The International Steam Pages

Drive trains for Steam-Turbine Locomotives: Direct (gear-train) vs electric.

Harry Valentine, Transportation Researcher. writes (9th March 2001, click here for a June 2001 update.)

The following note was added on 24th September 2001:

I was contacted by a steam loco enthusiast who inquire about using eccentric cams on driveshafts, inside roller bearing assemblies which activate pushrods. I was referred to a site which featured a Wankel steam engine, as well as an X-4 reciprocating layout (fully animated) using pistons, and eccentric cam with a roller bearing instead of a crankshaft and connecting rods which were restrained to in translation mode only (no rotation movement). After examining this layout, it became obvious that high-torque can be transmitted at low speeds by this method,which enables forward, reverse as well as transmitting power efficiently through right angles (or any other angle). Hence,in future steam turbine locos, a compact and efficient mechanical system is possible, saving the high cost of all-gear drive systems or an electrical transmission.

With renewed interest in future steam locomotive traction, an interesting debate has evolved with regard to the respective merits of direct drive via gears (The Penn. Central S2 locomotive; the LMS Turbomotive and the Swedish turbine 2-8-0) as compared to an electrical transmission (Norfolk&Western, Great Northern). Technology has changed and improved since the days of the original steam-turbine locomotives. Some of the technology used on the original steam-turbine locomotives is now quite obsolete, such as the Direct Current Generators and DC motors which experienced frequent commutator problems, or the need for forward and reversing turbines on the direct drive variants.

Steam-Turbine-Electric Variant:

Proposed, modern steam-turbine-electric locomotives can use sealed, oil-cooled AC alternators driven by triple or quad turbines, in a 1-2-4 or 1-2-4-8 power ratio. The wheels would be driven by 3-phase AC induction motors, using power coming via electronic control gear such as inverters, from the turbine driven alternators. Most of this equipment is already proven in railway operation, in modern diesel locomotives. So a modern steam -turbine-electric variant would share much componentry with existing diesel locomotives, enabling much repair and maintenance to be undertaken by existing railway technical staff. Given the extreme high power capability of a proposed modern steam-turbine-electric locomotive, the bogies (trucks) under the locomotive as well as the tender unit could be powered, in the fashion of a slug unit and enabling superior tractive capabilities hauling heavy freight trains in extreme, heavy-haul service.

Steam-Turbine-Direct-Drive Variant:

The direct drive concept has merit, in that it is lighter, less costly and may involve less complexity. The concept used in Sweden, Britain and the USA all delivered power to the rails using conventional steam locomotive driving wheels, complete with side-rods. A separate reversing turbine was installed, probably due to the lack of availability of a suitable bi-directional gearbox. The advent of diesel railway traction saw the development of hydraulic gearboxes, incorporating a built-in reverser to enable bi-directional operation. Diesel hydraulic locomotives such as those built by Krauss-Maffei in Germany, have a successful service record on smooth, well-maintained track in Western Europe. In the USA with its rougher track, diesel hydraulics experience problems in heavy-haul freight service. Unlike a steam-locomotive which used side-rods to connect the drive wheels, the diesel hydraulic locomotives used drive-shafts (cardan-shafts) and gears to power and lock the driving wheels together in unison. While this practice assured high traction under ideal conditions, very high stresses were placed on the gears on each bogie, resulting in accelerated gear wear and high costs of drive-gear replacement. The older system of using the turbine to power only one axle, which in turn powered the remaining drive-axles using side rods, avoided the problem of gear wear and costly gear replacement.

Direct-Drive using side-rods:

Freight and Goods service:

The system of using large wheels and side-rods had merit in that it avoided a complex and expensive inter-axle gearing system. Gear life is reduced as power is increased, or heavier and more expensive gearing needs to be used to transmit very high power levels to multiple axles. A side-rod accomplishes this at lower expense and with very little complexity. Access and maintenance are usually easy and straightforward. The side-rod system does favour low-speed, heavy-haul direct drive steam-turbine locomotives, such as a 2-10-2 + 2-10-2 Garrat layout. Each 10-wheel coupled set can be powered from 4-turbines each, in the 1-2-4-8 power ratio, to assure maximum efficiency at 15-power settings which can cope with a variety of rail power requirements. In freight, bulk and heavy-haul service, extreme power delivery would be needed at relatively low speeds. The gear system driving the coupled drive-wheelsets needs to be capable of handling extreme power over very prolonged periods of time and over a long service life. One successful method of transmitting high mechanical power through small gear systems, is to use double-intermediate idler-type gears. The twin-countershaft truck gearboxes are an example of this .... a powered input gear drives two others which are directly meshed to it as well as a driven output gear; i.e; the power is divided to two gears then back to one gear. A similar approach may be workable in a direct-drive, heavy-haul steam turbine locomotive.

