The Definition of Knuckle Boom Crane

Definition of Knuckle Boom Crane

A knuckle boom crane, also called an articulating boom crane, is a hydraulic crane with a boom made of two or more sections connected by joints, or knuckles, that bend much like a human finger. The sections fold and unfold to reach over obstacles and place loads precisely, then fold into a compact position for travel and storage.

Knuckle boom cranes are most often mounted on trucks behind the cab or at the rear of the bed, where they are used to deliver building materials such as drywall, lumber, roofing, block and pipe. They are also mounted on ships, docks, rail cars and in industrial settings. Many are operated with a remote control, which lets the operator stand where they have the best view of the load.

Because they lift heavy loads near people, structures and power lines, knuckle boom cranes carry the same core hazards as other cranes, including tip-over, structural overload, struck-by injuries and electrocution from power line contact.

Regulations and Standards

OSHA construction crane standard

In construction, OSHA's Cranes and Derricks in Construction standard, 29 CFR 1926 Subpart CC, lists articulating cranes, such as knuckle-boom cranes, among the equipment it covers in 1926.1400(a). However, 1926.1400(c)(17) excludes articulating or knuckle-boom truck cranes that deliver material to a construction site in two situations:

• Ground delivery: transferring materials from the truck crane to the ground without arranging them in a particular sequence for hoisting.
• Sheet goods and packaged materials: transferring building supply sheet goods or packaged materials, such as drywall, plywood, bags of cement or roofing shingles, onto a structure using a fork or cradle at the end of the boom, but only when the crane has a properly functioning automatic overload prevention device.

The exclusion does not apply when the crane holds, supports or stabilizes material while it is attached to the structure, or when it handles prefabricated components such as roof trusses, wall panels or precast concrete, or structural steel members. In those cases, Subpart CC applies in full.

Operator certification

When Subpart CC applies, 29 CFR 1926.1427 requires operators to be trained, certified or licensed, and evaluated by the employer before operating the equipment. Certification may come from an accredited testing organization, an audited employer program, a qualifying government license or, for military employees, a military qualification. Certification alone does not make an operator qualified, and the employer must also evaluate and document the operator's ability to safely operate the specific equipment.

Consensus standards

ASME B30.22, Articulating Boom Cranes, is the national consensus standard covering the construction, installation, inspection, testing, maintenance and operation of these cranes. Manufacturers' manuals and load charts also set requirements for each model.

Key Safety Practices

Load charts and capacity

• Know the load weight: confirm the weight of the load and rigging before lifting.
• Use the load chart: capacity drops as the boom extends outward, so the chart for the configuration and radius in use governs the lift.
• Overload protection: keep overload prevention and limit devices working and never bypass them.
• Attachments: account for the weight of forks, grapples, clamshells or jibs, which reduce net capacity.

Setup and outriggers

• Ground conditions: set up on firm, level, drained ground, and check for soft soil, backfill, vaults and underground utilities.
• Outriggers: extend and deploy outriggers as the manufacturer requires, using pads or cribbing to spread the load.
• Level: level the truck within the manufacturer's limits before lifting.
• Traffic and public: use cones and barricades to keep vehicles and pedestrians out of the work area.

Power line safety

Power line contact is one of the leading causes of fatal crane incidents. Under 29 CFR 1926.1408, before work begins the employer must identify the work zone and determine whether any part of the equipment, load line or load could get closer than 20 feet to a power line rated up to 350 kV. If so, the employer must take steps such as de-energizing and grounding the line, keeping the equipment at least 20 feet away using measures like a dedicated spotter and warning lines, or keeping it outside the minimum clearance distances in Table A. For lines over 350 kV and up to 1,000 kV, the 20-foot distance becomes 50 feet under 1926.1409. Remote control operation does not protect the operator or nearby workers if the crane becomes energized.

Operation and rigging

• Inspect the crane before each shift, checking hydraulics, hoses, controls, hooks, safety latches and outriggers.
• Keep workers out of the fall zone and away from pinch points between the boom, load and structure.
• Use proper rigging and tag lines. See knots (rigging) for rules on knots and slings.
• Avoid sudden movements that cause loads to swing, and stop work in high winds.
• Stow the boom and outriggers fully before driving.

Example of Knuckle Boom Crane

A building supply company delivered roof trusses to a residential construction site using a truck-mounted knuckle boom crane. The site supervisor asked the driver to hold each truss in place on the top plate while carpenters nailed it off. A safety coordinator noticed and stopped the work, explaining that holding trusses for installation fell outside the material delivery exclusion and required a certified operator and full compliance with the crane standard. She also noted an overhead distribution line along the street side of the house.

The builder rescheduled the truss setting with a crane service that provided a certified operator. Before the lift, the team held a pre-job briefing, measured the distance to the power line, set up warning lines and assigned a dedicated spotter to keep the boom more than 20 feet from the line. The operator deployed outriggers on cribbing, confirmed the load chart for the planned radius and used tag lines to control each truss.

Frequently Asked Questions: Knuckle Boom Crane

Is a knuckle boom crane covered by OSHA's crane standard?

Yes, articulating cranes are covered by 29 CFR 1926 Subpart CC in construction, except for certain material delivery activities described in 1926.1400(c)(17). Holding material for installation or handling prefabricated components or structural steel is covered.

Do knuckle boom crane operators need certification?

When the work is covered by Subpart CC, the operator must be certified or licensed and evaluated by the employer under 1926.1427. Operators doing excluded delivery work still need proper training to operate the equipment safely.

What is the difference between a knuckle boom crane and a telescopic boom crane?

A telescopic boom extends in a straight line by sliding sections out of one another. A knuckle boom bends at joints, which lets it reach over obstacles, fold compactly and place loads in tight spaces.

How close can a knuckle boom crane get to a power line?

For lines up to 350 kV, OSHA requires planning when equipment could come within 20 feet, and in general equipment must be kept at least 20 feet away unless the line is de-energized or the Table A distances are applied. Larger distances apply to higher voltage lines.