Eddy Current Validates Product Integrity

Eddy current material evaluation was historically used for the testing of bar, tube and wire materials.  Given those successes, it was also used to inspect the inside of tubing for corrosion and voids in critical applications such as pipelines and heat exchangers.  The benefits of eddy current testing methods are accuracy and speed of test.  This allows manufacturers to test 100% of their product at production line rates typically associated with bar, tube, and wire manufacturing.

Eddy current has also made its way onto the automotive and medical component manufacturing floor.  The testing technology has been proven to be fast and accurate, and better suited for testing production volumes of components compared to traditional magnetic particle and dye penetrant testing.  Eddy current testing can be easily integrated into manufacturing lines providing checks and balances to production processes.  While originally used for crack testing, eddy current is now used extensively for testing of material structure which includes validation of heat treat processes.

Eddy Current Crack and Flaw Component Testing

While an encircling or rotating eddy current probe works well for finding cracks and flaws in bar, tubing, and wire, more sophisticated designs are needed for testing complex components.  As an example, a major wheel bearing spindle manufacturer wanted to check for cracks and flaws in 8 different locations on a component.  A custom eddy current probe as seen in Figure 1 was designed with 8 individual eddy current coils in the appropriate testing locations.  A production line material handling station properly positioned the probe next to the wheel bearing spindle which was then rotated to achieve 100% testing coverage.  If the test found a flaw, the material handling system sent the part to a reject bin.  Figure 2 shows a similar eddy current probe designed to test an automotive CV joint.

Figure 1 - Wheel Bearing Spindle and Eddy Current Crack Probe

Figure 2 - Eddy Current CV Joint Inspection Probe

To test the bowl area on the top of an aluminum piston, the eddy current probe shown in Figure 3 was designed.  The four spring-loaded probes have coils in slightly different positions to ensure coverage of the area of concern.  For this in-line test, the probe comes down over the cylinder and spins to ensure complete coverage of the part.

Figure 3 – Eddy Current Piston Testing Probe

Eddy Current Heat Treat Process Verification

While not an absolute hardness test like a Rockwell test, eddy current testing can achieve sorting results on par with Rockwell testing.  The advantages of eddy current testing are the speed of the test and the integration with manufacturing processes.  Figure 4 shows a manufacturing test fixture used by a major automotive supplier to validate heat treat processes on water pump shafts.  Three different coils are used to simultaneously evaluate the condition of three places on the pump shaft.  The fixture was able to accommodate multiple shaft lengths and diameters, and testing rates up to 40 shafts per minute were obtained.

Figure 4 - Eddy Current Pump Shaft Testing Fixure

For testing complex shaped components, custom probes are often required.  Figure 5 shows an eddy current probe used to testing the hardness of the valve seat area of an engine block.

Figure 5 - Eddy Current Valve Seat Hardness Probe


Eddy Current Feature Verification

Because eddy currents are sensitive to the structure of a component, they can easily be used to detect anomalies like missing or misplaced threads, or incorrectly installed components.  As an example, a major automotive bearing supplier wanted an automated way to tell whether all the ball bearings were installed in a bearing race.  Figure 6 shows the eddy current probe with the ball bearing race on top.  If a ball bearing was missing, the eddy current system would set off an alarm.

Figure 6 – Eddy Current Missing Ball Bearing Probe

Conclusion

Eddy current is a clean testing technology that can provide 100% in-line testing at production line speeds.  Manufacturers that incorporate eddy current testing into their production lines benefit from both reduced scrap and warranty costs.  Modern eddy current instruments have industrial I/O capabilities which allow for ease of integration into production material handling stations.  For manufacturers still using magnetic particle or dye penetrant testing for the detection of cracks and flaws, eddy current offers a great alternative that doesn’t require chemicals or depends on human interpretation.