When manufacturers run into machining challenges, a typical first response is to call the tooling supplier to figure out why tool life, part quality, or machining speeds aren’t where they should be. But sometimes, the tool isn’t the problem.
Michael Burnett, a technologist at TimkenSteel, says a customer making finished ring gears for automatic transmissions recently approached the company trying to figure out what was wrong with the broaching bars that it was using to cut teeth into the inner diameters (ID) of gear blanks. Instead of getting the typical 1,500-to-2,000 pulls for each of the expensive broaching tools, it was only getting 500 pulls.
“Broaching is a very efficient way to cut gear teeth, but it can be expensive. The broaching bars are 6ft to 7ft long and can cost $80,000 to $100,000 per bar,” Burnett says. “You need to get the longest tool life out of those bars as possible to keep your per-part costs down.”
After looking at the customer’s tools, machines, and processes, Burnett’s team found the culprit was the steel blanks.
“They were using our material, but it wasn’t optimized for the task,” he adds.
After optimizing the material heat treatment that TimkenSteel had been supplying for an alternative that was better suited for the broaching process, the customer started getting 3,000 pulls from each broaching tool. Per-part costs on the finished transmission ring gears went to $2 from $6.
Specialty material opportunities
For Raymond V. Fryan, vice president of technology and quality at TimkenSteel, it was an Aha! moment. The supplier of highly specialized materials, often tailored to specific tasks, had an opportunity to provide value to a growing segment of the automotive industry.
“The cost of tooling was exceeding the cost of gear-blank supplies. When we saw that opportunity to lower tooling costs, we dedicated more resources to the problem,” Fryan says.
Staring down federal mandates to achieve 54.5mpg by 2025, automakers are slashing weight from vehicles wherever they can and exploring powertrain options that demand higher-precision parts. Both trends favor the sorts of specialty steels developed by TimkenSteel and its competitors. On automotive bodies, companies are cutting steel volume by selecting higher-strength materials to maintain structural performance with less weight. In powertrain side, they’re cramming 8, 9, and even 10 speeds into automatic transmissions. Those multi-speed gearboxes are typically the same size, if not smaller, than the 5- and 6-speed models they replace, so each gear has to be smaller and more precise to get the work done.
“To get that level of precision, you need higher-end, specialty steel that can handle more stress” in a smaller package, Fryan says. “With the drive for component lightweighting, our clean steel is a power-density enabler.”
Becoming a partner to powertrain component manufacturers, he adds, requires TimkenSteel to understand the capabilities of its materials and the process needs of its customers.
When gear makers approached TimkenSteel to find ways to extend broach-bar tool life, Burnett’s development group built a small-scale test rig to simulate the process. The lab broaching machine uses a tool with three teeth to cut gear shapes into the ID of rings at the company’s Canton, Ohio, technology center.
“We use this to understand how different variables affect the life of the broach bar,” Burnett says. “To do this in the plant with production tooling would be prohibitively expensive.”
A key discovery was that broaching difficulty had little to do with the hardness of the gear blanks. Customers and engineers had assumed that the material was too hard for the broaching bars and a softer alloy was needed. But Burnett says using lower strengths also can lower the performance of the finished gears, so that wasn’t a preferred solution to the tool-wear problem.
“Going softer can get more pulls on a bar, but you can’t go too soft or the material gets gummy and can clog the broach teeth,” Burnett adds.
After testing, Burnett’s team recommended an alternative heat treatment that had the same strength ratings but a modified microstructure. He notes that as TimkenSteel engineers studied the process, they found that microstructure was a major factor in broaching operations, more important in many cases than hardness.
Fryan says it’s that kind of result that makes it important to work with materials suppliers as closely as manufacturers work with tooling suppliers.
“It was counterintuitive that you could have two grades with the same hardness with very different tool lives,” Fryan says. “But it’s that kind of expertise that’s really opened up design flexibility for our customers so they’re not making the same compromises they were 10 years ago.”
Future automotive gears
The drive toward fuel economy has many automakers pushing the capabilities of each component, forcing manufacturers to increase precision and strength for every part. E. Buddy Damm, a scientist in TimkenSteel’s Advanced Steel Solutions division, says designers are putting higher loads on mechanical parts, making it vital that they understand the limits of the materials. His group uses computer simulations and other tools to study how much stress gears made from its steel can take before they fail.
“We can take the geometry of a gear tooth and model the stress of that tooth under load. We can then simulate inclusions [impurities about 10µm] in the steel to model the performance of different levels of steel cleanliness,” Damm says.
A typical gear failure occurs when a tooth breaks off of the main body, reducing contact with other gears and creating a loose piece that can jam the system. A broken tooth typically starts as a small crack that can be traced back to an inclusion in the steel. That means it’s important for TimkenSteel to be able to identify how many inclusions are in the steel so it can predict the likelihood of a crack and how the material will react if a crack forms.
“Designers can choose steels that won’t allow cracks to grow at desired load levels,” Damm says.
Fryan adds that specialty materials allow designers to improve performance without going through complete system redesigns. For example, an automotive manufacturer could spec a smaller, turbocharged 4-cylinder engine to replace a V6, running the engine and transmission faster to make up the displacement difference. That would put more stress on transmission gear sets designed for a traditional 4-cylinder engine; but, if the company replaced the older gears with cleaner steel, cogs would be less likely to fail. The manufacturer could use an existing transmission design with upgraded materials instead of going through prove-out for a new system.
“Lots of great engineering work has gone into improving sheet steel to cut vehicle weight. Engine and transmission steel components are behind body-in-white components in getting those upgrades,” Fryan says. “That’s where we see opportunity in the future.”
About the author: Robert Schoenberger is the editor of TMV and can be reached at 216.393.0271 or email@example.com.