
Radial Growth Testing @ Test Devices, Inc.

Radial Growth Testing

Introduction

Due to higher speed requirements, it has become a technical challenge to design high-speed rotating equipment to withstand the rotational stresses imposed on spinning components. Test Devices has the ability to quantify the diametrical or radial growth of high RPM rotating parts. The forces on rotating parts grow alarmingly fast as rotational speed increases. This can lead to substantial diametrical growth which must be taken into consideration during the design, particularly, if touching a nearby stationary component is possible.

Electric motors, for example, are particularly sensitive as the air gap between the rotor and stator is critical to the performance of the motor. Designers must ensure that the rotor/stator air gap is not consumed due to growth of the rotor from centrifugal force, thereby threatening mechanical contact and motor failure. Conversely, too much gap and one reduces motor performance. An example of Test Devices’ centrifugal growth measurement capability, known simply as Radial Growth Testing, is a recent test done for a high speed motor manufacturer. TDI’s measurement capability enabled the customer to document the growth their component experienced under centrifugal load at various speeds on upto operating speeds and optimize the distance between the rotor and stator. The Test Devices’ supplied data enabled the customer’s engineers to balance the flux density, manufacturing tolerances and material selections to optimize the motor/stator air gap. The result was a more efficient motor design with reduced warranty costs.

Fig 1 - View of motor gap

Figure 1
Motor Gap

Increasing speeds, thus, increased centrifugal stress, increases the diametral growth. Test Devices’ centrifugal growth measurement capability gives designers precise information on the amount of growth experienced during rotation. This growth is comprised of two components which Test Devices can determine separately. Growth shown only at speed is said to be "elastic growth" which fully recovers. Similar to a rubber band returning to its original shape after being stretched; rotating parts experiencing elastic growth return to their original dimensions after rotation is stopped. Plastic growth, or growth which results in a permanent deformation in the component's dimensions, occurs when the centrifugal force overcomes the component's material strength that is trying to hold its original shape. After undergoing plastic growth a rotating component can never return to its original size.

Radial growth of rotating equipment also affects the axial length of whole assemblies. Due to Poisson contraction, when the outside diameter (OD) of a rotating component grows it may at the same time axial contract to accommodate the radial growth. The resulting change in axial length can loosen lamination stacks and deteriorate motor performance.

Test Devices Inc. has developed test methods to assist designers in measuring these critical deflections (radial growth) during rotation at actual design speeds and temperatures. Test Devices' engineers can define the elastic and plastic components of centrifugal growth up to and including the burst speed. By using this data our customer's product engineers can improve equipment designs to obtain better performance and/or durability.

Procedure

Test Devices’ spin facilities utilize magnetic, fiber optic, or laser proximity sensors, to measure static/dynamic radial growth of high speed rotating components. Using several sensors enables growth monitoring of multiple rotor components to identify which elements experience the most extreme growth and at which speeds growth becomes critical to the detriment of the rotor/stator air gap or failure of the rotor. Figure 2 - rotor w/proximity sensors

This technique can be employed in testing rotor assemblies comprised of differing materials and interrupted surfaces. For instance, many magnetic rotor assemblies include not only stacks of laminations but also retaining assemblies designed to restrain the growth of the laminate stacks at speed. 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 be measured to ensure accurate measurements. Sensor to rotor gap is typically determined based on the customers expected growth. Once the probes are installed, the rotor is spun to speeds to elicit radial growth. Growth measurements are taken not only during acceleration but also during deceleration to determine the extent of permanent deformation.

The picture (above right) shows a typical radial growth test setup with multiple growth sensors monitoring different sections of the electric motor rotor.

Test Examples

The following data graphs from a radial growth test are examples of work done at Test Devices' facility. Radial growth testing has been performed on a wide variety of products such as electric motor rotors, composite flywheels, alloy material samples, jet engine rotors, and automobile starter armature comutators.

Example 1

The graph below shows a growth test of an automotive electric motor rotor at 392° F. The red line indicates the growth of the rotor OD at speed. Notice that there was little appreciable growth until 3000 rpm. From 3000 rpm to approximately 7000 rpm growth was consistent vs. speed. However, above 7000 rpm the growth becomes exponential as the rotor material is unable to overcome the centrifugal load. For this customer, rotor "failure" happened when the OD grew more than 8 mils, indicating that the rotor performance in service would have been substantially reduced.

Growth Chart

Figure 3
Growth Chart

Example 2

The four graphs below are growth data from a characterization test of a new steel alloy, tested in a rotor coupon configuration. The graphs are growth data taken from four sequential tests of the same rotor. With increased speeds the steel alloy yielded substantially, resulting in considerable "plastic" growth of the OD. This material was very ductile (easily stretched), allowing it to continuously yield even when substantially higher centrifugal loads were encountered. Many less ductile alloys would have failed earlier from the increased stress. The customer was able to use this test data to calibrate an electronic model to better predict the growth that actual engine components would see during operation.

Fig 4 - motor gap growth

Figure 4
Run #1
10 mils total growth and 2 mils of permanent growth

Fig 5 - motor gap growth

Figure 5
Run #2
14 mils total growth and 5 mils of permanent growth

Fig 6 - motor gap growth

Figure 6
Run #3
20 mils total growth and 17 mils of permanent growth

Fig 7 - motor gap growth

Figure 7
Run #4
14 mils total growth and 5 mils of permanent growth

Benefits and Applications

Radial growth testing can be an invaluable tool whenever the growth of any type of high speed rotating component is of concern. This technique can be used to verify the growth caused by centrifugal stress, not only to understand rotor/stator interaction but also to verify material properties for improvement of predictive models. With the data provided by Test Devices' Radial Growth test, a product's important parameters can be optimized for cost, performance and reliability.

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