Tuesday, July 19, 2011

Deck Machinery: A Twist in the Story of Rope Strength

By Merry Schnell and Frank Choltco-Devlin

July 2011

Application-induced twist is a very common thing to see in a rope, though it often goes unnoticed. While the twisted rope may seem fine, it actually experiences a loss of strength and its chance of failure is increased. With such serious consequences, it is important for those who routinely handle ropes to be able to identify twist, take appropriate actions to remove it, and prevent it from continuing to happen.

Rope Construction 101
The story of twist begins with the very basic understanding of rope construction. All ropes are made with twisted components. Simply stated, fiber is twisted into yarns, yarns are twisted into strands, and strands are either twisted or braided into rope. Strands are created in what is called “S-strands” or “Z-strands.” S-strands are twisted to the left, and Z-strands are twisted to the right. When these strands are used in combination, torque, which is the force that tends to produce a twisting motion, is balanced.

Braided vs. Laid Ropes
Laid rope constructions include 3-strand, 6-strand, and wire ropes. Because of the construction of these ropes, they often kink, or hockle. This happens as a load is applied to the rope, causing it to untwist until it reaches a “torque-balanced” state. When the load is removed, the strands try to resume their twisted state, but their lay has been dislocated and they become deformed (see Figure 1). During manufacture, laid ropes can be torque balanced at a specific load range, but this will not eliminate all untwisting while the rope is loaded outside its load range.

All braided ropes, including 8-strand, 12-strand single braids, double braids, and core-dependent double braids, are constructed from an equal number of S-strands and Z-strands. This creates a balanced or torque-neutral construction that will not naturally twist while under load.

In order for a rope to maximize its full-strength potential, all strands of the rope must share the load equally. This load sharing (and the strength of the rope) is reduced when a braided rope is twisted, or the twist levels of a laid rope are altered.

Twist in a Braided Rope: The Bad News
As twist is induced in to a braided rope, depending on the direction of the twist, either the S- or Z-strands will take more of the load.

Figure 2 illustrates this phenomenon, where all of the Z-strands are tight and all of the S-strands are loose. The loose S-strands will not bear the same load as the tight Z-strands, causing the tight strands to be overloaded and resulting in a significantly weaker rope.

Figure 3 uses 24-mm (1-in.) diameter AmSteel®-Blue, a 12-strand single braid construction, to illustrate how little twist it takes to affect the strength of the rope. It is important to note that larger diameter ropes will be affected more severely at lower levels of twist than smaller diameters. However, the first step toward preventing twist is to inspect for it.

Identifying Twist
Identifying twist in a braided rope is relatively easy. Simply follow a single line of picks (or crowns) down the length of the rope (see Figure 4). If the picks form a straight line parallel to the length of the rope, there is no twist. If the line of picks spirals around the circumference of the rope, that section of rope is twisted. Although a braided rope will not twist on its own under load like a laid rope, there are various ways a braided rope can become twisted. For example, when it is attached to a laid synthetic or wire rope, twist is transferred to the braided rope as it is loaded.

Managing Twist: The Good News
Twist can be managed, but in order to do so, it needs to be managed early on and as a continued part of the rope’s maintenance.

Twist can be introduced in to the rope when it is installed; therefore, it is important to handle the rope correctly from the beginning. Tips to managing twist include the following:

Improper reeling or unreeling of a rope can cause twist; rope should never be taken over the top of a reel (see Figure 5).
  • Alternate directions each time the line is wrapped on to an H-bitt.
  • Twist is induced in a rope when a load is lifted or pulled and allowed to spin. Once the load is removed, remove the twist.
  • Avoid using a laid rope as a messenger line for a braided mainline or pendant (see Figure 6).
  • Avoid using a laid rope or wire rope mainline with a braided mooring pendant.
  • Avoid using a laid-rope mooring pendant on a braided mainline
  • Avoid connecting a braided rope sling to a wire rope winch line.
  • Use a swivel to connect the messenger line to the mainline or pendant
  • to reduce twist in the mainline (see Figure 7).
As the old adage says, “An ounce of prevention is worth a pound of cure.” If the right steps are taken to manage twist from installation through the entire use of the line, the life of the line will be extended significantly.