Complexity marks the design of modern naval gears

Naval gears distinguish themselves from their civilian counterpart in two respects. Firstly, transmissions are often very complex since they are required to couple and uncouple different prime movers such as diesel engines and gas turbines, depending on operational requirements and to offer redundancy to maintain propulsion after battle damage.

Secondly, the gears themselves have to be shock resistant, ie, resistant to explosives and mines rather than gear train shock under ice conditions, and they have to run as silently as possible. This requires an expensive tooth type and a high degree of precision in manufacturing. Added to that is a requirement that the gears should produce as little noise as possible to limit a ship’s acoustic signature.

The phasing out of steam turbine powering of warships in favour of gas turbines created the demand for more flexible gearing arrangements. Nevertheless, the known low efficiency of gas turbines at part and low load forced navies to use small gas turbines, diesel engines or diesel electric prime movers for slow speed operation.

The most effective way to achieve this is to use various combinations: diesel engines for slow or cruising speeds, and diesel engines or gas turbines for high speed, when the diesel engines can be disconnected if necessary. This was practical and it worked, although it was not the most efficient use of the machinery on board. For example, the diesel engines did not contribute to powering the ship at high speed; thus the acronym CODOG was evolved – Combined Operation Diesel Or Gas turbine.

An important task of many warships is submarine hunting – ASW or Anti Submarine Warfare. This requires loitering as silently as possible at slow speed to locate a submarine, then if necessary chasing it at full speed to engage the target. The UK Royal Navy’s Type 23 Duke class frigates, for example, were created to do this; they entered service in 1989, employing a novel engine configuration to achieve their goal.

Instead of using diesel engines mechanically connected to the propeller shaft and thereby transmitting noise, the four 1.5MW Paxman Valenta 12RP2000CZ models are located in acoustic enclosures as gensets, and a 1.5MW electric motor turns each of the twin propeller shafts. The motor armature was part of the shaft, and a Rolls-Royce Spey SM1A gas turbine (derated to 11.6MW) was mounted via a reduction gearbox on to the end of each of the two shafts, via a clutch.

Running on electric power, the ship was very quiet. When speed was required the gas turbines were fired up and connected to the propeller shafts through the clutch arrangement. Both electric and gas turbine power could be run simultaneously, hence this arrangement is called CODLAG (Combined Operation Diesel eLectric And Gas turbine). Running on electric motors, ships of this class have a speed of 15 knots but with the gas turbines added the maximum speed is 28 knots.

Except for the newer Rolls-Royce WR21 model, gas turbines running below their maximum power have poor fuel economy. There are, however, some warships using on each shaft a small gas turbine for low speed and a larger gas turbine for higher speeds. Such arrangements can be found, for example, in the UK Royal Navy’s Type 22 frigates. This is a particularly interesting example because, although there were 14 ships in the class, the first ten vessels used a COGOG (Combined Operation Gas turbine Or Gas turbine) arrangement of twin Rolls-Royce Olympus TM3B sprint gas turbines, each of 16.6MW, or twin Rolls-Royce Tyne RM1C cruise gas turbines, each of 3.6MW each.

The last four ships had a COGAG (Combined Operation Gas turbine And Gas turbine) arrangement with twin Rolls-Royce Spey SM1A sprint turbines, each of 14MW, and twin Rolls-Royce Tyne RM3C cruise gas turbines, of 3.6MW each. Both versions had the same performance of 30 knots maximum and 18 knots cruising, despite the last four ships being longer and 500 tons displacement heavier than the original ten. Obviously, the efficiency of using all the turbines together in the COGAG arrangement is considerably better than the COGOG arrangement where two turbines are not used.

There are very many examples of diesel and gas turbines used in CODOG arrangements; however, the more ideal CODAG configuration has been slow to be realised. One of the main reasons is that a very much more complex gearing arrangement is required, together with engine controls and switching procedures. In ‘analogue days’ it was very difficult to achieve the necessary engine power output control and gearbox sequencing automation; if procedures were not correctly followed, severe damage could be inflicted on the plant.

