Jan. 2019

Gears Are
at the Core of
Aircraft Technology The Aircraft Gear Business
at Kawasaki Heavy Industries

One business in particular at Kawasaki Heavy Industries is gaining momentum-
the aircraft gear business. Aspiring to be an excellent company in this sector,
Kawasaki offers an impressive array of products, including transmission systems for helicopters,
accessory gearboxes, and aircraft power generation systems.
In this issue, we bring you a story featuring the core technology behind aircraft gears,
for which Kawasaki is known as a vital provider in the aircraft business.

  • Masahiro Gouhashi
    Assistant Manager, Engine Production Engineering Section 1, Engine Production Engineering Department Production Engineering Division, Aerospace Systems Company Kawasaki Heavy Industries, Ltd.
  • Kenta Ogasawara
    Assistant Manager, Drive System Engineering Section 1, Drive System Engineering Department Commercial Engine Project Division, Aerospace Systems Company Kawasaki Heavy Industries, Ltd.
  • Kazuhiro Sato
    Drive System Engineering Section 1, Drive System Engineering Department Commercial Engine Project Division, Aerospace Systems Company Kawasaki Heavy Industries, Ltd.

Unrivaled Reliability of
Kawasaki Gear Products

The Aerospace Systems Company of Kawasaki has been steadily increasing its gear-related product portfolio. Starting with the development of helicopter transmission systems, it expanded into accessory gearboxes (AGB) that drive auxiliary hydraulic, electric, and air-conditioning equipment, and proprietary traction-drive integrated drive generators (T-IDG™), a power generation system for aircraft. Kawasaki now boasts the industry’s most extensive lineup of aircraft gear products.
Most aircraft engine manufacturers work collaboratively with gear manufacturers in developing engine systems. In this regard, Pratt & Whitney has been partnering with Kawasaki in developing advanced systems.

In the 1960s, Kawasaki embarked on the development of helicopter transmission systems as part of its larger project to develop helicopters in-house, which later became one of the mainstays of its aircraft business. In a joint development project with Messerschmitt-Bölkow-Blohm (present-day AIRBUS) that began in 1977 for the BK117 helicopter, Kawasaki was tasked with the development of the most critical component–the transmission system. Thanks to its reliability and durability, this system contributed to making the BK117 one of the best-selling helicopters in the world and helped establish the foundation for Kawasaki’s growth in the aircraft business.

Interior of the transmission unit. There are two input shafts, as the BK117 D-2 is a twin-engine rotorcraft, and various gears, which are configured to drive auxiliary equipment.The transmission for a helicopter is placed atop the cabin, connecting the horizontally-installed engine and the vertically-configured rotor.

A helicopter transmission is installed atop the cabin and performs the following operations: 1) transfers power from the engine to the main rotor (rotor blade) and the tail rotor after stepping down the speed; 2) drives accessories; and 3) transfers the lift generated by the main rotor to the helicopter body, and receives the thrust force as well as the drag force that acts in opposition to the direction of movement.
In the BK117 D-2, the transmission steps down an engine speed of 6,000 rpm to 380 rpm (a reduction ratio of about 16:1), and at the same time increases the torque to 19,600 N・m to drive the main rotor. This is a remarkable torque capacity, sufficient to lift two passenger cars attached to the end of a 1-meter-long bar.

To be more specific, the rotary shafts from a pair of engines that generate a total of 1,000 horsepower are coupled to spiral bevel gears that change the direction of rotary motion by 90 degrees, and simultaneously reduce the engine output speed. After the directional change, the rotational speed is reduced by a helical gear on the second stage, resulting in an optimal rotor speed. Other Kawasaki designs use planetary gears to make them more compact, which can achieve a weight-to-horsepower ratio (weight divided by net horsepower) of less than half of an automobile transmission.

From Machining to
Final Inspection: The World’s
Top Provider of One-Stop
Gear Manufacturing

Achieving lightweight gears demands the utmost effort at thinning without compromising durability. The gears can be as thin as 2 mm, and yet transfer a tremendous amount of power. Gears with lightening holes are inevitably prone to deformation, and yet their tooth shape requires an accuracy at the level of micrometers (μm, which equals 1/1000 millimeter).
Masahiro Gouhashi of the Production Engineering Division comments: “The way gears deform varies, depending on their shape and type of machining process. Therefore, the key to producing accurate gears is minimizing deformation, so that the complex, delicate shapes the designers had first envisioned can be achieved.”

Gear-manufacturing process: 1) Gear is machined from a gear blank, 2) undergoes carburization and quenching processes for surface hardening, 3) is ground to an accuracy of 1 μm, and 4) is inspected for finishing accuracy using special equipment.

