Automotive Gears and Their Opportunities and Challenges

Automotive Gearmaking
Automotive Gears and Their
Opportunities and Challenges
Business is booming, but that
doesn’t mean it’s simple or easy
Bruce Morey
Contributing Editor
I
t’s a good time to be supplying the automotive powertrain sector,” said David Goodfellow,
CEO of Star SU (Hoffman Estates, IL), reflecting the impact of record setting vehicle sales in
2015. Gears are Star SU’s business. The company provides cutting tools, machine tools, and
servicing for practically any industry needing gears.
“About 50% of our business today is supplying the
automotive powertrain sector,” he said, not including off-highway customers.
While business is good, it does not mean it
is easy. “The trends in the automotive sector are
Machines such as those
available from Mazak that cut
gears as part of multitasking
machine are ideal for low to
medium volume applications,
according to the company.
achieving higher performance and better fuel
economy,” he explained. He has observed high
horsepower engines in the 300- to 500-hp range
mated to transmissions boasting up to 10 speeds.
More speeds translate into better fuel economy
by allowing the engine to operate at its most efficient load point more of the time. There is also
an increasing demand for all-wheel drive, which
means more transfer cases with even more gears.
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Competitive pressures require all these
gears to be quiet and robust over a long
time. That puts pressure on machine
tool makers to deliver machines that cut
gears faster to an almost unbelievable
tolerance level.
A good example are automatic transmissions. These often use planetary
gear sets, composed of a set of ring,
sun, and pinion gears. “We’re producing some of these pinion gears in four or
five seconds today,” he said. “Ten years
ago, they were being produced in a
minute or two.” That translates into obvious savings for the customer, requiring
fewer machines. Faster cutting means a
much more rigid machine tool combined
A gear that has just been hobbed (cut) on the Kashifuji KE201 Gear Hobber,
available through Involute Gear & Machine.
with efficient automation and associated gaging. “Robust, noise-free gears
mean cutting them to a very tight tolerance,” he said. Where
even just a few years ago the industry spoke of tolerances in
the thousandths of an inch, today it is expected to be 3–5 μm,
according to Goodfellow. “That is a pretty dramatic change in
the fundamentals of gearcutting,” he said.
Process Changes, Quality Gears Made Cleaner
Another dramatic change in the process itself is the move
away from first hobbing a gear, then shaving it and then heat
treating it. Hobbing is a multipoint cutting process that rotates
both tool and workpiece in precise relations to each other.
It is used to rough cut multiple gear teeth at once. “Today,
automakers are hobbing gears, heat treating them and then
grinding them post-heat treat. This provides a harder, more
robust gear. Post-heat treat finishing of these gears eliminates
distortion or changes of geometry in that gear set. The tolerance requirement after grinding is pretty critical,” he said.
The push for environmentally friendly factories is adding yet
another challenge for machine tool builders—eliminating potentially toxic coolants. “Nowadays, everybody wants to machine
dry to get rid of oil, contamination, smoke, and the potential
harm to operators as well as simply keeping a clean shop floor,”
Goodfellow said. Towards that end, Star SU partner Samputensili (Bentivoglio, Italy) introduced a new dry grinding machine
in October, 2015, in response to the pressure to eliminate the
mess and bother of coolants while grinding after heat treating.
Called the SG160 Sky Grind, it features two spindles, one for
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Automotive gearmaking
skiving hobbing and the other for generating grinding. Generating grinding is becoming a common
process for gears post heat treating. The grinding
tool resembles a worm gear with a grinding coat.
Like hobbing, both the tool and the workpiece rotate
rapidly in precise relation to one another.
Speed and Cost
“Cost is always a concern with automakers,” said
Loyd Koch, vice president and founder of Bourn and
Koch (Rockford, IL), citing another challenge amidst
the boom. Bourn and Koch supplies a wide range
of machine tools under 25 different brands, including gear shaping machines. “Each time they add a
speed to the transmission, that translates into them
buying a lot more machine tools and may require
setting up a new plant to make those transmis-
GMTA provides accurate gear cutting through their trademark scudding
process, described as a cross between hobbing and shaving by the company.
sions,” he said. However, he stressed that even after a contract
requiring incremental year-over-year reductions. “They figure
is awarded, there is usually a cost improvement rate written in
once you get out of the engineering stage and into production,
This is what power, precision,
and innovation looks like.
Over fifty years in the industry and 7,000+ items to show
you should be able to get better tools, better tool materials,
better coatings, and faster machines. We try to accommodate
them by building ever faster machine tools,” he said.
Star SU is a supplier of the cutting tools used by gear
shapers like the Fellows 10-4 that Bourn and Koch supplies.