In 1961 the German Navy was the first in the world to introduce a CODAG arrangement in its Köln class frigates with twin MAN V8V 24/30 diesel engines, each of 2.2MW at 872 rpm, and twin BBC gas turbines, each developing 8.8MW at 3,600 rpm. Diesel engines provided power up to 23 knots but by using the gas turbines alone 28 knots were achieved, and using all the engines the speed increased to 34 knots. This, however, required very complex gearing that not every navy wanted to use.

Germany’s current frigates, the F-124 Sachsen class, also use a CODAG arrangement. Marine Propulsion is fortunate to having a rare insight into the selection considerations for such a warship since details of the decision making process and its conclusions are no longer restricted.

The procurement agency considered CODOG, CODAG and CODAD proposals for the F-124 programme. Basic ship parameters were: twin screws and a weight limitation on the propulsion system of 170 tonnes. An operating cycle was used for the comparison as follows: 15 per cent at 10 knots, 70 per cent at 18 knots, 12 per cent at >26 knots and 3 per cent at 27 knots. Maximum speed was to be >28knots, with a cruising speed 18 knots.

Based on this profile, the CODOG configuration considered was two MTU 12V1163 diesel engines and two GE LM2500 gas turbines. The CODAG arrangement comprised two MTU 20V1163 TB93 diesel engines and one GE LM2500 unit, which allowed both screws to operate using only one diesel engine, yet propelling the ship at 18 knots.

A CODAD (Combined Operation Diesel and Diesel) arrangement was also considered, based on similar plant supplied to the Meko class vessels, which use four MTU 20V1163 TB93 diesel engines. Other models besides MTU were considered but the CODAD option was quickly eliminated due to weight: the four MTU units would exceed the weight limit by 56 tonnes and with four SEMT-Pielstick 18PA6V280 BTC units by 120 tonnes, although they offered 15 per cent more power.

Three gas turbines were considered: the General Electric LM2500, the Rolls-Royce Spey and the Rolls-Royce WR21. The Spey did not have sufficient power so was removed. There was interest in the WR21; however, a complete unit with recuperator is around 32 tonnes heavier than the GE model. A further benefit of the WR21 was its higher efficiency at lower power outputs; nevertheless, this is not really a factor as the diesel engine(s) are used for slower speeds and the turbine only used at full power. A decision was therefore made to select GE’s LM2500.

Comparing the capital costs of CODOG against CODAG, the former required two gas turbines, so by opting for CODAG the cost saving of the single turbine more than outweighed the cost of the more complex gearbox and the larger 20 cylinders against 12 cylinder diesel engines. The decisive factor was, however, the fact that the ship would be able to run for at least 85 per cent of the operation time with only one diesel engine running!

The reasons for adopting the CODAG solution were listed as: lower capital cost, less weight, lower operating costs, greater operational flexibility, lower installation volume and lower maintenance costs. Reliability, service life and acoustic emissions were viewed to be equal in both systems. The all important component in this package is the gearbox. The German manufacturer, Renk, submitted a design that was subsequently selected to produce the transmission, which was required to operate in four modes:

• the 23.5MW gas turbine turning both propellers
• the two 7.4MW diesel engines turning two propellers
• a single diesel engine turning two propellers
• CODAG mode, with the gas turbine and two diesel engines turning both propellers.

The reduction gear ratio for the diesel engines running alone is not the same as when the gas turbine is also running; therefore each diesel engine transmission is, in fact, a two speed selectable gearbox. This allows the propellers to be driven at the best efficiency in the optimal speed range for single engine cruise propulsion and at elevated speed in CODAG mode.

To that is added a cross over facility, whereby either one of the diesel engines can drive both shafts. When the gas turbine is running, the gas turbine power is equally distributed to two intermediate shafts by means of a cross connect gear. A combination of multi-disc clutches and flexible couplings on these intermediate shafts connect to both main gear inputs. The diesel engines deliver their power via flexible joints shafts and fluid couplings to the secondary pinion via selectable gear stages.

Achieving these mechanical connections is complex. That, however, is not the end of the story, because a control system is required that can change the modes automatically, as demanded by the bridge. The F-124 is the first reported application of bus technology in combination with programmable logic controls (PLC) for the whole ship control system, including the main reduction gears. For this reason, there are several clutches in the design to permit high speed reaction intervals, giving smooth changeover from one mode to the other without affecting ship performance or damage to the components.

