The Dual Clutch Transmission (DCT), also known as the Direct Shift Gearbox (DSG), is a revolutionary advancement in automotive technology. Although DCTs have been used in racing cars for nearly 70 years, their integration into mass-produced vehicles is relatively recent. This technology combines the best features of manual and automatic transmissions: it offers the responsiveness and efficiency of a manual, along with the comfort and ease of an automatic. Additionally, DCTs are more fuel-efficient, making them an attractive option for modern vehicles.
In China, the rapid growth of the automotive industry and the vast consumer market have significantly accelerated the adoption and commercialization of DCT technology. As a result, DCTs are becoming increasingly common in both domestic and international car models.
**DCT Structure Features**
As shown in Figure 1, the external structure of a DCT consists of two separate outer casings, which are fully enclosed to protect internal components. The clutch shell and transmission case are made from lightweight aluminum alloy, enhancing performance while reducing weight. The assembly process involves precision CNC machining to ensure accuracy, and all parts are carefully assembled to form a complete unit. Despite its advanced design, the overall shape of a DCT is similar to that of traditional manual and automatic transmissions.
Figure 2 illustrates the internal structure of a DCT, highlighting its key difference from conventional transmissions: it has two input shafts and two output shafts. These hollow shafts work together to transmit engine power, while two clutches control gear engagement. This dual-clutch system allows for faster and smoother gear shifts, as one clutch can be engaged while the other prepares for the next gear.
Figure 3 shows the inner input shaft assembly, where the inner and outer input shafts are arranged in a nested configuration. Each shaft is responsible for different gear sets, with the inner shaft handling gears 1, 3, and 5, and the outer shaft managing gears 2, 4, and 6. Shift forks move the gears into place, ensuring smooth operation.
Figure 4 displays the internal projection and physical layout of the DCT, showing how the output shafts connect to the differential. This component helps manage the rotational speed difference between the wheels during turns, improving traction and reducing tire wear.
**Shaft Machining Challenges**
One of the most complex aspects of DCT manufacturing is the machining of its hollow shafts. The inner input shaft, for example, is long and thin, with a total length of 380.5mm and an outer diameter ranging from 22mm to 26mm. Its inner hole is 12mm in diameter and extends 293.3mm deep. Achieving high precision is critical, as the straightening runout after heat treatment must not exceed 0.03mm. To address these challenges, manufacturers use advanced German technologies such as TBT deep hole drilling and MAE automatic straightening systems.
**Inner Input Shaft Machining Process**
The inner input shaft undergoes a detailed manufacturing process, starting with forging and normalizing. After rough machining, the part goes through hobbing, rolling teeth, and spline processing. Drilling deep holes and preheating are also essential steps. Following heat treatment—carburizing, quenching, and tempering—the shaft is cleaned, shot-blasted, and finally straightened. High-precision grinding ensures the final product meets strict tolerances.
**Inner Input Shaft Alignment Process**
The inner input shaft is made from low-carbon alloy steel (20MnCrS5), which is carburized and quenched to achieve high surface hardness and strength. To prevent cracks during straightening, two process options were tested. Option 2, which includes tempering before straightening, proved more effective in reducing crack formation. A MAE automatic straightening machine equipped with a crack detection system ensures quality and reliability.
**Conclusion**
The DCT represents a significant leap forward in transmission technology, offering improved performance and efficiency. While the manufacturing process presents unique challenges, especially with slender hollow shafts, ongoing advancements in machining and heat treatment are helping to overcome these obstacles. As the technology matures, DCTs are expected to play a major role in the future of automotive engineering. By addressing current limitations and fostering collaboration across the industry, we can ensure that DCTs continue to deliver superior performance and reliability on the road.
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