“From our standpoint, the shaping end of it, they are looking
for high speeds, but the tools have a big impact on that. It
does not make any sense for the machine to go faster than
the tool can tolerate. As tool life improves, then that pushes
the speed of the machines,” explained Koch. To make
advanced machines that can drive the improved tools, he
explained, it is important to have linear motors replace ball
screws. Using advanced CAE tools to provide the lightest yet
strongest machine movements is just as vital. “It is important to not overdesign gear shaping machine tools. That is
especially true with linear motors. Those devices are fast and
they do not like having to move a lot of weight,” he said. He
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with honeycombs and special shapes to reduce weight.
Lighter Gears, Better Fuel Economy
Most of the original processing methods and innovations for
gear manufacturing date to the early 1900s (or before), according to Scott Yoders, vice president of sales for Liebherr (Saline,
MI), including the invention of hobbing, shaping, and skiving.
However incremental improvements and attention to detail can
have profound impact on the quality of automotive transmis-
sions. As noted above, improved tooling and grinding wheels
included,” he said. He noted that the method was developed in
have made some processes economical only recently. Other
conjunction with LMT-Fette, with its ChamferCut tooling solu-
techniques seem truly revolutionary and Yoders believes now is
tion, highlighting the need for a cutting tool that can keep up
the time for automotive to consider them.
with machine speeds.
One example is lightweighting. “In recent years, to reduce
weight, manufacturers have reduced the face width of their
transmission gears,” he said. This makes the chamfer on the
Fuel Economy with Better NVH
While engineered chamfers are important for weight,
teeth ends, that runs the length of the involute, especially critical
Liebherr has also developed a new gear grinding technology
as manufacturers reduce the width of the face. The common
that improves fuel economy and NVH directly. Gear manufac-
way of producing that chamfer was to plastically deform the
turers are increasingly designing asymmetric gear teeth—dif-
sharp edge with a roll/press process. “A better way to engineer
ferent pressure angles from left-flank to right-flank. To the
that chamfer is through cutting, but that can eat into cycle
layman’s eye, these appear to be misshapen examples of
time without the right process. Lost seconds are real money in
gears, but in fact account for different loadings in power
automotive,” he said. Starting in 2014, Liebherr began offering
transmission on one flank of a tooth compared to the other;
a method of both hobbing and precisely cutting the end relief
drive-flank compared to coast-flank. “This increases load
chamfer simultaneously while roughing the gear tooth. Offered
carrying capacity while reducing NVH,” said Yoders.
on the LC ChamferCut series of machines, Yoders describes
The challenge, according to him, was developing a gen-
this as a parallel operation. “We believe it is five times cheaper
erating grinding process that specifically addressed dressing
than the roll/press method when all tooling and expenses are
of the grinding worm and the complex machine movements
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Automotive gearmaking
to accurately grind asym-
shorter face widths (i.e.,
metrically. “We believe we
lighter weight) but still carry
are the first to market to do
the same loads.
this for asymmetric gears,”
Perhaps the most
he said. Available on the
exciting new technology
Liebherr LCS- and LGG-
for addressing NVH is what
platforms, he also noted
Yoders describes as Noise
that these machines can
Excitation Optimization, or
also provide the completely
NEO, developed in a part-
new development called
nership with the Technical
generated end-relief (GER).
University of Munich “This
Until now, end relief in gear
is similar to noise cancel-
grinding was only available
lation in that we grind into
with the much slower profile
the teeth a specific wave-
grinding method, where
length and amplitude of
each tooth is ground individually. The GER grinding
has a similar effect—gears
can now be designed with
Gear manufacturers are increasingly designing asymmetric
gears, allowing higher loads with less NVH. Liebherr developed
a generating grinding process that solves the complex machine
movements needed to grind such gears.
the form error, both in lead
and profile direction on the
gear tooth,” he explained.
This introduces, on pur-
pose, a defined and engineered “waviness” in the profile form
deviation (commonly defined as ffα) and lead form deviation
(commonly defined as ffβ). Yoders presented audio data in
an interview with SME that dramatically demonstrated a gear
with significantly less gear whine after NEO correction.
Other gear manufacturing equipment suppliers have
noticed the increasingly stringent technical requirements in
automotive. “I think if you were to speak to an automotive
engineer about transmissions, they would say the difference
between the quality of the transmission [today] and one from
seven or eight years ago is something like 500%,” said Scott
Knoy, vice president of sales for German Machine Tools
of America (GMTA; Ann Arbor, MI). A unique process from
GMTA that helps provide that accuracy is their trademark
scudding process.
