An Automatic Balance Crane is a lifting machine that uses a moving counterweight or ballast system to keep the load moment and counter-moment matched as the boom radius changes. Unlike a fixed-counterweight tower crane that relies on a static ballast block sized for a worst-case lift, this design shifts mass along a track in real time. The purpose is to hold the resultant moment near the slew ring centre regardless of jib reach, which prevents tipping and reduces structural fatigue. Liebherr's EC-B series and the Kroll K-series have used variants of this idea to lift 12-tonne loads at 70 m radius without oversizing the base.
Automatic Balance Crane Interactive Calculator
Vary hook load, load radius, position lag, and slew rating to see load moment, balancing counter-moment, lag error, and the allowable LMI band.
Equation Used
The calculator follows the worked load-moment calculation: hook load times horizontal radius gives the overturning moment in tonne-metres. An automatic balance crane commands an equal counter-moment, while the lag term estimates the extra transient moment from a small position error.
- Tonnes are treated as tonne-force for tonne-metre moment calculations.
- Counter-moment is assumed to be actively commanded equal to the load moment.
- Lag error estimates the transient unbalanced moment from a position mismatch.
- Target balance band is +/-2% of slew bearing rated moment.
How the Automatic Balance Crane Actually Works
The core problem any crane fights is the load moment — the load weight multiplied by its horizontal distance from the slew centre. A 5-tonne load at 40 m gives you 200 tonne-metres of overturning torque, and that torque has to be cancelled by something on the other side of the pivot, or the crane tips. A fixed-counterweight crane just hangs a heavy concrete block at a fixed radius behind the mast and accepts the inefficiency. An Automatic Balance Crane, by contrast, drives the counterweight along a rail on the counter-jib using a ball screw or a chain drive, and a load moment indicator (LMI) feeds boom radius and hook load into a PLC that commands counterweight position to keep net moment within a target band — typically ±2% of the slew bearing's rated moment.
The geometry matters down to the millimetre. If the counterweight position lags the boom trolley by more than about 150 mm at typical trolley speeds of 40 m/min, you get a transient overturning moment that loads the slew ring asymmetrically — and slew bearing raceways do not forgive that. Position feedback usually comes from an absolute encoder on the counterweight drive shaft with 0.1 mm resolution, and the load cell on the hoist rope reads to ±0.5% of full scale. Get either of those wrong and the system either chases its tail (encoder drift) or trips the LMI on every lift (load cell noise).
Failure modes are predictable. The counterweight drive jams from corrosion or debris on the rail, the system fails safe by locking the boom trolley until manual override clears it. Encoder loses sync after a power cycle and homes incorrectly, putting the counterweight 200 mm off true zero — every subsequent lift then runs with a baseline moment offset. Or the operator overrides the LMI to lift slightly over rated load, and the counterweight runs to its end stop with no margin left. The mechanism is only as good as its sensors and its commissioning.
Key Components
- Counter-jib rail and trolley: A precision-machined rail running the length of the counter-jib that carries the moving counterweight. Straightness tolerance is typically 0.5 mm per metre because any sag adds an unwanted vertical load component on the trolley wheels. Length sets the maximum balancing reach — a 25 m counter-jib lets you balance a 60 m main jib at full load.
- Ballast block: A concrete or cast-iron mass, commonly 8 to 40 tonnes depending on crane class. The block must be sized so the maximum counter-moment matches the rated load moment plus a 1.25 safety factor at the design slew bearing rating, otherwise the LMI will derate every long-radius lift.
- Counterweight drive: Either a ball screw with a brake-equipped servomotor or a heavy roller chain driven by a gearmotor. Travel speed sits around 0.5 to 1.0 m/s — fast enough to track a trolley moving at 40 m/min, slow enough to avoid inertial overshoot. The drive holding brake must release in under 200 ms or the trolley command will outpace it.
