Overhead Bridge Crane

What is an Overhead Bridge Crane

An overhead bridge crane is a material-handling machine that lifts a freely suspended load with a hoist, traverses it sideways with a trolley running across a bridge, and moves it lengthwise by driving the whole bridge along two parallel elevated runway beams. The three motions together, hoisting (vertical), cross-travel (trolley), and long-travel (bridge), give the hook access to a rectangular working volume defined by the span between runways, the runway length, and the lift height. Because the runways sit on the building columns or on a free-standing steel structure, the crane occupies no floor space, which is its defining commercial advantage over forklifts and mobile cranes inside a fixed footprint.

Structurally a bridge crane has four assemblies: (1) the bridge, one or two main girders that carry the load across the span; (2) the end carriages (end trucks) at each end of the bridge, housing the travel wheels and drives that run on the runway rails; (3) the crab or trolley, carrying the hoisting machinery that traverses the bridge; and (4) the runway and its electrification, the rails, conductor bars or festoon cable, and the controls. The term EOT crane, electric overhead travelling crane, is the common purchase-order name in Asia and Europe, while ASME B30.2 in North America uses the formal description "top running bridge crane".

The lineage of the overhead crane runs from manually operated jib and bridge hoists of the early industrial era to the first electrically powered travelling cranes built in the 1880s, which let steelworks and foundries handle ladles and ingots that no gang of workers could move. The 20th century added box-girder fabrication, antifriction wheel bearings, and squirrel-cage travel motors. The decisive modern shift came with the standardized electric wire rope hoist and, from the 1990s onward, variable frequency drives that replaced contactor stepped control, plus radio remote control that moved the operator off a fixed cab and onto the floor beside the load.

The application scale is wide. Standard single girder cranes commonly cover 1 to 20 tonnes over spans of roughly 5 to 30 m; double girder process and mill cranes routinely reach 100 to 500 tonnes over spans up to 50 m and beyond, and purpose-built steel-mill and shipyard cranes exceed those figures. A single crane therefore does not exist for all duties: the engineering task is to map the lift weight, frequency, span, headroom, and environment of a specific bay onto a structural type and a duty class. Those two decisions, made in the first hour, determine most of the lifetime cost of the machine.

The four engineering levers that decide whether a bridge crane is correctly specified are capacity headroom (rated load plus a margin for off-center and dynamic loads), duty class (how the structure and mechanisms are sized for fatigue), span and lift (which fix deflection and rope length), and serviceability (how fast a hoist, a wheel, or a VFD can be swapped on a production line). Under-specifying any one of them transfers cost downstream into early fatigue cracking, drifting trolleys, or unplanned downtime that dwarfs the original purchase saving.

Structural Types and Configurations

Bridge cranes are first divided by how the trolley relates to the bridge, single girder underhung trolley versus double girder top-running trolley, and second by how the bridge relates to the runway, top-running versus under-running. These two binary choices, plus the related gantry and semi-gantry variants, cover the great majority of installations. The table below compares the two dominant girder configurations on the parameters that drive selection.

Single girder cranes use one main beam, usually a rolled wide-flange or a fabricated box welded to a top channel, with the hoist hanging from an underhung trolley that grips the bottom flange. They are lighter, install faster, and need the least headroom, which makes them the default choice for general workshops at 1 to 20 tonnes and spans under roughly 30 m. In practice you rarely see a single girder crane rated above 15 to 20 tonnes, because beyond that the single beam grows uneconomically heavy and its lateral and torsional stiffness become limiting. CMAA Specification 74 is the dedicated design code for single girder top-running and under-running cranes.

Double girder cranes use two parallel main girders bridged by the end carriages, with a top-running crab trolley riding rails laid on top of both girders. This raises the hook higher (the trolley sits between rather than below the steel), gives superior hook approach for very wide loads, carries far heavier capacities, and provides a stable platform for a service walkway, a second auxiliary hoist, or a magnet/grab control panel. The penalty is more steel, more headroom, and a 40 to 50 percent cost premium for equal capacity. CMAA Specification 70 governs multiple girder top-running bridge and gantry cranes and is the reference for almost all heavy-duty machines.