In gear production, the first step is to cut a round gear blank using special machining equipment with a cutter exactly the same shape as the desired teeth. The gear then undergoes a carburizing and quenching process, through which the gear surface absorbs carbon and hardens, followed by a grinding process whereby the teeth are ground to a μm level of precision. The skillful operation of this special equipment is another key expertise required by the manufacturer. Finally, to improve resistance to fatigue, small steel beads are blasted against the surface-a process called “shot peening.”
In the inspection process, nondestructive tests are performed repeatedly. These include “nital etch testing,” which uses a special chemical solution to ensure that the strength of the gear has not decreased due to excessive temperature rise during the machining and grinding processes, and “magnetic particle inspection,” which, by magnetizing the gears, detects micro-cracks at the surface that cannot be found in a conventional visual inspection.
Speaking of the comprehensive capabilities Kawasaki exhibits, Gouhashi says, “The most notable feature of our aircraft gear manufacturing is that we are in possession of expertise encompassing the entire process of gear manufacturing-from machining and heat treatment to final inspection-to make sure that no defects in hidden areas go undetected. In the aircraft gear sector, this comprehensive expertise is what gives us a competitive edge in the global arena.”

Outlook Bright on Achieving
60-Minute “LOL (Loss of
Lubrication)” Operation

The development divisions experience different challenges in helicopter transmissions than those faced by the manufacturing divisions. This is because the design engineers are tasked not only with improving the precision and performance of individual gears, but also with upgrading the entire transmission into a high-precision, high-performance component of the helicopter, all of which requires a variety of technological innovations. This includes development of bevel gears that can handle faster rotational speeds and greater horsepower, and mechanisms to minimize the vibration that light, thin gears are prone to generate.

Scenes from the transmission assembly. A shaft in the upper part drives the rotor.

In recent years, demand has been on the rise for improved LOL performance, which is the capability of continuing to fly after loss of lubricant in the transmission. This was originally required for military helicopters, to enable them to reach a safe location in the event that they experienced ballistic damage and were faced with a sudden loss of lubricating oil. However, this safety measure is now vital for media and emergency helicopters as well, as they often fly in urban areas where landing space may not be readily available. Commercial aircraft are required to have a 30-minute LOL capability, but an even longer operation is called for to ensure the safety of aircraft flying between offshore oil fields and onshore terminals. According to Kenta Ogasawara of the Commercial Engine Project Division, “Improved technology is providing a good outlook for Kawasaki to achieve a 60- to 70-minute LOL capability, and a demonstration test is planned for the end of fiscal 2018.”

Key to achieving such duration is cooling. A transmission configuration and structure which will preclude the overheating and seizing of gears and bearings is being developed. In addition, materials with greater seizure resistance are being selected. At Kawasaki, it is now possible to optimize gear shape and bearing configuration by means of computer simulation, which allows the simulation of temperature changes in transmission gears in the event of LOL.
“We hope that this technology will lead to future transmission development with innovative concepts," adds Ogasawara.

Gear meshing and cleanliness are rigorously checked during an inspection conducted before assembly of the accessory gearbox. The inspection process is recorded by a camera attached to the cap of the inspector.
Improvement in computer simulation technology directly enhances the company’s competitiveness. The shapes of the transmission’s interior components, the flow of lubricants, deformation of gear teeth, and other factors are simulated to achieve optimal design of the transmission.

Fan Drive Gear System (FDGS)
Is “Heart” of New Engine

At Kawasaki, proactive strategies are being implemented to tap into a promising market involving an FDGS for a geared turbofan (GTF) engine that was developed by Kawasaki’s partner, Pratt & Whitney.
Fans for passenger aircraft turbofan engines have been increasing in size in order to attain higher fuel efficiency. However, due to differences in the rotational speeds of the fans and the optimal rotational speeds for the turbines and compressors which drive them, further enhancements in engine performance were deemed challenging. A breakthrough in this stalemate was the GTF engine, which inserts a gearbox between the fan and the turbine, allowing each component to achieve optimal rotational speeds.

In the GTF engine, the FDGS uses planetary gears to accommodate its coaxial input and output shaft design, and to make it more compact and lightweight. The engine is also designed to produce 20,000 horsepower (hp), which far exceeds the engine output of helicopters, and to attain longer than the required 30,000 hours of reliable operation.
Kazuhiro Sato, who leads the development project, comments, “With such huge horsepower, even a 1% loss of energy, about 200 hp, will generate 150 kW of heat. This not only compromises the most important factor-fuel efficiency-but also poses serious challenges involving the oil for cooling and the weight and size of heat exchangers.”

