Aero-engine impellers are imperative key parts of engines, which are experienced by aerodynamic loads and structural stresses during high-speed running. Deviations in geometric dimensions as slight as a few microns could drastically influence the overall performance, stability, and even safety of the engine. Therefore, conducting high-precision dimensional inspection before impeller assembly is a necessary process to ensure assembly quality and reliability.
Introduction
With the rapid development of aviation technology, the demand for thrust, efficiency, and reliability of aero-engines becomes increasingly demanding. The impeller, being an essential part of the engine, bears unimaginable aerodynamic loads and structure stresses. Under the condition of high-rotating speed of the impeller, the slightest dimensional deviation will lead to severe aerodynamic failure, dynamic balance imbalance, or even operational shutdown. In order to obtain high accuracy and stability after assembly of the impeller, strict inspection of various geometric sizes should be conducted. Dimensional inspection not only specifies the assembly precision of the impeller but directly affects the optimization of engine’s overall performance and control of life. Therefore, possessing proficiency in advanced dimension inspection technologies, especially high-precision inspection before assembly, is the guarantee for the efficient operation of impellers.
As a result of the intricate internal structure of aero-engine impellers, dimensional inspection involves several technical challenges. This article will emphasize the discussion of dimensional inspection technologies for aviation impellers prior to assembly, comparative analysis of advantages and disadvantages of current inspection technologies, and anticipating future development trends.
Key Points and Difficulties of Impeller Dimensional Inspection
Key Points of Impeller Dimensional Inspection
The accuracy of impeller assembly would have direct effects on the operating efficiency and reliability of engines. To ensure the quality after impeller assembly, strict inspection on key dimensions should be conducted. The following are the key contents of dimensional inspection before impeller assembly:
(1) Shaft Hole Dimensions and Coaxiality
The size accuracy of the impeller shaft hole and the main shaft hole size will directly affect the correctness of impeller installation and rotation. If the shaft holes’ size accuracy or coaxiality is beyond the specified range, it will lead to bad matching between the main shaft and the impeller, therefore affecting the rotational balance and power output of the engine. Therefore, the shaft hole size and coaxiality must be as designed.
(2) End Face Flatness and Hub Roundness
End face flatness and hub roundness of the impeller directly relate to contact quality with other components. If the end face is not flat or the hub roundness is significantly different, it will affect the assembly connection quality and therefore the engine’s aerodynamic performance and running stability.
(3) Blade Spacing and Hub Symmetry
Hub symmetry and blade spacing of the impeller are important parameters that affect aerodynamic performance. A non-uniform blade spacing or inadequate hub symmetry would cause uneven airflow, increased vibration, and other issues, thereby decreasing the thrust efficiency and stability of the engine.
(4) Matching Surface Dimensions and Datum Repeatability
The dimensions of the mating surfaces of the impeller with other components (e.g., shafts, casings, etc.) and the consistency of their datum will directly impact the total assembling dimension accuracy. Especially in multi-stage assembly, even a slight dimension deviation will result in assembly error being magnified and hence affect the overall performance of work.
Difficulties in Impeller Dimensional Inspection
Although there have been significant achievements of classical dimensional inspection technologies, due to the complex geometric shapes and special process requirements of impeller parts, dimensional inspection is still faced with the following major technical challenges:
(1) Complex Free-Form Surface Structures
Impellers often have complex free-form surfaces with complex shape changes. It is hard for classical measurement technology to completely and accurately measure these surfaces, demanding the development of new measurement technologies corresponding to the geometric characteristics of impellers.
(2) Cumulative Effect of Measurement Errors
The geometry of an impeller is usually a combination of several components, and the assembly accuracy among those components will influence the dimensional errors of the total impeller directly. Particularly in multi-stage assembly, the dimensional error of any one component can be compounded at assembly time, and it will affect the overall assembly accuracy and quality.
(3) Limitations of Measurement Range
In dimensional measurement of the impeller, it’s usually necessary to scan free-form surfaces of a large area with evenly distributed measurement points. But traditional contact measurement technology can hardly cover all the measurement points with the physical dimension of the probe and the limitation of the contact mode, especially in the regions of complicated geometric characteristics, easily leading to inadequate measurement accuracy.
(4) Interference Factors in the Measurement Process
During the actual measurement process, factors such as the gloss of the surface material and the reflectivity of the surface of the impeller, and geometric surface imperfections at the workpiece surface, will compromise the measurement values, especially for optical measuring technology, as the factors of interference will be more effective there.
Common Dimensional Inspection Technologies and Digital Integration Methods
As technology has improved, new dimensional inspection technologies have been more diversified, such as contact, non-contact, optical, and non-destructive and others. The following are some general dimensional inspection technologies.
Coordinate Measuring Machine (CMM)
Coordinate Measuring Machine (CMM) is a traditional contact-type measuring instrument for dimensional inspection of parts with wide application in the measurement of aero-engine components. It possesses high precision and fine stability, capable of measuring big, complex impeller pieces. CMM collects high-accuracy measurement point data mainly by means of probe-workpiece surface contact and derives the dimensional error from the relationship between measurement points and coordinate systems.