- Load moment indicator (LMI): Microprocessor unit that reads hook load, boom radius, slew angle, and counterweight position, then computes net moment in real time. Cuts the hoist and slew commands if computed moment exceeds 95% of slew bearing rating. Required by EN 14439 and ASME B30.3 on tower cranes built since 2000.
- Slew bearing: The triple-row roller bearing the entire crane rotates on. Rated by static moment capacity in tonne-metres — a typical EC-B 200 model rates around 280 tm. The Automatic Balance Crane's job is to keep the applied moment well inside this rating despite varying load and radius.
- Position feedback encoders: Absolute encoders on the boom trolley drum and the counterweight drive shaft. 0.1 mm resolution typical. Both must home off the same datum at startup or the LMI's moment calculation will carry a constant error term that compounds over the shift.
Who Uses the Automatic Balance Crane
You see Automatic Balance Cranes wherever the lift radius varies a lot during a single shift, where the structural base cannot accept worst-case fixed counterweight loading, or where the crane sits on a barge or floating platform that cannot tolerate static heel. The shifting ballast lets you trade off footprint, dead weight, and operational flexibility in ways a fixed-ballast crane cannot match.
- High-rise construction: Liebherr 280 EC-B 12 tower cranes on the Shard construction in London used moving counterweight logic to balance long-radius lifts of cladding panels onto the upper floors without oversizing the tower foundation.
- Shipyard and offshore: Huisman 4400-tonne mast cranes on the Sleipnir semi-submersible vessel use automatic ballast water transfer between hull tanks to keep the vessel level as load moment changes during heavy lifts on offshore platforms.
- Bridge erection: Sarens SGC-250 ring crane variants used during the Hinkley Point C reactor lift use computer-controlled counterweight wagons running on the ring rail to track the load through a 360° slew.
- Port container handling: Konecranes ESP.10 ship-to-shore cranes use auto-balancing trim weights on the counter-jib to manage the moment swing as the spreader trolley moves between ship-side and yard-side at 240 m/min.
- Wind turbine erection: Mammoet PTC 200-DS heavy ring cranes use a moving superlift counterweight on a rear tray to maintain balance during nacelle and blade lifts at radii up to 100 m on onshore wind farms.
- Nuclear decommissioning: Custom Demag CC 8800-1 TWIN crawler cranes at Sellafield use automated counterweight positioning to manage moment during long-duration suspended lifts of contaminated reactor components.
The Formula Behind the Automatic Balance Crane
The fundamental balance equation tells you where the counterweight has to sit so the net moment about the slew centre stays inside the bearing's rating. At the low end of the typical operating range — say a short-radius lift of 15 m with a 2-tonne load — the counterweight barely needs to move off its home position, and the LMI mostly idles. At nominal mid-range conditions, 30-40 m radius and 5-8 tonne loads, the counterweight tracks the trolley continuously and the drive runs at 60-80% duty. At the high end, lifts beyond 60 m radius near rated capacity, the counterweight runs near its end stop and any encoder drift or load-cell noise eats into your safety margin fast. The sweet spot for these cranes lives in that middle band — that is where the auto-balance pays back its complexity.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Rcw | Required counterweight radius from slew centre | m | ft |
| WL | Hook load weight | kN | lbf |
| RL | Load radius from slew centre | m | ft |
| Wcw | Counterweight ballast weight | kN | lbf |
| Mnet | Net moment about slew centre after balancing | kN·m | lbf·ft |
Worked Example: Automatic Balance Crane in a 200 tm tower crane on a logistics warehouse
You are commissioning a Liebherr 200 EC-B 10 class tower crane on a logistics warehouse build in Calgary. The crane has a 60 m main jib, a 20 m counter-jib, and a 12-tonne ballast block on the moving counterweight trolley. The lift schedule includes hoisting precast roof beams of varying mass, and you need to verify the auto-balance counterweight position commanded by the LMI at three operating points across the day's lift sequence.