Top-running versus under-running describes the bridge, not the trolley. A top-running bridge rolls on rails fixed to the top of the runway beams and is the standard for medium and heavy duty because the wheel loads transfer cleanly into the column line. An under-running (underhung) bridge hangs its end trucks from the bottom flange of the runway beams, which allows very low headroom and easy interlocking with monorail spur tracks, but limits capacity (typically to around 10 to 15 tonnes) and demands a tighter runway flange tolerance. The table below summarizes the broader family of bridge and gantry configurations.

Beyond geometry, cranes are also classified by duty application: standard hook cranes for general loads, magnet cranes carrying a battery-backed electromagnet panel for steel plate and scrap, grab cranes for bulk material, ladle and casting cranes for molten metal, and explosion-protected cranes for hazardous areas. Each duty type layers additional requirements (redundant brakes, hot-metal heat shielding, ATEX components) onto the base structure, which is why the application must be declared at the request-for-quote stage, not added later.

Hoist, Trolley, and Drive Technologies

The hoist is the heart of the crane and the component most often replaced over its life. Two families dominate: electric wire rope hoists for higher capacity and longer lift, and electric chain hoists for lighter, shorter-lift duty. The choice of hoist, plus the drive technology on the three motions, sets the crane's precision, shock loading, and maintenance profile. The table below compares the main hoist technologies on engineering metrics that matter at selection.

Electric wire rope hoists spool steel rope onto a grooved drum through a multi-fall reeving (2/1, 4/1) that multiplies capacity and stabilizes the hook against rotation. Modern packaged units such as the Konecranes CXT and the Demag DH and DR series integrate the drum, gearbox, motor, brake, and limit devices into a compact frame, reducing headroom by a useful margin and shortening hook approaches. They cover roughly 1 to 80 tonnes off the shelf and are the default for standard cranes. Above that range, or for mill and process duty, built-up open winches with separate drum, gearbox, and motor are used because they can be specified for very long fatigue life and serviced component by component.

Electric chain hoists lift the load on a calibrated link chain over a pocketed sprocket. They are simpler, cheaper, and tolerant of dust, but the chain limits practical capacity (commonly up to about 5 tonnes) and lift height, and their hoisting speed is modest. They suit light assembly, maintenance bays, and low-budget single girder cranes. Hoist duty itself is rated separately from crane duty: ASME HST and the older HMI scheme define hoist duty classes H1 to H5 by load spectrum and starts per hour, where H4 demands operation up to 50 percent of each hour and at least 300 motor starts per hour for heavy warehousing and machining service.

Drives and controls have shifted decisively to variable frequency drives (VFDs) on all three motions. A VFD ramps the motor smoothly through a wide speed range, typically delivering a creep speed near 0.5 m/min for precise spotting and a full speed of 10 to 40 m/min on the hoist, with anti-sway logic that damps load pendulation on travel. This protects the structure from the shock loading that stepped contactor control imposes and extends rope, wheel, and gearbox life. Two-speed pole-changing motors remain common on lighter, lower-duty cranes where a VFD is not cost-justified. Typical motion speeds: hoisting 3 to 8 m/min for heavy main hooks and up to 40 m/min for light hoists, trolley cross-travel 10 to 30 m/min, and bridge long-travel 20 to 60 m/min.

Braking and safety is layered. Each motion carries a holding brake (commonly an electromagnetic disc or shoe brake) that sets when power is removed, so a power failure stops and holds the load rather than dropping it. Hoist motions add an upper and often a lower geared or weighted limit switch, an overload protection device (load cell or torque limiter) that prevents lifting beyond rated capacity, and on critical lifts a redundant second brake. End stops and rubber buffers limit travel, and anti-collision sensors are added where two cranes share a runway. These devices are mandated, not optional, under ASME B30.2 and OSHA 1910.179.

