Overhead Bridge Crane Product

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.