Evaluating bridge and gantry cranes
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%%MDASSML%%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.
Bridge 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.
|Top running, double-girder cranes provide the greatest lifting capacities, highest tonnages, widest spans, and heaviest duties.|
|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.|
|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.|
|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.|
|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.|
|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, email@example.com
For more information on cranes, visit the “Material handling” channel at plantengineering.com.
Daily visual checks
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 classes and cycles for CMAA service duties
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
Safe crane operation is an issue of paramount importance. Components such as redundant brake concepts for hoists, more reliable controls, and economical overload detection systems are engineered into the cranes to help in the safety process.
Even with this equipment available, safety starts with the operator. Whenever there is doubt as to safety, the operator should stop the crane, report the problem to a supervisor, and not operate the equipment until satisfied it is safe to do so, or is directed to proceed by a supervisor.
Operators should be familiar with the principal parts of the crane. Employees should receive hands-on training, read all instruction materials, and have a thorough knowledge of crane control functions and movements. The operator should test all crane controls at the beginning of each shift, and should perform a walk-around check to look for loose or damaged parts before commencing work.
There is a variety of other safety factors to consider.
When the load approaches the rated capacity, the operator should test the hoisting brakes by raising the load a few inches and applying the brakes.
The load should not be lowered below the point where less than two full wraps of wire rope remain on the hoist drum.
The operator should land any attached load and place the controllers in the “off” position when leaving a crane unattended.
Loads should never be carried over workers’ heads.
Cranes should never be used for side pulling.
Hand signals between the operator and hooker should be clearly agreed upon and understood before moving a load.
The operator should never lift two separately rigged loads at the same time.
The operator should approach the desired position as far as possible at the main speed and use creeping for final positioning.
Actual load handling requirements and duty cycles are the key to crane selection. To facilitate the selection process, the Crane Manufacturers Association of America (CMAA) has identified six duty requirements. Matching actual requirements with the proper crane is critical, because underspecifying could create excessive maintenance, and overspecifying increases the capital investment associated with the plant operation.
Operating speeds for the hoist, trolley, and bridge, based on duty classification and type of operation, are then selected. The picked figures are important, because excessive speed increases original and energy costs due to the larger motors, and too slow a movement could create a production bottleneck.
CMAA specifications provide detailed information and requirements for all crane components. In addition to service classifications and speed, suggested requirements for the bridge (girder, end trucks, wheels, bumpers, and rails), runways, hoists, electrical equipment, and controls are presented in various CMAA specification publications.
The working environment of the crane should also be considered. Heat, corrosives, moisture, and explosive potential should be investigated before component packages are assembled.
Selection of gantry cranes above 15 tons requires consideration of most bridge crane factors. In the smaller sizes, where the overwhelming number are found, several other points enter the equation, including aluminum or steel construction, adjustable or fixed height, span, type of casters, and height from floor to I-beam. Spans from 8%%MDASSML%%40 ft and heights from 6%%MDASSML%%35 ft are available.
Crane service classifications
The Crane Manufacturers Association of America, Inc., defines six duty service classifications for overhead traveling cranes.
Class A (Standby or infrequent service) — These cranes are used in installations such as power houses, public utilities, turbine rooms, motor rooms, and transformer stations where precise handling of equipment at slow speeds with long idle periods between lifts are required. Capacity loads are handled for the initial installment of equipment and infrequent maintenance.
Class B (Light service) — These cranes are used in repair shops, assembly operations, service buildings, warehousing, and other locations where service requirements are light and the speeds are slow. Between 2%%MDASSML%%5 lifts, averaging 10 ft, are made per hour. Loads range from zero to occasional full rated.
Class C (Moderate service) — These cranes are used in machine shops, paper mill machine rooms, and other locations where service requirements are moderate. Cranes handle loads averaging 50% of rated capacity. Between 5%%MDASSML%%10 lifts, averaging 15 ft, are made per hour. No more than 50% of the lifts are at the rated capacity.
Class D (Heavy service) — These cranes are used in machine shops, foundries, fabricating plants, steel warehouses, container yards, lumber mills, standard-duty bucket and magnet operations, and other locations where heavy-duty production is required. Loads approaching 50% of the rated capacity are handled constantly during the working period. High speeds are desirable. Between 10%%MDASSML%%20 lifts averaging 15 ft are made per hour. Not over 65% of the lifts are at the rated capacity.
Class E (Severe service) — These cranes are used in scrap yards, cement mills, lumber mills, fertilizer plants, and other locations where severe duty is encountered. Magnet, bucket, and magnet/bucket combination cranes typically handle these applications. Over 20 lifts per hour at or near the rated capacity are common.
Class F (Continuous severe service) — These custom designed cranes are used continuously to perform critical work tasks affecting the total production facility where severe service is experienced. Cranes must provide the highest reliability, with special attention to ease of maintenance features. Loads approaching full-rated capacity are handled on a continual basis.
Crane inspection checklist End truck
Check wheels for wear
Drive unit bolts
Check wheel bearings for lubrication if not lifetime sealed
Grease open gearing
Power pick up arm installation
Check end truck bumper
Check oil level in gearboxes
Check tagline and trolley end stop installation
Check trolley rail for wear
Inspect condition of walkway/platform
Check contactor points
Tighten electrical connections on motors and terminals
Examine pendant and taglines
Verify proper function of master switches and electrical accessories
Ensure proper fitting and condition of cables
Ensure supply voltage within
Ensure amperage on motors equal on all phases
Torque drive unit bolts
Check oil level (grease) in drive gear
Check oil level in hoist gear if not lifetime sealed
Check bottom block for deformation/cracks and free sheave movement
Adjust hoist, trolley, and bridge travel brakes
Check trolley bumper
Lubricate and inspect rope guide, pressure ring, and wire rope
Check limit switch bar movement (if applicable)
Check limit switch up and down
Check sheaves for wear
Check trolley wheels for wear
Check bridge wheels for wear (end truck)
Check power feeds and connectors
Examine runway end stops, rail alignment, and splices
Test hoist brakes with full load
Run trolley in low and high speed full length of girder
Test accessories for proper function
Run cranes in low and high speed full length of runway