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A turnbuckle is a three-piece metal coupling device designed to increase or decrease tension gradually without twisting the ropes or cables to which it's secured. It consists of a rectangular or oblong body with screw holes threaded in opposite directions at either end, and two threaded inserts. The threaded inserts usually have either a screw-eye or hook at the non-threaded end. When the inserts are threaded into the body and it's turned in one direction, the screws are each forced out of the body, relieving tension. When the body is turned in the opposite direction, the screws are drawn toward the center, increasing tension. The name itself, in fact, is very descriptive of its function: a turnbuckle is a buckle that turns.
Turnbuckles' bodies can range in size from about two inches (five cm) to over 12 inches (30.5 cm) in length. The threaded inserts that screw into the body are slightly less than half the length of the body. Thus, a turnbuckle assembly, when installed as a component of a length of rope or cable, can shorten it by almost the length of the turnbuckle body. Turnbuckles are very effective because the shortening or lengthening process, which directly affects the tension, can be done in extremely fine increments.
Turnbuckles are used in a wide variety of applications. In residential construction and renovation, for instance, turnbuckles are used in suspended ceilings to fine-tune the suspension adjustment, and on screen doors to keep them from drooping or sagging. Turnbuckles are also used extensively in watercraft and aircraft to regulate the lengths and tensions of riggings and rope and cable lines. In all applications, turnbuckles should be selected to be at least as strong as the cable or rope to which they're secured.
Turnbuckles are built primarily to regulate tension, and can pose a threat of injury if improperly used. Prior to removing a turnbuckle assembly from a cable or rope, it's necessary to relax the tension entirely, putting some slack into the rope or cables it's attached to. Otherwise, the sudden release of tension could cause the cable or rope to flail about. Additionally, turnbuckles that have become misshapen due to the pressure and tension they've been under should never be re-shaped by applying heat, as this will reduce the turnbuckle's strength.
One of the most well-known uses of turnbuckles is in boxing and professional wrestling rings. These turnbuckles are relatively large, with bodyies around 12 inches (30.5 cm) long and inserts of about six inches each. They secure the ropes that define the “ring" to the four corner posts. For safety's sake, the entire setup is covered with sturdy canvas or vinyl, so the turnbuckle itself isn't generally visible. Nevertheless, media reporters will frequently refer to it when describing a match because it's so integral a part of the contest. The turnbuckles used in athletic applications must be very strong because they endure a great deal of pressure and stress in the course of the competition, especially in professional wrestling rings, where wrestlers frequently climb atop the turnbuckle assembly.