(1) Advantages
- High precision, suitable for precise measurement of impeller features such as shaft holes, hubs, and end faces;
- Capable of measuring complex geometric shapes in three-dimensional space, with strong adaptability;
- Able to compare with CAD models and output error analysis reports automatically.
(2) Disadvantages
- Slow measurement speed, especially for large-area free-form surface measurement;
- Probing contact on the workpiece surface will cause local deformation, which affects the precision of measurement.
- In case of smooth or highly reflective workpieces, contact measurement is extremely susceptible to disturbance.
Blue Light Scanning Technology
Blue light scanning technology is a non-contact 3D measuring technology with the capability of obtaining high-density point cloud data in a short time. Its principle of working is illuminating the workpiece surface by a blue light source and obtaining the deformation of light with a camera in order to obtain the workpiece’s 3D data.
(1) Advantages
- No contact measurement, no physical deformation or damage to the workpiece surface;
- Fast measurement speed, quick capture of point cloud data, high efficiency for mass measurement;
- Large surface areas of the impeller are covered, particularly with free-form surface measurement.
(2) Disadvantages
- Measurement results are significantly affected by surface reflectivity, requiring special treatment of the workpiece surface;
- For parts with complex surface textures, smooth or highly reflective areas, the scanning effect may be affected;
- The measurement accuracy is limited by equipment performance, with declining accuracy for precision parts or small-sized parts.
Optical and Non-Destructive Testing Technologies
Optical testing technologies and non-destructive testing technologies have become more and more important dimensional inspection tools, especially in the detection of complex geometric forms and inner structures, where they have unbeatable advantages.
(1) Optical Inspection
Optical measurement methods include laser scanning, fringe projection, and interference, etc., which can be used for impeller contour and surface morphology measurement. Subtle surface details of the impeller can be quickly captured and measured by high-accuracy optical devices.
(2) Non-Destructive Testing (NDT)
Non-destructive inspection technologies like ultrasonic inspection and X-ray CT scanning are appropriate for the detection of internal flaws of impellers, including cracks and air bubbles. From the measurement of ultrasonic reflection signals or the measurement of X-ray attenuation, the structural integrity and dimensional deviations of the impeller can be identified.
Digital Inspection Path Planning and Post-Processing
With the development of digital technology, dimensional inspection’s data processing and path planning have also evolved towards intelligence and automation. Following are some important chains of digital inspection technology.
Inspection Path Planning
Inspection path planning is also one of the key parts of the automatic inspection system. With the help of digital path planning, the measuring system can automatically recognize the significant features of the impeller and rationally schedule the measurement sequence and the measurement point arrangement, so that the significant dimensions can be measured precisely.
(1) Single-Feature Path Planning
Single-feature path planning refers to measuring path planning for a single geometric feature of the impeller. This method can be used for relatively simple dimensional measurement work, and by quickly positioning and measuring the target feature, efficiency during inspection can be increased.
(2) Multi-Feature Path Planning
In complex parts with many features and surfaces like impellers, multi-feature path planning is particularly important. With regard to the relationship between each feature and optimal measurement path, probe interference and redundant measurement can be eliminated, thereby improving measurement accuracy and efficiency.
Code Post-Processing and Data Reporting
During the inspection process, raw data collected has to be analyzed and processed through post-processing. Measurement system automatically codes (for example, DMIS), verifies the measurement data against the CAD model, and generates error analysis reports that exactly represent the workpiece’s deviation in dimensions.
(1) Automated Data Analysis
Through automated data analysis, dimensional mismatches among each piece of the impeller can be quickly found and errors can be displayed graphically. Thus, engineers can quickly identify problems and take the right correction measures based on the report.
(2) Result Report Generation
Automated generation of error reports is one major advantage of digital inspection technology. Expert software reports can be docked directly into the assembly and manufacturing links, and hence errors are detected and corrected at the earliest stage, eliminating any subsequent quality issues.
Support Role of Inspection Technology in Assembly Quality and Life
Accurate dimensional checking not only improves assembly precision but also directly impacts overall performance and engine life. The following are the impacts of dimensional inspection on assembly quality and life:
Improving Assembly Precision and Dynamic Balance
By accurate dimensional measurement, matching precision between the impeller and other components can be ensured, preventing assembly errors, improving rotor coaxiality and dynamic balance, thereby avoiding unbalanced vibration under running conditions of the engine and ensuring optimal performance.
Enhancing Aerodynamic Performance and Efficiency
Impeller dimensional accuracy is very important in setting aerodynamic performance. When geometric dimensions of the impeller are optimized, aerodynamic performance can be maximized, air resistance minimized, thrust increased, and hence engine efficiency and fuel economy maximized.
Improving Quality Traceability and Life Control
The use of dimensional inspection technology guarantees product quality traceability. With the help of accurate dimensional information and inspection reports, a basis can be laid for future maintenance and remanufacturing that will guarantee the stability and long service life of the engine during its operational life.
Conclusion
With the gradual progress of aero-engine manufacturing processes, technologies for the inspection of impeller dimensions are similarly developing and progressing. Intelligent and digital inspection methods will be the direction of future development. Combining sophisticated inspection methods with automated path planning is bound to significantly improve the quality of impeller assembly and production efficiency. In the future, dimensional inspection will not only aim to improve inspection accuracy but also become an integral part of comprehensive quality control and intelligent manufacturing systems.
