EN1337-7:2004 Explained – Spherical & Cylindrical PTFE Bearings for Structural Stability

EN1337-7:2004 is a European Standard (EN) developed by the European Committee for Standardization (CEN) to regulate the design, manufacturing, and performance of spherical and cylindrical PTFE bearings. Published in March 2004, it replaced the 2000 edition (EN1337-7:2000) and is now implemented as a national standard across all CEN member states (including the UK as BS EN 1337-7:2004).

A key role of EN1337-7 is to align with the EU's Construction Products Directive (89/106/EEC), meaning bearings compliant with this standard can carry the CE mark-mandatory for market access in the EU. The standard exists in three official languages (English, French, German), with translated versions approved by CEN members deemed equally valid.

1, Scope & Key Exclusions

EN1337-7 focuses exclusively on spherical and cylindrical PTFE bearings, which are critical for structures requiring controlled rotation and displacement. However, it has clear boundaries to ensure precision:

• Angle Restrictions: Spherical bearings with a total included angle (2θ) > 60° and cylindrical bearings with 2θ > 75° are excluded (see Figure 6 in the standard for geometric references).
• Temperature Limits: The standard specifies a minimum operating temperature of -35°C, with plans to extend this to -40°C in future amendments. Applications outside this range require custom design and are not covered by EN1337-7's performance criteria.
• Torque Risks: Cylindrical PTFE bearings are prone to unexpected moments around their transverse axis-designers must account for this to avoid instability.

Notably, EN1337-7 does not cover flat sliding elements; these are regulated by EN1337-2:2004, which must be referenced alongside EN1337-7 for complete system design.

2, Core Design Requirements

Compliance with EN1337-7 starts with adhering to strict design rules, which prioritize load-bearing capacity, rotation flexibility, and long-term durability. Below are the most critical requirements:

1. Material Specifications

All materials must meet the criteria outlined in EN1337-2:2004 (Section 5), with key focus on:

• PTFE Sheets: The heart of the bearing, PTFE must have a low friction coefficient (≤0.12 dry, ≤0.03 lubricated) and sufficient compressive strength (refer to EN1337-2 Table 10 for characteristic values).
• Metal Backing Plates: Convex/concave plates (for spherical/cylindrical shapes) must use steel with a yield strength ≥235 MPa. Bonding between PTFE and metal plates (if used) requires a shear strength ≥1.5 N/mm².
• Mating Surfaces: Hard, smooth metallic surfaces (e.g., stainless steel or chrome-plated steel) are mandatory to pair with PTFE, ensuring minimal wear and consistent performance.

The standard also specifies material combinations for curved sliding surfaces (PTFE + metal) via EN1337-2 Table 9-these pairings are validated for durability and compatibility.

2. Structural Details

• Curved PTFE Sheets: Can be attached to either convex or concave backing plates, but must follow EN1337-2 Section 6.2.1 for thickness, protrusion (measured per EN1337-2 Figure 2), and embedding depth.
• Concave Backing Plates: Must undergo strength verification per EN1337-2 Section 6.9 and meet dimensional limits (see EN1337-7 Figure 7) to prevent stress concentration.
• Restraining Rings (Spherical Bearings Only): Free spherical bearings (Figure 4a in the standard) can be converted to fixed bearings using steel restraining rings. These rings must follow the design rules for pot bearings in prEN1337-5:1996 (Section 6).

3. Performance Validation

EN1337-7 requires two critical performance checks to ensure safety:

• Compressive Stress Control: At the ultimate limit state (ULS), the design axial force (Ns,d) divided by the reduced contact area (Ar) of the curved sliding surface must not exceed PTFE's design compressive strength (fd). The reduced area Ar is calculated as λ×A (A = projected area of the curved surface; λ = reduction coefficient from EN1337-7 Annex B).
• Rotation Capability: Metal mating surfaces must fully cover PTFE sheets to avoid metal-to-metal contact. Designers must also account for additional rotation specified in EN1337-1:2000 (Section 5.4) to prevent edge stress.

4. Eccentricity Management

A unique requirement of EN1337-7 is controlling total eccentricity (et) to prevent sliding surface separation (a risk to durability). Eccentricity arises from three sources:

• Friction (e1): Calculated using EN1337-2 Table 11's friction coefficients.
• Horizontal loads (e2): Zero if loads are transferred via guides/restraining rings.
• Rotation (e3): Acts opposite to e1 and depends on PTFE attachment (convex vs. concave).

Total eccentricity (et) must lie within the "kernel" of the projected sliding area to ensure PTFE remains in compression (σp ≥ 0) at the serviceability limit state (SLS). Annex A of EN1337-7 provides formulas for these calculations.

To market bearings in the EU, manufacturers must complete conformity assessment per EN1337-7 Annex ZA, which links to the Construction Products Directive. Two main certification systems apply:

• System 1: For bearings in critical applications (e.g., bridges where failure risks structural safety). Requires a notified body to audit factory production control (FPC) and issue an EC Certificate of Conformity.
• System 3: For non-critical applications. Manufacturers conduct initial type testing (via an approved lab) and issue a self-declaration of conformity.

CE-marked bearings must display key information: manufacturer details, certification body number (System 1 only), EN1337-7 reference, and design values (load capacity, rotation angle).

Spherical and cylindrical PTFE bearings compliant with EN1337-7 are ideal for structures needing multi-directional rotation or precise displacement control-common in:

• Long-span bridges (e.g., cable-stayed or suspension bridges).
• High-rise buildings (to accommodate wind-induced sway).
• Seismic zones (when integrated with isolation systems).

For practical implementation, industry leaders like Luzetech offer EN1337-7-compliant PTFE bearings tailored to seismic isolation and vibration control. Their solutions align with the standard's durability and performance demands, making them a go-to for projects across Europe and beyond.

EN1337-7 is more than a technical document-it ensures:

• Safety: Rigorous testing and design rules prevent structural failure.
• Interoperability: Standardized dimensions and performance let bearings work across EU projects.
• Market Access: CE marking opens doors to the EU's $100B+ construction market.

For engineers and buyers, specifying EN1337-7-compliant bearings is a non-negotiable step toward building resilient, code-compliant structures.

EN1337-7:2004 sets the benchmark for spherical and cylindrical PTFE bearings, balancing technical precision with real-world usability. By following its guidelines and partnering with trusted suppliers like Luzetech, project teams can deliver safe, durable, and EU-compliant structures.