Liebherr has been developing, designing and manufacturing gearboxes for construction machines and other sophisticated applications for more than 60 years and has also been working on the development of hydrostatic-mechanical powersplit transmissions. The first version to be developed is a gearbox designed specifically for mobile machinery that can fulfil strict requirements in terms of dynamic performance and reversing. Liebherr investigated the specific requirements for an optimum powersplit transmission and then designed a customized transmission based on this.
Hydrostatic-mechanical powersplit transmissions have been used in tractors for years. Construction machinery is now also set to benefit from the advantages of these systems, which is why a great deal of development work is currently being undertaken in the supply industry. Improved diesel consumption is the primary objective of this development. Alongside the harmonious interaction between driver and machine, which itself has a huge impact on performance, the requirements in terms of dynamic performance play an equally important part and must therefore also be taken into account when designing transmissions. Detailed knowledge about the respective machine's deployment conditions is a must here.
State-of-the-art drivetrains
State-of-the-art mobile machinery drivetrains, with strict requirements in terms of reversing, represent the basis for development. Hydrostatic drivetrains, powershift transmissions with hydrodynamic converter and recently also hydrostatic-mechanical powersplit transmissions are used here.
Hydrostatic transmissions
The strengths of hydrostatic gearbox concepts primarily lie in low losses during reversing and insertion into the pile of loose or bulk material. This ultimately leads to lower energy or fuel consumption during the load cycle. Another advantage of these concepts is the fact that they use non-wearing hydrostatic brakes. However, systems of this kind also display weaknesses compared to converter transmissions with lock-up clutch due to lower efficiency at high speeds.
Powershift transmissions with hydrodynamic converter
Powershift transmissions with hydrodynamic converters turn the advantages of hydrostatic transmissions into disadvantages and their disadvantages into advantages. They can impress with high efficiency by closing their lock-up clutch, in particular at high speeds. This higher gearbox efficiency allows lower diesel consumption to be achieved at constant speeds. During reversing and when filling the bucket, on the other hand, converter transmissions do not perform as well in terms of diesel consumption. This is due to the fact that the converter needs a high engine speed in order to achieve a high torque level, which therefore also requires a high power output from the diesel engine. When operating at low speeds, some of this power output is dissipated in the form of heat during the filling process due to slippage in the converter.
Hydrostatic-mechanical powersplit transmissions
Hydrostatic-mechanical powersplit transmissions represent the third principle now being used. The goal here is to combine the advantages of the gearbox principles described above.
One thing that all concepts already presented and available on the market have in common is the fact that they are operated with two hydrostats and drive with a power split in second gear. However, there are differences regarding first gear. Three different options are available here. First gear can either be fully hydrostatic or employ a power split. When using the power-split first gear concept, the gearbox can also be designed as an input-coupled or output-coupled unit, whereby the reversing gearbox can be located at the input or the output.
With the first of these options, a hydraulic unit is connected directly to the diesel engine, which makes first gear a fully hydrostatic gear. This then leads to an input-coupled power-split transmissions in second gear, which can generate idle power in certain operating conditions. This is the case if the hydraulic unit on the diesel engine switches to motor operation, which then feeds additional hydraulic power to the gearbox's input shaft alongside the power supplied by the diesel engine. When idle power flows, the power flowing in the gearbox's input shaft must be higher than that which the diesel engine itself can generate. However, this additional power also leads to additional losses that compromise efficiency and therefore have a negative impact on the gearbox concept.
The clutches which need to be engaged/disengaged during reversing represent a challenge for transmissions which already employ a powersplit concept in first gear. If the reversing unit is located at the gearbox's input, the entire planetary gearbox – including the hydrostat that is connected to the gearbox's input – must change its direction of rotation extremely quickly. The key here is to strike a balance between high dynamic performance during the change of direction and achieving a level of rotary acceleration that the hydrostat can handle in the long term without overloading the clutches. However, this version offers better efficiency in first gear than the hydrostatic version.
Innovative new concept from Liebherr
Liebherr also believes in the principle of hydrostatic-mechanical powersplit transmissions. The Liebherr gearbox has been specifically designed for applications requiring high dynamic performance. Within a development project of this kind, the engineers at Liebherr collaborate very closely with specialists from the respective vehicle manufacturer from the outset, which allows them to address the specific requirements of the application in detail. All components within the drivetrain are also supplied by Liebherr.
In this configuration, one hydrostat is connected to the diesel engine shaft, while two additional hydrostats are connected to the planetary gearbox. This offers the advantage that no hydrostat needs to be switched over to a different gearbox shaft. The complex clutching/declutching operations of the hydrostats, which require additional shafts, clutches and bearings, are thus eliminated altogether.
A further big advantage is that the gearbox output shaft doesn’t have to be switched with clutches. This means that the system does not require large clutches, which would otherwise be needed to transfer the high level of torque. These clutches employ a larger friction lining diameter than the clutches on the input side, as they must handle greater torques. The drag losses in the clutch increase disproportionately to the clutch diameter. The fact that these large clutches are not required therefore allows efficiency to be further optimised.
The displacement of the hydrostats must not be altered while changing from first gear to second gear. The change from first gear to second gear is performed at a relative speed of zero for the forward clutch (Cf) or backward clutch (Cb), which means that the clutches generate very little heat and are subject to minimal wear. This makes it easier to change gear under load, both with regard to clutch load and gear change logic.
By using three hydrostats, Liebherr also offers further advantages over existing concepts. For example, the newly developed concept also allows vehicles to be driven output coupled when using a split-power gear. This makes it possible to bypass drops in efficiency associated with idle power. In addition to this, only one clutch operation needs to be performed between 0 and 40 km/h at a synchronous switching point. All other changes in speed are performed purely by adjusting the hydrostats.
Outlook
The gearbox is currently in the testing phase. Various simulation tools and test bench experiments are being used to record various parameters and test all the functionalities.
The mid- to long-term goal is also to develop further hydraulic-mechanical powersplit transmissions for other application profiles.
Liebherr's overall approach and objective is to optimally match powersplit transmissions with their target vehicle. In this vein, gearbox designs are optimized to meet the requirements of their respective deployment in close cooperation and coordination with customers. This is the only way to ensure that the individual requirements of a specific application are fulfilled as effectively as possible.
