Section 3 Gearing

3.1 Construction and welding

3.1.1 Where castings are used for wheel centres, any radial slots in the periphery are to be fitted with permanent chocks before shrinking-on the rim.

3.1.2 Where welded construction is used for the manufacture of wheels and gearcases, welding is to be in accordance with the requirements specified in Ch 13 Requirements for Welded Construction of the Rules for Materials.

3.1.3 Welded constructions are to be stress relief heat treated on completion of welding.

3.1.4 Bolted attachments within the gear case are to be secured by locking wire or equivalent means.

3.2 Accuracy of gear cutting

3.2.1 The machining accuracy (Q grade) of pinions and wheels is to be demonstrated. For this purpose records of measurements are to be available for review.

3.3 Non-destructive testing

3.3.1 Magnetic particle or liquid penetrant testing is to be carried out on the teeth of all surface hardened forgings. This examination may also be requested on the finished machined teeth of through hardened gear forgings.

3.3.2 The manufacturer is to carry out an ultrasonic examination of all forgings where the finished diameter of the surfaces, where teeth will be cut, is in excess of 200 mm, and is to provide LR with a signed statement that such inspection has not revealed any significant internal defects.

3.3.3 On gear forgings where the teeth have been surface hardened, additional test pieces may be required to be processed with the forgings and subsequently sectioned to determine the depth of the hardened zone. These tests are to be carried out at the discretion of the Surveyor, and for induction or carburised gearing the depth of the hardened zone is to be in accordance with the approved specification. For nitrided gearing, the full depth of the hardened zone, i.e. depth to core hardness, is to be not less than 0,5 mm and the hardness at a depth of 0,25 mm is to be not less than 500 Hv.

3.4 Dynamic balancing

3.4.1 All rotating elements such as pinion and wheel shaft assemblies and coupling parts, are to be appropriately balanced.

3.4.2 The permissible residual unbalance, U, is defined as follows:

where

m	=	mass of rotating element, in kg
R	=	maximum service rev/min of the rotating element

3.4.3 Where the size or geometry of a rotating element precludes measurement of the residual unbalance a full speed running test of the assembled gear unit at the manufacturer's works will normally be required to demonstrate satisfactory operation.

3.5 Meshing tests

3.5.1 Initially, meshing gears are to be carefully matched on the basis of the accuracy measurements taken. The alignment is to be demonstrated in the workshop by meshing in the gearbox without oil clearance in the bearings. Meshing is to be carried out with the gears locating in their light load positions and a load sufficient to overcome pinion weight and axial movement is to be imposed.

3.5.2 The gears are to be suitably coated to demonstrate the contact marking. The thickness of the coating to determine the contact marking is not to exceed 0,005 mm. The marking is to reflect the accuracy grade specified and end relief, crowning or helix correction, where these have been applied.

3.5.3 For gears without crowning or helix correction the marking is to be not less than shown in Table 2.3.1 No load tooth contact marking.

Table 2.3.1 No load tooth contact marking

ISO accuracy grade	Contact marking area
Q ≤ 5	50% b x 40% h _w + 40% b x 20% h _w
Q ≥ 6	35% b x 40% h _w + 35% b x 20% h _w
Note 1. Where b is the face width and h _w is the working tooth depth. Note 2. For spur gears, the values of h _w should be increased by a further 10%.

3.5.4 Where allowance has been given for end relief, crowning or helix correction, the normal shop meshing tests are to be supplemented by tooth alignment traces or other approved means to demonstrate the effectiveness of such modifications.

3.5.5 For gears with crowning, or helix correction the marking is to correspond to the designed no load contact pattern.

3.5.6 A permanent record is to be made of the meshing contact for purpose of checking the alignment when installed on board the craft.

3.5.7 The full load tooth contact marking is to be not less than shown in Table 2.3.2 Full load tooth contact marking.

Table 2.3.2 Full load tooth contact marking

ISO accuracy grade	Contact marking area
Q ≤ 5	60% b x 70% h _w + 30% b x 50% h _w
Q ≥ 6	45% b x 60% h _w + 35% b x 40% h _w
Note 1. Where b is the face width and h _w is the working tooth depth. Note 2. For spur gears, the values of h _wshould be increased by a further 10%.

3.5.8 Where, due to the compactness of the gear unit, meshing tests of individual units cannot be verified visually, consideration may be given to the gear manufacturer providing suitable evidence that the design meshing condition has been attained on units of the same design.

3.5.9 The normal backlash between any pair of gears should not be less than:

where

α_n	=	normal pressure angle, in degrees
a	=	centre distance, in mm