Duty Classification and Design Standards

Duty classification is the single most consequential number on a crane order. It tells the manufacturer how heavily and how often the crane will be worked, which sets the fatigue design of the structure, the size of the motors and gearboxes, and the rated life of the mechanisms. Two cranes of identical 20-tonne capacity can differ by a large margin in steel weight and price purely because one is a Class C machine shop crane and the other a Class E scrapyard magnet crane. CMAA Specification 70 defines six classes, A through F, by lifetime load cycles and load spectrum, summarized below.

The class is derived from two inputs: the load spectrum (how often the crane lifts near full capacity versus light loads) and the number of lift cycles over its design life. A Class C crane handles an average around 50 percent of rated capacity up to roughly 10 times per hour; a Class D crane handles 50 percent of capacity constantly through the working period; Class E and F cranes lift at or near rated capacity for twenty or more cycles per hour, continuously. Choosing a class one step too low saves money on day one and pays it back as fatigue cracking and mechanism wear; one step too high wastes capital. The class should reflect the real, documented duty cycle of the bay, not an optimistic guess.

The same physical duty is described by three regional standard families that buyers must reconcile on international projects. In North America, structural and mechanical design follows CMAA Specification 70 for multiple girder cranes and 74 for single girder cranes; operation, inspection, and maintenance follow ASME B30.2 for top-running bridge cranes; and OSHA 29 CFR 1910.179 makes the ASME design code legally binding for general-industry cranes installed after August 1971. In Europe, machines are CE marked under the Machinery Directive 2006/42/EC, with structural design to EN 13001 and classification to FEM 9.511. In China, design follows GB/T 3811 with mandatory TSG Q type inspection. ISO 4301-1 provides the international classification reference.

The FEM and ISO systems differ from CMAA by splitting the rating into two independent numbers: a structural group (FEM A1 to A8, ISO A1 to A8) for the crane as a whole, and a mechanism group (M1 to M8) for each drive. The mechanism group is defined by total full-load running hours: M3 corresponds to about 400 hours, M4 to about 800, M5 to about 1,600, and M6 to about 3,200 hours of cumulative full-load operation. As a working approximation, CMAA Class C maps near FEM 2m / ISO M5, Class D near FEM 3m / ISO M6, and Classes E and F near FEM 4m to 5m. The mapping is approximate, not one to one, so the exact group should always be confirmed against the manufacturer's design calculation rather than assumed from a conversion chart.

Key Specification Parameters

A bridge crane data sheet typically lists 15 to 30 lines, but only a handful truly drive selection: rated capacity, span, lift height, duty class, the three motion speeds, deflection limit, wheel load, and the electrical and protection ratings. Reading them in the right order, and knowing which are constraints versus preferences, is the core skill of crane procurement. Each is decoded below.

Rated capacity (SWL) is the maximum working load the crane may lift, declared in tonnes and marked visibly per ASME B30.2 and OSHA 1910.179. It must cover the heaviest planned load plus the weight of the lifting device (spreader beam, magnet, grab) and a margin for off-center picks. The capacity rating presumes a freely suspended load; side pulling and dragging are prohibited because they impose loads the structure is not designed for. For magnet and grab cranes the dead weight of the attachment is significant and must be subtracted from the available hook load.

Span is the centre-to-centre distance between the two runway rails, and lift height is the vertical hook travel from lowest to highest position. Span fixes the girder size because deflection grows steeply with span: doubling the span roughly multiplies the deflection by eight for a given section, which is why long-span bays move to double girder or deeper sections. Lift height fixes the rope length and drum size and influences how much the load can swing, which is what makes anti-sway VFD control valuable on tall lifts.

Deflection limit is a hard structural constraint. CMAA 70 limits the main girder's vertical deflection under rated load plus trolley weight, without impact, to L/888 of the span. The supporting runway under CMAA 74 commonly uses L/600 for top-running and L/450 for under-running cranes, and heavy mill cranes per AISE Technical Report 6 tighten this to L/1000 to limit rail wear. Excessive deflection lets the trolley roll toward midspan under its own weight, accelerates wheel and rail wear, and can stall the bridge drive, so it is enforced, not advisory.

