Types of Gearing
These are the most common form of drive. They are cylindrical and have the teeth cut straight and parallel to the axis of rotation. The tooth form can be one of several, but there is no axial thrust component on the bearings as the teeth are straight. The efficiency of the spur gear can be as high as 98% and capable of practical speed ratios of 10:1 although 6-7:1 is more common.
The main disadvantage of spur gears lies in the fact that they tend to be noisy at over 1000ft/min. If made to exceptionally fine limits of accuracy, plain spur gears can be used at far higher speeds in turbine drives.
Single Helical gears
Helical gears are produced by cutting the teeth at an angle to the gear axis and the teeth follow a spiral path thus making for gradual tooth engagement and load distribution. Efficiency is as great as for spur gears.
Ratios of 10:1 are possible with increased load over spur gears. A degree of axial thrust is produced which must be catered for in the bearing design. Angular contact or tapered roller bearings are employed. On larger designs where plain bearings are fitted a thrust block arrangement must be fitted.
Single helical gears can be used at speeds up to 4000 ft/min
Double Helical gears
Commonly specified where the axial thrust from a single helical design would be too large or where there plain bearings are used. To balance the side thrust the teeth are formed on each gear in helices of identical angle but opposite hand. For cast commercial gears the teeth are sometimes of the uninterrupted type, cut by the planing process. For hobbed gears a 3 in wide gap is left for the hob clearance.
Single reductions of 10:1 with double reductions of 75:1 and triple reductions of 350:1 are used
Pitch line velocities from 4000 to 20000ft/min are possible depending on the accuracy of manufacture.
Bevel gears are used in situations where it is desired to transmit motion between two shafts whose axis intersect. The most common type is that in which the teeth are radial to the point of intersection of the shaft axes or apex and these are known as straight bevel gears.
The tooth action is similar to that of spur gears, being in line contact parallel to the pitch line. There is no longitudinal sliding between the teeth, but there is an end thrust developed under tooth load which acts away from the apex, thus tending to separate the gears. Thrust bearings must therefore be provided. The maximum gears ratio is 4:1. The maximum speed at pitchline is 1000ft/min.
Spiral bevel gears
The spirally cut gears like the helical gear in its relationship to the spur gear, can withstand higher speeds than the straight cut bevel and is quieter in action. Unlike the straight cut bevel gears which can be shaped or precision forged the spiral bevel gears must be made on a special machine ( made by Gleason Co). Pitchline velocities of 4000ft/min maximum can be handled.
Similar in appearance to the spiral bevel gear it is distinguished by having the pinion axis offset to the wheel axis. They are mainly used in the automotive back axle drives where they provide smooth tooth engagement at the high speeds combined with high load carrying capacity.
Spiral or crossed axis gears
These gears are identical in every way to helical gears, the only difference is that they are used to transmit power between shafts that are not parallel. Mating gears must have the same base pitch but their helix angled may vary. The contact made by the pitch cylinders of spiral gears is point contact only and there fore spiral gears are suitable for light duties only.
Zero Bevel gears
These gears have teeth that are curved in the same general direction as straight teeth. They are spiral gears of zero spiral angle.
Skew Bevel gears
In this form the pinion shaft is offset in relation to the wheel. The pinion may have straight teeth or it may have skew teeth similar to a helically cut bevel gear. The object is to obtain more gradual tooth engagement than with a straight tooth bevel. An additional advantage is that it sometimes makes possible the provision of bearings at both ends of the pinion shaft. Skew bevels are seldom used as they are difficult to set up.
The meshing condition of this sort of gear are said to be better than those of the external gears for the reason that the contact area is between a concave and a convex surface, while also making better conditions for lubrication.
Other advantages include shaft direction is the same for input and output, Greater load capacity is possible, increased safety as the teeth are guarded.
Disadvantages include difficulty in supporting the shaft, range of gear cutting processes is reduced and tooth interference is a common problem.
