Ultimate Guide to Electric Power Engineering: Transmission System: Sag and Tension of Conductor (part 2)

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4. Ruling Span Concept

Transmission lines are normally designed in line sections with each end of the line section terminated by a strain structure that allows no longitudinal (along the line) movement of the conductor. Structures within each line section are typically suspension structures that support the conductor vertically, but allow free movement of the conductor attachment point either longitudinally or transversely.

4.1 Tension Differences for Adjacent Dead-End Spans

Tables 11 and 12 contains initial and final sag-tension data for a 700 ft and a 1000 ft dead-end span when a Drake ACSR conductor is initially installed to the same 6300 lb tension limits at 60°F. Note that the difference between the initial and final limits at 60°F is approximately 460 lb. Even the initial tension (equal at 60°F) differs by almost 900 lb at -20°F and 600 lb at 167°F.

4.2 Tension Equalization by Suspension Insulators

At a typical suspension structure, the conductor is supported vertically by a suspension insulator assembly, but allowed to move freely in the direction of the conductor axis. This conductor movement is possible due to insulator swing along the conductor axis. Changes in conductor tension between spans, caused by changes in temperature, load, and time, are normally equalized by insulator swing, eliminating horizontal tension differences across suspension structures.

TBL. 12 Sag-Tension Data 795 kcmil-Type 16 ACSR/SD with NESC Heavy Loading 300 and 1000 ft Spans

4.3 Ruling Span Calculation

Sag-tension can be found for a series of suspension spans in a line section by the use of the RS concept. The RS for the line section is defined by the following equation:


RS is the ruling span for the line section containing n suspension spans S1 is the Span length of the first suspension span S2 is the Span length of the second suspension span Sn is the Span length of the nth suspension span

Alternatively, a generally satisfactory method for estimating the RS is to take the sum of the average suspension span length plus two-thirds of the difference between the maximum span and the aver age span. However, some judgment must be exercised in using this method because a large difference between the average and maximum span may cause a substantial error in the RS value.

As discussed, suspension spans are supported by suspension insulators that are free to move in the direction of the conductor axis. This freedom of movement allows the tension in each suspension span to be assumed to be the same and equal to that calculated for the RS. This assumption is valid for the suspension spans and RS under the same conditions of temperature and load, for both initial and final sags. For level spans, sag in each suspension span is given by the parabolic sag equation:


Di is the sag in the ith span Si

is the span length of the ith span HRS is the tension from RS sag-tension calculations The sag in level suspension spans may also be calculated using the ratio:

...where DRS is the sag in RS.

Suspension spans vary in length, though typically not over a large range. Conductor temperature during sagging varies over a range considerably smaller than that used for line design purposes.

If the sag in any suspension span exceeds approximately 5% of the span length, a correction factor should be added to the sags obtained from the previous equation or the sag should be calculated using catenary Equation 29. This correction factor may be calculated as follows:

where D is the sag obtained from parabolic equation w is the weight of conductor, lb/ft H is the horizontal tension, lb

TBL. 13 Sag-Tension Data 795 kcmil-37 Strand AAC "Arbutus" with NESC Light Loading 300 and 1000 ft Spans

S is the span length, ft H is the horizontal tension, lb w is the resultant weight, lb/ft

4.4 Stringing Sag Tables

Conductors are typically installed in line section lengths consisting of multiple spans. The conductor is pulled from the conductor reel at a point near one strain structure progressing through travelers attached to each suspension structure to a point near the next strain structure. After stringing, the conductor tension is increased until the sag in one or more suspension spans reaches the appropriate stringing sags based on the RS for the line section. The calculation of stringing sags is based on the preceding sag equation.

TBL. 13 shows a typical stringing sag table for a 600 ft RS of Drake ACSR with suspension spans ranging from 400 to 700 ft and conductor temperatures of 20°F-100°F. All values in this stringing table are calculated from RS initial tensions, shown in TBL. 12 using the parabolic sag equation.

5. Line Design Sag-Tension Parameters

In laying out a transmission line, the first step is to survey the route and draw up a plan-profile of the selected right-of-way. The plan-profile drawings serve an important function in linking together the various stages involved in the design and construction of the line. These drawings, prepared based on the route survey, show the location and elevation of all natural and man-made obstacles to be traversed by, or adjacent to, the proposed line. These plan-profiles are drawn to scale and provide the basis for tower spotting and line design work.

