How Tower Cranes Work: Slewing, Trolleys, Hoists, and Climbing Explained

Walk onto any high-rise job and the tower crane is the first thing you notice. It’s the machine everything else waits on. Most people on site never think about how it works, though. How does a steel tower hold a heavy load out at the end of a jib longer than a football field? And how does it get taller as the building rises?

If you supervise near one, you should understand the basics. Knowing how the crane works makes it easier to see when something is going wrong.

Tower crane vs. mobile crane

Most mobile cranes, like hydraulic truck cranes and telescopic boom cranes, lift with hydraulic pressure. A tower crane works differently. It uses electric motors driving wire rope drums, and it’s fixed in one spot for the whole project.

	Telescopic / mobile crane	Tower crane
Power for lifting	Hydraulic system	Electric motors and wire rope drums
Mobility	Drives or is trucked between sites	Fixed on a concrete foundation for the life of the job
Height	Limited by boom length	Can exceed 250 m (820+ ft) on climbing setups
Horizontal reach	Boom angle and length	Jib up to roughly 70 m (230 ft)
When it’s on site	As needed	Erected before the structure goes up, removed after it’s done

With either type, capacity drops as height and radius increase. Keep that in mind for every lift.

The foundation and the mast

A tower crane is only as good as what it’s standing on. Before anything goes up, crews pour a massive reinforced concrete foundation with anchors cast into it. This is what keeps the crane from moving or tipping.

A mobile crane then sets the first stack of mast sections. These are the lattice blocks that make up the tower, bolted one on top of another. A mobile crane can usually only reach about 15–20 m (50–65 ft), so it builds the crane to a working height and stops there. From then on, the tower crane raises itself.

Remember: Erection, climbing, and dismantling are high-risk operations. Under 29 CFR 1926.1404, assembly and disassembly must be directed by a qualified A/D director who follows the manufacturer’s procedures. Section 1926.1435 adds tower-crane requirements, including a pre-erection inspection.

How the crane slews (rotates)

The upper crane sits on a large ring gear called the turntable (or slewing ring). A slip ring induction motor drives the rotation. It’s used because its torque can be increased by adding external resistance, which gives the operator strong, smooth control.

There’s also a slewing limit switch. Power and control cables run down through the mast, so the crane can’t spin endlessly. After about 1.5 turns the limit switch stops the motor before the cables twist and break.

Jib, counter-jib, and balance

The top of the crane has four main parts:

• Jib: the long working arm out front, up to about 70 m long
• Counter-jib: the shorter arm on the back, which carries the counterweights (often 15–20 tons)
• Tower peak: the top point that the pendants (tie bars) run from to support both jibs
• Operator cab: usually just below the turntable

The goal is to keep the crane’s center of mass over the mast. If it shifts too far either way, the crane becomes unstable. The counterweight is sized to offset a working load. That means when the hook is empty, the counterweight pulls the crane slightly off balance toward the back. This matters again during climbing.

The trolley and why radius matters

A trolley rolls along a track on the jib to move the load in and out. A motor turns a drum carrying two ropes. One rope pays out while the other winds in, so the trolley gets pulled one way or the other. A limit switch stops the trolley before it hits the end of the jib. That’s needed because the operator, sitting up in the cab, can’t judge the trolley’s exact position.

The trolley position also controls how much the crane can lift:

Trolley position	Approximate capacity (example crane)
Close to the mast	Maximum rated load
Mid-jib	Significantly reduced
Jib tip	As little as 5–6 tons

Tip: Never estimate capacity. Check the load chart at the actual radius, including rigging weight. Most tower crane overloads happen when the load is moved outward after the hook is already set.

The hoist: motors, brakes, and the master pulley

The hoisting unit sits on the counter-jib. It’s another slip ring induction motor driving a rope drum through a gearbox. It has a brake next to it, which is critical. Without the brake, the load would drop back down as soon as the motor stopped.

