Resonant acoustic method (RAM) nondestructive testing (NDT) offers an approach to product testing that uses sound to completely and reliably test product quality.

Capstan Atlantic (Wrentham, MA) uses a RAM NDT unit from The Modal Shop (Cincinnati) to test parts it manufactures for the automotive industry. The company’s niche is in making multi-level, high-performance precision gears for automotive power train systems, supplying to the Big Three in the United States, as well as Japanese transplant facilities.

“The RAM NDT unit we’ve employed in our manufacturing process has enabled us to provide 100% product testing,” says Capstan Atlantic Vice President of Engineering Rich Slattery. “To put it simply, it provides a peace of mind that lets us sleep at night.”

Newly formed gears are sintered on 65-foot continuous belt furnaces at more than 2,000 F. Source: The Modal Shop Inc.

The Need for 100% Inspection

Capstan Atlantic’s production facility involves a complex metallurgy process that begins with an iron-based, powdered alloy. Additions such as chrome, nickel, molebdenum and graphite are added for strength, ductility and wear resistance. The powder blends are compressed to varying densities, depending on the requirements of the application. The newly formed gears are then sintered on 65-foot continuous belt furnaces at a temperature of more than 2,000 F. Although the methods Capstan Atlantic employs are robust and minimize the chance of defects, the process still requires a 100% test for structural integrity. Therefore, product testing is an essential element in the quality assurance program.

Prior to investing in the RAM NDT unit, the company used a nondestructive torque test method. This approach did not deliver the level of performance and reliability required; it was subject to human interpretation, very slow and, therefore, costly. Also, it did not guarantee 100% conformance.

“We felt that the manual testing approach was too uncertain for us,” says Senior Development Engineer Eric Day. “Because of the demanding, highly competitive nature of our industry, we wanted a testing method that qualified every single component that left our facility. We found it with the RAM NDT unit.”

“Our previous method tested around 40 to 50 parts an hour,” says Slattery. “But the RAM NDT unit tests 600 to 700 parts an hour. The improvement in the level of effectiveness and efficiency we’ve experienced with the RAM NDT is phenomenal.”

The RAM NDT unit has been incorporated on Capstan Atlantic’s assembly line. Components pass through it before packaging. Product that does not pass inspection is automatically removed from the line. Source: The Modal Shop Inc.

A Sound Approach to Product Testing

To understand how the RAM NDT unit works, consider the operation behind a bell or tuning fork. When either instrument is struck, it vibrates, emitting a sound. An instrument that rings true produces a consistent sound, and this consistency in sound reveals the structural integrity of the instrument. That is the basis for RAM NDT technology.

When struck by a tiny anvil, components such as gears emit a natural frequency as part of their structural response. This unique and measurable signature is then compared and analyzed against both good and bad product. Just as a cracked bell will not ring true like a structurally sound bell, components can be tested in the same manner. If a gear is cracked, lacks the correct density or misses other characteristics of a structurally sound product, the flaw will be exposed when their signature deviates from what has been identified as good product.

The RAM NDT unit tests the whole part for both external and internal flaws and provides an objective, quantitative analysis that eliminates errors involving human interpretation and judgment through the use of sophisticated equipment. A dynamic sensor captures sound and a high-speed analog to digital convertor translates the sound into measurable data. Because a defective part will shift in its structural resonance, this shift is identified when compared to pre-defined data. In effect, the RAM NDT listens to the structural response of a part and evaluates it against the statistical variation from a control set of good parts in order to screen defects.

The criterion used to represent pre-defined data is established by way of templates. The resonant signature of both good and bad product is captured in order to provide objective measuring variables for comparison. After these templates have been established through up-front programming, the RAM NDT is a self-regulating unit that involves little maintenance and eliminates the need for a trained operator.

“We’ve incorporated the RAM NDT unit right on our assembly line,” says Slattery. “It doesn’t slow our production one bit. Components pass through it right before packaging. Any product that doesn’t pass inspection is removed from the line automatically. And gears that pass inspection are immediately packaged and shipped in a streamlined operation.”

Furthermore, the RAM NDT is suited to plant-floor, high-volume test applications. From the rugged microphone and industrial electric impactor to the National Electrical Manufacturers Association (NEMA) digital controller, the unit has been designed for industrial environments.

In its 100,000-square-foot facility staffed by 230 employees, Capstan Atlantic produces a variety of structural parts for the automotive industry. Source: The Modal Shop Inc.

On-Pitch Benefits

For Capstan Atlantic, the RAM NDT not only contributes to peace of mind, it also contributes to the bottom line. According to Slattery, the RAM NDT system saves the company 33 cents on each gear produced over the previous method. And because the company ships nearly 5,000 gears every day, that amounts to a total savings of $1,660 per day. Furthermore, because the unit eliminates the need for a specially trained operator, it also reduces personnel costs.

“When I go home at the end of the day, I don’t spend the night worrying about whether or not a gear slipped through our quality system and was shipped to a customer because I know every single gear produced on our line has been thoroughly and completely tested. It’s hard to put a price tag on that,” says Slattery. “But it is easy to figure the consequences involved in a field failure. That’s every manufacturer’s worst nightmare.”

Although there was an initial learning curve, after employees at Capstan Atlantic mastered the new system, they found it relatively easy to use. Also, the data captured by the RAM NDT provided feedback that contributed to other quality improvements. “We discovered that, additionally, the unit has enabled us to recognize things about our process and make improvements upstream that have increased our product yield by reducing process variation,” says Slattery.

The Modal Shop Inc.
(800) 860-4867


- With the RAM NDT unit, Capstan Atlantic estimates a savings of $1,660 per day.

- The RAM NDT unit provides Capstan Atlantic with 100% inspection of its gears.

- Data captured by the RAM NDT unit provided feedback that contributed to other quality improvements at Capstan Atlantic.