Worm reducer gears
conditions for worm gears include the following;
shafts at right angles
large speed reductions in smallest compass
smallest number of gears
A worm drive comprises a cylindrical worm having helical teeth or threads, similar to a helical gear, meshing with wheel with a concave face. The tooth contact is a line one and heavy loads can be handled. Efficiencies claimed for worm gears are 97% and above. Ratios of 1000's to one is possible with double worm drives and it is the most popular form of industrial drive.
In addition to the above bevel gears there is a special type which has right angle non intersecting gears having tapered pinions with threads of constant axial lead meshing with face type gears. This is used mainly in the automotive industry.
Gearbox casing 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
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.
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 misalignment to exist and allow for any machining errors in the fully floating sun wheel.
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 accommodated ( the shuttling caused by machining errors in the rim)
Principal of Operation
If dia 'A' = dia 'B' then for one rotation of 'A' a point on the surface of 'A' would move through a distance equal to 2 x Pi x Ra;
the distance that would be travelled by a point on 'B' would be 2 x Pi x Rb and as Ra=Rb. the ratio is 1:1.
One rotation of 'A' causes one rotation of 'B'
If the gear 'A' is fixed and 'B' allowed to rotate freely around it constrained within
an annulus; then for one rotation of 'A' and corresponding rotation of 'B' the point
of contact on the annulus would have moved through a distance equal to 2x P x Ra.
The circumference of the annulus would be equal to 2 x P x (Ra + Rb), hence for
one revolution of 'A' then 'B' would have only traveled half way round the annulus.
By varying the size of the sun and planets the gear ratio can be altered. The outlet drive could be taken either off the bar 'c' or if 'c' was fixed off the rotating annulus.
Comparison of Epicyclic gearing to Tandem gearing
The Star annulus has teeth on the inner rim. A resilient mount is provided when the star annulus is fixed. This allows a certain degree of distortion to occur reducing tooth loading. The planet wheels are located by a planet carrier ring, on fitted at each end
The system may be constructed in three different ways
The fixed member is called the torque reaction member. The number of wheels is determined by tooth loading
Epicyclic gearing alignement
In normal operation epicyclic gear designs the planet pins are straddle mounted on a rigid carrier and are precisely aligned to each other.
If they are not the load distribution across the face is affected, but not the load sharing.
The sun pinion and flexible annulus are centered by the planet wheels when under load
With the ideally supported annulus, load sharing between the planets is ensured by the radial flexibility and uniform loading across the teeth by the self correcting toroidal twisting of the annulus and by the high accuracy of the gearing.
Toroidal twisting of annulus
The effect on tooth loading depends of on the supporting method of the annulus.
Introduction of Annulus flexibility
Toroidal twisting effect on the annulus is reduced to a minim by having the tubular extension thin, and nearly in line with the axial thrust from the teeth.
Other designs include the Allen-Stoeckicht where the split annulus of a double epicyclic gear are given a degree of movement within the carrier fo the two rings, this carrier itself is given a degree of axial movement by being fixed to the outer casing by a straight cut tooth coupling.
Also the Renk design has the annulus supported by a series of leaf sleeve spring packs. The annulus is split into two separate annuli. This design permits both torsional and radial movement and to a lesser degree angular movement in the diametrical plane. All movement is dampened by the oil and friction within the spring packs
Introduction of flexible pin
Plane wheel spindle (vickers)
For this design the annulus is made radially stiff.
Standard involute double helical tooth arrangements are used.
The planet/annulus centres and pressure angles are standard
Changing the diameter of the base circle within the tooth height does not effect the gear ratio. However, matching the root circle to the base circle makes the tooth all addendum and hence all the tooth is on the involute curve and no undercutting exists. This is especially used for the highly loaded teeth of the sun wheel.
The sun/planet ring used slightly increased diameters so as much as the tooth depth is used as possible.