Motion speeds define productivity and shock. Typical figures: hoisting 3 to 8 m/min for heavy main hooks and up to 40 m/min for light wire rope hoists, with creep near 0.5 m/min; trolley cross-travel 10 to 30 m/min; bridge long-travel 20 to 60 m/min. Faster is not always better: higher speeds raise dynamic loads, so heavy or fragile loads favor lower speeds with a wide VFD range for fine spotting.

Maximum wheel load is the value the building structural engineer needs, the largest vertical force any one end-carriage wheel transmits to the runway, used to size the runway beam, columns, and foundations. It depends on capacity, bridge dead weight, span, and the trolley's closest approach to one end. Getting this number wrong overloads the building, which is why it appears prominently on every reputable data sheet. The table below lists the spec parameters and how each is expressed.

Electrical and protection ratings close the data sheet: supply voltage and frequency, motor insulation class (F or H for hot environments), enclosure ingress protection (IP54 indoor up to IP65/IP66 for washdown or dusty service), control method (pendant, radio remote, or cab), and the runway electrification type (rigid conductor bar, festoon cable, or energy chain). Outdoor and foundry cranes add wind loading, rail clamps or storm anchors, and heat shielding. These ratings are where a crane priced for a clean machine shop quietly fails in a cement plant or a galvanizing line, so the environment must be stated explicitly.

Selection Decision Factors

To turn the preceding five chapters into a specific crane order, follow the decision sequence below. Most selection errors come not from a single wrong line but from deciding a downstream parameter (brand, hoist) before the upstream constraints (duty, span, building capacity) are fixed. These eight steps double as a request-for-quote template that lets several manufacturers bid on identical terms.

1. Capacity and lifting device: Define the heaviest load, then add the weight of the spreader, magnet, or grab and a margin for off-center picks. State whether a second auxiliary hoist is needed. Capacity sets the entire structure, so set it first and do not under-round it.
2. Duty class: Document the real duty cycle, lifts per hour, fraction of capacity per lift, and hours per day, then map it to a CMAA class A to F or an FEM / ISO mechanism group. This is the parameter buyers most often understate; treat it as a fatigue-life decision, not a formality.
3. Span, lift, and headroom: Measure rail-to-rail span, required hook lift, and the clear headroom between the hook at top position and the roof steel. Tight headroom pushes toward single girder, low-headroom hoists, or under-running designs.
4. Single vs double girder: Choose single girder for roughly 1 to 20 tonnes and spans under 30 m with no walkway or magnet panel; choose double girder above that, or where higher hook lift, wide-load hook approach, a service walkway, or a second hoist is required.
5. Building and wheel load: Confirm whether the building frame can carry the maximum wheel load, or whether a free-standing runway, gantry, or semi-gantry is needed. Engage the building structural engineer before finalizing the configuration, not after.
6. Drives, controls, and speeds: Select VFD on all three motions for precise or heavy loads, two-speed motors for light duty; set hoist, trolley, and bridge speeds to the cycle time; choose pendant, radio remote, or cab control to suit the operator's position and sight lines.
7. Environment and protection: Declare indoor or outdoor, temperature, dust, washdown, hot metal, or explosive atmosphere. Match motor insulation class, IP rating, heat shielding, wind anchors, and ATEX or equivalent certification to the site, not to the catalog default.
8. Standards and total cost of ownership (TCO): Specify the governing code for the destination (CMAA / ASME / OSHA, FEM / EN / CE, or GB / TSG), then weigh purchase price against installation, periodic inspection, spare hoist and wheel availability, and downtime cost. A crane that saves capital but lacks local service support can cost far more over a 20 to 30 year life.

One dimension consistently underweighted at purchase is manufacturer serviceability: local spare-part inventory for hoists, wheels, brakes, and VFDs; availability of field inspection and load testing to ASME B30.2 and OSHA cadence; remote diagnostics and condition monitoring; and the ease of swapping a packaged hoist versus rebuilding a built-up winch. Konecranes (CXT hoists), Demag (DH and DR hoists, part of the Konecranes group), ABUS, Street Crane, Gorbel, and major Chinese makers such as Weihua and Nucleon all maintain service and parts networks; the right choice is the one with proven response time in the crane's region, because availability of a replacement hoist on day one of a breakdown matters more than a small difference in the original quote.