Customer expectations for the durability and comfort of cars, vans and trucks are always increasing. But, there’s a spectrum. Manufacturers must strike a balance between producing vehicles with the highest possible quality and vehicles that are affordable and profitable. Consumers want vehicles that are reliable and nice to be in, but they will also weigh price against features until they find the level of performance and quality they are willing to pay for.

Sound is one aspect where this dynamic comes into play. In some cases, manufacturers may spend a lot of money to ensure their vehicles produce a specific sound—the growl of a sports car, for example. In other cases, manufacturers strive for the quietest interior they can get.

When manufacturing automotive transmissions—or any product, for that matter—engineers must strike a balance between price and performance. Photo courtesy Volkswagen

“You may want to hear the engine, but nobody wants to hear the transmission; it’s annoying to the customer,” says Nils Krohn, Ph.D., head of acoustics quality assurance at the Volkswagen Group.

Manufacturing quiet, high-quality transmissions is the job of VW’s assembly plant in Kassel, Germany. With 15,500 employees from 52 nations, the plant is the company’s largest, most diverse production facility for gears and transmissions. The factory makes transmissions from start to finish: from milling the gears to final assembly.

It produces some 3 million transmissions and electric drives annually. In addition to producing parts and assemblies for its own brands—Volkswagen, Volkswagen Commercial Vehicles, Seat, Audi, Skoda, Lamborghini and Porsche—Kassel also supplies components for third-party customers.
 

Gears and other transmission components are machined to strict tolerances. Photo courtesy Volkswagen

Testing to Increase Productivity

Gears and other transmission components are machined to strict tolerances. But nicks, surface ripples and other flaws in the parts are unavoidable, and tolerances can be exceeded due to tool wear. With such high production volume over the course of a single day, even a small percentage of substandard gears or transmission assemblies is a substantial number that must be reduced to an absolute minimum. This makes testing, finding the causes of defects, and predicting tolerance errors critical for quality.

At VW, testing has become more rigorous and fine-tuned over the past several years, and the success rate for completed transmissions is better than it used to be. The reason for that, according to Krohn, is the company’s emphasis on testing for the right reasons, establishing a control loop, and identifying problems earlier and earlier in the process. 

Reworking a single gear pulled off the production line is one thing. Pulling a gear out of an assembled transmission is costly, but not the end of the world. However, pulling a faulty transmission from a completed vehicle is expensive and unacceptable. A faulty transmission reaching the customer is even worse.

To prevent any of that from happening, VW’s testing process is focused on identifying durability issues and ensuring customer acceptance. Durability issues, such as nicks and surface ripples in gears, can lead to a failure in an assembled transmission.

According to Krohn, gears and transmissions that function perfectly and produce insignificant noise can be made, but they would not be cost-effective. On the other hand, noisy “vibration factories” jammed into the drivetrain could be easily be made for little more than the cost of materials. But would they even be worth that cost? Somewhere in-between those two extremes are reliable, unobtrusive and affordable transmissions. This is the conundrum VW engineers face on a daily basis.
 

VW’s assembly plant in Kassel, Germany, produces 3 million transmissions and electric drives annually. Photo courtesy Volkswagen

The Control Loop

Establishing a control loop with testing for each transmission family is the key element. VW’s basic control loop comprises cycles of testing, comparison and evaluation, refining test parameters, and retesting throughout the developmental stages of the production line.

To get the job done, VW uses the DISCOM noise analysis system from Hottinger Brüel & Kjær (HBK), a manufacturer of test and measurement technology for noise, vibration and harshness (NVH); durability and fatigue; electrical power; reliability; asset and process monitoring; and weighing. HBK has been working with VW for many years.

The DISCOM system consists of transducers, data acquisition hardware and software for NVH testing. At VW, the DISCOM system is used both to test gears at the end of their production lines and also to test the completed transmissions at the end of their lines. The end-of-line transmission test simulates vehicle conditions and uses order-synchronous resampling, so the noise sources from different rotors inside the transmission can be separated.

The DISCOM system consists of an industrial PC that has been enhanced with a unique USB-based data acquisition system. Two key elements of the system are a parameter database and a results database. Due to the order-synchronous resampling process, a root cause-analysis of the production defect is possible. The result database and associated tools in the DISCOM system help develop predictions on tool wear and related tolerance mismatch problems.

When creating a new production line, the first task is producing a preliminary group of around 20 transmissions. Krohn’s team worked with HBK to analyze data and find identifying characteristics. This provides a baseline to identify potential defects on the line.

For example, a seven-gear transmission (six forward and one reverse) would typically have around 700 characteristics (two speed ramps per gear, which works out to 50 characteristics per test step or ramp). Examples of characteristics include spectral values, energy values and order tracks.

Once a baseline is established, the completed transmissions are put in vehicles and tested using a mobile test system looking for correlation with the test stand. The data and drive feedback are used to refine the test limits. This is an iterative process that leads to a much more refined set of limits for the next trip through the end-of-line test stand.

The next step in the process is the pre-series. Here, the major goal is to find a subset of those characteristics that are of specific relevance for the current transmission model and where the limits are most important. In the end, out of those 700 characteristics, the VW team will identify around 200 that are relevant to the particular gear set and transmission. This process is a more refined version of the pre-series end-of-line test and mobile test, where they will continue to adjust parameters and limits, looking for that best-fit for durability, customer acceptance and cost-effectiveness.

A transmission undergoes function testing at the end of the assembly line. Photo courtesy Volkswagen

Production line testing is the final step. In practice though, the production line is just one long stage in the test-loop refining process. All of the data for each gear and transmission test, both pass and fail, are stored and available for retrieval for later reference and analysis. This aids in developing predictions concerning tool wear (which can lead to surface ripples) and identifying root causes for faults that can be eliminated in the future.

When machining and material issues can be found early in the process, they are not so expensive to deal with. The testing process developed by VW and HBK doesn’t just weed out defective transmissions, it improves customer acceptance and cost effectiveness by stopping the vast majority of problems before they can become increasingly expensive.

For more information on noise, vibration and harshness testing, read these articles:
System ‘Crushes’ Force Measurement Application
Quiet Please! NVH Is Becoming More Important Than Ever
Motor Manufacturer Opens State-of-the-Art NVH Testing Lab