Once the plan-profile is completed, one or more estimated RSs for the line may be selected. Based on these estimated RSs and the maximum design tensions, sag-tension data may be calculated providing initial and final sag values. From these data, sag templates may be constructed to the same scale as the plan-profile for each RS, and used to graphically spot structures.

5.1 Catenary Constants

The sag in a RS is equal to the weight per unit length, w, times the span length, S, squared, divided by eight times the horizontal component of the conductor tension, H. The ratio of conductor horizontal tension, H, to weight per unit length, w, is the catenary constant, H/w. For a RS sag-tension calculation using eight loading conditions, a total of 16 catenary constant values could be defined, one for initial and final tension under each loading condition.

Catenary constants can be defined for each loading condition of interest and are used in any attempt to locate structures. Some typical uses of catenary constants for locating structures are to avoid over loading, assure ground clearance is sufficient at all points along the right-of-way, and minimize blowout or uplift under cold weather conditions. To do this, catenary constants are typically found for (1) the maximum line temperature, (2) heavy ice and wind loading, (3) wind blowout, and (4) minimum conductor temperature. Under any of these loading conditions, the catenary constant allows sag calculation at any point within the span.

5.2 Wind Span

The maximum wind span of any structure is equal to the distance measured from center to center of the two adjacent spans supported by a structure. The wind span is used to determine the maximum horizontal force a structure must be designed to withstand under high wind conditions. Wind span is not dependent on conductor sag or tension, only on horizontal span length.

5.3 Weight Span

The weight span of a structure is a measure of the maximum vertical force a structure must be designed to withstand. The weight span is equal to the horizontal distance between the low points and the vertex of two adjacent spans. The maximum weight span for a structure is dependent on the loading condition being a minimum for heavy ice and wind load. When the elevations of adjacent structures are the same, the wind and weight spans are equal.

5.4 Uplift at Suspension Structures

Uplift occurs when the weight span of a structure is negative. On steeply inclined spans, the low point of sag may fall beyond the lower support. This indicates that the conductor in the uphill span is exerting a negative or upward force on the lower tower. The amount of this upward force is equal to the weight of the conductor from the lower tower to the low point in the sag. If the upward pull of the uphill span is greater than the downward load of the next adjacent span, actual uplift will be caused and the conductor will swing free of the tower. This usually occurs under minimum temperature conditions and must be dealt with by adding weights to the insulator suspension string or using a strain structure ( FIG. 9).


Min. sag Max. sag Uplift at tower Min. sag Max. sag

Above: Fig. 9 Conductor uplift.


5.5 Tower Spotting

Given sufficiently detailed plan-profile drawings, structure heights, wind/weight spans, catenary constants, and minimum ground clearances, structure locations can be chosen such that ground clearance is maintained and structure loads are acceptable. This process can be done by hand using a sag template, plan-profile drawing, and structure heights, or numerically by one of several commercial programs.

6. Conductor Installation

Installation of a bare overhead conductor can present complex problems. Careful planning and a thorough understanding of stringing procedures are needed to prevent damage to the conductor during the stringing operations. The selection of stringing sheaves, tensioning method, and measurement techniques are critical factors in obtaining the desired conductors sagging results. Conductor stringing and sagging equipment and techniques are discussed in detail in the IEEE Guide to the Installation of Overhead Transmission Line Conductors, IEEE Std. 524-1992. Some basic factors concerning installation are covered in this section. Because the terminology used for equipment and installation procedures for overhead conductors varies throughout the utility industry, a limited glossary of terms and equipment definitions excerpted from IEEE Std. 524-1992 is provided in the section appendix. A complete glossary is presented in the IEEE Guide to the Installation of Overhead Transmission Line Conductors.

6.1 Conductor Stringing Methods

There are two basic methods of stringing conductors, categorized as either slack or tension stringing.

There are as many variations of these methods as there are organizations installing conductors. The selected method, however, depends primarily on the terrain and conductor surface damage requirements.

6.1.1 Slack or Layout Stringing Method

Slack stringing of conductor is normally limited to lower voltage lines and smaller conductors. The conductor reel(s) is placed on reel stands or "jack stands" at the beginning of the stringing location.

The conductor is unreeled from the shipping reel and dragged along the ground by means of a vehicle or pulling device. When the conductor is dragged past a supporting structure, pulling is stopped and the conductor placed in stringing sheaves attached to the structure. The conductor is then reattached to the pulling equipment and the pull continued to the next structure.

