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Rotor Testing Services:

Growth Measurement at Speed

A better understanding of a components dynamic response to stress and temperature can lead to improvements in manufacturing efficiencies, reduced production costs, reduced component weight, improved performance/reliability/efficiency, and reduced life cycle costs to the end user.

In response to this need, Test Devices has developed a method to measure elastic and plastic radial growth of components in real-time during a spin test. This capability allows for the measurement of elastic/plastic radial growth at high stress (high speed), up to and including component failure, as well as low stress (low speed). Measurements can also be performed at representative engine temperatures.

Growth measurement at speed allows customers to identify the individual radial growth components (elastic and plastic) that are experienced during a test. Our method enables total growth of components to be "dialed-in", meaning that acceleration can be stopped once a predetermined growth has been reached. As an example, Test Devices has "pre-grown" finished rotors to specific growth levels prior to installation in actual test engines. This pre-growth avoids the vibratory difficulties resulting from the growth of a single stage during engine testing.

Predictive models based on non-rotational (static) plastic growth measurements at ambient temperatures can be inaccurate. Elastic and plastic radial growth measurement at appropriate speeds and temperatures provides customers with the ability to determine a component’s position on the stress/strain curve during a test. This is helpful in validating computer models for new and exotic aerospace alloys or when material performance of an existing component does not match the predictive models.

Radial growth testing is beneficial for any component which experiences growth, elastic or plastic, during normal operation at high speed. Below is a list of components for which growth is commonly measured at speed.

Traditional Growth Measurement

Mechanical measurements (e.g. calipers) are traditionally employed to measure the plastic radial growth component only. The method employs an iterative approach whereby components are repeatedly accelerated to predetermined speeds, decelerated to 0 rpm, and then measured. This approach is time consuming and relies heavily on predicted growth curves not actual growth data. Since component growth is not linear with speed, unexpectedly large amounts of growth can result from modest speed increases based on models. There is no way to know how much growth a component is experiencing while at speed, and "over-growth" may result in a out of tolerance component.

Procedure

Test Devices' spin facilities utilize magnetic, fiber optic, or laser proximity sensors, to measure elastic/plastic radial growth of high speed rotating components. Multiple sensing methods gives us the flexibility to choose the optimal mix of probe types in order to measure growth at various locations of components that are made of more than one material type Components that are all one homogeneous material often also require multiple sensing elements as well in order to record growth along variations in geometry or interruptions in surfaces along the part.

For instance, many magnetic rotor assemblies include not only stacks of laminations but also retaining assemblies of differing materials designed to provide axial load to the laminate stacks during operation. Thus, it is important to not only measure the growth of the laminate stacks, but also the performance of the retaining assemblies.

The sensors are individually calibrated and aligned with the rotor surface to ensure accurate measurements. Sensor to rotor gap is typically established based on customer supplied predictive growth. Once the probes are installed, the rotor is accelerated to predefined speeds to initiate growth. Measurements are taken not only during acceleration but also during deceleration to determine the extent of permanent deformation.

An important point to remember is that the measurements recorded by the proximity sensors include vibration occurring in the rotating assembly. Therefore Test Devices has developed a method to remove the effect of the vibration from the recorded data in order to avoid misrepresenting the growth measurements.

Data Acquisition and Reporting:

Test Devices uses an on-line high speed data acquisition system to record all test parameters throughout the duration of radial growth tests. All test data are included in the final report together with all other project paperwork, including growth measurement data, certificates of conformance, balance, heat calibrations, and tooling material certificates. Test data is also archived at Test Devices for the customer’s future reference.