Nearly always in the form of a short hollow cylinder .
having the following advantages
Renk Compound Gear
Offers 17-1 reduction capacity. The sleeve pack is adjustable to give the required torsional characteristics. The springs also give some bending flexibility and dampening through oil and friction.. This resilience from the secondary pinions gives greater isolation to the gear
By application of either the ahead or astern brake the direction of the output shaft can be controlled. This system act as an alternative to a reversing engine or CP propeller
Shown below are various layouts for a two stage reduction gearbox
Interleaved (split secondary)
Interleaved (split secondary)
Locked Dual Tandem
Locked Dual Tandem (articulated)
The connection between the rotor and pinion shaft is always via a flexible coupling
The dual tandem arrangement has the advantage that there are two pinion contacts on the secondary wheel. This halves the tooth load and allows a much smaller wheel.
To achieve this, however, requires very accurate setting uo so that one pinion does not sit in its backlash whilst the other is loaded.
This may be achieved by setting one pinion so that it gives the correct contact then slightly rotating the other pinion until it is fully contacted and then 'Locking' the whole arrangement. One method of doing this is by taper fit flexible couplings which can be moved relative to the shaft by application of hydraulic pressure between the mating surfaces.
Extensive use of quill shaft and flexible couplings is made to negate effects from pitch errors creating high dynamic tooth loading. Great care must be taken with the alignement of the primary pinion and primary wheels as this is very highly stressed.
Triple/Double reduction steam plant gearbox
The main wheel pinions are free to move axially because of the axial freedom of the planets on their bearing oil film
The first stage of the HP turbine is a start gear. This due to the high speed of
the HP turbine causing centrifugal stress to distort a free planet carrier causing
meshing problems. With a star gear the plane carrier is fixed.
Sun wheels are connected via flexible couplings to allow for manufacturing and alignment errors
Commisioning and Inspections
Should a fault be found it may be necessary to check alignment, the condition of the flexible couplings, bearings and mounting arrangements.
Checking for mis-alignment
This can be done by blueing one of the teeth then viewing the complimentary mating teeth. Where the blue has transferred this is where the teeth have meshed and this can be compared to the polished area of the on load contact areas.
There will be some difference to the on load polished area as the displacement component tending to push the two centres apart, pushes the pinion up in its bearing. For very accurate alignment this force can be represented by pulling the pinion away from the wheel
If damage to the Bull wheel is suspected , say due to rapid decelleration of the propeller, and the Bull wheel may have slipped its shrink fit then alignment should be checked in a number of positions.
Operation and Maintenance
A gear set will operate satisfactory provided;
Oils should have anti-rust additives, water content should be kept below 0.2%
Excessive rust and sludge can lead to failure due to corrosion fatigue particularly
in gears suffering from pittings
Blued tapes taken on inspection may be kept to record wear
IME recommends inspection periods no more frequent than 6 months to prevent undue contamination.
Clutches are generally designed to engage at minimum load and engine speed. Operation above this can lead to excessive gearbox and clutch loading and can shorten life or lead to catastrophic failure
Oil forces the friction plates, generally made from a suitable steel alloy material or leaded bronze, together. These loose plates are alternately splined to drive or driven shaft.
The oil is supplied under a controlled flow via an accumulator so allowing a gradual engagement over a short period. The oil is generally supplied via a solenoid valve from the gearbox lube oil system
Emergency drive is allowed by fittings screws which jack the plates firmly together
Takes the form of an inflatable tyre on which is mounted ferrodo clutch lining. Air is supplied via a slipper arrangement to the tyre segments which inflate forcing the clutch material into contact with the driven inner circumference.
Emergency drive is via though bolts which pass radially though drive and driven wheel circumferences
Fluid friction clutches
Operate using the shear resistance of the clutch fluid. For marine use this is generally a fine grade mineral oil although synthetics may be used.
A pumped control flow is delivered to the drive assembly and allowed to flow to the driven assembly. As the flow increases so more of the assemblies become available for driving and slippage reduces eventually reaching a maximum.