Marine Gearbox of Highest Accuracy and Highest Power Density
■Outline
High power marine gearboxes for use with main steam turbine engines to propel LNG carriers are obliged to be enlarged in both gear diameter and tooth width by the increase in engine output and the decrease in propeller speed in recent years, resulting in increased difficulty to keep the gears including tooth contact satisfactory in precision and quality.
Kawasaki Heavy Industries, Ltd. (KHl) has completed research for smaller size and increased reliability by mobilizing its most advanced original techniques regarding design, materials and machining, and succeeded in incorporating the fruits of research into new products. A gearbox of 29,400kW in power output embodying the new technology is 20% less in mass and 30% less in external cubic measure than any conventional product of a comparable class. The new product is the highest in the world in power density, and its gear diameter and tooth width are reduced by 10 to 15%, resulting in enhanced accuracy and uniformization of tooth contact and dramatically increased reliability.
■Principal Particulars
The principal particulars of the recently developed high-power marine gearbox of 29,400kW in output are listed in Table 1 in comparison with an earlier model.
Table 1 Comparison of Principal Particulars
| Item |
New Type |
Former Type |
| Type |
Double-reduction tandem articulate double-helical gears |
| Transmitted power (kW) |
29,400 |
| Revolutions (rpm) |
High pressure turbine: 5,000
Low pressure turbine: 3,350
Propeller: 90 |
Gear material
(Pinion/wheel) |
Ni-Cr steel/
Ni-Cr-Mo-V steel |
Cr-Mo steel/
Cr-Mo steel |
Tooth surface finish
(Pinion/wheel) |
Ground/shaved |
Shaved/shaved |
External dimensions:
W x H x L (mm) |
7,400 x 5,300 x 5,600 |
8,600 x 6,100 x 5,800 |
| Overall mass (tons) |
130 |
163 |
■Salient Points of R&D and Commercialization
The salient points of the R&D work for reduced size and increased reliability and the ensuing project for commercial production include the following.
(1) In order to reduce the size and weight of the gearbox within the permissible contact stress on the tooth surface determined by the strength of the material, the tooth contact should be uniformized, the nominal designed contact stress increased and the impact of errors in gear fabrication reduced. For these purposes, ultra-precision modification of the tooth surface was carried out, with elastic deformation, centrifugal deformation and thermal deformation taken into account. The result was the successful development of suitable design techniques and ultra- precision machining techniques.
(2) With a view to improving precision and quality including tooth contact, it was decided to use grinding, instead of the previous shaving, for finishing work on pinions, and a high-precision tooth surface grinding technique was developed, which was to use an ultra-precision gear grinding machine.
(3) To increase the fatigue strength of wheels, a nickel- vanadium-added material was developed for use on the rim, which was 30% harder than what was previously available. A new welding technique was also worked out to match this harder material, and a continuous tooth cutting technique was also developed to maintain precision against the wear of tools.
(4) It was decided to increase the fatigue strength of pinions by thermal treatment for carbo-quenching of the tooth surface, which had been inapplicable to large gears previously, and a technique to control the quantity of distortion by carburizing was developed to optimize the grinding margin.
Already six units of the recently developed gearbox are in operation, all in a fully satisfactory state. A total of 20 main steam turbine engines for LNG carriers, including those equipped with these new type gearboxes, are highly appreciated by the owners and builders of the vessels. KHI will continue to further develop its business in every sector of marine propulsion engines in addition to such main turbine engines for LNG carriers.
Kawasaki Heavy Industries, Ltd.
Address: World Trade Center Building, 4-1, Hamamatsu-cho 2-chome, Minato-ku, Tokyo 105-6116