Turbines where shaft power may be taken off either end, i. e; the steam inlet end or the exhaust end, would be preferable. Two turbines may be mounted parallel to the drive axles, on either side of an input gear, the exhaust-end on one facing the inlet end of the other, on the same input shaft. A 500-Hp turbine and a 1, 000-Hp turbine may be mounted on one input shaft, on either side of an input (bidirectional) gear, whereas a 2000-Hp and 4, 000-Hp turbines may be similarly mounted. The input gear would drive two-intermediate gears, which in turn would drive another single gear, mounted to a frame-mounted quill-drive. The two low-power turbines could be mounted transversely on one side on the main drive axle, whereas the two high-power turbines could be mounted on the other side, in similiar transverse fashion. In extreme power situations, parallel gears seem preferable to a system involving bevel or spiral bevel gears, which have a higher sliding friction.

The main drawbacks of the transverse turbines include maintenance access to the turbines as well as the input gears and their reversing units. In heavy-haul service, a direct-drive system would stretch their gear systems to the limits, which could ultimately become a major maintenance area, involving high costs and downtime.

The Turbomotive & Passenger service:

There is a proposal in the UK to develop a replica of the LMS turbomotive, a high-speed, direct-drive, steam-turbine locomotive using very large wheels as well as the classic side-rods to power the coupled set. In the high-speed case, a multi-turbine system could drive through a modified Voith-type railway transmission (gearbox), which features the built in reverser for full bi-directional operation. Power to one axle could be achieved via a quill-drive powering one-axle, with side-rods transmitting power to the other axles. The largest turbine of the 1-2-4 or 1-2-4-8 power ratio set would drive directly into the modified gearbox (its fluid flywheel would not be needed), while the smaller-turbines may drive into the same input via a gearbox connection. Several turbine types can deliver shaft power from either the steam-inlet end, or the steam exhaust end ...... this allows 2-turbines to drive into the same input gear, while being mounted on either side of the gear, back to front to reduce gearing complexity and expense. One-way roller clutches may be used so that out-of-service turbines may remain idle, until preheated prior to being engaged. A triple turbine layout (500-Hp + 1, 000-Hp + 2, 000-Hp) could work well in a modern replica of the LMS turbomotive. In its original form, it was powered by a single 2, 600-Hp turbine, while a modern replica could yield 3500-Hp from 3-turbines, or 7500-Hp from 4-turbines. In the modern replica, the middle-drive axle may have to carry the quill-drive, so as to make room for the bi-directional gearbox.

Whereas maximum power may be needed in the forward direction, 3, 500-Hp should suffice as being sufficient in the reverse direction. Hence, the 4, 000-Hp turbine in the 500+1, 000+2, 000+4, 000-Hp power-pack could drive direct, bypassing the gearbox altogether. The same frame-mounted quill-drive could carry 2 x 90-degree drives and ring-gears, that is, power from the turbines goes to one axle via gears and to the rest via the side-rods. This layout could save the cost of an additional gearbox, while enhancing performance and reliability in the forward direction. Running in reverse, it is not likely to ever need more than 3, 500-Hp pulling a passenger train, or rather, such occasions would likely be extremely rare. The gearing of the 4, 000-Hp turbine could be set to enable the best efficiencies to be realised at speeds of excess of 70-miles-per-hour.

Among the drawbacks of the longitudinally mounted multi-turbine system, are access and maintenance to the turbines, the gearbox and the final drive gear. The final drive gear would be a 90-degree system comprising close-ratio bevel or spiral-bevel primary gearing, driving helical secondary gearing. Running high levels of power through gears usually involves accelerated wear and tear, making them a certain maintenance item.


A direct drive steam turbine locomotive using side rods may ultimately also use gears. Gear wear will be major maintenance item, requiring repair/replacement costs and added downtime. The high power that will be sent through the reduction and reverse gears will be higher than that involved in diesel-electric, all-electric or steam-turbine-electric traction. Gear wear has the potential to become a cause of concern in heavy-haul freight operations. A direct drive system may also require multiple gear ratios, to maximize operating efficiency, since turbines ultimately yield their optimal efficiency when running at maximum power and over a limited speed range. A steam-turbine-electric system may cost more initially than a direct-drive side rod steam-turbine locomotive, yet it can operate at higher levels of reliability over longer durations, using modern technology.

