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Test Patterns
NDT Talk

The significance of zero defects

Let's say a defect rate of one part per million (1ppm) has been set. Is it possible to achieve this via random sampling or SPC? Not likely.

But is 1 ppm sufficient for production? Using needle bearings for rocker arms in a 6-cylinder 4-valve engine as an example:

  • 1 ppm bad needles for the arm allows
  • 24ppm bad bearings, which--at 24 bearings per engine-allows
  • 576ppm wrong engines, which means
  • ONE IN EVERY 1,736 CARS has a bad rocker arm bearing

If it is a 14-million-car year, for example, 1 ppm needles trans-lates to 40 bad cars per day!

Projecting like this, you can see that bearing components having very high volumes, and being critical to both function and safety, really have allowable defect rates of only 0.1 ppm or 0.01 ppm.

100% testing of bearings becomes especially important in order to achieve zero defects from such processes as heat treatment, forming, machining and finishing, all of which cause unpredictable flaws, both in time and in type.

In this issue of Test Patterns, we present a number of bearing components systems, appropriately including ones for hardness testing and for crack testing on needle bearings. We hope you find the information presented to be useful and informative.

The pressure for defect-free parts

Bearing manufacturers, in particular, face increasing demands for higher quality by automotive and other industry OEMs requiring them to pay special attention to material testing. Defect quotas within a ppm range no longer are acceptable (see column, left). Reliable, easy-to-use and very fast test methods for 100% inspection for correct micro-structure from heat treatment or alloy and for cracks and surface flaws have become a requirement in many production lines.

ibgNDT Systems has responded to these demands by developing test instruments and systems for many bearing components, including balls, needles, rollers, tapered rollers, inner rings, outer rings, shafts, pinion pins, hubs, spindles and more. Inspections commonly are for material properties such as hardness, case depth, hardness pattern, soft areas,decarburization, retained austenite, temper, annealing, grinder burn, wrong material, wrong pre-heat treat condition, carbides and for cracks, seams and other surface flaws.

Pressures of defect-free 1
NDTalk 1
Testing pinion pins 2
Detecting cracks 3
Sharon Kendra-Routsaw 3
High speed ball testing 4
Testing on needles 4

For material properties, a newtest protocol called "Preventive Multi-Frequency Testing" (PMFT) was developed by ibg. PMFT allows for ultra-fast testing with multiple frequencies (typically eight within 163 milliseconds). The method is easy to set up, operate and maintain in the production line environment.

Illustration represents partially soft area on bearing race for which systems have been specifically developed to test.

Very importantly, all heat treat or other microstructure errors detectable by eddy current are found with the PMFT test protocol, even those that are not expected to happen. Thus, zero defects for material properties can be achieved by the eddy current test system.

In addition to reliability gained from PMFT, features in the test instrumentation eliminate drifting of voltage vector values over time and over plant temperature changes. Thus, the testing also is very repeatable over time, even under factory floor conditions.

Pictures being worth thousands of words, this edition of Test Patterns presents pictures of systems that test pinion pins, needle bearings (for hardness), needle bearings (for cracks) and ball bearings.

Are you feeling the pressure for defect-free parts?

Testing pinion pins for complaince

Induction hardening of pinion pins is a process commonly monitored via 100% inspection by an eddy current system. Production volumes are very high, and the hardening process is complex, requiring the production of a hardened center area and soft end areas.

Pictured here is a system developed by ibg for automatic inspection of pinion pins. It tests a range of part sizes from 12 to 25mm diameter and 25 to 45mm length, each at up to 60 parts/minute. Testing at two positions was required to assure sensitivity to all the hardening factors, including the critical pattern runout on each end.

soft end shardened

Correct surface hardness, hardness depth and hard­ness pattern are critical. The center area is a bearing surface, and the ends must be soft for the following assembly processes. So, verifying correct hardening is critical both for the end use and for the following assembly process.

Tested pins escape the system via a very gentle-but high-speed-escapement mechanism that prevents denting the soft ends.

This is an example of how a high-speed test system can be designed to fit unique criteria.

Crack detection in needle bearings--at 10 parts/second

Needle bearings are driven through the eddyscan® H2/10 rotating eddy current scanner as it rotates at 6,000 rpm. Guided through via precision guide tubes, and tracked individually as they pass through the system, each individ­ual needle bearing is tested and sorted individually into OK or NOK parts chutes. Cracks bigger than 0.002" deep x 0.120" long must be detected and sorted out. Throughput speed is 100mm/second. Thus, 10mm long needles are tested at 10 parts/second, or 36,000 pph. Needles with diameters from 1.5mm to 3.0mm can be tested.

This system easily can be generalized to test almost any size of cylindrical bearing component.

Sharon Kendra-Routsaw now on ibg NDT staff

As applications engineer, Sharon is involved in the technical aspects of NDT installations, including engineering feasibility, testing and as an information source for questions, production integration and mechanical handling equipment.

A specialist in nondestructive testing of heat treat­ed material for the automotive industry, Sharon is certified for Level III Eddy Current testing.Sharon has been involved in NDT testing for 12 years, and has additional experience in metallography and corrosion testing.

A member of the ASNT, including past senior board membership, Sharon attended Henry Ford Community College and the University of Michigan. She has a degree in Metallurgy.

ibg NDT is pleased to have her know-how on board to implement our NDT test systems.

High speed testing and high-speed sorting-vital to ball bearing testing

Changeover to another size requires only an exchange of the indexing disc.

Each ball is tracked through the system and individually sorted to OK or NOK parts chutes, the default sorting gate position being the NOK chute.

This is another fast one!

In the past, testing ball bearings with single-frequency eddy current systems has allowed "bad" balls (wrong material or wrong hardness) to escape detection. "Zero defects" status was not being achieved.

Accomplishing highly reliable, 100% testing at very high speeds was a big challenge. Overlaps in permeability of the balls were being missed. ibg's Preventive Multi-Frequency Testing (PMFT) was applied.Testing with six frequencies (800 to 80 kHz) provided highly reliable testing with test times under 50 milliseconds.

Next, a high speed mechanical test and sort system was developed to keep up with the test speed. The one pictured with the eddyliner®P tests and sorts balls individually at 36,000 balls per hour. This one tests balls from 1.5mm to 6.0mm diameter.

Fully automated testing of needle bearings

Totally automatic inspection of needle bearings for hardness and material mix has been developed by ibg. Designed to run at a test rate of 3,600 parts per hr., the system can be upgraded to double-or even quadruple-tnis rate. This system inspects needles with diameters

1.5mm to 3.0mm and lengths 5mm to 26mm. The system concept can also be applied to roller bearings, tapered rollers and balls- and to larger size ranges of parts.