English 
Spanish 
Accurate seal selection during the hydraulic cylinder design phase is crucial. The core of this process lies in translating systematic operating conditions into specific design parameters and material choices. In addition to considering conventional factors such as operating environment, temperature, medium, and pressure, the selection process also requires in-depth consideration of more complex engineering details such as structural compatibility, failure prevention, and dynamic performance.
I. In-depth and Systematic Input
The design input of a product determines its design output. If the design input is not comprehensive enough or not quantified as much as possible, the likelihood of failure feedback is extremely high. Therefore, systematic design input analysis is the cornerstone of component selection.
Pressure Analysis: It is crucial to not only determine the rated working pressure but also to consider peak pressure and pressure surges (such as those caused by directional changes). This is essential for selecting the appropriate pressure rating of the seal and determining whether to install anti-extrusion backup rings.
Motion and Cycling: Analyze the speed range, especially low-speed stability (anti-creeping) and frictional heat at high speeds. Also, clarify the operating frequency and total stroke cycle, which directly relate to the wear life of the seal.
Medium and Temperature: Specify the type of hydraulic fluid (e.g., HM46 anti-wear hydraulic oil, aviation hydraulic oil, blue oil, etc.), water content, cleanliness level (e.g., ISO 4406), and temperature. The fluid will cause varying degrees of volume expansion in the rubber material, affecting sealing performance.
Load and Installation: Are there continuous lateral forces or eccentric load moments? This will determine whether the guiding system needs to be reinforced (e.g., by increasing the number and width of guide strips). Also, assess whether the installation space is limited and whether it allows for easy maintenance and replacement later on.
II. Transformation Based on Input Information
The design input information is analyzed and transformed into specific design choices, as exemplified below:
1. Pressure Rating and Sealing Form:
Low pressure (<16MPa): Single-acting seals (such as Y-rings, U-rings) can be considered.
Medium to high pressure (16-31.5MPa): A combination seal is usually required (such as Glyd ring, Step seal), which consists of an elastic O-ring providing pre-tension and a wear-resistant PTFE sliding ring as the main seal.
Ultra-high pressure (>31.5MPa) or with impact: Designs with high-strength backup rings must be used, and the clearance must be strictly controlled.
2. Speed and Friction:
Low speed (<0.05 m/s): Materials with stable friction coefficients and anti-stick-slip characteristics should be selected, such as filled PTFE.
High speed (>0.5 m/s): The thermal conductivity and temperature resistance of the material should be the main considerations to prevent heat accumulation leading to seal hardening or failure.
3. Material and Operating Condition Compatibility Decisions:
Nitrile rubber (NBR): High cost-effectiveness, suitable for most mineral oils and operating conditions below 80°C.
Polyurethane (PU): Excellent wear resistance, suitable for medium to high pressure, clean systems, but sensitive to hydrolysis and high temperatures.
Fluorocarbon rubber (FKM): High temperature resistance (>150°C), resistant to various media, but poor low-temperature elasticity and high cost.
Polytetrafluoroethylene (PTFE): Extremely low friction coefficient, resistant to almost all media, but needs to be used in combination with an elastomer.
4. Structure:
Lateral load: An independent guide ring/band must be designed. Its material (such as wear-resistant plastic, bronze) and arrangement must be able to withstand all lateral forces to protect the main seal.
Extrusion Gap Calculation: Based on the maximum working pressure and possible deflection, calculate the maximum single-sided gap between the piston and cylinder, and between the piston rod and guide bushing. This value must be less than the extrusion resistance of the selected sealing material; otherwise, a backup ring must be added.
III. Key Design Details
The seals are installed in sealing grooves and slide within the hydraulic cylinder, therefore, the machining quality of the sealing grooves and sliding surfaces must be specified. The design of the sealing structure must also be evaluated.
Friction Pair Surface: The hardness, roughness (Ra 0.1-0.4 μm), and plating (hard chrome or more advanced ceramic coating) of the sealing mating surfaces (piston rod, cylinder) must be clearly specified.
Groove Design: Strictly follow the dimensions, tolerances, radii, and surface finish requirements in the seal manufacturer’s manual. Improper grooves are a common cause of seal failure.
Installation Friendliness: The design should consider the installation path to avoid sharp edges scratching the seals, and installation guides should be designed if necessary.
Redundancy and Pressure Relief: For critical applications, a dual-seal series design can be used, with a pressure relief hole between the two seals to monitor the failure of the primary seal and prevent pressure trapping.
IV. Design Verification and Improvement
Perform a final review before finalizing the design.
Simulation Analysis: For complex or high-pressure conditions, finite element analysis (FEA) can be used to simulate the stress and strain state of the seal under compression and pressure, optimizing the compression ratio and groove design.
Design Checklist:
1. Have all extreme operating conditions (highest/lowest temperature, maximum pressure shock, maximum side load) been considered?
2. Is the compatibility of the sealing material with the system fluid and temperature documented (e.g., manufacturer’s compatibility table)?
3. Are the surface treatment requirements of the friction pair clearly marked on the drawings?
4. Has the extrusion gap been calculated, and have anti-extrusion measures such as backup rings been implemented?
5. Is the load-bearing capacity of the guiding system sufficient, and does it prevent the seals from bearing lateral forces?
6. Is the seal installed in the correct direction, and is there sufficient space?
Sample testing: For brand new or particularly critical designs, building a prototype and conducting durability testing is the most reliable method for verifying the design choices.
Successful seal selection follows a core logic: comprehensively collect and quantify input conditions → perform initial type screening based on pressure and speed → select materials based on media, temperature, and lifespan → design based on structural load and gap protection → ensure reliable implementation through detailed design and verification.
★★★Review of selected articles in our hydraulic cylinder engineering newsletters:
- Three working conditions of the hydraulic cylinder!!! What are the three?
- Choosing the Right Cylinder for Stamping Equipment
- How to perform daily maintenance and care on the hydraulic station?
- Working principle of single-acting booster cylinder
- Working principle of double-acting booster cylinder
- Several practical repair methods for hydraulic cylinders
- Causes and solutions for the creeping phenomenon of hydraulic cylinders during operation
- Six basic laws of hydraulic systems: from pressure formation to flow distribution
- Complete guide to hydraulic cylinder failure repair!
- A question: Why do hydraulic cylinder seals fail?