What is a clevis?

I. Core Definition: The Connecting Core of Transmission Lines

The power transmission clevis (known as “U-shaped hanger” in Chinese) is a critical fitting in transmission lines that connects insulator strings to towers or conductors. This specialized U-shaped connector uses pins or bolts to achieve rigid connections, undertaking the important mission of transmitting mechanical loads and maintaining electrical clearance in overhead lines.

In power industry standards, the clevis is classified as “hanger hardware”. Its main function is to reliably connect suspension or tension insulator strings to tower crossarms while ensuring line flexibility and adjustability.

II. Structural Features: Precision Engineering for Power Applications

Power specialty clevis features special structural design:

1. U-shaped body: Made of high-strength forged steel or malleable iron, 30-50% thicker than industrial clevis

2. Reinforced pinholes: Increased wall thickness with rounded edges to avoid stress concentration

3. Corrosion protection: Hot-dip galvanized coating ≥86μm, passing 3000-hour salt spray test

Special pins: Anti-rotation design with cotter pin holes to prevent disengagement

III. Core Functions: Three Missions of Power Connection

• Mechanical load transfer:

• Withstands comprehensive mechanical stresses including conductor weight, wind loads, ice loads
• In UHV lines, a single clevis must withstand over 10 tons of continuous tension

• Electrical insulation assurance:

• Maintains sufficient air insulation distance
• Prevents corona discharge (with smooth surface treatment)

• Operational maintenance convenience:

• Provides flexible connection angle adjustment

Facilitates insulator replacement during line maintenance

IV. Types and Applications: Adapting to Various Voltage Levels

1. Standard U-hangers:

   • For 110-220kV lines

   • Material: Malleable iron KT-35

   • Nominal load: 70-100kN

2. Reinforced hangers:

   • For 330-500kV lines

   • Material: High-strength forged steel Q345

   • Nominal load: 150-200kN

3. UHV specialty hangers:

   • For 750-1000kV lines

   • Material: Alloy steel 35CrMo

   • Nominal load: 300-400kN

4. Tension string hangers:

   • For line terminals, angle towers

   • With adjustment holes for length fine-tuning

   • Failure load exceeding 500kN

V. Technical Standards: Strict Power Industry Specifications

Power clevis must meet the following standards:

• Mechanical performance: Failure load ≥4 times maximum working load

• Electrical performance: No visible corona (at 1.1 times maximum working voltage)

• Corrosion resistance: Uniform hot-dip galvanizing, no missed areas or peeling

• Dimensional accuracy: Hole diameter tolerance ±0.2mm, hole distance tolerance ±0.5mm

Main standards:

• DL/T 757-2009 “Tension Clamps”

• GB/T 2315-2008 “Nominal Failure Load Series for Power Fittings”

• ANSI C119.4-2004 “Overhead Line Hardware Standards”

VI. Installation & Maintenance: Key Aspects of Power Safety

Installation requirements:

1. Use torque wrenches with design-specified values

2. Cotter pins must be symmetrically bent at 60-90 degrees

3. Pin ends should protrude 2-5 threads

Maintenance points:

1. Regular inspection of zinc coating integrity (every 2 years)

2. Check pins for loosening or corrosion

3. Measure residual mechanical strength (every 10 years)

Danger alerts:

• Zinc coating loss >10%: Replace immediately

• Cracks or deformation found: Replace immediately after power outage

• Pin wear exceeding 10% of diameter: Replace immediately