This stringing method is typically used during construction of new lines in areas where the right-of way is readily accessible to vehicles used to pull the conductor. However, slack stringing may be used for repair or maintenance of transmission lines where rugged terrain limits the use of pulling and tensioning equipment. It’s seldom used in urban areas or where there is any danger of contact with high-voltage conductors.

6.1.2 Tension Stringing

A tension stringing method is normally employed when installing transmission conductors. Using this method, the conductor is unreeled under tension and is not allowed to contact the ground. In a typical tension stringing operation, travelers are attached to each structure. A pilot line is pulled through the travelers and is used, in turn, to pull in heavier pulling line. This pulling line is then used to pull the con ductor from the reels and through the travelers. Tension is controlled on the conductor by the tension puller at the pulling end and the bullwheel tension retarder at the conductor payout end of the instal lation. Tension stringing is preferred for all transmission installations. This installation method keeps the conductor off the ground, minimizing the possibility of surface damage and limiting problems at roadway crossings. It also limits damage to the right-of-way by minimizing heavy vehicular traffic.

6.2 Tension Stringing Equipment and Setup

Stringing equipment typically includes bullwheel or drum pullers for back-tensioning the conductor during stringing and sagging; travelers (stringing blocks) attached to every phase conductor and shield wire attachment point on every structure; a bullwheel or crawler tractor for pulling the conductor through travelers; and various other special items of equipment. FIG. 10 illustrates a typical stringing and sagging setup for a stringing section and the range of stringing equipment required. Provision for conductor splicing during stringing must be made at tension site or midspan sites to avoid pulling splices through the travelers.

During the stringing operation, it’s necessary to use proper tools to grip the strands of the conductor evenly to avoid damaging the outer layer of wires. Two basic types or categories of grips are normally used in transmission construction. The first is a type of grip referred to as a pocketbook, suitcase, bolted, etc., that hinges to completely surround the conductor and incorporates a bail for attaching to the pulling line. The second type is similar to a Chinese finger grip and is often referred to as a basket or "Kellem" grip. Such a grip, shown in FIG. 11, is often used because of its flexibility and small size, making it easily pulled through sheaves during the stringing operation. Whatever type of gripping device is used, a swivel should be installed between the pulling grip and pulling line or running board to allow free rotation of both the conductor and the pulling line.

A traveler consists of a sheave or pulley wheel enclosed in a frame to allow it to be suspended from structures or insulator strings. The frame must have some type of latching mechanism to allow insertion and removal of the conductor during the stringing operation. Travelers are designed for a maximum safe working load. Always ensure that this safe working load won’t be exceeded during the stringing operation. Sheaves are often lined with neoprene or urethane materials to prevent scratching of conductors in high-voltage applications; however, unlined sheaves are also available for special applications.

Above: Fig. 10 Tension stringing equipment setup.

Above: Fig. 11 Basket grip pulling device.

Travelers used in tension stringing must be free rolling and capable of withstanding high running or static loads without damage. Proper maintenance is essential. Very high longitudinal tension loads can develop on transmission structures if a traveler should "freeze" during tension stringing, possibly causing conductor and/or structure damage. Significant levels of rotation resistance will also yield tension differences between spans, resulting in incorrect sag.

Proper selection of travelers is important to assure that travelers operate correctly during tension stringing and sagging. The sheave diameter and the groove radius must be matched to the conductor.

FIG. 12 illustrates the minimum sheave diameter for typical stringing and sagging operations.

Larger diameter sheaves may be required where particularly severe installation conditions exist.

Above: Fig. 12 Recommended minimum sheave dimensions.

Above: Fig. 13 Clipping offset illustration.

6.3 Sagging Procedure

It’s important that the conductors be properly sagged at the correct stringing tension for the design RS. A series of several spans, a line section, is usually sagged in one operation. To obtain the correct sags and to insure the suspension insulators hang vertically, the horizontal tension in all spans must be equal. Figures 15.13 through 15.18 depict typical parabolic methods and computations required for sagging conductors. Factors that must be considered when sagging conductors are creep elongation during stringing and pre-stressing of the conductor.

Above: Fig. 14 Nomograph for determining level span equivalents of non-level spans.

