Detecting Surface Cracking in Gears to Extend Their Useful Lives

• Develop an alternative to MT, PT, and UT to reliably inspect gears for surface cracking.

Over time, gear teeth are prone to surface cracking because of the constant torque they are submitted to. Surfacebreaking cracks are typically found in the addendum, dedendum, and fillet of gear teeth, although they can also occur elsewhere.

Depending on the size and type of gears, they are usually tested for defects using a variety of techniques, most notably magnetic-particle (MT), dye-penetrant (PT), and ultrasonic (UT) testing, or any combination of the three.

MT and PT are time-consuming, can easily miss small defects, and only offer crude repor ting/record-keeping capabilities. Moreover, they are messy and require pre-inspection and postinspection clean-ups. At best, these two testing methods can help detect the existence of flaws, but they only offer rudimentary length information.

UT, on the other hand, is faster than the above two inspection methods, but because of the near-surface “dead zone” intrinsic to the technology, it is somewhat blind to surface-breaking cracks. UT also requires couplant, also making a postinspection clean-up necessary.

A better inspection method was therefore necessary, one that would:

• be more sensitive to surface cracks;
• be capable of scanning the addendum, dedendum, and root of a gear tooth in one pass;
• take less time to perform than MT, PT, and UT;
• be capable of reliable surfacebreaking crack detection and record keeping;
• be easy to handle by gloved operators, in a dirty environment.

• Compact ECA probes designed for gear teeth profiles, that are rugged, and easy to handle.

Eddy Current Testing (ECT) is an electromagnetic technique well known for its surface-breaking crack detection capabilities. Eddy Current Array (ECA) technology is the next stage in ECT evolution. It is already being used in the aerospace and nuclear industries, among others, for its capacity to quickly scan surfaces and accurately detect surface-breaking defects.

ECA technology uses multiplexing—several individual coils are grouped together in one probe and excited sequentially—to eliminate interference from mutual inductance and to enable scanning wider areas.

Grouping multiple coils also gives the acquisition and analysis software the ability to display data in the form of 2D and 3D C-scans. C-scans obtained with ECA technology are much simpler to analyze and much more accurate than other display methods. The software’s high-performance acquisition and analysis capabilities make computerized record-keeping and reporting possible, which are also great advantages over other inspection technologies.