Section 7 Design

7.1 General

7.1.1 Gears are to be designed to ISO 6336 Method B, or equivalent, and are to comply with the requirements of this Chapter.

7.1.2 It is the manufacturer's responsibility to provide, together with the requirements of Vol 2, Pt 3, Ch 1, 5.1 Plans 5.1.1, results of the calculations with all relevant input data.

7.2 Fatigue limits

7.2.1 Acceptable limiting fatigue bending stresses for various materials are given in Table 1.7.1 Values of endurance limit for bending stress, σ_{F lim} , where σ_B is the ultimate tensile strength, in N/mm², and HV is the Vickers hardness number.

Table 1.7.1 Values of endurance limit for bending stress, σ_{F lim}

Heat treatment	σ_{F lim} N/mm²
Through-hardened carbon steel	0,09σ_B + 150
Through-hardened alloy steel	0,1σ_B + 185
Soft bath nitrided (tuffrided)	330
Induction hardened	0,35HV + 125
Gas nitrided	390
Carburised A	450
Carburised B	410
Note 1. A is applicable for CR Ni Mo carburising steels. Note 2. B is applicable for other carburising steels.

7.2.2 Acceptable limiting fatigue surface stresses for various surface treatments are given in Table 1.7.2 Values of endurance limit for Hertzian contact stress, σ_{H lim} . Material mechanical properties are based on the gear wheel values.

Table 1.7.2 Values of endurance limit for Hertzian contact stress, σ_{H lim}

Heat treatment		σ_{H lim} N/mm²
Pinion	Wheel	σ_{H lim} N/mm²
Through-hardened	Through-hardened	0,46σ_B2 + 255
Surface hardened	Through-hardened	0,46σ_B2 + 415
Carburised, nitrided or induction-hardened	Soft bath nitrided (tufftrided)	1000
Carburised, nitrided or induction-hardened	Induction-hardened	0,88HV₂ + 675
Carburised or nitrided	Nitrided	1300
Carburised	Carburised	1500

7.3 Tooth form

7.3.1 The fundamental tooth profile in the transverse section is to be of involute shape, and the roots of the teeth are to be formed with smooth fillets of radii not less than 0,25 of gear normal modulus (mn).

7.4 Tooth loading factors

7.4.1 For values of application factor, K _A, see Table 1.7.3 Values of K _A .

Table 1.7.3 Values of K _A

Main and auxiliary gears	K _A
Main propulsion – electric motor or gas turbine, reduction gears	1,15
Main propulsion – engine reduction gears: Hydraulic coupling or equivalent on input High elastic coupling on input Other coupling	1,10 1,30 1,50
Auxiliary Gears:
Electric, gas turbine and engine drives with hydraulic coupling or equivalent on input	1,00
Engine drives with high elastic coupling on input	1,20
Engine drives with other couplings	1,40

7.4.2 Load sharing factor, K _g. The value for K _g is to be taken as 1,15 for multi-engine drives or split torque arrangements. Otherwise, K _g is to be taken as 1,0. Alternatively, where measured data exists, a derived value will be considered.

7.5 Factors of safety

7.5.1 The required factors of safety are shown in Table 1.7.4 Factors of safety, where S_{H min} relates to surface stress and S_{F min} to bending stress.

Table 1.7.4 Factors of safety

	S _{H min}	S _{F min}
Main propulsion gears	1,40	1,80
Auxiliary gears	1,15	1,40

7.6 Gear wheels

7.6.1 In general, arrangements are to be made so that the interior structure of the wheel may be examined. Alternative proposals will be specially considered.

7.7 Clutch actuation

7.7.1 Where a clutch is fitted in the transmission, normal engagement shall not cause excessive stresses in the transmission or the driven machinery. Inadvertent operation of any clutch is not to produce dangerously high stresses in the transmission or driven machinery.

7.8 Gearcases

7.8.1 Gearcases and their supports are to be designed to be sufficiently stiff such that movements of the external foundations and the thermal effects under all conditions of service do not disturb the tooth contact.

7.8.2 Inspection openings are to be provided at the peripheries of gearcases to enable the teeth of pinions and wheels to be readily examined. Where the construction of gearcases is such that sections of the structure cannot readily be moved for inspection purposes, access openings of adequate size are also to be provided at the ends of the gearcases to permit examination of the structure of the wheels. The attachment to the shafts is to be capable of being examined by removal of bearing caps or by equivalent means.

7.8.3 Access arrangements for examination and maintenance are to be provided for internal clutches, brakes, bearings and couplings where applicable.

7.8.4 Electrical equipment fitted within the gear case is to be suitable for installation in an oil-laden atmosphere and consideration is to be given to areas where oil sprays play directly onto electrotechnical equipment and/or cable fastenings. All electrical items are to be earthed to meet the requirements of Vol 2, Pt 9 Electrotechnical Systems. Gearcases are to be earthed to the ship's structure.

7.8.5 For gearcase security arrangements, see Vol 2, Pt 7, Ch 3, 8.14 Security 8.14.2

7.9 Alignment

7.9.1 Reduction gears with sleeve bearings are to be provided with means for checking the internal alignment of the various elements in the gearcases.

7.9.2 In the case of separately mounted reduction gearing for main propulsion means are to be provided by the gear manufacturer to enable the Surveyors to verify that no distortion of the gearcase has taken place, when chocked and secured to its seating on board ship.

7.9.3 See Vol 2, Pt 5, Ch 4 Shaft Alignment for further requirements.

7.10 Turning gear

7.10.1 The gearbox is to be provided with a means of turning the gearing for inspection and maintenance purposes. The turning gear is to be designed to turn the shaft at a rate to allow visual inspection without risk to bearings when the oil supply is off. The turning gear arrangements are to provide means that enable ready operation by hand.