6.3.1 Creep Elongation during Stringing

Upon completion of conductor stringing, a time of up to several days may elapse before the conductor is tensioned to design sag. Since the conductor tension during the stringing process is normally well below the initial sagging tension, and because the conductor remains in the stringing sheaves for only a few days or less, any elongation due to creep is neglected. The conductor should be sagged to the initial stringing sags listed in the sag tables. However, if the conductor tension is excessively high during stringing, or the conductor is allowed to remain in the blocks for an extended period of time, then the creep elongation may become significant and the sagging tables should be corrected prior to sagging.

Creep is assumed exponential with time. Thus, conductor elongation during the first day under tension is equal to elongation over the next week. Using creep estimation formulas, the creep strain can be estimated and adjustments made to the stringing sag tables in terms of an equivalent temperature. Also, should this become a concern, Southwire's Wire and Cable Technology Group will be happy to work with you to solve the problem.

6.3.2 Prestressing Conductor

Prestressing is sometimes used to stabilize the elongation of a conductor for some defined period of time. The prestressing tension is normally much higher than the unloaded design tension for a conductor. The degree of stabilization is dependent upon the time maintained at the prestress tension.

After prestressing, the tension on the conductor is reduced to stringing or design tension limits. At this reduced tension, the creep or plastic elongation of the conductor has been slowed, reducing the permanent elongation due to strain and creep for a defined period of time. By tensioning a conductor to levels approaching 50% of its breaking strength for times on the order of a day, creep elongation will be temporarily halted (Cahill, 1973). This simplifies concerns about creep during subsequent installation but presents both equipment and safety problems.

Above: Fig. 15 Nomograph for determining control factor for conductor sagging.

6.3.3 Sagging by Stopwatch Method

A mechanical pulse imparted to a tensioned conductor moves at a speed proportional to the square root of tension divided by weight per unit length. By initiating a pulse on a tensioned conductor and measuring the time required for the pulse to move to the nearest termination, the tension, and thus the sag of the conductor, can be determined. This stopwatch method has come into wide use even for long spans and large conductors.

The conductor is struck a sharp blow near one support and the stopwatch is started simultaneously.

A mechanical wave moves from the point where the conductor was struck to the next support point at which it will be partially reflected. If the initiating blow is sharp, the wave will travel up and down the span many times before dying out. Time-sag tables such as the one shown in TBL. 14 are available from many sources. Specially designed sagging stopwatches are also available.

Above: Fig. 16 Conductor sagging by calculated angle of sight. (fig. to be added)

The reflected wave can be detected by lightly touching the conductor but the procedure is more likely to be accurate if the wave is both initiated and detected with a light rope over the conductor. Normally, the time for the return of the third or fifth wave is monitored.

Traditionally, a transit sagging method has been considered to be more accurate for sagging than the stopwatch method. However, many transmission-line constructors use the stopwatch method exclusively, even with large conductors.

6.3.4 Sagging by Transit Methods

IEEE Guide Std. 524-1993 lists three methods of sagging conductor with a transit: "Calculated Angle of Sight," "Calculated Target Method," and "Horizontal Line of Sight." The method best suited to a particular line sagging situation may vary with terrain and line design.

Above: Fig. 17 Conductor sagging by calculated target method. (fig. to be added)

6.3.5 Sagging Accuracy

Sagging a conductor during construction of a new line or in the re-conductoring of an old line involves many variables that can lead to a small degree of error. IEEE Std. 524-1993 suggests that all sags be within 6 in. of the stringing sag values. However, aside from measurement errors during sagging, errors in terrain measurement and variations in conductor properties, loading conditions, and hardware installation have led some utilities to allow up to 3 ft of margin in addition to the required minimum ground clearance.

Sag is based on parabolic functions. If sag exceeds 5% of span, don’t use this chart.

Above: Fig. 18 Conductor sagging by horizontal line of sight. (fig. to be added)

TBL. 14 Sag-Tension Data 795 kcmil-Type 16 ACSR/SD with NESC Heavy Loading 300 and 1000 ft Spans

6.3.6 Clipping Offsets

If the conductor is to be sagged in a series of suspension spans where the span lengths are reasonably close and where the terrain is reasonably level, then the conductor is sagged using conventional stringing sag tables and the conductor is simply clipped into suspension clamps that replace the travelers. If the conductor is to be sagged in a series of suspension spans where span lengths vary widely or more commonly, where the terrain is steep, then clipping offsets may need to be employed in order to yield vertical suspension strings after installation.

