Gearing Up for Greater Efficiency
More gears made faster and with higher quality are needed to meet upcoming fuel economy and emission regulations
By James D. Sawyer
This is something of a golden era for those who provide the machines that manufacture gears for automotive transmissions. Not only is auto production increasing in the Western Hemisphere and the BRIC countries, but—at least in North America—the number of speeds in the average transmission is proliferating (even as the number of cylinders in the average engine is shrinking). This is because of impending federal mandates on fuel economy and emissions.
In short, the US has gone from a time when engines had six or eight cylinders and transmissions had four or five gears to one in which the average engine will have four cylinders and the average automatic transmission—the most popular choice of consumers—will have eight (or more) gears.
To illustrate the current demand for gears, Hyundai subsidiary Powertech America (West Point, GA) is set to produce approximately 650,000 automatic transmissions this year. It will do so with fewer than 500 employees, including administrators, and a high level of automation.
With such demand, the need to produce transmission gears quickly and efficiently is great. And because these transmission designs will need to fit more gears in a case no bigger than the cases currently used, the gears will be smaller.
Quality is Even More Important Now
“This means,” said Karl Schaeferling, Director Product Management—Honing, Shaving & Chamfering Solutions of Gleason Corp. (Rochester, NY), “that the torque density for the gearboxes will be higher compared to the past. This implies a higher quality of … gearboxes in general and, of course, for the gears, too. The use of fine finishing processes will increase, specific gear flank modifications will be increasing and finally the surface quality has to be higher.
“In general, the need for gear grinding and gear honing will increase to fulfill these tasks.”
Peter Loetzner, president and CEO of EMAG LLC (Farmington Hills, MI), agreed that there is a need to increase precision: “The higher fuel economy requirements drive the need for more gears or gear combinations. The quality and precision of each gear component is directly linked to the subsequent fuel economy of the vehicle. Plus the transmission has not become any larger, requiring more precise components to do the job. We definitely see a tightening of the specs and tolerances in gear manufacturing in the US following a trend we have been seeing in Europe for quite some time.”
There are other ramifications of the downsizing, according to Walter Friedrich, president of German Machine Tools of America (GMTA; Ann Arbor, MI), and Scott Knoy, GMTA vice president of sales.
"If indeed gears and gearboxes get smaller, tools and fixtures will get smaller, too,” said Friedrich. That likely would mean smaller machines and possibly reduced costs.
“Conversely,” Knoy, said, “the smaller gearboxes have to be robust and perform at higher levels than the older, larger boxes as far as durability, noise and length of service, so advanced steels will be used for these new gearboxes and the development of the metal products will result in increased tool costs to machine the exotic metal. New cutting tool steels will be required, which drives R&D cost as well as the increased cost of these new tool steels. Additionally, the new cutting processes will require stiffer and more rigid machinery that can handle the high torque and vibration associated with machining these new varieties of metal. These changes will also be necessary on the fixtures and tooling used to hold and handle these new smaller gears.”
The levels of precision and accuracy are not something with which gear machining manufacturers are unfamiliar.
“For a long while,” said David Goodfellow, president of Star SU LLC (Hoffman Estates, IL), “gears ground for accuracy have been prevalent in aerospace, turbine, and with marine propulsion gears. With the onset of eight and nine-speed transmissions running at much higher rpm, power density and sound issues have the automotive manufacturers calling for hard finishing.”
Goodfellow sees other trends in automotive gear production such as “carbide hobbing, higher speed machining and honing.” He added that the new generation of gears “will be more challenging to workholding fixtures because the smaller gears will be more difficult to clamp and maintain stiffness while maintaining high accuracy and productivity.”
An Emphasis on Throughput
Productivity is a topic that comes up often in discussing automotive gears.
“Automotive manufacturers will always prefer to hob a part if possible” to initiate the making of a gear, said GMTA’s Knoy. “This is due to the speed of the process and the tool life associated with the hobbing process.” GMTA, however, offers an alternative from Profilator, which uses a process called scudding.
