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One reason: Concentricity between the hollow shaft and the rotating body (roller) is not enough. Disposal measures: Check the size and accuracy, and adjust the fixed connection with the rotating body (roller).
The second reason is that the assembly machine has low precision (face straightness, radial, large axial clearance) and large vibration. Disposal countermeasures: Find out the reasons, repair and match equipment.
Cause 3: Slurry, dirt, and other foreign matter enter the frictional contact surface. Disposal measures: Add filter or decontamination device, modify the type of rotary joints.
Cause four: The use of pressure, temperature, and speed exceeds the range of optional rotary joints. Disposal measures: Study the operating conditions, ask the research unit or the manufacturer, and adapt the rotary joint type.
No matter what type of rotary joint, it must be properly maintained and maintained during daily use. The rotary joint will be durable and not easy to leak. 
Under normal circumstances, the leakage of the rotary joint is generally caused by the following reasons: