LIC. MARIA DE LOURDES RIVERA TRUJANO

The hardened and ground gearings of marine gears transmit great torques.

Operating data

Input power P = 5,475 kW; input speed 750 min–1_; output speed 209 min–1_{; operating temperature ca.} 50 °C.

Bearing selection Coupling shaft

The coupling shaft (upper right) is supported at the drive end by a spherical roller bearing 23248B.MB (locating bearing) and at the opposite end by a cylindri- cal roller bearing NU1056M (floating bearing). The shaft transmits only the torque. The bearings have to accommodate only the slight deadweights and minor gearwheel forces from a power take-off system. The bearing dimensions are determined by the design; as a result larger bearings are used than needed to accom- modate the loads. Consequently, a life calculation is not required.

Input shaft

At the input shaft the radial loads from the gearing are accommodated by two spherical roller bearings 23248B.MB. The thrust loads in the main sense of ro- tation during headway operation are separately accom- modated by a spherical roller thrust bearing 29434E. The bearing 23248B.MB on the left side also accom-

modates the smaller axial loads in the opposite direc- tion. It is adjusted against the spherical roller thrust bearing with a slight clearance and preloaded by springs. The preload ensures that the thrust bearing rollers do not lift off the raceways when the load changes but keep rolling without slippage. The hous- ing washer of the spherical roller thrust bearing is not radially supported in the housing to ensure that this bearing can transmit no radial loads.

Output shaft

At the output shaft, radial and axial loads are accom- modated separately. The radial loads are accommodat- ed by two spherical roller bearings 23068MB. In the locating bearing position at the output end a spherical roller thrust bearing 29464E accommodates the differ- ence from the propeller thrust during headway opera- tion and the axial tooth loads. The smaller axial loads during sternway operation are taken up by the smaller spherical roller thrust bearing 29364E. These two thrust bearings are also adjusted against each other with a slight axial clearance, preloaded by springs and not radially supported in the housing.

Bearing dimensioning

Based on the operating data, the following nominal fatigue lives are obtained for the different bearings. The minimum value of Lh= 40,000 hours required for clas- sification was not only reached but far exceeded.

Shaft Bearing Rolling bearing Equivalent Index of Nominal Viscosity Factor Attainable

location dynamic dynamic fatigue ratio life at

load stressing life k= a23= utmost

P fL Lh n/n1 a23II· s cleanliness

[kN] [h] Lhna[h]

Coupling shaft

Locating bearing 1 23248B.MB only slightly loaded by deadweight

Floating bearing 2 NU1056M only slightly loaded by deadweight

Input shaft

Radial bearings 3 23248B.MB 242 3.98 49,900 6.3 >114 »200,000

3 new 23048B.MB 242 1.88 4,100 5.8 >114 »200,000

4 23248B.MB 186 5.18 120,000 6.3 >114 »200,000

Thrust bearings 5 29434E 80 >6.03 >200,000 5.2 >114 »200,000

5 new 29334E 80 4.91 102,000 5.0 >114 »200,000

Output shaft

Radial bearings 6 23068MB 158 >6.03 >200,000 2.4 >84 »200,000

7 23068MB 293 4.64 83,500 2.4 >84 »200,000

7 new 23968MB 293 2.70 13,600 2.3 39 »200,000

Thrust bearings 8 29364E only briefly loaded during sternway operation

9 29464E 650 3.81 43,300 2.5 > 87 »200,000

The effects of basing the bearing dimensions on attain- able life become evident in the case of the two bearings dimensioned for the least load carrying capacity: the spherical roller bearing 23248B.MB (bearing location 3) at the coupling end of the input shaft and the spher- ical roller thrust bearing 29464E (bearing location 9) at the output end of the output shaft.

Based on the index of dynamic stressing fLa nominal life Lh= 49,900 h is calculated for spherical roller bearing 3 and Lh= 43,300 h for spherical roller thrust bearing 9. Due to the required minimum life of 40,000 h the transmission bearings would thus be sufficiently di- mensioned.

Attainable life

The actually attainable life Lhnais considerably longer than the nominal life Lh.

Lhna= a1· a23· Lhis calculated with the following data: Nominal viscosity of the oil: n40= 100 mm2_/s

Operating temperature: t = 50 °C Operating viscosity: n = 58 mm2_/s Spherical roller bearing 23248B (no. 3): C = 2,450 kN; C0= 4,250 kN; n = 750 min–1; dm= (440 + 240)/2 = 340 mm

Rated viscosity: n1= 9.2 mm2_/s Viscosity ratio: n/n1= 6.3 Spherical roller thrust bearing 29464E (no. 9): C = 4,300 kN; C0= 15,600 kN; n = 209 min–1_; dm= (580 + 320)/2 = 450 mm

Rated viscosity: n1= 23 mm2_/s Viscosity ratio: k = n/n1= 2.5 A stress index fs*= C0/P0*> 14 is obtained for both bearings; consequently, K1= 1 and K2= 1; therefore, K = 1 + 1 = 2.

From the viscosity ratiok and the factor K the following basic factors are obtained:

– for the radial spherical roller bearing a23II= 3.8 – for the spherical roller thrust bearing a23II= 2.9 Factor a23is obtained from a23= a23II· s.

The cleanliness factor s is determined on the basis of the contamination factor V. Both bearings operate under utmost cleanliness conditions (V = 0.3). Cleanliness is utmost if the particle sizes and filtration ratios of con- tamination factor V = 0.3 are not exceeded.

Taking into account the viscosity ratiok and the stress index fs*, a cleanliness factor of s > 30 and consequently

an a23factor = a23II· s > 114 and > 87, respectively, is obtained for the bearings under consideration. The attainable life is in the endurance strength range.

This means that smaller bearings could be provided for bearing locations 3, 5, 7 and 9 to accommodate the same shaft diameter (see table: 3 new, 5 new, 7 new, 9 new) and would, in spite of the now higher bearing loads, still be in the endurance strength range.

Machining tolerances

As all bearing inner rings in this application are sub- jected to circumferential load they are fitted tightly onto the shaft seats:

– Radial bearings to n6 – Thrust bearings to k6.

If the radial bearing outer rings are subjected to point load, the bearing seats in the housings are machined to H7.

As the spherical roller thrust bearings are to accommo- date exclusively thrust loads they are fitted with clear- ance, i.e. radially relieved, into the housing seats which are machined to E8.

Lubrication, sealing

To meet the high requirements on safety and reliabil- ity, adequate lubrication and cleanliness conditions are provided for marine gears. The circulating oil ISO VG 100, which is used to lubricate both gear wheels and rolling bearings, is cooled and directly fed to the bear- ings. By-pass filters with filter condition indicators and with an adequate filtration ratio ensure an oil condi- tion where no particles bigger than 75 µm are found and where, consequently, cleanliness is usually utmost (contamination factor V = 0.3).

For this reason, the oil cleanliness class should be 14/11 or 15/12 (ISO 4406).

Radial shaft seals protect the transmission from con- tamination.