Clipping offsets are illustrated in FIG. 19, showing a series of steeply inclined spans terminated in a "snub" structure at the bottom and a "dead-end" structure at the top. The vector diagram illustrates a balance of total conductor tension in the travelers but an imbalance in the horizontal component of tension.

7. Defining Terms

Block-A device designed with one or more single sheaves, a wood or metal shell, and an attachment hook or shackle. When rope is reeved through two of these devices, the assembly is commonly referred to as a block and tackle. A set of 4s refers to a block and tackle arrangement utilizing two 4 in. double sheave blocks to obtain four load-bearing lines. Similarly, a set of 5s or a set of 6s refers to the same number of load bearing lines obtained using two 5 in. or two 6 in. double-sheave blocks, respectively.

Synonyms: Set of 4s, set of 5s, set of 6s.

Bullwheel-A wheel incorporated as an integral part of a bullwheel puller or tensioner to generate pulling or braking tension on conductors or pulling lines, or both, through friction. A puller or tensioner normally has one or more pairs arranged in tandem incorporated in its design. The physical size of the wheels will vary for different designs, but 17 in. (43 cm) face widths and diameters of 5 ft (150 cm) are common. The wheels are power driven or retarded and lined with single- or multiple-groove neoprene or urethane linings. Friction is accomplished by reeving the pulling line or conductor around the groove of each pair.

Clipping-in-The transferring of sagged conductors from the traveler to their permanent suspension positions and the installing of the permanent suspension clamps.

Synonyms: Clamping, clipping.

Clipping offset-A calculated distance, measured along the conductor from the plum mark to a point on the conductor at which the center of the suspension clamp is to be placed. When stringing in rough terrain, clipping offset may be required to balance the horizontal forces on each suspension structure.

Grip, conductor-A device designed to permit the pulling of conductor without splicing on fittings, eyes, etc. It permits the pulling of a continuous conductor where threading is not possible. The designs of these grips vary considerably. Grips such as the Klein ( Chicago) and Crescent utilize an open-sided rigid body with opposing jaws and swing latch. In addition to pulling conductors, this type is commonly used to tension guys and, in some cases, pull wire rope. The design of the come-along (pocketbook, suitcase, four bolt, etc.) incorporates a bail attached to the body of a clamp which folds to completely surround and envelope the conductor. Bolts are then used to close the clamp and obtain a grip.

Above: Fig. 19 Conductor sagging for checking sag S.

Synonyms: Buffalo, Chicago grip, come-along, Crescent, four bolt, grip, Klein, pocketbook, seven bolt, six bolt, slip-grip, suitcase.

Line, pilot-A lightweight line, normally synthetic fiber rope, used to pull heavier pulling lines which in turn are used to pull the conductor. Pilot lines may be installed with the aid of finger lines or by helicopter when the insulators and travelers are hung.

Synonyms: Lead line, leader, P-line, straw line.

Line, pulling-A high-strength line, normally synthetic fiber rope or wire rope, used to pull the conductor. However, on reconstruction jobs where a conductor is being replaced, the old conductor often serves as the pulling line for the new conductor. In such cases, the old conductor must be closely examined for any damage prior to the pulling operations.

Synonyms: Bull line, hard line, light line, sock line.

Puller, bullwheel-A device designed to pull pulling lines and conductors during stringing operations.

It normally incorporates one or more pairs of urethane- or neoprene-lined, power-driven, single- or multiple-groove bullwheels where each pair is arranged in tandem. Pulling is accomplished by friction generated against the pulling line which is reeved around the grooves of a pair of the bullwheels. The puller is usually equipped with its own engine which drives the bullwheels mechanically, hydraulically, or through a combination of both. Some of these devices function as either a puller or tensioner.

Synonym: Puller.

Puller, drum-A device designed to pull a conductor during stringing operations. It’s normally equipped with its own engine which drives the drum mechanically, hydraulically, or through a combination of both. It may be equipped with synthetic fiber rope or wire rope to be used as the pulling line. The pulling line is paid out from the unit, pulled through the travelers in the sag section and attached to the conductor.

The conductor is then pulled in by winding the pulling line back onto the drum. This unit is sometimes used with synthetic fiber rope acting as a pilot line to pull heavier pulling lines across canyons, rivers, etc.

Synonyms: Hoist, single drum hoist, single drum winch, tugger.

