Evaluating bridge and gantry cranes
Ron Holzhauer, Managing Editor, Plant Engineering Magazine -- Plant Engineering, 4/1/2001
Properly sized, designed, and installed bridge and gantry cranes dramatically increase production, significantly reduce material handling costs, and substantially lower the risk of employee injury. A properly maintained crane lasts at least 20 yr, could reach 50 yr or more, and occasionally outlives the structure or company originally housing or using the equipment.
These overhead workhorses maneuver large, bulky loads through the plant for shipping and receiving, relocating and staging, or integrating with heavy-duty manufacturing operations. Computers and other control packages near the equipment or at a remote location allow the crane to closely match almost any size and type of load, where and when needed, and under all operational and environmental conditions.
Hoisting speeds over 200 fpm, bridge speeds to 1000 fpm, and capacities over 1000 tons are available, although slower movements and smaller loads are the norm. Cranes are 15–30-ft overhead, but could be up to 200 ft to clear floor-mounted equipment, to place material where needed in the manufacturing operation, or for safety reasons. A variety of mechanisms, such as hooks, magnets, or buckets, are available on the hoist to match particular grabbing or lifting requirements.
TypesBridge cranes are available in top running and underhung configurations, and single or double girder, to match plant structural and lifting requirements. Gantry cranes are constructed in single or double leg, single or double girder, and top running or underhung versions.
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| Top running, double-girder cranes provide the greatest lifting capacities, highest tonnages, widest spans, and heaviest duties. |
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| Top-running, single-girder cranes that have a one-beam bridge that rides on a rail atop the runway and handle loads up to 30 tons with spans up to 60 ft. |
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| Double-girder underhung cranes have the highest hoist mounted above the bridge to attain a bit more headroom and reach capacities up to 50 tons. |
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| Single-girder underhung cranes have the bridge and trucks running on the lower flanges of the runway beams and are usually limited to 10 tons. |
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| Double-leg gantry cranes move along floor rails or guidepaths with a capacity typically less than 30 tons, although units reaching several hundred tons are in service. |
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| Single-leg gantry cranes substitute a wall-mounted runway for the second leg and are usually designed to handle loads of less than 20 tons for a specific operation. |
Bridge cranes are usually used for high tonnages, long spans, and heavy duties. Although available in several forms, each crane design contains several pieces of common equipment.
- Hoists are used to lift and lower the load.
- Trolleys consist of a frame, end trucks or wheels with side frames, and drive. They suspend or support the hoist, rope, and load block; and travel on one or more bridge rails or beams.
- Girders are the principal horizontal structural steel beams holding the hoist and trolley. They are supported by end trucks and are perpendicular to the runway. Very wide or large capacity cranes require two or more girders.
- Runways consist of structural steel rails, girders, brackets, and framework. They support and allow movement of the crane through the plant.
- End trucks are an assembly of structural members, wheels, bearings, and axles that support the girders or trolley cross members.
- Bridges consist of girders, end trucks, walkways, cross bridge electrification controls, and drive mechanisms. They carry the trolley and travel along the runway rails.
Gantry cranes are often a practical alternative to bridge cranes, and are capable of serving many of the same lifting, traveling, and duty classifications. This floor-mounted equipment essentially "bridges" the lifting and moving service area as the crane rolls down a predetermined path on legs.
Gantries are suitable if the plant structure cannot handle the bridge loading, if the installation is temporary and may require relocation at a later date, or overhead runways are long, costly to erect, and difficult to maintain in alignment. The gantry is common in situations where the crane itself does little or no traveling, and material transfer is handled almost exclusively by the trolley.
Single and double-leg, single-girder cranes typically handle less than 20 tons, although models that accommodate up to about 60 tons are available with special construction features. The two double-girder types usually have capacities less than 30 tons, but again, exceptions exist and some versions moving several hundred tons are in service.
— Ron Holzhauer, Managing Editor, 630-320-7139, rholzhauer@cahners.com
For more information on cranes, visit the "Material handling" channel at plantengineering.com.
| Component | Detail |
| Operating mechanisms | Function |
| Limit switches | Operational for function |
| Hooks | Deformation and cracks |
| Chains | Wear, elongation, and twist |
| Wire rope | Wear, broken wire, and kinks |
| Slings | Wear, broken wire, and kinks |
| Rope reaving | Proper seating in drum and sheave grooves |
| Load class | Load Cycles 20,000200,000 | Load Cycles 200,000600,000 | Load Cycles 600,0002,000,000 | Load Cycles Over 2,000,000 | Load Cycles Mean effective load factor |
| L1 | A | B | C | D | 0.3500.530 |
| L2 | B | C | D | E | 0.5310.670 |
| L3 | C | D | E | F | 0.6710.850 |
| L4 | D | E | F | F | 0.8511.000 |
| Irregular occasional use followed by long idle periods | Regular use in intermittent operation | Regular use in continuous operation | Regular use in severe continuous operation |
L1 Cranes that hoist the rated load capacity exceptionally, and normally handle very light lifts
L2 Cranes that rarely hoist the rated load, and normally handle about one-third of capacity
L3 Cranes that hoist the rated load fairly frequently, and normally handle between one-third and two-thirds of capacity
L4 — Cranes that regularly hoist close to the rated load
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