The propulsion gear control design includes PLC panels with touch screens. These operate through a digital control system that also varies the engine throttles and propeller pitch as required.

The same CODAG configuration is also adopted for the US Coast Guard’s large maritime security cutter (WMSL), which features a propulsion suite of twin 7.4MW MTU 20V1163TB93 diesel engines and a single GE LM2500 gas turbine rated at 22MW. The cutter can cruise at 18 knots with one engine driving both propellers but running both diesel engines plus the gas turbine yields a sprint speed of 28 knots. Renk is supplying a transmission similar in type to the F-124 programme for these vessels.

Meanwhile, the next German frigate programme, the F-125, due to enter service in 2014, will deviate from the F-124 arrangement by employing a CODLAG configuration. This will replace the two diesel engines with two 4.7MW electric motors, powered by four 2.9MW diesel driven gensets; a single 20MW GE LM2500 gas turbine is connected to a gearbox that splits the power equally between both shafts.

The French-Italian consortium building the multi mission FREMM frigate has also specified a CODLAG configuration; this will comprise twin 2.1MW electric motors mounted directly on the propeller shafts and a GE LM2500 G4 gas turbine of 32MW connected to a cross connect gearbox that splits the power equally between both shafts. Four 2.2MW MTU 16V4000-driven diesel generators provide electric power for power generation and electric propulsion.

The FREMM arrangement differs from the F-125 layout in that it has a propeller clutch on each shaft, so it can be completely disconnected from the cross connect gearbox, thereby eliminating any gear noise. Under electric propulsion a speed of up to 15 knots can be attained but the maximum speed of the frigates with all engines running is 28 knots.

Naval transmissions usually employ double helix gears. Standard spur gears are far too noisy, and single helix gears result in the problem of thrust that must be absorbed by bearings and heavier casings. Double helix gears are more expensive to manufacture and usually receive a highly accurate finish grinding process to ensure as near perfect tooth forms as possible.

Many external influences affect the performance of power transmission gears: reaction loads from external couplings, foundation and casing distortion, dynamic mass forces due to heavy seas and heat expansion generated in the bearings and meshing teeth.

These factors and the need for quiet operation require careful consideration of the tooth design for both pinions and gears, which dictate overall transmission performance. Fundamental is the type of gear – spur, single helix or double helix.

Spur gears transfer the loading from one pair of teeth to the next with a jump, causing force impacts and increased noise. Any attempts to decrease impacts by profile correction grinding only produce marginal improvements, with the basic disadvantage of lack of contact ratios remaining and high sensitivity for misalignment and tooth edge overload. For this reason, spur gears are considered to be unsuitable for warships.

Single helix teeth offer some benefits over spur gears where tooth impacts are reduced by the helix angle; however, a number of new problems arise. Even with a shallow helix angle of <10 degrees, increased axial loadings require separate thrust bearings. Low transverse contact ratios reduce the opportunity for noise reduction.

Increased axial forces cause shafts in bearings to tilt, giving unpredictable non-contact areas and uneven load distribution across the width of the tooth face. At lower than maximum torque loadings, uneven tooth contact patterns cause noise levels to increase. Axial forces cause distortion of the casing, requiring heavier construction to compensate. To compensate for casing and foundation distortions after installation, bearings require onboard re-adjustment after delivery.

These problems can, however, be designed out or adjusted for, and noise levels can be reduced by increasing the helix angle. Increasing axial loadings, however, makes for a new round of difficulties, which in conclusion leaves only one alternative – the double helix.

Used in more than 80 per cent of marine transmissions, the double helix gear combines several benefits with few disadvantages. It is self centring and unaffected by external forces acting on the casing. Gear loading is consistent throughout the entire power range, with low noise generation.

The double helix advantages can be summarised as: maximum transverse (tooth) contact giving smooth engagement and lowest noise emission; no axial impact to bearings due to self centring effect; even tooth contact pattern throughout all loading conditions; no axial loads to casing generated, thus a lightweight casing is achievable; and only bending and torsional deflections of pinions need be corrected by grinding.