When it comes to moving extremely heavy or bulky items around your industrial facility, the best equipment to do the job will usually be an overhead crane. Because these machines avoid obstructions by lifting items above the floors on your site, they provide an unparalleled degree of safety and flexibility. And their control systems allow human operators to move the items with a high degree of control and ergonomic efficiency.
Overhead cranes are not all the same. In fact, it may well be said that no two are exactly alike because each one is specifically configured for the environment and application in which it will be used. Still, there are several different categories of cranes, each of which is best suited to particular kinds of uses. That's why selecting the right type of crane for your facility and application is critically important.
In this article we want to examine three of the most commonly used types of overhead cranes: the bridge crane, the gantry crane, and the monorail crane. We'll briefly discuss the defining characteristics of each of these systems, and how each one works. But our primary focus will be on understanding the particular environments and applications for which each type of crane is best suited.
The configuration of a bridge crane is easy to visualize. It consists of two elevated runways that run in parallel along the travel length of the crane. These runways may be built into the support structure of your facility or, in the case of freestanding bridge cranes, may be supported by floor mounted columns.
Connecting or "bridging" the runways is a movable single or double girder beam. Each girder is supported by an end truck on each side that can propel the girder back and forth along the runway. (You might picture this as a pair of elevated railroad tracks with one or two parallel ties between them that can move along the length of the tracks). Each girder beam supports a trolley that can move from side to side along the girder. The trolley, in turn, supports a hoist and hook that can lift a load vertically.
The choice between a single or double girder system depends mostly on the span between the runways, the amount of weight to be lifted, and the lifting height required. If the span between the runways is more than about 65 feet, or the weight to be lifted is more than 15 tons, a double girder system is probably the best choice. The hook height (how high above the floor the hoist is required to lift) is also important because with a double girder system the hoist is located on top of the girders, allowing for a greater vertical range of movement.
A gantry crane is very similar to a bridge crane. The major difference is that the girder, whether single or double, does not rest upon runways built into the structure, but is supported by rigid legs that can travel on the ground. The legs may have casters to roll along the floor, or in some cases may move along a track installed in the floor. So, while a bridge crane's position is fixed, a gantry crane can be moved around your facility.
The mobility of a gantry crane is determined in part by the material used in its construction. Gantry cranes constructed with lightweight aluminum may be truly portable, with the ability to be rolled from location to location even, in some cases, while carrying a load. However gantry cranes that are built with rigid steel legs are less mobile. They may, in fact, require disassembly and reassembly when they are moved.
There are also what might be called hybrid gantry cranes. These are single-leg configurations where the crane has a leg on one end, while the other end is attached to an overhead runway similar to what a bridge crane would use. The advantage of this type of crane construction is that it maximizes floor space and is particularly useful for covering large spans.
Gantry cranes may be fixed or adjustable height. The fixed height variety is the most common. They are best suited for applications in which the process is fixed and repetitive. An adjustable height gantry is more flexible since its height can be altered based on the application or configuration of the location in which it will be used. Of course, that flexibility comes at a price. Adjustable height gantry cranes normally are more expensive than the fixed height variety.
In general, a gantry crane has performance characteristics similar to those of an equivalent bridge crane. Gantry cranes are, however, usually less expensive than bridge cranes.
Unlike bridge and gantry cranes, a monorail crane has no movable girder. Instead, the trolley carries the hoist along a supporting I-beam. The load can be moved in only two dimensions – up and down, and along the track of the I-beam.
One of the great advantages of this crane configuration is that the I-beam need not be linear. It can, in fact, be curved, allowing the hoist to move from station to station along a complex path. The layout can also include branches and switches, and even changes in elevation.
Monorail crane installations are normally customized for specific applications. They are, for example, often used for transporting items along an assembly line or through a production process.
Bridge cranes are the heavyweights of the overhead crane universe. They typically are physically the largest crane type, and are used to carry the heaviest loads. The double girder configuration is particularly well suited to applications in which there is a wide span between the runways, or the load is exceptionally large or heavy.
The supporting runways of a bridge crane may be built into the structure of the facility, making it ideal for large, permanent installations where the design of the crane is integral to the design of the building. However, these cranes are not limited to fixed-location applications. Freestanding bridge cranes, which feature a modular design that allows them to be easily relocated to other parts of your facility, are also widely used.
Gantry cranes are well suited for portable or outdoor applications. They are often used for loading and offloading in storage yards, shipyards, rail yards, and on construction sites. Because of their mobility, they are also ideal for transporting loads between locations on a site, or between buildings.
The gantry is the crane of choice for circumstances in which the need for a crane at a particular location is temporary or, perhaps, when it is to be used in a facility that is leased rather than purchased. They are also at the top of the list when the workflow or application can be expected to change on a regular basis.
Monorail cranes work especially well in applications that are highly structured and the load's route of travel is fixed, even though it might not follow a straight line. Examples include production lines and continuous operations such as painting processes.
Because the topology of a monorail crane's route of travel is variable, special care must be taken with the design to ensure that the crane is properly supported and that appropriate margins of safety are maintained at every point in the workflow.
Your primary consideration is, of course, the applications in which the crane will be used. For example, if you will be moving heavy loads across a large fixed space, a bridge crane might be indicated. If the route of travel is fixed but non-linear, a monorail crane might be best. And if the workflow or application will frequently change, or if outdoor or portable operation is required, a gantry crane will probably be the best choice.
Beyond the application, the nature of the facility in which the crane will be used is of great importance in determining which type will best meet your needs. Your first concern must be safety. For example, OSHA requires that there be a three-inch overhead clearance and two-inch lateral clearance for your crane installation. And of course, the nature and location of structural elements of the facility, such as beams and columns, as well as the placement of other equipment on the floor of the building, may have a major impact on your choice of which type of overhead crane is appropriate for your situation.
Along with safety concerns, the layout of your facility will also determine the design features that will be necessary in your overhead crane installation. For example, if you are considering a bridge crane, is there room and the appropriate foundational support for the installation of runways along the length of the structure? Are there load-bearing columns or other non-movable features of the space that would limit that range of motion that would be available for your crane installation?
In terms of these considerations, it's actually an advantage that overhead cranes are generally not off-the-shelf purchases. In most cases, each crane sold for industrial use is carefully engineered to fit the intended environment and application. So you'll have a lot of flexibility in getting just the kind of crane that will meet your needs. Expert help is available, and should be brought into the project at as early a stage as possible.
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