72 Series Transmission Block


  • The 72 Multiversal Series stringing blocks offer flexibility in the field.
  • They can be converted in the field to accommodate one, two, three, three-offset, or four conductors.
  • With a few exceptions, 72 Series stringing blocks may be configured for 22″, 28″, 35″ and 42″ (OD) sheaves.
  • Available block grounds BDG and helicopter 7212-H



About Transmission Construction
Transmission-class line construction is very specialized. Compared to distribution construction, structures are taller, spans are longer, conductors are larger/heavier/stiffer, and pulls are longer and can extend over steep and very uneven terrain. In addition, aluminum conductors vary in construction requirements, calling for special handling techniques, tools, and equipment.

Transmission construction also involves different line and structure designs, as well as different conductor configurations (including simultaneous stringing of multiple conductors (‘bundles”) using a single pulling line). These configurations can range from single to bundles of two, three, four, and six conductors per phase. Some lines are strung using conventional, ground-based equipment and tools, while others require stringing using helicopters.

For all these reasons, the configuration and quality of the blocks in which sheaves are mounted are as important as the sheaves.

About 72 Series Transmission Blocks
The Sherman + Reilly 72 Multiversal Series is a series of blocks that may be used in multiple stringing configurations and afford the versatility of being reconfigurable in the field. That is, these blocks may be converted in the field to accommodate one, two, three, three-offset, or four conductors. With few exceptions, 72 Series blocks may be configured with 20″, 22″, 28″, 35”, and 42″ (OD) sheaves.

The key to the versatility of the 72 Series is that all blocks are assemble from three basic components:

• Open-side, single-conductor block-frames
• Center-drum sections
• Sheaves

These three components are mixed and matched to configure blocks to meet the specific needs of each project, then can be rearranged (in the field) to meet the needs of the next project.

The open-side single-conductor block-frame is a unique design that can support full loading from one side of a single-conductor frame. The “open” side of the frame can then be mated with other frame, drum, and sheave components to build out a complete block. This capability enables combining single-conductor frame components with center sections and sheaves in mix-and-match combinations to configure blocks to match a range of stringing needs.

When a 72 Series frame is assembled with only a throat guard and a single sheave, a single-conductor block is created, and stringing single conductors with such a block is identical to stringing with a conventional single-conductor block. The throat guard locks positively in position with the single-side frame forming a completely enclosed area for the conductor. Although the guard locks positively in place, it is easily unlocked for removal of the conductor after the sagging operation is complete and clipping in is to be done.

When 72 Series blocks need to be configured as bundle blocks, each block is assembled from two single-conductor frames, sheaves, and center-drum sections, all mounted on a solid shaft. Blocks may be configured that accommodate a separate steel pulling line, that have groove contours that accommodate reconductoring, that maintain balance when pulling three conductors simultaneously with a separate pulling line, and that have conductor groove contours that accommodate reconductoring.

For example, a 72 Series block may be configured as any of:

• A single-conductor block with throat-guard attachment.
• A two-conductor bundle block with a drum section to accommodate a pulling line.
• A three-conductor bundle block by assembling two single-conductor sections with a drum section with conductor groove in the center.
• A four-conductor conductor bundle with a double-center drum with two conductor grooves.

Regardless of configuration, all sheaves and center-drum sections turn on low-friction (high-efficiency) ball bearings, and each rotates independently from the others.

In addition to the versatility afforded by reconfigurability, 72 Series blocks are available in any of the three designs that Sherman & Reilly offers: symmetrical, slim-line, oroffset. More about the significance of these designs is below.

72 Series frames, sheaves, and center-drums are all cast from virgin A356-A6 Aluminum, an alloy that renders light weight, exceptional ruggedness, reliability, and durability. The properties and purity of the A356-T6 Aluminum used by Sherman & Reilly assure that the molten metal flows smoothly and evenly throughout the casting mold, assuring proper and uniform density throughout the finished part. Proper density and uniformity combined with the purity and metallic properties of the alloy assure that parts are strong but light-weight, and assure that there are no internal voids or brittleness that could cause sudden complete failure in the field. Combined with low-friction, lubricated-for-life bearings in the sheaves, the materials used for the parts and how the parts are made assure high-efficiency and smooth-stringing performance. The high block efficiency helps to maximize stringing distances while minimizing pulling tensions and reducing loads on stringing equipment.

