The assembly of the cardan shaft on a vehicle involves connecting the flanges at both ends to the output flange of the diesel generator set. When attaching the gearbox input flange to the auxiliary gearbox flange, it is essential to tighten the fastening bolts diagonally as per process requirements, ensuring that each bolt is tightened with uniform torque. However, in real-world scenarios, due to limited space during installation, operators sometimes skip proper tightening procedures, only securing the upper half of the bolts. This can lead to uneven tightening and poor contact between the two flange surfaces.
When the diesel engine is running, the crankshaft applies a torque 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 shaft, and F3, which introduces an additional bending moment on the cross pin. If the flange surfaces are not tightly joined—especially if there’s a gap on one side—and the tightening torque is inconsistent, the flange may shift slightly relative to the crankshaft flange. This movement creates an axial alternating impact force on the bolts, potentially loosening them. Additionally, under the influence of F2@R, this displacement generates shear forces on the coupling bolts, which, if left unaddressed, could eventually cause bolt failure.
Another key factor is horizontal misalignment. The excitation torque in the system typically comes from changes in the working torque transmitted through the universal joint, rather than from unbalanced inertia, which has already been addressed during dynamic balancing. According to the principle of universal joint transmission, when the input and output shafts are parallel, the dynamic torques from both joints cancel out, resulting in no excitation force. However, if the shafts are not aligned, the generated torques become unsynchronized, leading to vibration and increased stress on the system. The greater the offset or angle between the axes, the higher the excitation torque becomes.
In the DF7 locomotive's auxiliary drive system, the horizontal offset of the cardan shaft is 0.18 mm, which transmits the entire auxiliary system’s torque. This means even small deviations can create significant excitation forces. The design specifies that the horizontal deviation between the diesel engine’s crankshaft centerline and the universal joint shaft should not exceed 10 mm. However, during overhauls, it is often found that the deviation reaches up to 11 mm, contributing significantly to bolt loosening. The tolerance for flange alignment is 0.5 mm, but some flanges exceed this, causing increased vibration and premature loosening. Additionally, the lock washer is too thin, and the bolts lack sufficient strength and rigidity.
To address these issues, several improvements have been implemented:
1. **Strict Bolting Procedures**: Ensure all flange connection bolts are tightened diagonally according to the process specifications. After tightening, check each nut to confirm uniform torque and ensure the flange surfaces are properly aligned.
2. **Control Horizontal Misalignment**: Maintain the horizontal deviation within 4 mm. If the dimensions don’t meet the requirements, re-commission the unit or cut the gearbox base as needed. During pre-installation, ensure the diesel engine’s crankshaft and cardan shaft centerlines are aligned horizontally.
3. **Thicker Lock Washers**: Increase the thickness of the anti-loosening washers from 1.5 mm to 2 mm. This improves the washer’s resistance to deformation, helping maintain consistent bolt tightness.
4. **Stronger Bolts**: Replace the flange coupling bolts with 35CrMo steel to enhance their strength and ensure reliable transmission of force.
5. **Flexible Coupling Installation**: Some DF7B locomotives now use a flexible coupling between the universal joint and the front gearbox, reducing vibration. After testing nearly 20 units over two years, the incidence of bolt loosening dropped from 53.7% to 7.1%, showing a significant improvement.
These measures have greatly improved the reliability and performance of the cardan shaft system, reducing maintenance needs and increasing operational safety.
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