The Definition of Knots (Rigging)

Definition of Knots (Rigging)

In rigging, knots are ties, hitches and bends made in fiber rope to secure loads, attach tag lines, tie off materials, join ropes and hoist light items by hand. Common rigging knots include the bowline, clove hitch, round turn and two half hitches, timber hitch and sheet bend. Each has a specific purpose, and choosing the wrong knot or tying it incorrectly can cause a load to slip, shift or fall.

Knots are an everyday part of work in construction, tree care, stagecraft, shipping, utilities and industrial maintenance. They are used mostly with natural and synthetic fiber ropes. Knots are not a substitute for engineered rigging hardware, and they must not be used to shorten or repair lifting slings.

Understanding knots, their limits and the rules for when they may and may not be used is part of the training expected of riggers, signal persons and other workers involved in lifting and moving loads.

Common Rigging Knots and Their Uses

• Bowline: forms a fixed loop at the end of a rope that does not slip or tighten under load and is easy to untie afterward. It is widely used for tag lines and for hoisting tools or materials by hand line. It can loosen when not loaded, so it should be tied with a long tail and checked.
• Clove hitch: a quick hitch around a pipe, post or rail. It is useful for temporary tie-offs and starting lashings but can slip or roll if the load shifts, so it is often backed up with half hitches.
• Round turn and two half hitches: a secure hitch for tying a rope to a post, ring or anchor. The round turn takes the strain while the half hitches are tied.
• Timber hitch: used to lift or drag logs, poles and pipe. It grips tighter as tension increases and releases easily when the load is removed. It is often combined with a half hitch to keep long objects in line.
• Sheet bend: joins two ropes, including ropes of different sizes.
• Square knot (reef knot): a binding knot for bundling. It should not be used to join two ropes that will be loaded, since it can slip or capsize.
• Figure eight knot: a stopper knot that prevents a rope end from running through a block or hardware. A figure eight loop forms a secure fixed loop.

Knots and Rope Strength

Why knots weaken rope

Any knot reduces the strength of a rope, because the tight bends inside the knot concentrate stress on some fibers. The amount of strength lost depends on the knot, the rope material and construction, and how well the knot is dressed and set. Some common knots reduce rope strength by a substantial fraction of its rated breaking strength. For this reason, the working load of a knotted rope should be treated as lower than the rope's rated capacity, and manufacturer guidance should be followed.

Other factors that affect knot security

• Rope material: slippery synthetic ropes may need extra tucks or backup knots.
• Dressing and setting: a knot should be neatly formed with no crossed strands and pulled tight before loading.
• Tail length: a short tail can pull through, especially under changing loads.
• Condition: worn, wet, frozen, cut or chemically damaged rope should not be used.

Rules and Limits for Knots in Rigging

Slings

OSHA's general industry sling standard, 29 CFR 1910.184, states that slings shall not be shortened with knots or bolts or other makeshift devices. For fiber rope slings, knots shall not be used in place of splices. Practical rules include:

• Wire rope slings: never tie a knot in a wire rope sling. Knotting crushes and kinks the strands and can cause sudden failure.
• Synthetic web and round slings: never knot or tie slings together to shorten or extend them. Use slings of the correct length or approved hardware.
• Inspection: under 1910.184, slings and their fastenings and attachments must be inspected each day before use by a competent person designated by the employer. A sling that has been knotted should be removed from service and evaluated.

Tag lines and hand lines

Tag lines control the swing and rotation of a suspended load so workers can stay out from under it. They are usually attached with a bowline or other secure knot to a point on the load, not to the hook or sling in a way that could interfere with the rigging. Workers holding tag lines should never wrap the line around their hand or body, and they should stay clear of the fall zone to avoid struck-by hazards. Tools and materials raised by hand line should be secured with knots that will not slip, to prevent a dropped object hazard.

Qualified riggers

Selecting rigging, tying knots and attaching loads should be done by trained workers. In construction, OSHA's crane standard requires that loads be rigged by a qualified rigger when employees hook, unhook or guide a load within the fall zone. Cranes such as a knuckle boom crane rely on correct rigging just as much as larger cranes.

Example of Knots (Rigging)

On a commercial building project, a crew prepared to lift a bundle of conduit with a mobile crane. One worker found that the synthetic web sling was slightly too long and tied a knot in it to shorten it. The site's qualified rigger saw the knot during a pre-lift check, stopped the lift and explained that knotting a sling is prohibited and can severely reduce its capacity.

The crew replaced the knotted sling with a correctly sized sling from the rigging storage area and tagged the knotted one out of service for inspection. The rigger then attached a tag line to the end of the bundle using a bowline so a worker could control rotation from outside the fall zone. The supervisor added a short review of sling rules and tag line knots to the next morning's pre-job briefing.

Frequently Asked Questions: Knots (Rigging)

Why is the bowline so common in rigging?

The bowline forms a fixed loop that does not slip or tighten under load and is easy to untie even after heavy loading. That makes it practical for tag lines and hand lines.

Can I tie a knot in a sling to shorten it?

No. OSHA's sling standard says slings shall not be shortened with knots or bolts or other makeshift devices. Use a sling of the correct length or approved adjustable hardware.

Do knots reduce rope strength?

Yes. All knots reduce rope strength, and some reduce it significantly. The exact loss depends on the knot and rope type, so knotted ropes should be used well below the rope's rated capacity.

Who should tie rigging knots for lifts?

Workers trained in rigging should tie knots used for lifting and load control. In construction crane work, OSHA requires a qualified rigger in certain situations, such as when workers are in the fall zone while hooking, unhooking or guiding a load.