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Low Speed Concept—the innovative hydrostatic drive system
As the result of a profitability analysis of hydrostatic drive systems, Linde Hydraulics developed the Low Speed Concept and the large-displacement hydraulic motors that are required for this technology, which can even be installed directly in the drive train.
With the goal of improving the efficiency of hydrostatic drives in mind, Linde Hydraulics analysed the individual drive components and their interaction in mobile units. The analysis shows that the main losses stem from the hydraulic motor. The reason for this are the windage losses of the rotating group during the rotary motion in the oil. As the loss increases quadratically with the rotational speed, Linde Hydraulics analysed the traditionally high hydraulic motor speeds of conventional propulsion drives in further detail.
In order to achieve an overall conversion ratio of around 10:1 between the diesel engine speed and the wheel speed in propulsion drives, high hydraulic motor speeds with low torques are reduced over several transmission stages in the standard configuration, in order to achieve the low speeds that are required at the wheel or on the track chain at high torques. In this conventional configuration, the considerable power loss resulting from the windage loss at high engine speeds is further intensified by the mechanical loss in the required step-down gearing.
On this basis, Linde Hydraulics developed the Low Speed Concept. This concept reduces the speed levels and the number of components in the drive train, so that the diesel engine speed is converted as directly as possible into wheel speed.
Large variable displacement motors of the Series 02 (HMV 210-02 and HMV 280-02) form the basis of the Low Speed Concept from Linde Hydraulics. While the maximum displacement of the hydraulic motor is doubled, the machine achieves higher speeds and thus higher productivity in spite of lower rotational speeds.
The basic principle of the Low Speed Concept is that the power loss of a hydrostatic motor is more or less linear in relation to its nominal size at a given operating status. The result is a clearly reduced power loss with larger hydraulic motors and reduced speeds. Thus, the speed reduction in the hydraulic motor leads to an increase in overall efficiency.
This Low Speed Concept offers substantial potential for improving propulsion drives in terms of power consumption. A comparison shows that the maximum speed of the vehicle increases when the size of the hydrostatic motor increases and the rotational speed decreases accordingly. At the same time, fuel consumption decreases at all operating points. Further savings are possible from reduced acquisition costs for step-down gearing and the consequently reduced power loss. The vehicle drive train is therefore simpler and more compact and offers a more direct and immediate driving experience with less play and trail.
As there are fewer mechanical components and the rotational speeds are lower, the Low Speed drive is also significantly quieter than a conventional drive. This significantly reduces the costs for additional noise insulation, and can even eliminate them altogether depending on the design of the particular vehicle model. Furthermore, the overall maintenance costs in drive systems with fewer mechanical components are substantially lower. The rotational speed reduction in the hydraulic motor leads to an increase in the overall efficiency, despite a larger unit being used.
The innovative design principle of the pumps and motors from Series 02 supported this new approach and actually made it possible to begin with. With Series 02, Linde has already made a decision in favour of design features that are perfectly suited to the idea of a low speed drive and in particular for the high torques that are required for success with this concept. The hydraulic motors of Series 02 cover a nominal displacement range of 28 to 550 cm3/ref.
The successful interplay between rotational speed and control accuracy with respect to vehicle operation, which is one of the basic properties of the swash plate control system for Series 02 motors, has proven to be particularly beneficial.
During the development, great emphasis was placed on refined running properties, in particular at low rotational speeds. At very low speeds hydraulic motors from Linde Hydraulics provide a significantly more constant and smooth torque than conventional bent-axis motors, which have widely fluctuating torque characteristics. In practical applications, this means that the hydraulic motors of Series 02 have excellent start-up characteristics when starting heavy mobile machines. Moreover, they make it possible to control and manoeuvre precisely even at low rotational speeds, i.e. when moving heavy loads with precision.
The Through-Drive Motor from Linde Hydraulics that is integrated directly in the drive train makes it possible to omit the drop box and leads to maximum torque conversion.
The swash plate design of these motors with their 21 ° swash plate angle leads to extremely compact installation dimensions. The inline design of Series 02 motors makes it possible to bring the motor closer to the consumer during the installation and additionally allows for through-shaft versions:
Low Speed Integration
The Through-Drive Motor, a hydraulic variable displacement motor which delivers the torque at both shaft ends, can be installed directly in the drive train. The drop box, which is usually required in conventional drive concepts, can therefore be omitted. The diverse benefits of the Low Speed concept are even more evident with Low Speed Integration.
Optimum implementation of the Low Speed Concept means: direct conversion of the diesel engine speeds into wheel speeds.
Low Speed Direct
The direct drive is the simplest and most consistent design of the Low Speed Concept. In the new Linde truck series 39X, the direct drive has already been implemented in a highly effective manner with large fixed displacement motors in swash-plate design.
The Low Speed Concept has already been tried and tested many times. In close cooperation with the vehicle designers, Linde Hydraulics enters new territories that not only provide technically interesting systems, but above all make new vehicles more economical, efficient and environmentally friendly.
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