Powered Slewing and Free Slewing — Two Operating Modes That No Other Crane Requires
Unlike every other slewing drive application — where the drive either rotates the structure or holds it stationary — a tower crane slewing drive planetary gearbox must support a third mode: free slewing (also called weathervaning). When the crane is out of service, the slewing brake is released and the jib rotates freely under wind pressure — aligning itself downwind like a weathervane to minimise the wind load on the tower.
The motor drives the pinion against the slewing ring gear to rotate the jib. Speed: 0 to 0.7 rpm. Torque: 15,000 to 80,000 Nm at the pinion output. The operator controls the speed proportionally via the cab joystick. When the motor is de-energised, the slewing brake holds the jib at the current position against wind loads of 30,000 to 80,000 N at jib height.
The slewing brake is released and the jib rotates freely under wind action. The pinion remains in mesh with the ring gear but is not driven — it is back-driven by the jib rotation, and the motor freewheels. The slewing drive must have sufficiently low friction in the back-driven direction that the jib can weathervane freely in winds as light as 15 to 20 km/h — a minimum-friction requirement that opposes the high-holding-torque requirement of powered mode.
Free slewing is not a convenience feature — it is a structural survival requirement. A tower crane jib at 200 metres height in a 100 km/h storm wind experiences a lateral force of 30,000 to 80,000 N. If the jib is locked perpendicular to the wind, this force acts on the full jib projected area and generates a bending moment that can exceed the tower structural capacity — risking tower collapse. If the jib is free to weathervane, it aligns downwind, reducing the projected area to the jib cross-section (5 to 10% of the broadside area) and reducing the wind moment by 90 to 95%.
The slewing drive must therefore satisfy two contradictory requirements: high friction for powered slewing (to hold the jib against working-mode wind loads) and low friction for free slewing (to allow the jib to weathervane in light winds). The engineering solution is a two-stage brake: a spring-applied service brake that provides high holding torque during powered operation, and a mechanical release mechanism that fully disengages the brake for free-slewing mode. The planetary gearbox itself must have sufficiently low no-load torque (less than 2 to 3% of the rated torque) that the back-driven friction does not prevent weathervaning in moderate winds.
The transition between powered slewing and free slewing is managed by the crane control system — but the mechanical design of the slewing drive determines whether the transition is smooth or abrupt. When the operator engages the slewing brake after positioning, the brake must engage progressively — a sudden brake engagement at 0.5 rpm slewing speed generates a deceleration shock that can excite the jib natural frequency and cause the suspended load to swing. Conversely, when the operator releases the brake for free slewing, the release must be smooth enough that the jib does not lurch in the prevailing wind direction. The brake engagement and release characteristics are therefore part of the slewing drive specification — not just the holding torque capacity.
The free-slewing bearing friction also affects the overnight noise level. In urban areas, tower cranes that free-slew at night can generate a rhythmic metallic ticking from the pinion teeth clicking past the ring gear teeth under wind-driven rotation. This noise, while structurally harmless, can generate complaints from residents in adjacent buildings — and some urban sites now specify slewing drives with anti-noise pinion coatings (nylon-tooth pinions or polymer-lined ring gear segments) for overnight free-slewing in residential zones.
Jib Inertia and Wind Torque — The Two Loads That Size the Slewing Drive
The tower crane slewing drive must overcome two simultaneous loads: the inertia of the jib assembly (accelerating the jib + counter-jib + trolley + load from standstill to working speed) and the wind-induced moment (rotating the jib against or across the wind). On calm days, inertia dominates. On windy days, wind dominates. The drive must be sized for the worst-case combination.
| Crane Class | Jib (m) | Tip Load (t) | Inertia (kg·m2) | Coppia |
|---|---|---|---|---|
| Flat-top (40–55 m) | 40 – 55 | 1.5 – 2.5 | 200k – 500k | 15k – 30k Nm |
| Hammerhead (55–75 m) | 55 – 75 | 2.0 – 4.0 | 600k – 1.5M | 30k – 55k Nm |
| Heavy-lift (60–85 m) | 60 – 85 | 3.0 – 6.0 | 1.5M – 4M | 50k – 120k Nm |
Wind loading at height is not the same as at ground level: Wind speed increases with height — at 200 metres, the wind is typically 1.5 to 2.0 times the ground-level speed. The wind force scales with the square of the speed, so the jib at 200 metres experiences 2.25 to 4.0 times the wind force of the same jib at ground level. Tower crane slewing drives must be rated for the wind speed at the actual operating height, not the ground-level measurement reported by weather stations.
The wind torque calculation is more complex than a simple drag force multiplied by radius. The jib is a distributed mass — each section contributes a different drag force at a different radius from the slewing centre. The counter-jib (behind the tower) partially offsets the main jib wind torque when the wind is from ahead, but adds to it when the wind is from behind. The trolley position along the jib changes the wind moment arm dynamically during operation. And the suspended load acts as a pendulum that adds a time-varying dynamic component to the wind torque at the load pendulum period (typically 3 to 8 seconds for 20 to 50 metre hoist rope lengths).
The slewing drive must be sized for the maximum wind-plus-inertia-plus-pendulum combination — not for any single component. This combined load case is typically 1.3 to 1.5 times the wind-only torque and 1.5 to 2.0 times the inertia-only torque. The safety factor applied to the combined case is typically 1.25 to 1.5 (lower than for cranes that do not have the load-moment indicator protection that tower cranes provide).
