What is Cable Grip and Why Do We Use Them?

A cable grip is a device for propelling a vehicle by attaching to a wire rope (called a haul rope) running at a (relatively) constant speed. The vehicle may be suspended from the cable, as in the case of aerial lifts such as a gondola lift (télécabine), may be guided by rails, as in a cable traction railway, or may be self-guiding, as in a button lift. Typically, multiple vehicles will use the same cable; where just one or two vehicles are in use they will tend to be attached to the cable permanently such as in funiculars.

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While the cable grips used in the original cable railways were manually operated, requiring considerable skill and strength, modern cable grips tend to be automatic. Given that the cable runs at a relatively constant speed, accelerating the vehicle to match the speed of the cable presents a technical difficulty; possible methods are to apply the grip gradually, to accelerate the vehicle (e.g. by guiding wheels) prior to applying the grip, or to use a sprung linkage between the grip and the vehicle. The cable must also be able to handle friction from the grip and increased load while the vehicle is accelerating.

Cable grips have additional uses, such as supporting electric or structural cables for pulling or laying of the electric or structural cable itself. In such use they may also be called cable pulling grips, cable socks, or cable stockings.

This is the type of grip used in the cable cars of San Francisco. The grip is attached to the lead truck of the car (or both trucks, in the case of double-ended cars), and is a field-replaceable unit. While side grips and even top grips have been used, the sheer number of rope (cable) changes, crossings, and "let-go" curves make the bottom grip, i.e., one in which the jaws open directly downward, the most practical type.

The operating lever raises and lowers the center plate of the grip, which in turn operates the jaws. With the lever fully forward, the grip is open, and the cable is free to fall out, which is necessary when the cable crosses under another line, reaches the end of a line, or negotiates a "let-go" curve. With the cable lifted into the jaws, the operating lever is pulled back, forcing the jaws between two fixed rollers, and capturing the cable between the grip's replaceable mild-steel dies, but without applying pressure. To start moving the car, the operating lever is pulled back further, squeezing the cable between the dies.

The grip was designed by the German railway mechanical engineer William Eppelsheimer in around .[1]

See also Andrew Smith Hallidie.

• Gondola lift (télécabine)
• Detachable chairlift

• Cable traction railway
• People mover

• Button lift

• George Hilton: The Cable Car in America, Stanford University Press , ISBN 978-

Gail Leach Carvelli

Whether they are pulling cable through conduit or stringing it with overhead transmission lines, contractors face a basic challenge: getting the cable from point A to point B without snagging and damaging it. To overcome this hurdle, they rely on the cable-pulling grip. Usually constructed of stainless or galvanized steel, grips let contractors pull cable reliably in a variety of applications (see table, page 16).

The main purpose of a pulling grip is to give users a way to hold onto a cable as it is installed or removed from conduit and duct, or strung with overhead utility lines, according to Ken Hagemeyer, director of product management at Greenlee Textron. "Ultimately, you want to make sure you have a secure and strong grip on the cable," he says. "If not, you run the risk of damaging it."

Ensuring a secure grip starts by determining the type of application and the peculiarities of the cable run. If you are pulling through conduit, are there many bends? Will the cable be run indoors or outdoors? Are you pulling fiber-optic or copper cable? What is the grip`s breaking strength?

Use the proper grip

"Installers should always make sure they use a grip that is designed for the job they are doing and that the grip is strong enough," says Randy Minor, senior product manager for Hubbell Inc.`s Wiring Device-Kellems Div. "If you use a grip that is too weak, it may pop off the cable, interrupting the pull."

Grips come in several varieties, but the two most common are set-screw and wire-mesh. While the objective is the same, each works differently to pull the cable. Another type--the slack grip--pulls slack out of underground cables for final placement.

Generally used for long pulls of heavy copper cable, set-screw grips rely on two or more opposing screws to provide positive locking on each cable. These grips accommodate pulls of up to four cables by using individual locking heads. "Set-screw grips offer more flexibility for the size of cable being pulled," says Greenlee`s Hagemeyer. "Because of their size, they can pull just about anything."

Wire-mesh grips, on the other hand, use a different gripping action. Instead of relying on two or more points to grasp the cable, as does the set-screw grip, mesh grips depend on many points of mechanical gripping. "Each cross in a wire-mesh grip is grabbing hold of the cable," Minor explains. "This makes these grips very reliable."

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This is vital, Minor says, when pulling cable for an outside-plant application, such as stringing wire with overhead utility lines. "You don`t want to drop a cable and stop traffic because the pulling mechanism gave way," he says. "And when you are pulling cable underground, the grip needs to be reliable because you are pulling it out of sight. You can`t see what is going on."

The design of the wire-mesh weave--for example, single or multiweave--can also be specified. The design is often dictated by the size of the cables being pulled, which determines how much force can be used for the pull before the breaking strength is reached.

Pulling action

Michael Brigandi, product manager of industrial products for Pass & Seymour/Legrand, says contractors need to use whichever grips give them the best pulling action. "Choosing a grip type often depends on how heavy the cable is and how long the pull is," he says. "Single-weave may not be strong enough for the cable you are pulling. So you want to make sure that your grip is the strongest you can get so it doesn`t pop off during a pull."

Another type of grip, usually called a straight grip, is manufactured of brass or steel and has teeth that bite into the wire strand. "The more tension you put on it, the stronger the grip is supposed to be, provided the teeth are in good shape," says Gerry Gilbert, southeast regional manager for Harris-McBurney Co. (Brandon, FL).

When choosing a grip, you need to know from the manufacturer how much tension you can put on it before it breaks.

An eye for grips

A pulling eye--swivel, flexible, or breakaway--is usually attached to the top of a pulling grip. The application dictates the eye type. "If the cable will go around a lot of bends in the conduit, choose a swivel-eye type of product," Brigandi says. "This keeps your cord from getting tangled or twisted."

The commonly used swivel eye does as its namesake suggests: It lets the cable untwist during the pull. "When pulling conventional cable, I always look for a grip that has a swivel on it," Gilbert says. "Cable has a tendency to twist when you are putting it up."

Less-rigid flexible eyes allow the cable to follow the line of the pull around corners and bends.

A third type, the breakaway eye, combines features of the swivel- and flexible-eye types, but breaks away from the grip if too much tension is exerted during the pull.

"It is much more important to have a breakaway swivel when working with fiber," Gilbert says. "The tension you put on a fiber-optic cable cannot be as strong as that which you place on copper because you don`t want to break any of the glass inside the sheath. So we always look at the manufacturer`s tension or stress-breaking point for the cable and make sure we have a breakaway swivel on the end of our grip."

Eye type is not the only consideration when working with fiber-optic cable. According to Hubbell`s Minor, the grip`s design is also important. "Some grips are specifically made to pull fiber-optic cable," Minor says. "Because of the different construction of fiber-optic cable, it`s more delicate than electrical cable, so the wire-mesh grips tend to be longer than what we would normally use." He adds that a finer wire is used for the mesh material because "you don`t need the high strength that you do with electrical cable."

Safety concerns

Regardless of the pulling grip`s features, the bottom line for Harris-McBurney`s Gilbert is the product`s safety. "The first concern I always have when looking for any grip is safety," says Gilbert. "It is especially important to check out these grips from time to time. With the straight grips you have to make sure the teeth are in good shape." If not, he says, the teeth could let go during a high-tension pull and the wire could snap back, injuring those pulling the cable.

Gilbert does not hesitate to spend a little more money to buy grips from a well-established manufacturer. Grips can range from six dollars to hundreds of dollars, depending on the type. "I will pay more money for a grip that is going to last and be safe," he says.

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