Puller, reel-A device designed to pull a conductor during stringing operations. It’s normally equipped with its own engine which drives the supporting shaft for the reel mechanically, hydraulically, or through a combination of both. The shaft, in turn, drives the reel. The application of this unit is essentially the same as that for the drum puller previously described. Some of these devices function as either a puller or tensioner.

Reel stand-A device designed to support one or more reels and having the possibility of being skid, trailer, or truck mounted. These devices may accommodate rope or conductor reels of varying sizes and are usually equipped with reel brakes to prevent the reels from turning when pulling is stopped. They are used for either slack or tension stringing. The designation of reel trailer or reel truck implies that the trailer or truck has been equipped with a reel stand (jacks) and may serve as a reel transport or payout unit, or both, for stringing operations. Depending upon the sizes of the reels to be carried, the transporting vehicles may range from single-axle trailers to semi-trucks with trailers having multiple axles.

Synonyms: Reel trailer, reel transporter, reel truck.

Running board-A pulling device designed to permit stringing more than one conductor simultaneously with a single pulling line. For distribution stringing, it’s usually made of lightweight tubing with the forward end curved gently upward to provide smooth transition over pole cross-arm rollers. For transmission stringing, the device is either made of sections hinged transversely to the direction of pull or of a hard nose rigid design, both having a flexible pendulum tail suspended from the rear. This configuration stops the conductors from twisting together and permits smooth transition over the sheaves of bundle travelers.

Synonyms: Alligator, bird, birdie, monkey tail, sled.

Sag section-The section of line between snub structures. More than one sag section may be required in order to properly sag the actual length of conductor which has been strung.

Synonyms: Pull, setting, stringing section.

Site, pull-The location on the line where the puller, reel winder, and anchors (snubs) are located. This site may also serve as the pull or tension site for the next sag section.

Synonyms: Reel setup, tugger setup.

Site, tension-The location on the line where the tensioner, reel stands and anchors (snubs) are located.

This site may also serve as the pull or tension site for the next sag section.

Synonyms: Conductor payout station, payout site, reel setup.

Snub structure-A structure located at one end of a sag section and considered as a zero point for sagging and clipping offset calculations. The section of line between two such structures is the sag section, but more than one sag section may be required in order to sag properly the actual length of conductor which has been strung.

Synonyms: 0 structure, zero structure.

Tensioner, bullwheel-A device designed to hold tension against a pulling line or conductor during the stringing phase. Normally, it consists of one or more pairs of urethane- or neoprene-lined, power braked, single- or multiple-groove bullwheels where each pair is arranged in tandem. Tension is accomplished by friction generated against the conductor which is reeved around the grooves of a pair of the bullwheels. Some tensioners are equipped with their own engines which retard the bullwheels mechanically, hydraulically, or through a combination of both. Some of these devices function as either a puller or tensioner. Other tensioners are only equipped with friction-type retardation.

Synonyms: Retarder, tensioner.

Tensioner, reel-A device designed to generate tension against a pulling line or conductor during the stringing phase. Some are equipped with their own engines which retard the supporting shaft for the reel mechanically, hydraulically, or through a combination of both. The shaft, in turn, retards the reel.

Some of these devices function as either a puller or tensioner. Other tensioners are only equipped with friction type retardation.

Synonyms: Retarder, tensioner.

Traveler-A sheave complete with suspension arm or frame used separately or in groups and suspended from structures to permit the stringing of conductors. These devices are sometimes bundled with a center drum or sheave, and another traveler, and used to string more than one conductor simultaneously.

For protection of conductors that should not be nicked or scratched, the sheaves are often lined with nonconductive or semi-conductive neoprene or with nonconductive urethane. Any one of these materials acts as a padding or cushion for the conductor as it passes over the sheave. Traveler grounds must be used with lined travelers in order to establish an electrical ground.

Synonyms: Block, dolly, sheave, stringing block, stringing sheave, stringing traveler.

Winder reel-A device designed to serve as a recovery unit for a pulling line. It’s normally equipped with its own engine which drives a supporting shaft for a reel mechanically, hydraulically, or through a combination of both. The shaft, in turn, drives the reel. It’s normally used to rewind a pulling line as it leaves the bullwheel puller during stringing operations. This unit is not intended to serve as a puller, but sometimes serves this function where only low tensions are involved.

Synonyms: Take-up reel.

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