I. Core Concept: What is a Seal Ring?
In the field of engineering machinery, a seal ring typically refers to a ring-shaped sealing element with a circular or near-circular cross-section. It primarily relies on its own elastic deformation to generate contact pressure on the sealing contact surface, thereby achieving a sealing function.
The most important and typical example is the O-ring.
2. O-ring Details
1. Working Principle
The sealing principle of O-rings is based on two stages:
Static Sealing: When installed in a groove and compressed, the O-ring cross-section undergoes elastic deformation (typically with a compression rate of 15%-30%), generating initial contact pressure on the contact surface, thereby sealing the medium.
Dynamic Sealing (Limited): In low-pressure reciprocating or low-speed rotary motion, the medium pressure acts on the O-ring, pushing it towards the other side of the groove, thereby increasing its contact pressure with the sealing surface and achieving a “self-sealing” effect. However, please note that O-rings are generally not recommended for high-speed rotary sealing.2. Main Advantages
Simple structure, compact size: Easy to design and install.
Low cost: Low cost for mass production.
Reliable sealing: Excellent static sealing effect, can be used for bidirectional sealing.
Low frictional resistance: Suitable for reciprocating motion.
Abundant materials: A variety of rubber materials are available to suit different working conditions.
3. Main Disadvantages
High starting frictional resistance: After a long period of inactivity, the rubber may adhere to the metal surface, resulting in high friction during startup.Poor high-pressure resistance: When using O-rings alone, they are easily squeezed into the gaps between parts under high pressure (typically >10MPa), causing gap extrusion and permanent damage.
High requirements for groove dimensions and surface finish: Machining accuracy directly affects the sealing performance.
III. Main Applications of Seals in Construction Machinery
Seals (mainly O-rings) are ubiquitous in construction machinery:
Hydraulic System (most common):
Pipe Joints: Static sealing at threaded joints, flange joints, etc.
Valve Blocks: Static sealing between valve plates.
End Caps of Hydraulic Pumps/Motors/Cylinders: Static sealing of housing mating surfaces.
As Auxiliary Seals: As secondary seals or back-up rings in piston or piston rod seals.
Transmission System:
Static sealing of various end caps and inspection covers of gearboxes and drive axles.
Lubrication System:
Joint sealing of lubricating oil pipes.
Pneumatic System:
Static and low-speed dynamic sealing in systems such as brakes and control cylinders.
IV. Common Materials (Similar to general seals, but with different emphasis)
Materials used for making sealing rings must have good elasticity, mechanical strength, and media resistance.
Nitrile Rubber: The absolute mainstay for O-rings in construction machinery. Due to its excellent resistance to petroleum-based hydraulic oil and lubricating oil, as well as its cost advantages, it is suitable for over 80% of hydraulic static sealing applications in construction machinery.
Fluorocarbon Rubber: Used in harsh environments with high temperatures (>150°C) or special requirements for oil compatibility (such as phosphate ester hydraulic oil). Expensive, but with excellent performance.Hydrogenated Nitrile Butadiene Rubber (HNBR): Its performance is between that of nitrile rubber and fluororubber. It has better high-temperature resistance, ozone resistance, and abrasion resistance than ordinary nitrile rubber, and is often used in engine-related or more demanding hydraulic systems.
Ethylene Propylene Diene Monomer Rubber (EPDM): Resistant to hot water, steam, brake fluid, and cold climates, but not resistant to mineral oil. Commonly used in cooling systems and brake systems (some types) of construction machinery.
Polyurethane (PU): Excellent abrasion resistance, but poor elasticity. It is generally not used to make standard O-rings, but is more often used to make other types of seals (such as Y-rings, Glyd rings, etc.).
V. Key Challenges and Solutions: “Gap Extrusion” Under High Pressure
This is the biggest problem faced by O-rings when used in high-pressure hydraulic systems in construction machinery.
Problem Description: When the system pressure is very high, the soft O-ring is pushed by the high-pressure medium to one side of the mating gap between metal parts and is partially squeezed into this gap. When the pressure is released or changes, the squeezed-in portion may be cut off or suffer permanent compression deformation, leading to seal failure.
Solution: Use back-up rings
What are back-up rings?Typically a ring made of a relatively hard material such as PTFE or nylon, installed in the groove on the pressure-bearing side of the O-ring.
Operating principle: The back-up ring fills the gap, providing physical support for the O-ring and preventing it from being squeezed into the gap. It does not directly provide a sealing function itself, but rather protects the O-ring.
Usage principles:
In static seals, it is recommended to install a back-up ring on the pressure-bearing side when the pressure is >10MPa.
When the pressure is >30MPa, back-up rings should be installed on both sides of the O-ring.
In dynamic seals, it is recommended to install back-up rings to improve stability, even at lower pressures.
VI. Selection, Installation, and Maintenance Key Points
Correct Selection:
Material: Ensure compatibility with the working medium (refer to the chemical compatibility chart).
Dimensions: Strictly adhere to national standards (e.g., GB/T 3452.1) or international standards (e.g., AS568) when selecting inner diameter, cross-sectional diameter, and groove dimensions. Incorrect dimensions are one of the most common causes of failure.
Standard Installation:
Cleaning: Ensure the O-ring, groove, and all tools are absolutely clean.
Lubrication: Apply an appropriate amount of system oil or grease to the O-ring and sealing surface before installation to reduce friction and prevent twisting and scratching.
Use of Tools: Avoid using sharp tools (such as screwdrivers); use dedicated installation tools or sleeves. Any minor scratches can lead to leakage.
Original text from: pneumatic seal https://www.bszzseal.com/
