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Why there is the issue of seal scorching. Seal scorching is primarily attributed to the extreme high temperatures generated by gas under high-pressure compression.
The phenomenon of seal scorching is mainly categorized as being caused by the extreme high temperatures generated by gas under high-pressure compression.
We have attempted to provide the following description based on relevant information and NOK sample data:
I. Definition and Causes of Scorching Phenomenon
1. Phenomenon Description
In hydraulic cylinders (especially during the start-up phase), the U-shaped, Y-shaped, and other lip seals of the piston or piston rod, as well as their adjacent wear rings, exhibit localized scorching, carbonization, or material melting on the lip contact surface. This damage is characterized by its sudden, localized, and high-energy nature.
2. Core Mechanism: Adiabatic Compression Generates Instantaneous High Temperature
A) Air Entrapment: When the air in the hydraulic system (cylinder) is not completely expelled, air accumulates in the enclosed space formed by the seal design. For U-shaped seals, the valley (groove) on the inner side of the lip is a typical air entrapment area.
B) Rapid Compression: At the moment the hydraulic cylinder is activated, the piston moves too quickly, preventing the hydraulic fluid from displacing the air trapped in the valleys of the surface. This air is then rapidly compressed within microseconds.
C) Energy Conversion and Temperature Rise: This process is approximately adiabatic compression, and almost all of the compression work is converted into thermal energy. Theoretical calculations and experiments show that the local temperature can instantly reach 600-800℃ or even higher.
D) Material failure: This temperature far exceeds the heat resistance limits of conventional sealing materials (such as nitrile rubber NBR ≤120℃, polyurethane PU ≤110℃), leading to thermal decomposition (carbonization) or melting of the material’s contact surface. The adjacent wear rings will also be damaged by the high temperature.
3. Characteristics of the Failure
High-incidence timing: The vast majority of failures occur during the initial operation after the cylinder has been started or after a long period of inactivity. In normal operation, if the system is well-sealed, air is unlikely to accumulate again in large quantities.
Specific locations: Concentrated in the contact zone between the sealing lip and the cylinder barrel/piston rod, and on the surface of the anti-wear ring immediately adjacent to the seal.
II. Basic Preventive Measures
Based on the above analysis, to prevent scorching caused by adiabatic compression, the following requirements must be strictly followed during equipment operation and maintenance:
1. Thorough Air Bleeding:
Timing: This must be performed after the hydraulic cylinder is installed, repaired, or restarted after a long period of inactivity.
Method: Open the bleed valve at the highest point of the system and cylinder. Operate the cylinder at low speed and short strokes repeatedly under no-load or low-load conditions until the oil flowing from the bleed port is continuous and free of bubbles. Then, tighten the bleed valve.
2. Smooth Start-up:
High-speed, full-stroke movement immediately upon start-up is strictly prohibited. The piston should be moved slowly using low-speed commands first. After confirming smooth operation and complete oil filling, gradually increase the speed to the working state.
3. U-shaped Seal Groove Pre-filling Technology:
When installing U-shaped seals, fill the groove with an appropriate amount of high-temperature lubricating grease compatible with the hydraulic oil (such as lithium-based grease). The impact on cleanliness needs to be evaluated.
Function: ① Occupies air accumulation space; ② Provides initial lubrication during start-up; ③ Assists in cushioning and heat dissipation.
III. Fundamental Solutions
Basic measures rely on strict operational discipline, while design optimization can improve the inherent reliability of the system from the source.
1. Optimizing Piston Structure and Selecting High-Temperature Resistant Materials
Piston structure: Adopting a stepped piston or integrated seal combination design to reduce or eliminate enclosed cavities that can trap air. Small air guide grooves can be designed on the support ring.
Seal material upgrade: Using materials with excellent heat resistance (continuous temperature resistance > 200℃), low friction, and self-lubricating properties, such as high-performance PTFE composite materials. Their ability to withstand instantaneous high temperatures is significantly superior to traditional elastomers.
2. Structural Protection Principle of KZT Anti-Fouling Ring (Key Innovation)
Adding KZT-type anti-fouling rings (scraper rings) on both sides of the main piston seal is a core design in advanced sealing solutions such as those from NOK, preventing coking. Its function goes far beyond just “anti-fouling”:
Mechanism of action:
1. Cleaning and isolation: The sharp lip of the KZT ring effectively scrapes away the oil film, contaminants, and air-containing oil-gas mixture from the cylinder wall, ensuring that the main seal always operates in a clean oil environment.
2. Modification of the dangerous cavity: It transforms the “dangerous concave cavity” originally present in the sharp valley of the U-shaped seal into a “safe parallel gap cavity” between the KZT ring and the main seal.
3. Elimination of adiabatic compression conditions:
Dangerous cavity: Air has nowhere to escape within the concave corner and is instantly compressed adiabatically.
Safe cavity: Air is distributed in the gentle gap; the cavity changes linearly during piston movement, allowing air to be slowly expelled or dissolved. The compression process becomes gentler (approaching an isothermal process), and heat can be absorbed and dissipated by the oil.
The KZT anti-fouling ring eliminates the conditions that cause adiabatic compression by physically modifying the geometry and state of the sealing chamber, thus achieving immune-level protection against coking failure. However, it is crucial to remember to create a gap in the anti-fouling ring to prevent deformation and failure caused by pressure shock.
IV. Implementation Recommendations
1. For existing equipment: The enforcement of operation and maintenance procedures must be strengthened.
2. For new equipment designs or critical equipment modifications: The piston seal assembly with an added KZT anti-fouling ring is highly recommended, as this is the most reliable solution proven through engineering validation.
V. Related Reference Information
Usage and Maintenance
Failure Analysis of Seal Failures in Hydraulic Systems
（1）Cavitation
Cavitation refers to the bursting of bubbles in the system. Compressed air bubbles, insoluble in the hydraulic fluid, expand with considerable energy on the low-pressure side as they pass through the sealing gap, resulting in a jet effect.
Surface roughness exacerbates this process, not only causing scratches on the seals but also eroding the metal surfaces of adjacent grooves and cylinder bores. Once the sealing surface is damaged by pitting and pores, the hydraulic oil flows through the longitudinal scratches at very high speed and acceleration, intensifying wear. Initially, these surface scratches might be mistaken for abrasive wear. This phenomenon can be caused by vibrating fluid flow, high pressure, and high vacuum.
（2） Diesel Effect
Another consequence of air bubbles in hydraulic fluid is the Diesel effect. If the system pressure rises sharply within a very short time interval, the bubbles are heated to the point where the gas mixture within them spontaneously ignites. For example, an air bubble with a diameter of 25 mm, compressed from atmospheric pressure to 50 MPa in a few milliseconds, will have its center temperature rise to 2500°C. If this effect occurs near a seal or support ring, the seal and support ring will be scorched. In addition to direct component failure, the hard fragments produced by the scorching of the ring or seal will also cause system malfunctions. Therefore, special attention must be paid to the materials used, ensuring they do not leave harmful combustion residues in this application. For example, using modified PTFE materials will not produce combustion residues that affect normal operation. Figure 5 shows the damage to a plastic support ring caused by the Diesel effect.
The scorching failure of hydraulic cylinder seals is the result of a high-energy physical process, “adiabatic compression,” acting on weak points. By understanding this mechanism and building a multi-faceted protection system ranging from “standard operating procedures” to “optimized design,” particularly by modifying the seal chamber structure with KZT anti-fouling rings, such failures can be eliminated, ensuring the long-term stable operation of hydraulic cylinders.
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