An all-gear system:

A direct-drive steam-turbine locomotive using gears to transmit power, could have the appearance of a modern diesel-hydraulic locomotive. It would utilize gear-driven, powered bogies (trucks) and Voith type railway transmissions, which could offer a reduction gear (for starting a heavy train) as well as a built-in reverser for bi-directional operation. To be competitive against proposed modern piston steam locomotives, including those using uniflow piston systems, a power output at the drawbar will need to exceed that of a modern diesel locomotive.

In the Krauss-Maffei diesel-hydraulic locomotives, two-diesel engines drove into two bi-directional railway gearboxes, which in turn drove the powered bogies (trucks) at either end, using cardan-shafts and a bevel-gear system on each axle. While such layout may work on very smooth track, operation on rough track accelerates gear wear and wear on the cardan shafts. The drive train may need to be modified, using inter-axle torsen-type differentials to allow variations in axle speed. A quill-drive may be needed to transmit power between the final drives and the axles, so as to reduce wear and tear on the cardan shafts. Flexible couplings other than a universal joints may need to be used on the drivelines, to improve long-term reliability and longevity while running on rough track.

With a boiler system mounted above the frame, the turbines will have to be mounted longitudinally, below the frame. Several turbine-gearbox variations are possible:

  1. Each bogie truck) is connected to its own gearbox and turbine sets, in a 1-2-4 power ratio (2 x 500-Hp, 2 x 1, 000-Hp, 2 X 2, 000-Hp) giving 6-turbines and 7, 000-Hp max.

  2. A single quad-turbine powerpack drives into two gearboxes, one for each axle. There will be a total horsepower of 7500-Hp and 15-power settings at maximum efficiency.

  3. A single quad-turbine powerpack drives into a single gearbox, with power divided between both bogies (trucks). This system too will yield 7, 500-Hp max and 15-power settings .... and will also use very long drive-shafts which could give rise to harmonic vibrations.

Using some form of direct-drive between turbine and power-axle will involve some type of gearing, and even drive shafts. The shorter and the fewer the number of driveshafts, the less the maintenance and problems of such items. A steam-turbine direct drive may have some advantages hauling fast passenger trains along non-electrified routes. The concept may need much more research to develop it for use in heavy-haul freight services, where a modern steam-turbine-electric could have a competitive edge, given the present state-of-the-art in gearbox design.

Conclusions: An all-gear system using cardan shafts may work an very smooth track, however, based on past experience with this type of transmission on rough track, a steam -turbine-electric may be the more desirable option in the longer term. Mounting the turbines and the gearboxes directly into the bogies (trucks) in the fashion of a monomotor bogie (single large electric motor powering 3-axles), would add to the complexity of the bogie and cause a maintenance nightmare.

Harry Valentine, Transportation Researcher;

Harry has sent the following update:

Several discussions have been ongoing re steam turbine loco drivetrains. Electrical transmission equipment and associated gears does involve considerable expense. The old side-rod system, though considered obsolete, actually has considerable merit in terms of lower price and low friction when roller bearings are used, i.e, high efficiency in low-RPM applications and very high torque capabilities.

In view of this, some research was done, using as a basis an old Swedish steam turbine loco. Transmitting power in the same direction between parallel driveshafts using side-rods is well prove. However, it has been discovered that power can also be transmitted between parallel shafts rotating in OPPOSITE direction (a reverse without gears or electric motors). Even more, power can be transmitted using the side-rod approach, between driveshafts at 90-degrees to each other. This can be done quite simply if the two driveshafts are located at different levels, but a little more side-rod complexity if they're located at the same height (same plane).

Side-rod (connecting-rod) drive systems using roller bearings and levers, could be used in a new generation of lower-speed, heavy-haul freight/goods steam (turbine) locomotives. The concept may be possible for use in passenger steam locos running at speeds below 130-mi/hr for brief periods. The advantage is cost savings over complicated hydraulic or electrical transmissions. There is also a proposal from South Africa involving the use of chain drive between small steam engines (bogie mounted) and drive axles (the Steam Queen group).

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Rob Dickinson