Comparison of hydrostatic transmission and dynamic pressure transmission of tire loader
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As a mobile machine, the wheel loader is extensively used for earthmoving, loading, lifting, and transporting loose or piled materials. On one hand, it is a large traction tire loader known for its excellent maneuverability; on the other hand, it can quickly change work sites under high-speed driving conditions. This combination of strength and agility makes it both economical and practical for a wide range of applications.
The hydrostatic drive system, especially when combined with a subsequent load-shift transmission, offers several advantages over the traditional hydrodynamic torque converter system. It has already been successfully applied in wheel loaders with power ratings up to 60 kW. As the power increases, the application of hydrostatic transmission becomes more complex due to the broader torque range. Therefore, an additional shifting mechanism is often required, which may lead to brief interruptions in traction. However, in loaders with engine power ranging from 60 kW to 100 kW, the static drive system is becoming increasingly common.
In higher-powered models, such as those with 200 kW engines, 25,000 kg of load capacity, and a maximum speed of 45 km/h, more advanced transmission systems beyond the torque converter have been introduced. The hydrostatic transmission provides significant benefits over dynamic pressure systems. For example, it allows for a wide control range, enabling smooth or minimal gear shifts in the additional transmission. Its compact design also supports integrated transmission schemes that can be installed in modern wheel loaders in various orientations—either tilted or lateral to the direction of movement. Additionally, in a closed-loop system, torque can be transmitted in both directions.
This feature is particularly useful during braking, as it can generate a "support" force on the drive unit, reducing brake wear and lowering operating costs. When braking, the return flow energy can be redirected to other hydraulic pumps and actuators, further enhancing efficiency. One of the key advantages of hydrostatic transmission is its flexibility in open- and closed-loop controls, allowing it to efficiently utilize engine power at any speed. As a result, optimizing the transmission system for tire loaders has become a critical focus in current and future R&D efforts.
When comparing the traction and speed capabilities of torque converters and hydrostatic systems, it's clear that hydrostatic drives offer superior performance. Under full engine power, the hydraulic traction typically reduces by about 80% of the loader’s weight. This level of traction is not only sufficient but also prevents wheel slippage when cutting into piles of material—a common issue during loading operations.
Compared to dynamic pressure systems, where traction rises rapidly, hydrostatic systems significantly reduce tire wear. Moreover, they provide maximum traction while maintaining hydraulic pressure, making them ideal for heavy-load tasks. In contrast, dynamic pressure systems can only deliver around 50% of the original traction due to reduced motor speed.
The torque capacity of a torque converter follows a quadratic relationship with speed. This means it can handle all the torque at a specific speed—usually at the load speed—but drops sharply below that, leading to reduced traction and speed. In contrast, hydrostatic transmission can theoretically cover the entire engine characteristic curve, offering flexible speed and torque distribution. Combined with different control strategies, it also helps optimize noise and energy consumption.
By regulating the drive motor speed at the optimal point, the driver can simply adjust the accelerator pedal to request the desired acceleration, traction, or speed. The goal of transmission optimization is to ease the driver’s workload, improve working conditions, and allow them to focus on critical tasks. In addition to performance benefits, modern hydrostatic transmission systems are also cost-effective compared to torque converter designs, especially when paired with specialized drive power.