“Scudding,” added Friedrich, “is a process very similar to hobbing, but it uses shaper-cutter-style tools. Therefore scudding does not have the interference limits as a hob cutter, which allows for many more applications.” The advantage of scudding compared to shaping is that it reduces cycle time and allows dry cutting, he said.
“As a general rule,” Knoy added, “if a part can be hobbed or shaped, we can scud it.”
GMTA is not the only company to offer such a process.
“Profilator makes the only scudding machine,” said Knoy, “but others are developing similar processes and are using the name gear skiving. Gear skiving is widely known in the industry as the process of recutting a gear in the hardened state with a special skiving hob. Profilator has a two to three year lead on the development of this scudding process when compared to other companies.”
Quicker Grinding Times
Andreas Mehr, technology development engineer at Liebherr-Verzahntechnik GmbH (Kempten, Germany), said that his company’s customers are looking to improve their grinding times while maintaining or improving quality. If gears can be ground more efficiently, he noted, “for example with dressable CBN tools,” or if grinding a large module—instead of profile grinding—can be generated, “then these customers will have a big productivity advantage over their competition.”
In addition, Scott Yoders, vice president of sales at Liebherr’s US operations (Liebherr Gear Technology Inc.; Saline, MI), said that chamfering and deburring systems integrated within gear hobbing machines “have been expanded upon by Liebherr to include separate parallel-processing stations” for both chamfering and deburring that do not increase total cycle time.
As well as using higher speed machining, carbide hobbing and honing to increase productivity, Star SU offers a holistic approach to gear making. “Improvements come from the ability to understand machines, tools, applications and services as a completely integrated process,” the company’s Goodfellow said. To that end Star SU offers what it calls “Total Life Cycle Management, which includes resharpening and recoating of precision cutting tools, as well as maintenance services of gear related machinery.”
Is there a Material Difference?
While the companies are unanimous in seeing precision and increased productivity as the drivers of gear making today, there is little consensus on what—if any—impact the new generation of transmissions will have on the materials used to make gears.
Gleason’s Schaeferling, for instance, does “not see a clear trend for different steel materials [in gears] because manufacturers do not want to incur the higher costs of higher alloying.” He added that the size and shape of gears is not undergoing any remarkable changes “and therefore the existing solutions are a good basis” for the future. Schaeferling does, though, see the “constant development of new materials to increase the capabilities of tools for precutting and finish cutting operations.”
As noted above, GMTA’s Knoy sees the need for new materials for both tools and gears.
EMAG’s Loetzner views the issue of gear material much the same as Gleason’s Shaeferling: many automakers tend to be conservative and would prefer to stick with the tried, true and inexpensive unless faced with a compelling reason to change.
The ‘Gear Factory’
Loetzner believes EMAG has that compelling reason. It is a multiprocess machining line nicknamed the gear factory. An improved gear material is required because induction hardening is incorporated into the inline manufacturing process, allowing for streamlined material flow because the workpiece is not taken out of the workflow.
The gear factory incorporates all essential processes, from green turning to hobbing, to hardening, hard turning, laser welding and grinding. The first machine element is known as a pendulum machine with two spindles. The term pendulum is used because the turret “swings” between the two spindles. A blank is loaded in the first spindle and the turret machines it, Loetzner said, while the second spindle unloads and loads a blank. Once the blank in the first spindle is complete the turret moves in less than a second to the second spindle and machining begins there while the first spindle unloads and then reloads. Loetzner said this is the fastest process for machining in the green stage.
“The chip-to-chip time is below a second,” he said. “That is the initial throughput improvement.”
Automation moves the green turned part to the second machine element, a traditional hobbing machine. The third stop, Loetzner said, “is the induction hardening and quench unit and then [the workpiece] goes into a hard turn grind process, or a laser welding module, which allows you to put on synchronizing gears or other secondary components that are required, and after that you can add another turning unit. So that whole line allows the gear manufacturer to make gears in a very small footprint in a very flexible manner.”
