Overhead Bridge Cranes--creating new space for your factory

Up and away with overhead cranes
Whether a load weighs 25 pounds or 100 tons, overhead cranes gets them off the floor and to their destination.
By Gary Forger, Editorial Director -- Modern Materials Handling, 11/1/2001
Make best use of the cube. It's been said so many times, and has never had greater value than now. Yet, so many managers think only of higher racks and new mezzanines to minimize footprints and maximize vertical space usage. But their options don't stop there.
Quite literally, overhead cranes add a new dimension to the movement of materials in a facility. While conveyors and lift trucks are focused on moving inventory, work-in-process, and finished goods at or near floor level, cranes do most of their work well above the floor and often at ceiling height. This, in turn, frees up the floor for other activities, making overhead cranes a space saver in addition to being an efficient way to move loads.
Long known for their ability to transport loads of many tons from one end of a facility to another, cranes are equally adept at moving much lighter loads within the relatively small area of an individual workstation.
In fact, the latter has taken on increasing importance as the ergonomics of handling loads has received greater scrutiny. Twenty-five pounds is now considered to be at the high end of the manual lifting range, not 50 pounds. Similarly, it is estimated that nine out of 10 loads handled in industry weigh 300 pounds or less, considerably less than what was once the case.
The end result is that enclosed-track workstation cranes are now much more widely used than they once were. At the same time, jib and gantry cranes are often used at workstations as well as other applications. Bridge cranes, the fourth type, are the heavy lifters, handling not only the largest loads but those that need to travel the greatest distance in the least amount of time.
All four types of cranes can be used in warehousing/distribution operations as well as manufacturing. Except for steel service centers and similar operations, however, cranes are most often associated with manufacturing operations.
Regardless of the specific application, each crane type offers its own range of movement, both directionally and in distance, while handling loads of various dimensions and weights. In addition, the frequency of moves differs by crane type.
While most cranes are either electric or air powered, some are operated manually. As with other types of materials handling equipment, manual operation is generally confined to applications handling the lightest loads and the least frequency of use.
In all of this lifting, lowering, and moving of loads, cranes can't do it all. They are typically used with hoists tailored to the job at hand. See the Click-on at mmh.com for a rundown of the types of hoists and their applications. A crane for every load
Bridge cranes make the biggest impression of the four types because they move loads of many tons from one end of a large facility to another. The standard configuration is two steel girder runways attached to the building at ceiling height. In some cases, the crane is positioned further down the wall of the building. Either one or two additional girders span the two runways, creating the bridge. The bridge girders are attached on each side to an end truck that travels along the runway.
Applications run from heavy manufacturing, fabrication shops, and steel service centers to large paper roll handling. The various designs are suitable to specific requirements.
Top-running double-girder bridge cranes generally carry the heaviest loads, as much as 100 tons. True to its name, this design has two parallel girders mounted on top of the runway beams. The hoist is mounted on a trolley that runs atop the girders, maximizing the lifting height of loads. As with all other bridge cranes, the combination of hoist, runway, and bridge movement provides six directions of travel for any load.
Top-running single-girder bridge cranes are much the same but have only one bridge between runways. This design requires the hoist's trolley to run along the lower flange of the girder, reducing lifting height somewhat as well as carrying capacity.
There are also two underhung bridge crane designs. Rather than having the girder on top of the runway, these have the bridge girder trolley running along the lower flange of the runway. Capacity of both single- and double-girder underhung cranes are less than the top-running ones. Lifting height is reduced also.
Traditionally, overhead bridge cranes, especially top running ones, have been under the control of an operator that rides in a cab attached to the bridge. Unfortunately, the frequency of moves often leaves the operator idle a significant portion of the shift. With that in mind, many users are looking at other control options - pendant or radio remote control.
Both put the operator on the floor, and allow maximum use of that person's time. Pendant controls are hard wired to the crane, requiring the operator to walk with the crane as it moves. The drawback here is the inherent inflexibility of movement and potential for accidents as a result. Radio remote control eliminates that inflexibility and potential danger, allowing the operator to remain in one position as the crane moves. Both pendant and radio remote control can be used with other types of cranes too.
A cousin of the bridge crane is known as the gantry. This floor-mounted type uses either a single or double leg to support the bridge that the hoist trolley runs along. While the double leg design supports both sides of the bridge, a wall rail provides the support on the other side of the single leg. In both cases, the leg travels along a floor rail.
The enclosed-track workstation crane is another takeoff on the overhead bridge. Much like the double-leg gantry, these can be built anywhere in the plant or warehouse. It consists of four vertical columns that are anchored to the floor to create the four corners of the crane's work envelope. Two parallel runways connect the columns on each side. An underhung bridge in the form of an enclosed track with a hoist on a trolley moves from one end of the runway to the other, providing the range of motion for the workstation. Loads under 50 pounds or greater than 1,000 pounds can be easily lifted and moved with great precision within the work envelope.
The final category of cranes is the jib, which is not a derivation of the bridge crane. As with the other types, jibs come in several different designs.
The simplest is the wall-mounted jib crane. It consists of a horizontal girder that attaches to the wall with a pivot. A steel cable extends from the end of the horizontal beam at a 45 degree angle to an anchor point on the wall above the crane, providing support. The pivot allows the beam to swing 180 degrees. The range of motion of the hoist and its trolley as well as the load is limited to the length of the beam.
A floor-mounted jib replaces the wall with a vertical column that can be placed anywhere in the facility. Depending on the pivot mount, the jib can swing 180 or 270 degrees. A floor-mounted, free-rotating column design provides 360 degree range of movement, as does a ceiling-mounted rotating column jib. Another floor-mounted version places the vertical column on a floor-mounted runway, allowing the jib to travel some distance. Similarly, a wall-traveling design allows the horizontal beam and its load to move along a runway mounted above the beam.
As can be seen, not only do cranes add a new dimension to materials handling, but they are a match for a load of any size.