Given
- Wcw = 12,000 kg (≈ 117.7 kN)
- Counter-jib max travel = 18 m
- Slew bearing rated moment = 200 tm (≈ 1,962 kN·m)
- WL nominal lift = 5,000 kg (≈ 49.0 kN)
- RL nominal = 35 m
Solution
Step 1 — at the nominal mid-shift lift, a 5-tonne beam at 35 m radius, compute the required counterweight radius:
That sits 81% of the way along the 18 m counter-jib travel — comfortably inside range, and the drive can still react to a quick trolley-out command without bottoming the end stop.
Step 2 — at the low end of the day's lift range, a 2-tonne miscellaneous lift at 15 m radius:
The counterweight barely leaves its home zone. At this operating point the auto-balance system is essentially idle — a fixed-counterweight crane sized for the same job would impose roughly 1,470 kN·m of unbalanced reverse moment on the slew bearing, but the moving system holds net moment near zero. This is where the mechanism earns its keep on bearing fatigue life.
Step 3 — at the high end, the day's heaviest long-reach lift, a 7-tonne precast at 55 m radius:
That number exceeds the 18 m physical travel limit. The LMI will refuse the lift or derate it. To stay inside the 18 m rail you would either (a) increase ballast to roughly 21,400 kg, (b) reduce the load to 3.93 tonnes at 55 m, or (c) bring the radius in to 31 m at full 7 tonnes. This is the ceiling of the auto-balance envelope, and the moment you ignore it the crane stops being self-balanced.
Result
At the nominal 5-tonne, 35 m lift the LMI commands the counterweight to 14. 57 m from the slew centre, holding net moment within ±2% of zero. The 2-tonne short-reach lift sits the trolley at 2.5 m — almost at home — while the 7-tonne long-reach lift exceeds the 18 m rail and the system correctly refuses it; the operational sweet spot lives in the 25-45 m radius band where the trolley uses 50-90% of available travel. If your measured counterweight position differs from the predicted value by more than 100 mm, the three usual culprits are: (1) the absolute encoder on the counterweight drive lost its home reference after a power cycle and is reporting an offset, (2) the hook load cell drifted out of calibration so the LMI is computing the wrong load moment in the first place, or (3) the boom-radius angle sensor on the trolley drum has slip in its coupling, feeding a stale radius value to the controller.
When to Use a Automatic Balance Crane and When Not To
The decision between an Automatic Balance Crane, a fixed-counterweight tower crane, and a guy-derrick comes down to how much the load moment varies during the work cycle, how much you can spend on controls and commissioning, and how forgiving your foundation or barge is to dynamic moment swings. Here is how those three options stack up on the dimensions that matter when you are sizing a job.
| Property | Automatic Balance Crane | Fixed-Counterweight Tower Crane | Guy Derrick |
|---|---|---|---|
| Net moment on slew bearing during a typical lift cycle | Held within ±2% of zero | Swings ±40-60% of rated | Swings ±50-70%, manually adjusted |
| Maximum useful boom radius for a given ballast | Up to 70-100 m | 40-60 m | 30-50 m |
| Capital cost relative to fixed-counterweight equivalent | +25-40% | Baseline | -15-30% |
| Commissioning time | 3-5 days (LMI calibration) | 1-2 days | 2-4 days (rigging) |
| Slew bearing fatigue life at typical duty | 20,000+ hours | 8,000-12,000 hours | Not bearing-limited |
| Failure mode if controls fail | Locks out — fail safe | Continues, operator-limited | Operator-dependent |
| Best application fit | Variable-radius high-rise, offshore, ring lifts | Repetitive same-radius lifts | Low-budget short-duration jobs |
Frequently Asked Questions About Automatic Balance Crane
The LMI does not just check counterweight position — it checks net moment against slew bearing rating with a 5% margin. If your slew bearing is rated at 200 tm and you are lifting at 95% of that rating, any small load-cell noise spike or wind gust pushes the computed moment over the trip threshold even though the trolley has room to travel.