The hoist rope runs up over the tower peak and through a sensor-equipped master pulley. It then passes through the trolley sheaves, down to the hook block, back up, and is dead-ended at the jib tip. The load sensors in that pulley feed the overload protection system.

OSHA requires these protective systems. Under 29 CFR 1926.1415 (safety devices) and 1926.1416 (operational aids), cranes need working devices such as load moment or rated capacity indicators, anti-two-block protection, and hoist and trolley limits. If an operational aid isn’t working, the standard requires specific temporary measures and a timeline for repair. Taping over a warning light or bypassing a limit switch is never acceptable.

Communication: the cab and the ground

The operator in the cab often can’t see the load, especially when it’s landing on a deck below or behind the structure. That’s why every tower crane lift depends on a signal person on the ground or at the landing point, usually talking to the operator by radio.

Subpart CC covers this in detail:

• 1926.1419–1926.1422: signal requirements, including hand and voice/radio signals
• 1926.1428: signal person qualifications
• 1926.1427: operator certification and qualification

Keeping people out from under the load is just as important. A dropped or swinging load is a classic struck-by hazard, and 1926.1425 requires keeping workers clear of suspended loads. Struck-by is one of the Focus Four hazards for good reason.

How a tower crane climbs

This is the clever part. The crane raises itself using a climbing frame, also called a telescoping cage, that wraps around the mast:

1. Unbolt the turntable from the top mast section and connect it to the climbing frame. The weight of the whole upper crane now rests on the frame.
2. Balance the crane. The crane lifts a new mast section and parks it so that, together with the trolley position, the upper crane is balanced over the mast. This is needed because the counterweight leaves an empty crane tail-heavy.
3. Jack it up. A hydraulic cylinder inside the frame pushes against climbing points on the mast and lifts the entire upper crane.
4. Lock and reset. At the end of its stroke, the frame locks onto the mast. The cylinder retracts, resets to the next set of climbing points, and jacks again. Two strokes usually open enough room for a new section.
5. Insert the new mast section. The section is rolled into the gap on small wheels or rails, then bolted in place.
6. Repeat or reconnect. Climb again, or bolt the turntable back to the mast and return to normal operation.

On tall buildings, the mast is tied into the structure at intervals with collars and struts anchored to the building’s columns or floors.

Did you know? Climbing is one of the most dangerous moments in a tower crane’s life. Many serious tower crane incidents have happened during erection, climbing, or dismantling, not during normal lifts. Clear the area, follow the manufacturer’s procedure exactly, and don’t let anyone improvise.

Wind: the crane’s biggest enemy

The higher the crane, the more wind it takes. The video gives about 40 km/h (around 25 mph) as an operating cutoff. In practice, the manufacturer’s in-service wind limit applies, and the operator and competent person decide when to stop. Large, flat loads like wall panels, formwork, and glazing catch wind like a sail and should stop well before the crane’s limit.

When the crane is shut down, it’s left in free slew (weathervaning). The slewing brake is released so the jib can turn with the wind instead of fighting it. A tower crane locked in place during a storm takes the full side load of the wind on its structure.

How SafePath OSHA Fits Into This

Cranes and rigging is one of the elective topics in the OSHA 30-Hour Construction course. Tower crane sites are where supervisors earn their pay: lift planning, signal person coordination, wind calls, and keeping crews out from under the load. If you’re the one making those calls, the 30-hour card is the one you need. See The Foreman’s Guide to OSHA 30 for what comes with that role.

The SafePath OSHA 30 app has 1,200 questions across all 24 topics, 9 required and 15 electives, from scaffolding to steel erection. It also includes a supervisor toolbox with toolbox talks, incident reports, audit checklists, and crew certification tracking. That’s useful for keeping operator, rigger, and signal person qualifications on record.

Across the full SafePath suite there are 2,200+ practice questions in 2 apps, all available in Spanish. Start with the study lessons and the free practice quizzes, then walk into your Outreach class already knowing the material.