that range the shaft will bend, which should excite a mode shape producing vibration. Accordingly, a different balancing strategy should be adopted within the length of that range.

[0034] A purpose of the circles is to give a user of the graph an indication of the relative desirabilty of having a particular component (and therefore its circle) close to the line X.

[0035] More particularly, the locations available on the HPC module for dynamic two plane low-speed balancing are the front and rear balancing planes. The graph has a range of axial positions (a “rigid response range”—shaded grey on the graph and located towards the front of the HPC) in which the data points lie on the straight line X. If an unbalance (a “rigid response imbalance”) occurs somewhere in the response range, it can be balanced at the front and rear balancing lands, and the bending moment induced will not significantly excite the resonant vibration because that region of the spool is behaving rigidly with respect to the resonant vibration. Thus, in this case a suitable balancing strategy to address the rigid response imbalance can be defined in which, to the maximum extent possible, mass is added or removed from the HPC module at the balancing planes to correct the on-line imbalance.

[0036] However, the graph also has a range of axial positions (a “flexible response range”—unshaded on the graph and located towards the rear of the HPC and into the HPT) in which the data points do not lie on the straight line X. When an unbalance of the HPC module occurs somewhere in the flexible response range (e.g. due to reason 3) discussed above), the bending moment induced by balancing that imbalance at the balancing lands will induce flexing and drive the resonant vibration. Accordingly, if the balancing land positions are not moved, options to balance the HPC against imbalances in the flexible response range include using compressor component balancing, straight build for the compressor and/or blade distribution procedures to ensure that there is no imbalance induced in the HPT by the HPC. Such approaches may be implemented, for example, to reduce the likely major imbalance from the component indicated Y towards the rear of the HPC. In effect, the result of adopting the approaches is to shrink the diameters of the HPC component circles.

[0037] However, these approaches will not address the likely major imbalance caused by, e.g. geometric error at the HPC/HPT interface, from the component indicated Z in the HPT. In this case, therefore, another option is to move the rear balancing land of the HPC rearwards so that it is adjacent the HPT. The straight line X will then be shifted (indicated by dashed line X′) so that it passes close to the centre of circle Z. With the major response indicated by circle Z induced by the HPC in the HPT then falling close to the line that passes through the balancing lands, this unbalance can be safely addressed at those lands without inducing bending. However any compressor internal component imbalances will not fall on the line X′, and so component balancing, straight build, and/or blade distribution approaches may be needed to eliminate any imbalances within this region (i.e. to shrink the HPC component circles).

[0038] Another option is to take an intermediate approach, and place the balancing lands such that the straight line passes somewhere between the HPC component and HPT component responses, and then adopt a balancing strategy with a degree of component balancing/straight build/blade distribution etc. for the HPC, but to a lesser extent than might have been otherwise been required.

[0039] Other possible options are to more substantially reconfigure the modules, such as altering where module interfaces lie. As will be appreciated, any approach involving component reconfiguration (even if just movement of a balancing land), may require repetition of the procedure outline above to define an appropriate balancing strategy.

[0040] Having defined a balancing strategy for the HPC, a similar procedure can then be performed to define a balancing strategy for the HPT. In this way both modules of the spool can be balanced independently.

[0041] Advantageously, the procedure allows the rotor dynamics of a given rotor configuration and low speed balancing requirements to be optimised at the same time. It also allows a multitude of balancing options to be considered with reference to just one graph.

[0042] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.