Check the wind speed input first — most LMIs add a wind moment term that scales with boom area and gust speed. A 12 m/s gust on a 60 m jib can add 8-15 tm of phantom moment. Second, look at the load cell zero drift — if the empty-hook tare reads 200 kg instead of zero, every lift carries that error.
Work the balance equation backwards. Load moment is 78.5 kN × 60 m = 4,710 kN·m. To hold that with a counterweight at the maximum useful rail position — typically 90% of the 20 m counter-jib, so 18 m — you need Wcw = 4,710 / 18 = 261.7 kN, or about 26.7 tonnes of ballast.
Add 25% for the 1.25 safety factor on slew bearing moment rating and you are at 33-34 tonnes. Also reserve at least 10% of rail travel as headroom for dynamic overshoot during fast trolley moves — sizing the ballast to use 100% of travel at rated load leaves no margin when the trolley accelerates.
Overshoot comes from two places: the counterweight drive's velocity loop chasing trolley velocity instead of trolley position, and the holding brake releasing late. If the brake takes 300 ms to drop instead of the 150-200 ms spec, the drive winds up commanded torque while the brake still holds, then the brake releases and the trolley snaps to position with momentum.
Diagnostic: log encoder velocity during a step move and look for a velocity spike at brake release. Fix: replace the brake coil if release time is over 200 ms, then retune the velocity loop with a feed-forward term proportional to trolley command velocity rather than measured trolley velocity.
You can, but only with an inclinometer in the LMI loop. The standard land-based balance equation assumes the slew axis is vertical. Once the platform heels by more than about 1°, the load weight produces a horizontal component the controller cannot see, and the moment calculation drifts.
Huisman and Liebherr Maritime cranes solve this with a dual-axis inclinometer feeding a tilt correction term into the moment calculation, plus a list-compensation routine that biases the counterweight to counter platform heel directly. Without that, the crane will think it is balanced while actually loading the slew bearing asymmetrically — and slew bearings on floating cranes are not cheap to swap.
Almost always thermal drift in the load cell strain gauges or the boom-radius angle sensor. Strain gauge load cells can drift 0.05% of full scale per °C of temperature change. On a 200 tm crane that's 1 tm per 10°C — and a tower crane jib in direct sun easily swings 30°C between dawn and noon.
Best practice: tare the load cell with empty hook every 4 hours during temperature transitions, or specify a temperature-compensated load pin (typical drift 0.005% per °C). If drift is more than 5 tm in 4 hours with empty-hook tare in spec, suspect water ingress in the load pin connector — that is the common field failure on cranes older than 5 years.
Three drivers: variable-radius work cycles, foundation cost sensitivity, and bearing life. If your lift schedule swings between 15 m and 60 m radius continuously — typical on a high-rise where you place steel inboard and cladding outboard — the auto-balance keeps slew bearing fatigue inside an acceptable envelope and you get 2-3× the bearing life.
If your foundation is a piled raft on poor soil, the reduced peak overturning moment lets you specify a smaller foundation. Rule of thumb: if the variable-radius cost saving on foundation plus bearing life exceeds about $80,000-120,000 over the project, the +25-40% crane premium pays back. On short repetitive jobs at one radius, stick with fixed counterweight.
Every subsequent moment calculation carries a constant offset equal to ballast weight times the homing error distance. A 200 mm homing error on a 12-tonne ballast adds 23.5 kN·m of phantom counter-moment — the LMI thinks it is balanced when it is actually 23.5 kN·m biased toward the load side.
Defence: use absolute multi-turn encoders with battery-backed position memory rather than incremental encoders that re-home on power-up. If you have to use incremental, the home proximity switch must be a hard mechanical datum repeatable to ±2 mm, not just a magnetic sensor that can drift with metal swarf accumulation.
References & Further Reading
- Wikipedia contributors. Tower crane. Wikipedia
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