How to Select the Right Polyurethane O-Ring Size and Hardness?
July 19, 2026
Selecting the correct polyurethane O-ring size and hardness is essential for achieving reliable sealing performance and preventing premature seal failure.
Even when using high-performance materials like polyurethane, an incorrect cross section, inside diameter, or hardness grade can lead to problems such as leakage, excessive friction, extrusion, and reduced service life.
This guide explains how to select the right polyurethane O-ring size and hardness for hydraulic, pneumatic, and industrial sealing applications.
1. Understand Polyurethane O-Ring Dimensions
The size of a polyurethane O-ring is normally defined by two main dimensions:
- Inside Diameter (ID)
- Cross Section (CS)
The standard size format is:
Inside Diameter × Cross Section
Example:
34 × 1.5 mm Polyurethane O-Ring
means:
- Inside Diameter: 34 mm
- Cross Section: 1.5 mm
The outside diameter can be calculated as:
OD = ID + (2 × CS)
Common Polyurethane O-Ring Size Parameters
Inside Diameter (ID)
The inside diameter determines how the O-ring fits into the groove.
A correct ID ensures:
- Proper stretching during installation
- Stable positioning
- Effective sealing compression
Too much stretch may cause:
- Increased stress
- Faster wear
- Reduced service life
Too little stretch may cause:
- Movement inside the groove
- Leakage risk
Cross Section (CS)
The cross section refers to the thickness of the O-ring.
Common polyurethane O-ring cross sections include:
| Cross Section | Typical Applications |
|---|---|
| 1.5 mm | Small precision equipment |
| 1.78 mm | Compact hydraulic and pneumatic sealing |
| 2.0 mm | General industrial sealing |
| 2.4 mm | Medium-duty applications |
| 2.62 mm | Standard hydraulic sealing |
| 3.0 mm | General hydraulic systems |
| 3.5 mm | Heavy-duty sealing |
A larger cross section generally provides:
- Better gap filling
- Higher sealing reliability
- Improved resistance to extrusion
However, larger O-rings may require more installation space.
2. Select the Correct Polyurethane O-Ring Hardness
Polyurethane O-rings are commonly available in different Shore A hardness grades.
The most common options are:
| Hardness | Application |
|---|---|
| 70 Shore A | General industrial sealing |
| 80 Shore A | Hydraulic and dynamic sealing |
| 90 Shore A | High-pressure and heavy-duty applications |
70 Shore A Polyurethane O-Rings
70A polyurethane O-rings provide:
- Good elasticity
- Easy installation
- Reliable compression sealing
Recommended for:
- General machinery
- Low to medium pressure systems
- Static sealing applications
Advantages:
✔ Better flexibility
✔ Good sealing at low pressure
✔ Easier assembly
80 Shore A Polyurethane O-Rings
80A is one of the most commonly selected hardness grades for industrial applications.
It provides a balance between:
- Elasticity
- Wear resistance
- Pressure resistance
Suitable for:
- Hydraulic cylinders
- Pneumatic equipment
- Industrial automation
- Mobile machinery
For many hydraulic applications, 80 Shore A polyurethane O-rings are the preferred choice.
90 Shore A Polyurethane O-Rings
90A polyurethane O-rings are designed for demanding conditions.
They provide:
- Maximum extrusion resistance
- Higher pressure capability
- Improved mechanical strength
Recommended for:
- High-pressure hydraulic systems
- Heavy machinery
- High-load sealing applications
Trade-off:
Higher hardness means:
- Lower flexibility
- More installation force may be required
3. Choose O-Ring Hardness Based on Pressure
Operating pressure is one of the most important factors when selecting polyurethane hardness.
| Working Condition | Recommended Hardness |
|---|---|
| Low pressure | 70A |
| Medium pressure | 80A |
| High pressure | 90A |
For hydraulic systems above normal operating pressure, selecting a harder polyurethane grade can help reduce:
- Extrusion damage
- Seal deformation
- Premature failure
4. Choose Size Based on Application Type
Static Sealing Applications
For stationary sealing:
Examples:
- Hydraulic blocks
- Flanges
- Covers
Recommended:
- Standard cross section
- 70A–80A hardness
Dynamic Sealing Applications
For moving applications:
Examples:
- Hydraulic pistons
- Rod seals
- Pneumatic cylinders
Recommended:
- Higher wear resistance polyurethane
- 80A–90A hardness
- Proper lubrication
5. Polyurethane O-Ring Size Selection Mistakes to Avoid
Choosing a Smaller Cross Section
A small CS may cause:
- Insufficient compression
- Leakage
- Faster wear
Choosing Excessive Hardness
A harder O-ring is not always better.
Too high hardness may result in:
- Difficult installation
- Reduced flexibility
- Poor sealing on uneven surfaces
Ignoring Groove Design
O-ring performance depends not only on the seal itself but also on:
- Groove width
- Groove depth
- Clearance gap
- Surface finish
Always verify the O-ring groove dimensions before final selection.
Polyurethane O-Ring Size and Hardness Selection Guide
| Application | Recommended PU O-Ring |
|---|---|
| Pneumatic equipment | 70A–80A |
| Hydraulic cylinders | 80A |
| Heavy hydraulic machinery | 80A–90A |
| High pressure systems | 90A |
| General industrial sealing | 70A–80A |
Polyurethane O-Ring vs Other Materials
When selecting O-ring materials, polyurethane is often chosen because it provides better mechanical performance compared with standard elastomers.
Compared with NBR O-rings:
Polyurethane offers:
✔ Higher wear resistance
✔ Better tear strength
✔ Better extrusion resistance
NBR may be preferred when:
✔ Oil resistance is the main requirement
✔ Cost efficiency is important
Need Help Selecting the Right Polyurethane O-Ring?
Choosing the correct polyurethane O-ring requires considering:
- O-ring dimensions
- Cross section
- Hardness
- Pressure
- Temperature
- Application environment
We supply polyurethane O-rings in metric and custom sizes, with different hardness grades for hydraulic, pneumatic, and industrial sealing applications.
Contact us for PU O-ring specifications, samples, and customized sealing solutions.
