Overhead Bridge Crane

Overview

An overhead bridge crane, or electric overhead travelling (EOT) crane, is the standard machine for moving heavy loads inside a factory bay. A bridge of one or two steel girders spans the building width and rolls along elevated runway rails on wheeled End Truck carriages. A Hoist Trolley carrying a Wire-Rope Hoist traverses the bridge, so the hook can reach any point in the rectangle defined by the runway length and the bridge span. Three motions, long travel, cross travel and hoisting, together give full three-axis coverage of the floor below.

Double-girder cranes in the 5–40 t class dominate general manufacturing, steel service centres and power-plant maintenance bays. The double-girder layout places the trolley on rails atop the girders, which maximises hook height under the roof and allows a service walkway between the girders. Capacities run from sub-tonne workshop cranes to 500 t turbine-hall machines built on the same architecture.

Structure

The Bridge Girder Assembly uses two welded Welded Box Girder sections. A box girder is torsionally stiff, which matters because the trolley loads one girder eccentrically as it travels. Internal diaphragm plates every 1.5–2 m stop the webs from buckling, and the girder is fabricated with an upward camber of roughly span/1000 so it deflects to flat under rated load; EN 15011 limits live-load deflection to span/750 for typical duty. Each girder ends in a machined Girder End Plate bolted to the end-truck frame with preloaded HV bolts, which lets the bridge be split for transport.

Each End Truck carries two forged, surface-hardened Travel Wheel units running on the runway rail. Wheel diameter, typically 315–630 mm, is set by wheel-load and rail-hardness limits in FEM 1.001. One wheel per truck is driven by a Long-Travel Drive Unit; the two drives are fed from synchronised inverters so the bridge does not skew and bind its wheel flanges against the rail.

Hoist and trolley

The Wire-Rope Hoist is a drum machine. The Hoist Motor drives a machine-grooved Rope Drum through a three-stage helical Hoist Gearbox; the grooves keep the Hoist Wire Rope in a single layer, since multi-layer spooling crushes and fatigues rope. A polymer Rope Guide rides the drum to hold the rope in its groove at every hook position. The rope reeves through the Hook Block, usually 4/1 on cranes above 10 t, multiplying drum pull and halving or quartering hook speed. Sheave diameter must exceed about 20 times rope diameter (ISO 4308) to keep bending fatigue acceptable. The Forged Hook is a single forging to DIN 15401, sitting on a thrust bearing so a suspended load can rotate freely.

A spring-applied Hoist Holding Brake on the motor shaft is the primary load-holding device: it is engaged by springs and released electrically, so any power loss sets the brake. It is sized for at least 1.5 times rated hoisting torque.

The Hoist Trolley is a welded frame on four flanged wheels riding the Trolley Rail sections welded to the girder top flanges, moved by a compact Cross-Travel Drive at 10–25 m/min.

Power and control

Power reaches the moving crane through an insulated Conductor Bar System running the runway length; spring-loaded Current Collector shoes slide inside it to pick up the three phases and earth. Along the bridge, a Festoon Cable System of flat cables on rolling trolleys feeds the moving hoist trolley. A lockable Main Isolator Switch on the bridge dead-isolates everything for maintenance.

All three motions run on variable-frequency Motion Inverter drives housed in the bridge Control Panel. Inverter control replaced the older two-speed pole-changing motors because ramped acceleration sharply reduces load swing, the pendulum oscillation of the load on the rope, and eliminates the mechanical shock of direct-on-line starts. Many controllers add electronic anti-sway, shaping the speed ramp against the pendulum period calculated from rope length. The operator drives the crane from a Radio Remote Control, with a wired Pendant Station retained as backup; both provide a category-0 emergency stop.

Protection

A strain-gauge Load Measuring Cell in the rope anchorage trips hoisting at 110 % of rated load. A geared Hoist Limit Switch switch on the drum shaft stops the hook before the top and bottom extremes, with the top limit backed by a second independent switch. Lever-type Travel Limit Switch switches slow and stop bridge and trolley before the End Buffer units, which absorb only residual energy. Where two cranes share a runway, an Anti-Collision Sensor rangefinder enforces a minimum separation. Crane structures are classified by duty (ISO 4301 groups M3–M8) because fatigue from load cycles, not static strength, usually governs the design life of girders, gearing and rope.