Dynamic Loading

1. Phenomenon Description:

In large, multi-stage telescopic hydraulic cylinders, a dynamic phenomenon commonly referred to as "arching" or "caterpillaring" can be observed at the moment the final stage reaches its full extension. As the moving mass of the fully extended stages undergoes sudden deceleration, the stored kinetic energy is converted, resulting in a visible deflection or "arching" of the entire cylinder assembly. This is not merely an aesthetic anomaly; it is an indication of significant internal stress and transient loading conditions that are imposed on the system's components, most critically, the rod seals.

2. The Root Cause: Hydraulic Pressure Intensification & Shock Loading

The root cause is a combination of factors that occur at the end of the stroke:

• End-of-Stroke Deceleration: The hydraulic fluid, which is pushing the final stage, must rapidly decelerate the entire moving mass of the extended stages.

• Pressure Spike (Intensification): As the final stage "bottoms out" or hits its internal stop, the hydraulic flow is abruptly halted. This creates a powerful pressure wave, often called a hydraulic shock or pressure spike, that can exceed the system's normal operating pressure by a significant margin. This is a classic example of pressure intensification.

• Elastic Deformation: The cylinder tubes themselves are not infinitely rigid. Under this sudden shock load, the metal tubes will undergo a momentary elastic deformation, causing them to flex or bow. This is the "arching" that can often be observed.

3. Effects on Rod Seals: 

The effects on rod seals in particular, can be profound. Here is a more detailed breakdown:

• Uneven Radial Loading & Eccentricity: The "arching" of the cylinder introduces what engineers call eccentric loading on the seals. The centerline of the rod is no longer perfectly concentric with the seal housing. This causes the radial load on the seal to become highly uneven.

  • High Compressive Loads: On the concave side of the arch, the seal is subjected to extreme compressive force, which has the potential to exceed the seals' design limits. This can lead to material extrusion, permanent deformation (compression set), and premature wear.

  • Gapping and Loss of Contact: On the convex side of the arch, the rod pulls away from the seal lip. This creates a gap, however momentary, that breaks the sealing line. This directly compromises the seal's "rod-following" ability.

• Seal Distortion (Ovality): the uneven forces push the seal into an out-of-round or oval shape. When distorted:

  • Leakage Path: The gap created on the low-pressure side becomes a direct path for external contaminants to enter the hydraulic system.

  • Reduced Performance: Even after the arching event subsides, the seal may not return to its original shape, leading to persistent weeping or leakage and reduced scraping efficiency.

Conclusion:

The common acceptance of cylinder "arching" as normal overlooks a critical failure mode for rod seals. This transient shock loading introduces severe eccentric forces, leading to seal distortion, uneven wear, and a compromised sealing interface. For applications requiring high reliability and long service life, selecting a seal with superior resilience, a strong energizer to improve rod-following under eccentric conditions (like a Style 2 or 3), and robust geometry, is critical to mitigating the damaging effects of this phenomenon.