The assembly of the cardan shaft on a vehicle involves connecting flanges at both ends to the output flange of the diesel generator set. When attaching the gearbox input flange and the auxiliary gearbox flange, bolts must be tightened diagonally as per process specifications to ensure even torque distribution. However, in real-world scenarios, due to limited installation space, operators sometimes skip proper tightening procedures. They may only tighten the upper half of the bolts initially and then tighten the lower half later, leading to uneven clamping forces and gaps between the flange surfaces. This improper tightening can cause misalignment and uneven stress distribution during operation.
When the diesel engine is running, the crankshaft applies a torque of F1@R (where F1 is the force from the crankshaft to the cardan shaft, and R is the radius of the cross pin). This force is split into two components: F2, which rotates the universal joint, and F3, which creates an additional bending moment on the cross pin. If the flange surfaces are not fully engaged and the bolt torques are inconsistent, the cardan shaft flange may shift relative to the crankshaft flange under the influence of F3. This movement generates axial alternating forces on the bolts, potentially loosening them over time. Additionally, the shear force caused by this displacement could lead to bolt failure if not addressed promptly.
Horizontal misalignment is another critical factor. The excitation torque in the auxiliary system may originate from either unbalanced inertia or changes in the transmitted torque through the universal joint. While dynamic balancing tests address the former, the latter remains a key concern. When the input and output shafts of the universal joint are not parallel, the additional torques generated by the two joints do not cancel out, creating vibration and stress on the system. The greater the misalignment, the higher the excitation torque, directly affecting the stability of the system.
In the DF7 locomotive's auxiliary drive system, the horizontal offset between the cardan shaft’s input and output shafts is 0.181 mm. This small deviation can generate significant excitation torque, especially since it transmits the entire auxiliary system’s power. The design allows for a maximum horizontal deviation of 10 mm between the diesel engine’s crankshaft centerline and the universal joint shaft. However, during maintenance, deviations often exceed this limit, reaching up to 11 mm. This misalignment contributes significantly to bolt loosening. Additionally, some flanges exceed the tolerance of 0.5 mm, increasing vibration and reducing bolt retention. The thin lock washers and insufficient bolt rigidity further exacerbate the issue.
To address these problems, several improvement measures have been implemented:
1. **Strict Bolt Tightening:** Bolts must be tightened according to process guidelines, ensuring diagonal tightening and uniform torque. After tightening, each nut should be checked to confirm even clamping and proper flange contact.
2. **Limit Horizontal Deviation:** The horizontal deviation should not exceed 4 mm. Any dimensions that don’t meet the requirements must be corrected, such as repositioning the gearbox base. During installation, efforts should be made to align the diesel engine’s crankshaft and cardan shaft centerlines horizontally.
3. **Thicker Lock Washers:** Increasing the thickness of the anti-loosening washer from 1.5 mm to 2 mm improves its strength and resistance to deformation, ensuring better bolt retention.
4. **Stronger Bolts:** Replacing standard bolts with 35CrMo alloy bolts enhances their strength and ensures reliable transmission of torque.
5. **Flexible Coupling Installation:** Some DF7B locomotives now use a flexible coupling between the universal joint and the front gearbox, effectively reducing vibration. After testing nearly 20 locomotives over two years, the incidence of bolt loosening dropped from 53.7% to 7.1%, showing significant improvement.
These adjustments have proven effective in improving the reliability and performance of the cardan shaft system in diesel locomotives.
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