Importantly, the load-moment indicator (LMI) on a tower crane monitors the hook load and radius continuously — and can automatically limit the slewing speed when the crane is operating near the rated capacity. This LMI-driven speed limiting reduces the peak inertia torque during high-load lifts by restricting the maximum angular acceleration. The slewing drive must respond to these variable speed-limit commands with proportional, smooth torque delivery — any cogging or dead-zone in the drive response at low joystick inputs makes the reduced-speed operation jerky and reduces operator confidence in the precision-placement capability.
Anti-Collision and Zoning — How the Slewing Drive Interfaces with the Site Safety System
On multi-crane construction sites, two or more tower cranes may have overlapping working radii — meaning their jibs can potentially collide during slewing. Anti-collision systems (ACS) monitor the position of every crane jib on the site and automatically limit the slewing range to prevent contact. The slewing drive must interface with the ACS through an angular position encoder and must respond to automatic slewing-stop commands within 1 to 2 seconds.
The ACS requires continuous angular position data — typically from a 12 to 16-bit absolute encoder mounted on the drive output or the slewing ring gear. The encoder must be mechanically robust (construction-site vibration, temperature, moisture) and electrically compatible with the ACS controller. The slewing drive must provide a dedicated encoder mounting interface on the output shaft with zero-backlash coupling.
When the ACS detects a potential collision, it commands the slewing drive to stop within 2 to 5 degrees of angular travel — smooth enough to avoid shock-loading the tower, but fast enough to prevent the jib from entering the exclusion zone. This requires a brake with controlled deceleration characteristics — not an abrupt emergency stop that would generate dynamic loads exceeding the tower design limit.
Some sites restrict the crane from slewing over neighbouring properties (hospitals, railways, public roads). The slewing drive and ACS must enforce hard angular limits — the jib physically cannot rotate beyond the permitted sector. This requires software-configurable angular limits with hardware-backed failsafe stops (redundant encoders or mechanical limit switches).
The ACS encoder is a single point of failure with site-wide consequences. If the encoder fails on one crane, the ACS cannot determine that crane jib position — and must either shut down the affected crane (safe but costly: USD 5,000 to 15,000 per day of lost crane time) or allow the operator to continue without collision protection (unsafe and prohibited on most sites). On a multi-crane site, an encoder failure on one crane can force the shutdown of all cranes sharing the overlap zone — because the ACS cannot verify that the failed crane jib is not in the collision path.
This is why redundant encoders (two independent units on the same slewing drive output) are increasingly specified for tower cranes on multi-crane sites. The cost of a second encoder (USD 500 to 1,500) is negligible compared to the daily cost of shutting down 2 to 4 cranes waiting for an encoder replacement that may take 24 to 48 hours to source and install at the tower top.
Three Failure Modes Specific to Tower Crane Slewing Drives
The slewing brake must hold the jib against wind loads during working operation. If the brake fails (pad wear, spring fatigue, hydraulic leak), the jib rotates uncontrolled under wind pressure with a suspended load — an immediate collision and dropped-load hazard. The brake must be spring-applied (failsafe — engages on power loss) with sufficient holding torque for the maximum in-service wind speed (typically 72 km/h). Brake capacity degradation is invisible externally and can only be detected by periodic holding-torque measurement. Monthly brake testing is the standard interval — any extension of this interval increases the risk of undetected degradation below the minimum holding capacity. The test procedure is straightforward: with the jib positioned perpendicular to the prevailing wind direction (maximum wind torque), the operator releases the motor and verifies that the brake holds the jib stationary for a minimum of 60 seconds. If the jib creeps even 0.5 degrees during this hold test, the brake capacity has fallen below the required level and the brake pads or springs must be replaced before the crane returns to service.
During free-slewing, the ring gear teeth slide past the pinion teeth at variable, uncontrolled speeds — without the benefit of driven-side hydrodynamic oil film. The non-driven contact condition produces higher friction and faster surface wear than powered slewing at the same speed. Over months of accumulated free-slewing (weekends, holidays, storm periods), the pinion teeth experience thousands of contact cycles under boundary-lubrication conditions. Cranes left in free-slewing for extended periods without re-greasing develop tooth wear at 2 to 3 times the powered-slewing rate — a finding that was not recognised in early specifications and led to premature pinion replacement on many tower cranes with high free-slewing exposure.
The ACS depends on accurate angular position data from the slewing drive encoder. If the encoder fails, malfunctions, or loses calibration, the ACS cannot determine the jib position and must shut down the affected crane — and potentially all cranes sharing overlap zones. Encoder failures can be caused by vibration, moisture ingress, cable damage, or electromagnetic interference from the slewing motor. The cost of a single encoder failure on a 4-crane site can reach USD 20,000 to 60,000 per day in lost crane time across all affected units — far exceeding the cost of the encoder itself (USD 500 to 1,500).
Slewing Drive Planetary Gearbox for Tower Cranes — Frequently Asked Questions
Korea Ever-Power provides tower crane slewing drive planetary gearboxes from 15,000 to 120,000 Nm with free-slewing capability, integrated brakes, and ACS encoder provisions.
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