What is scudding? “Think of it as a cross between hobbing and shaving,” explained Knoy: A continuous generating process using a multipoint tool that eliminates issues
such as the spacing error known as drop tooth. “There are
no idle strokes on the machine tool, as you get with the
gear shaping process,” he said. He also noted that it probably will not completely replace shaving, since it requires
a cross-axis angle that prevents the process from getting
as close to a shoulder as shaving. “It is especially useful in
increasing the quality of the green machining, so that after
heat treating you do not have to do as much honing or
grinding,” Knoy said. “Presenting a more accurate part after
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heat treat keeps costs down.” Knoy also
hinted that hard scudding might be available in the near future, replacing a grinding
or a honing operation after heat treating and
further reducing costs while producing higher
quality, lighter gears.
There are a number of applications where
it could be the process of choice, including ring gears, sliding sleeves, and annulus
gearing. Synchronizer parts and hubs are
also ideal, according to Knoy. For internal ring
gears, he notes that a special advantage of
scudding is in making lead corrections along
the face or flank of the tooth. “You can crown.
You can taper. This is something that the industry hasn’t been able to do with broaching,
which was the major way of making internal
ring gears for 40 years. In some instances, we
are replacing broaching,” he said.
The Fellows 10-4 Gear Shaper available through Bourn and Koch boasts an
infinitely variable stroking range of 50 to 1300 strokes/minute.
Special Applications
The search for a quality finish is, in some cases, leading automotive manufacturers to look at honing after heat
treating and grinding gears, or as an alternative to grinding.
“We are seeing some desire for honing after gear grinding,”
explained Rodney Soenen of Involute Gear and Machine
(Chesterfield, MI), though he observes it is more popular
in aerospace. Involute is a provider of gear hone tools and
accessories, as well as a distributor of gear hobbing and
gear inspection machines. “We have seen some companies
experimenting with honing a shaved gear to increase quality,
or in some cases to repair gears. They may have a thousand
gears that perhaps for whatever reason were slightly below
expectations” and honing brings them back into compliance,
he said. He also noted that honing is popular for finishing
pinions and sun gears, critical and numerous gear parts in
automatic transmissions. “The honing process can be very
fast, depending on the style, and improved surface finish correlates to increased durability,” he explained.
Machines that provide multitasking operations is another
niche area of gear and spline cutting. “Multitasking is aimed
at low- to medium-volume production,” explained Mike Finn
development engineer at Mazak Corp. (Florence, KY). Multitasking machines that power skive gears and splines using
the synchronized rotation of a multitasking machine’s milling
spindle and C axis, as is done on some Mazak machines,
offers a lot of flexibility for low-to medium-volume production
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runs. “This would be useful for building prototype parts and
titasking machine that power skives a gear one day can ma-
aftermarket applications, as well as off-road equipment,” said
chine completely different, nongeared components the next.
Finn. “We successfully did that with crankshaft prototypes
The same machine can also cut the part’s mating surfaces on
many years ago and the same mind set could be made
the same machine that cuts the gear teeth.
towards gearcutting.”
For its skiving process development,
Mazak offers its Integrex Series MultiTasking machines, according to Finn.
“Many machines can cut gears, depending on the gear type, but our most
popular models are the Integrex series of
The Ultimate Multitasking Mill/Turn Centers
machines, ranging from the Integrex 100,
200, 300, 400, and e420,” he remarked.
“These cut gears and splines from 1 to
12" [25.4–304.8 mm] in diameter.”
Mazak is currently developing cutting
processes for power skived gears and
splines of all sizes and types, both OD
and ID in either straight and angled teeth
patterns, said Finn. “These machines
allow for easy part and tool changeover
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Designed with four machining stations and two dedicated
loading/unloading stations, the ICON 6-150 is equipped
with a six position table, it shuttles pallets to four
machining modules for fast, ultra-precision cutting.
cross-axis angles,” he said. He noted
that flexibility is the key advantage of a
?
multitasking machine. The same mul-
The ICON 8-150 has six machining stations and two dedicated loading/
unloading stations. The eight position table shuttles the pallets to the
machining modules. The 8-150 was engineered for maximum flexibility
in a small footprint.
Bourn & Koch
815-965-4013 / www.bourn-koch.com/
GMTA
734-973-7800 / www.gmtamerica.com/
Involute Gear and Machine Co.
586-329-3755 / www.involutegear
machine.com/
Liebherr
734-429-7225 / www.liebherr.com/
Equipped with four machining stations, two dedicated loading/unloading
stations, and simultaneous vertical and horizontal axis capabilities, the
ICON 6-250 was developed around flexible machining principles.
Mazak
859-342-1700 / www.mazakusa.com/
Star SU LLC
847-649-1450 / www.star-su.com/
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