There is a growing tendency of using casecarburized, hardened and ground gears as reduction gears for marine main engines. In designing casecarburized and hardened gears, the most suitable case thickness which has an effect upon the casecrushing strength of the tooth surface and the bending strength of the tooth root is of prime importance. Under present conditions, however, there is no established method of determining the case thickness.
This paper deals with the most suitable case thickness which balances the fatigue strength between the tooth surface and the tooth root. The rolling contact fatigue strength of the tooth surface was examined with the use of a two-disc machine, and the bending strength of the tooth root by means of an unidirectional pulsating load machine. As a result of the examination, the following two methods were obtained.
(1) A method of obtaining the critical contact pressure (Hertzian maximum pressure) from the results of the rolling contact fatigue test on the assumption that t=αm (m: module, α: constant determined according to module), which is widely used in determining the case thickness, expresses the most suitable case thickness against the bending stress.
(2) A method of determining the most suitable case thickness with the use of the test results and calculations made of the bending stress at the tooth root and also of the shear stress created by the rolling contact in consideration of the residual stress.

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This report gives some load distribution along face width of main gear from tooth root strain measured by strain gauges. Measurement was made under 1/4, 2/4, 3/4 and 4/4 load of main propulsion turbine with the vessel at both ballasted and fully loaded draft.
Consequence of measurement were follows.
(1) Max. load sharing ratio was 1.7.
(2) Pattern of load distribution along face width was a little different from the calculated one.
(3) The draft of vessel or the load of main propulsion turbine reflected scarcely on the pattern of load distribution along face width.
(4) Max. tooth root stress of main gear was about 30kg/mm², and this value was about 23% larger than that calculated by AGMA formula and about 9% larger by HAND BOOK OF MECHANICAL INDUSTRIES (fifth edition) .
While, in order to examine the influence of line shaft alignment to the load distribution along face width, measurement was made at the condition of the steady bearing shifting 0.7mm upward and 0.3mm downward from the original position. In consequence, it seemed that the position of steady bearing reflect scarcely on the load distribution along face width.

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The newly developed reduction gear is called “Simple Locked Train Type” or “SLT Type” for short. This reduction gear has two idler gears arranged between a pinion and a wheel, and a special power equalizer is provided for perfect tooth load sharing to these two idler gears.
Test running was carried out by using an experimental reduction gear of SLT type, SLT 2, 000, which can transmit 2, 000 BPS×2 at input speed of 720 rpm in order to confirm the performances of this type of gear including the function of tooth load sharing. Test results obtained were quite satisfactory as expected.
Further a larger scale SLT type reduction gear, SLT 9, 000, which can transmit 9, 000 BPS×2 at input speed of 370 rpm, was made for tests. Performance tests were carried with a great success. SLT 9, 000 is still under endurance test.

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