The structural parameters and performance characteristics of the imported universal UN seals meet the design requirements. The UN-type seal features a Y-shaped cross-section, classifying it as a lip seal. One of its key advantages is low friction resistance—only 40% of that of a standard “Oâ€-ring. The initial friction during startup is not affected by the length of time the system has been idle or by internal pressure, ensuring smooth and stable operation. It also offers excellent stability, reliable sealing performance, and strong pressure resistance with a long service life and a wide operating pressure range. Additionally, its simple structure allows for single-turn sealing, reducing manufacturing costs, and it’s easy to install without requiring special tools.
Safety precautions are essential when using the seals and guards, which are equipped with an internal “Oâ€-ring. The piston's travel must be limited due to structural constraints. Since bolt elongation is much smaller than the designed piston stroke, installing a limit device is not practical. While the limit device serves as a safety function, it can cause more damage in case of misoperation. For example, at the Urumqi Locomotive Depot, a fixed tensioner was damaged and broke loose due to a misoperation. During each stretching process, the pull body, cylinder head, and main bearing bolts are tightened until the piston reaches the bottom dead center before stretching begins.
In October 2001, the first prototype was tested at the Hami Railway Locomotive Depot. During testing, the high-pressure hose connecting the two hydraulic cylinders was bent forward, coming into close contact with the crankshaft gear near the ninth main bearing. This created potential safety hazards. To address this, the joint position was adjusted so that the hose bent backward, improving visibility. However, this change led to the scrapping of the main bearing tensioner. After careful analysis and considering feedback from on-site operators, the design was modified: the lower part of the cylinder was raised, while the slot height was reduced. The high-pressure hoses were replaced with rigid straight tubes, and a safety shield was installed.
During testing, the four hydraulic cylinders used in the cylinder head bolt tensioner were connected in the hydraulic system. It was observed that after the system was unloaded, the piston in the last cylinder moved downward less than the other three, indicating poor return performance. To ensure synchronization among the four cylinders, reduce manual compression workload, improve efficiency, and enhance safety, the appropriate diameter of oil pipes and pipe joints were determined. Measures taken included repurchasing high-pressure hoses with a 6mm diameter, designing and manufacturing oil pipes of different diameters, conducting comparative experiments, and ultimately selecting the best option: a rigid oil pipe with a 5mm diameter.
In practice, it was found that the lever provided with the accessory for turning the nut had a short service life. This was caused by oil residue and sand between the nut and screw during disassembly, making the lever difficult to use. The lever material was either too hard, leading to breakage, or too soft, causing deformation. As a result, the lever eventually failed on the Diesel Engine. Although the valve stem was improved, the service life remained unsatisfactory, and this issue still needs to be resolved.
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