Configuring 72 Series Blocks
Bundle blocks are used on high-capacity, long-distance, high-voltage transmission-line construction projects. These are projects that are planned many months in advance of construction in order to take into consideration the distance and terrain of the route, the voltages, and the capacity of the line. All of these factors (and others) affect the type, size, and number of conductors used and how they will be configured and installed. All of these factors further affect the configuration of the blocks that will be used. Since almost all projects are a bit different from others, bundle blocks are usually custom-configured and engineered specifically for each project.

Bundle blocks do not lend themselves to being configured from a set of tables in a catalog or on a website. Rather, bundle blocks must be configured in consultation with Sherman + Reilly. As preparation for that consultation, it is useful to understand the steps involved, which are to determine:

1. Sheave size.
2. Sheave lining, if any.
3. Number of conductors.
4. Separate pulling-line sheave, if any.
5. Frame type & design.
6. Fittings & hanging hardware.
7. Attachments.

Sheave size is largely a function of the size of the conductor being strung and of span length and terrain over which the conductor will be installed. Sheave lining is largely a function of the type of conductor being used and/or requirements from the conductor manufacturer.

Whether a separate sheave for the pulling line is needed is largely driven by the conductor manufacturer having a requirement for a separate sheave, so as to avoid contamination and/or damage to the conductor caused by damage done to, or dirt/debris left on, the sheave by a steel pulling line.

Frame size/type and fittings/hanging hardware are almost entirely a function of sheave size, the manner by which the block will be suspended, by conductor spacing, and the manner by which the conductors will be affixed (“clipped in”) to the insulators. The addition of an aluminum center drum may be required to achieve the desired conductor spacing. Depending on configuration, the pulling-line groove of that center-drum may be either a groove made of steel inserts or a machined groove lined with urethane.

Attachments are a function of how the conductor will be installed and grounded.

The three design variations (symmetrical, slim-line, and offset) allow for matching the block configuration to the number of conductors, to whether there is a separate pulling-line sheave, and to the separation between conductors when attached to the insulators on the tower.

The symmetrical design accommodates two-, three-, and four-conductorconfigurations. The sheaves are equidistant from each other regardless of the number of conductors. Usually, these blocks are configured for 18″ outside centers. Depending on the number of conductors and the their configuration, these blocks may have a center-drum with a center groove for the pulling line to pass through. This center groove can be either high-carbon steel inserts or urethane lined. If the center-drum is a one-piece design, then the groove is lined with urethane. If the center-drum is a two-piece bolt-on design, then the center groove is made of steel inserts.

The slim-line design is limited to three-sheave blocks that can be used for stringing two or three conductors that are clipped in on closer centers. This design is ideal for use where minimum weight and cost are prime factors.

The offset design makes use of special geometry to achieve a balance of two conductors with one. Such a balanced design facilitates the simultaneous stringing of three conductors while having a separate sheave for the pulling line. Therefore, this design greatly reduces the risk of damage to any conductor or conductor sheave caused by the pulling line, while minimizing twisting and misalignment caused by imbalance. This design is offered in a 28″ sheave size, only. (This design block may also be used to string two conductors on 18” centers by simply disassembling the block frame on one side and rotating the inter-flange sheave 180º to create a two-conductor configuration.)

Ordering 72 Series Blocks
Bundle blocks do not lend themselves to being ordered from a set of tables on a catalog or website. Rather, configuring and preparing an order for bundle blocks should begin by consulting Sherman & Reilly months in advance of the blocks’ being needed on-site.

Complete Dimensions



Sheave OD in. Rim Width in. groove bottom dia. in. Connection point to groove bottom in. Throat Dimensions in. Height OVERALL in. width overall in. groove radius in.
Other Other Height Width
22 22 3 1/8 18 1/8 13 8 1/4 4 33 15/16 9 1/4 13/16
28 28 3 1/4 24 13 1/32 8 3 5/16 39 15/16 9 1/4 1
35 35 4 30 1/4 15 5/8 10 1/4 4 23/32 48 15/16 11 1 1/8
42 42 4 1/2 36 13/32 16 29/32 11 5 1/16 55 15/16 12 5/32 1 13/32


Size (NOM) maximum conductor od, in. maximum working load lbs. block weight (NOM) lbs. Material Efficiency
frame sheave
22 Refer to IEEE Standard 524 (2016) 12,000 36 A356-T6
Aluminum- (virgin)
28 12,000 65
35 12,000 78
42 17,000 113

Please refer to the Product Sheet found on this page under Downloads.