Eddy-Current Testing Chops False Rejects

At Delphi Automotive's Chassis Div. plant (Sandusky, OH), the inspection of automotive wheel spindle bearings used to be a slow and costly process. "We had people sitting in booths for three shifts around the clock, checking 100% of the parts," relates Bablu Ratnaparkhi, senior metallurgist, NDT, at the plant.

Workers relied on a magnetic particle inspection process, by which parts were magnetized and sprayed with a magnetic particle-based solution. Because particles accumulated in cracks or other surface flaws on the magnetized parts, the flaws could be visually detected when parts were inspected under a blue light. The 0.1-inch-deep keeper groove on each spindle bearing was particularly difficult to inspect, however, and small, hairline cracks in the groove area were often hard for workers to spot, Ratnaparkhi says.

A thorough magnetic particle inspection of each spindle bearing required about three minutes, and when a worker found a problem, the part had to be set aside for later examination by a metallurgist before being scrapped. About 20% proved to be false rejects, Ratnaparkhi estimates.

In search of a better way, Delphi set out several years ago to look into other nondestructive testing (NDT) methods for checking the wheel spindle bearings. The company found what it was looking for in eddy-current testing technology supplied by Zetec Inc. (Issaquah, WA). When a specially designed eddy-current probe and associated instrumentation was combined with custom material handling equipment, the result was an automated, in-line testing system that dramatically improved throughput, while lowering costs and boosting inspection accuracy, Ratnaparkhi says.

Compared to the three minutes per part required for magnetic particle inspection, the eddy-current system tests one spindle bearing every six seconds, according to Ratnaparkhi. The system also reduced labor costs by almost $1 million a year, while all but eliminating false rejects, he says.

Eddy-current testing relies on a probe with an electrical test coil that produces alternating magnetic fields. When the probe is moved close to an electrically conductive material to be tested, the probe induces circulating electrical currents, or eddy currents, within the test piece. The eddy currents in turn create magnetic fields that interfere with the magnetic field of the coil, changing its electrical signal. Interruptions in the eddy current flow caused by imperfections such as cracks in the test piece can then be detected with instrumentation that monitors the test coil electrical signal.

Ratnaparkhi notes that the Zetec eddy-current system consistently detects small spindle bearing cracks that may be only 0.004 inch wide and 0.005 inch deep. The cracks can result from original forging defects, or may be created during heat treating or grinding processes, he says.

Major challenges in developing the eddy-current test system involved the design of the probe, as well as the mechanical system for handling the spindle bearings, says Ratnaparkhi. The underside of the keeper grooves is a key inspection area, and in order to inspect the entire bearing, the part has to be rotated 360 degrees while the probe maintains contact with the groove underside.

Zetec probe designers worked closely with Delphi to come up with a multicoil probe that can simultaneously test the keeper groove area as well as seven other places on the spindle bearing, Ratnaparkhi says. Small ceramic standoff wheels are built into the probe to correctly position the active part of the eddy-current sensors during the test. A walking beam transport system moves the parts in and out of the test stations, where the bearings are nested, gripped and rotated at 120 rpm during the test.

Delphi installed its first eddy-current system for spindle bearing testing in 1995 and currently has 14 such systems that are used for 100% in-line testing at the Sandusky plant.

Zetec Inc.
(800) 643-1771

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