Aircraft Gear Development Chronology for Kawasaki Heavy Industries

This is why, in addition to being lightweight and compact, the FDGS must be designed to minimize losses fro s mix and flow in a very complex way inside the FDGS, and Kawasaki possesses the world’s most sophisticated technology to computer-simulate this flow. Moreover, the company has developed a proprietary technique that not only optimizes the flow, but also maximizes the lubrication and cooling effects. As a result, an energy efficiency of 99.6% has been achieved.

“Kawasaki’s proprietary technologies were all developed in conjunction with the Corporate Technology Division in the course of a research project to develop a gear system for open rotor engines whose commercialization is expected to take place after 2030: in other words, in an engine development project already focused on the next generation after GTF engines. Based on the expertise gained, we would like to continue to upgrade our technological prowess and propose further innovations,” adds Sato.Transmissions and FDGS are behind-the-scenes systems. However, without them, the safe flight of aircraft is impossible. Gears are, indeed, at the core of aircraft technology.

An FDGS (a gear system for a geared turbofan,(right) for a next-generation open rotor engine will be driving more complex counter-rotating propellers (left).

Looking Forward to Tomorrow

T-IDG™ Supports Progressive
Electrification of Aircraft

In an aircraft, electricity for flight equipment and facilities is supplied from a generator driven by the engine. The generator, which is installed on the accessory gearbox of the engine, is an integrated drive generator (IDG) which houses a constant-speed drive unit: in other words, a CVT (continuously variable transmission). Being integrated with the CVT, the generator always rotates at a constant speed and provides the aircraft with constant-frequency power even if the engine speed changes.

A hydro-mechanical CTV is commonly used for IDGs. However, Kawasaki’s newly-developed T-IDG™ has a high-speed traction-drive CVT, which is the first in the world to be applied as a constant-speed drive unit for aircraft. T-IDG™ was selected as the main generator for the P-1 maritime patrol aircraft and the C-2 transport aircraft of the Japan Ministry of Defense. To transmit power, a traction-drive CVT utilizes the viscous resistance of oil film, reducing losses as compared with conventional hydraulic CVTs. It also offers better durability because no contact friction occurs between parts. To meet the increasing demand for “more electric aircraft” (MEA), Kawasaki is accelerating its efforts to develop T-IDG™ with greater capacity, in order to introduce products to the commercial aircraft market.

From the Project Team

ByBy Dr. Tatsuhiko Goi

Fellow, Gear System Technology, Aerospace Systems Company, Kawasaki Heavy Industries, Ltd.

Aiming to Become the Dominant Player
in the Global Aviation Market
by Leveraging Our Aircraft Gear Technologies

Fundamentally, gears are used for efficient transfer of energy. The more we try to enhance that efficiency, the more important the roles of gears become. They are parts that demonstrate The aircraft gear business at Kawasaki was launched with the licensed production of helicopter transmissions. After that, in conjunction with the Corporate Technology Division, the Aerospace Company tackled various challenges to develop unbeatably high-performing and reliable products based on proprietary technologies. These achievements are attributable to our integrated manufacturing process, covering all phases from development to manufacturing, and allowing the sharing and solving of problems that arise in between phases. A proactive corporate culture that has encouraged engineers to take on new challenges was another contributor.
As a result, technologies originally associated with helicopter transmissions have evolved to a level that is now applicable to the accessory gearboxes that drive auxiliary equipment, the traction-drive integrated drive generator (T-IDG™) that is driven by the aircraft engine to generate electrical power, and now, future technology-the FDGS. These technologies are indispensable to the operation of aircraft, and, considering the future growth of the aviation industry and the expansion of gear applications (“gearification”*), our gear business has great potential for growth and is expected to become one of the pillars of the Aerospace Systems Company.

In the aircraft sector, continuous efforts are being made to reduce fuel consumption and emissions, and eventually there will come a time when more advanced GTFs, open rotor engines, and other next-generation engines are the only options. Even when such trends become prevalent, gears will still be pivotal parts. Without gears, there would be no next-generation engines.
The drivers of new aircraft industry trends are prominent U.S. and European manufacturers. However, in terms of gear technology that can turn these trends into reality, or which serves as the backbone of innovation and demonstrates great potential in creating technological breakthroughs, Kawasaki is the world leader. In my opinion, it is not an overstatement to say that Kawasaki’s technological innovations drive forward the new trends. Indeed, those who rule the gears, rule the world.

* “Gearification” is a term coined to mean “using gears to drive fans.”