Bundle Conductor Stringing Sheaves

Bundle conductor stringing sheaves (also known as bundle blocks or multi-sheave blocks) are specialized stringing blocks designed specifically for the unique requirements of stringing bundled conductors on high-voltage (HV), extra-high voltage (EHV), and ultra-high voltage (UHV) transmission lines.

What are Bundled Conductors?
As previously discussed, bundled conductors consist of two or more individual sub-conductors (e.g., 2-bundle, 3-bundle, 4-bundle, 6-bundle) that make up a single phase of a transmission line. They are used to:

Reduce corona losses and radio interference at high voltages.

Increase current-carrying capacity.

Improve voltage regulation and power transfer capability.

How Bundle Conductor Stringing Sheaves Work:
Unlike a single-sheave block, a bundle block features multiple sheaves (pulleys) arranged to simultaneously accommodate and guide each sub-conductor of the bundle while maintaining their designed spacing.

Key aspects of their operation and design:

Multiple Sheaves in a Single Frame:

A single robust frame houses two, three, four, or even more sheaves.

For a 2-bundle conductor, you'll typically see a block with three sheaves (two outer sheaves for the conductors and a central sheave that might guide the pulling rope or act as a support).

For a 3-bundle conductor, a block with three to four sheaves (each for a conductor) is common.

For a 4-bundle conductor, a block with five sheaves (four for conductors and often a central support sheave) is typical.

For 6-bundle, blocks with seven sheaves or more are used.

Precise Spacing: The sheaves are positioned within the frame to maintain the precise design spacing between the individual sub-conductors as they are pulled. This ensures the bundle's integrity and prevents twisting or clashing.

Large Sheave Diameter: Similar to large-diameter single blocks, bundle block sheaves are always of large diameter (e.g., 500mm to over 1000mm, or even larger) to minimize bending stress on the conductors, which is even more critical when multiple conductors are involved.

Specialized Sheave Materials/Linings:

Urethane-Lined Aluminum Alloy: Very common. The polyurethane lining protects the conductor's surface from damage and reduces friction.

Nylon/MC Nylon: Also used, offering good conductor protection and sometimes being lighter.

Polished Aluminum Alloy: Used for some applications, but linings are preferred for sensitive conductors.

High Load Capacity Bearings: Each sheave is mounted on heavy-duty, sealed, anti-friction bearings (ball bearings or tapered roller bearings) to ensure smooth rotation and handle the significant combined weight and tension of multiple large conductors.

Robust Frame Construction: Made from high-strength, often galvanized steel or robust aluminum alloys, designed to withstand the very high pulling forces and maintain structural integrity.

Passage of Joints and Spacers: The design of bundle blocks (especially the throat and groove profiles) allows for the smooth passage of conductor compression splices, mid-span joints, and especially bundle spacers. Spacers are critical components that keep the sub-conductors separated in the final configuration, and they must be able to pass through the blocks during stringing.

Importance of Bundle Conductor Stringing Sheaves:
Maintains Bundle Integrity: Ensures that all sub-conductors are pulled simultaneously and evenly, preventing them from twisting, snarling, or getting damaged.

Conductor Protection: Minimizes bending stress and surface abrasion on multiple expensive conductors.

Accurate Sagging: Contributes to a uniform and accurate sag across all sub-conductors in the bundle, which is crucial for electrical performance and clearances.

Efficiency: Allows for the rapid stringing of an entire bundled phase in one pass, significantly reducing project time compared to stringing individual conductors.

Safety: Reduces the risk of uncontrolled conductor movement, snags, and potential hazards to ground crews.