Gearbox casing and layout

These are subjected to a complex array of forces from all the components. It is preferable that all these are dealt with within the gearcase and little or no residual forces act on the supports. Also it is preferable that there is no transfer of load from an external source, say the propeller.

The gear casing is generally constructed of fabricated steel plates, the casing must have a certain degree of flexibility internally in the planes in which the bearing loads act to allow for incorrect tooth contact.

The residual weight and turning moment is supported by as small an area as possible to negate forces transferred by the movement of the ships hull

Turning moment (generally found on systems using Tandem style gearing)

However, to prevent undue movement in the oil clearance of the mid component, the pitch of the primary pinion and secondary wheel had to be the same. The pitch on the secondary wheel was limited to commercial viability. This makes for a coarse pitch on the primary ,all addendum teeth where encompassed on the primary pinion for strength

Gear oil sprays

The position of the oil sprays within a gear casing are of paramount importance. Power losses and overheating in high speed gears may be reduced by applying some of the oil to the teeth as they disengage. This being the side where the cooling effect is greatest. This helps to prevent scuffing and shows the importance of reducing the bulk temperature of the oil.

Lubricating oil is supplied to flexible couplings, bearings and the line of contact between pinion and wheel.
For the gearing oil is sprayed under pressure diectly into the line of contact from a distance of 25 - 50mm. there may be three or four sprays per mesh. For bothahead and astern directions. Oil must be supplied under sufficient pressure to ensure total wetting before being flung off by centrifugal force.
Vents as fitted to the crankcase as the oil at the point of contact becomes hout leading to increased vaporisation. Sight glasses or indicators may be provided to ensure positive flow of oil

QUILL SHAFTS

The turbine is connected to the pinion by a torque tube. Here two flexible couplings are used ; this may be dynamically balanced before fitting

Quill shafts are fitted to increase the length of the shafting without increasing the overall length. This has the advantage that gear teeth may be brought to mesh at the node point and hence point of minimum vibration.

Teeth hence have a steady load instead of a fluctuating cyclically with vibration. Hence, the drive is sometimes called a nodal drive.

Plant layout

Layout of large stal laval propulsion plant

The gearing fitted to the SS Leonia and other large turbine propulsion plants is of the Articulated type, this is indicated by the fitting of the flexible couplings to the Epicyclics allowing a certain degree of misalignement to exist and allow for any machining errors in the fully floating sunwheel.

The first stage reduction is that of Start type Epicyclic, Star rather than Planetary is used due to the problems of distortion of the Planet carrier ring under centrifugal stress can lead to uneven tooth contact and loading.

The Pinion is allowed free axial movement by the planets on their oil film, this allows for the shuttling of the main wheel to be accomodated ( the shuttling caused by machining errors in the rim)

The Brown Boveri Thrust cone

The disadvatage of using a single helical gear is that there is a resultant axial thrust. Traditionally this would be counteracted by using an oversize thrust block. A simpler method is shown below where resultant axial forces are reacted out by thrust cones mounted on the pinion and wheel schematic of brown boveri thrust cones

With the cone system there is a line of contact and a very large relative radius of curvature with a large oil entraining velocity of 220 ft/s .There is thus considerable axial resilience with the large radius of curvature, a small radial width of cone is sufficient to take the thrust