According to Loetzner, the gear factory takes up just one third the space of a more traditional process that requires off-line hardening and annealing ovens. “The process,” he said, “significantly reduces the investment for the gear manufacturer and is pretty much cycle-time neutral.”
While the new process does not directly increase throughput, said Loetzner, by eliminating the offline hardening steps—removing the gears from the machine tool so the gears can be hardened and then returning them for further machining—the process does remove non-value-added time from the gear making process.
“It is,” Loetzner said, “a technology that does require a change in gear material. Some Asian automakers are using this technology now. We believe the industry as a whole will move toward this technology.
“The change in materials does not require a change in the machine tools used or in the machining technique. Tool life is the same. Once the transmission designers catch up to it and design to manufacture with this option in mind, it’s just a matter of time before the gear factory will drive cost savings in manufacturing, through value stream improvement and reduction in floor space requirements.”
Gleason also has an innovation that provides solutions across a spectrum of gear manufacturing. In October 2012 the company and Smart Manufacturing Technology (SMT; Nottingham, England) announced a global strategic partnership to provide gear manufacturers a complete design to manufacturing system. This seamlessly integrates SMT’s Masta system design and analysis software with Gleason’s software for gear design and manufacturing, which is called Cage. The partners say customers will benefit from having a fully integrated workflow when designing not just automotive transmissions but also other types of gearboxes, powertrains and other devices.
Not Just for Automotive
The integrated package provides a system design loop between the system design and simulation and system testing capabilities of Masta and the gear design and analysis, manufacturing and gear testing solutions of Cage. In addition to automotive applications it is well suited to other industries that use gear-shaft-bearing systems such as aerospace, marine, industrial and energy production, including wind turbines.
The first release of the integrated software is scheduled for June 2013.
Looking at the big picture, GMTA’s Friedrich said, “More gears mean more production, which means more machinery and capital equipment. The challenge for us is to reduce cycle time in order to minimize the total number of machines required.”
And while the driver of this is the trend toward greater fuel economy, it is not necessarily tied solely to smaller engines. Transmissions with a large number of gears also allow larger, more powerful engines to be more efficient.
“Even if fuel consumption is discussed in many ways,” Schaeferling of Gleason said, “the market for powerful cars is not shrinking as might be expected. In fact, it is growing.”
He does not believe that gear proliferation will continue much longer.
“The maximum number of speeds for manual transmissions will be six or seven,” he said, “and for automatic transmissions the maximum will be eight or nine speeds. For emerging markets it will be two speeds lower.”
How Many are Enough?
Stefan Sommer CEO of transmission maker ZF Friedrichshafen (Friedrichshafen, Germany), believes nine forward speeds is the natural limit for automatic transmissions. He said that going beyond that number increases transmission weight and adds complexity that outweighs whatever fuel economy advantage extra gears might bring. The first application of ZF’s 9HP transmission will be in models of the Range Rover Evoque with four-wheel drive.
“There is no hard line, but you have to consider the law of diminishing returns. The question is whether adding even more gears makes sense,” he told the Automobilwoche Congress in Berlin in November 2012.
Yet even before he spoke a press report claimed that Hyundai was developing a 10-speed automatic. Also, a month before the Berlin event word circulated that Ford and General Motors had signed a memorandum of understanding to develop nine and 10-speed gearboxes. The two US automakers had previously collaborated on six-speed automatics, and GM has confirmed that the companies are holding discussions.
Whether the proliferation beyond nine-speeds is truly beneficial in providing improved fuel efficiency or whether it is a marketing ploy for car companies to brag that “Mine’s bigger” remains to be seen. ME
This article was first published in the April 2013 edition of Manufacturing Engineering magazine. Click here for PDF.