Causes and Solutions for Piston Slipping or Crawling in Automotive Interior Hydraulic Press

(1) Internal Sticking of Hydraulic Cylinder:

• Incorrect assembly, deformation, wear, or exceeding positional tolerances of components inside the hydraulic cylinder may lead to excessive action resistance. This, in turn, causes variation in piston speed with different stroke positions, resulting in slipping or crawling. The reasons are often poor quality of component assembly, surface damage, or iron debris from sintering, increasing resistance and decreasing speed. For example, misalignment between the piston and piston rod or bending of the piston rod, offset installation of the hydraulic cylinder or piston rod to guide rails, and tight or loose fitting of sealing rings. The solution is to rework or adjust, replace damaged components, and remove iron debris.

(2) Poor Lubrication or Oversized Hydraulic Cylinder Bore:

• Since there is relative motion between the piston and cylinder, as well as between the guide rails and piston rod, poor lubrication or an oversized hydraulic cylinder bore can intensify wear, reducing the linearity of the cylinder bore centerline. This results in fluctuating frictional resistance during the piston's operation, causing slipping or crawling. The corrective measure involves grinding the hydraulic cylinder, preparing the piston according to matching requirements, grinding the piston rod, and fitting guide bushings.

(3) Entry of Air into Hydraulic Pump or Cylinder:

• Compression or expansion of air can cause piston slipping or crawling. The remedy involves inspecting the hydraulic pump, installing dedicated exhaust devices, and quickly operating the full stroke several times to purge air.

(4) Direct Relationship between Seal Quality and Slipping or Crawling:

• The quality of sealing components directly influences slipping or crawling. O-ring seals, when used under low pressure, tend to experience slipping or crawling more easily than U-ring seals due to higher face pressure and a greater difference in dynamic and static frictional resistance. The face pressure of U-ring seals increases with rising pressure, improving sealing effectiveness, but it also leads to a larger difference in dynamic and static frictional resistance. As internal pressure increases, it affects rubber elasticity, and increased contact resistance of the lip edge may cause the seal to flip or elongate, contributing to slipping or crawling. To prevent flipping, supporting rings can be used to maintain stability.