Crane basics

When it comes to moving product overhead, cranes maximize floor space while increasing productivity.

By Bob Trebilcock, Editor at Large -- Modern Materials Handling, 4/1/2008

• Workstation cranes
• Overhead cranes
• Control systems
• Cranes and controls manufacturers

Mention materials handling, and most people think of moving materials at floor level, using lift trucks, conveyors and automatic guided vehicles.

Warehouse and plants, however, are three dimensional. Sometimes it makes more sense to use the whole cube of the facility, including the space above the floor, to move the goods. That's where cranes come into play. They are available in a variety of types and load capacities, from workstation cranes that enable operators in a work cell to safely manipulate relatively light loads to massive overhead cranes capable of moving loads weighing more than 100 tons.

“Cranes move loads that are not practical to move with a forklift,” explains John Kennedy, general manager for Konecranes America (800-231-0241, www.konecranesamerica.com). “Not only can you move heavy loads safely, a crane allows you to efficiently move loads from one spot to another without adding extra floor space for aisles to accommodate a lift truck, or without the worry of your driver backing into a column or a rack.”

Regardless of the type, a basic crane is comprised of three components:

1. The bridge, which is made of rails that carry a trolley or trolleys. Depending on the type of crane, a bridge may be stationary or it may move forward and backward.
2. The trolley, which is the unit that travels from side to side on the bridge rails and carries the hoist.
3. The hoist, or the lifting device, that hooks onto and manipulates the load.

Cranes can be manual or power driven, depending on the application and the size of the load that has to be moved. What's more, along with operator-controlled cranes, some applications involve completely automated cranes whose movements are directed by software.

Workstation cranes

Unlike large overhead cranes, workstation cranes are free-standing units used to handle product in a relatively limited area. “You'll most often find workstation cranes in production environments,” says David Butwid, vice president of sales and marketing for Gorbel (800-821-0086, www.gorbel.com).

Several types of workstation cranes are commonly used.

Gantry crane

A gantry crane looks like an overhead bridge crane, except it's not part of the structure of a building. Instead, a gantry consists of two uprights connected by an I-beam that serves as a bridge for the trolley. In a powered gantry crane, the uprights run on a track at the floor level; in a lighter duty crane, the uprights are on wheels and can be manually repositioned around a facility. With load capacities of up to 5 tons and spans of up to 30 feet, gantry cranes are most commonly used in maintenance operations. They are a cost-effective solution for applications that require infrequent duty.

Jib crane

A jib crane is also used for spot handling. These units include a single bridge that rotates to cover a circular area. Because the crane rotates, loads are easy to position. Jib cranes are often used in conjunction with an overhead crane to improve throughput in areas with more traffic and production than a single crane can handle. Free-standing jib cranes are bolted to the floor, and require a strong enough foundation to support the load without the crane toppling over. Because they are free-standing, they provide 360 degrees of rotation. Wall- or counter-mounted jib cranes are installed above the floor and provide 180 degrees of rotation; with these designs it's important that the wall or column is sturdy enough to handle the force that's exerted when a load is moved.

Articulating-arm jib crane

These are jib cranes with a pivoting two-piece arm. This allows the operators to more precisely position loads.

Monorail

A monorail consists of an I-beam track that hangs from the ceiling. Carriers travel along the I-beam to transport loads from one station to another. Each carrier has its own motor and drive unit as well as onboard intelligence. That allows every carrier on the system to act independently of the others.

While a monorail can be used to carry loads weighing several tons, they are more often used to transport lighter loads over long distances at high speeds. A carrier, for instance, typically travels at speeds of 200 feet per minute and can reach 600 feet per minute. Monorails are often used to cover large areas in a factory or to connect different areas of a facility serviced by different overhead cranes. “The limitation of a monorail is that your work cell layout is fairly permanent since it's expensive to change the layout and path of a monorail system,” says Gorbel's Butwid.

Overhead cranes

Overhead cranes are often called the workhorses of overhead materials handling. While jib and workstation cranes are used to improve ergonomics in small areas, bridge and gantry cranes do the heavy lifting over long distances.

Bridge cranes are ceiling-mounted and allow movement of loads in six directions: up and down, forward and backward, and sideways in both directions. Often, they are part of the building structure and are built at the same time the facility is being put up. “A bridge crane allows you to move very heavy loads safely,” says Konecrane's Kennedy. “You're hooking to the load and you get very precise movement of loads that are impractical to move with a lift truck.”

Although there are several types, bridge cranes consist of a trolley that runs across a bridge (which can have one or two girders) that moves across a bay along a runway. The largest capacity units, known as top-running, double-girder cranes, mount the bridge on top of the runway and have two girders for the bridge. With capacities of up to 600 tons, a top running crane gives you the maximum headroom of any crane. Bridge cranes are always powered.

In addition to top-running cranes, there are under-hung cranes where the trolley runs on the bottom of the support I-beams. While this design allows a user to get loads much closer to the facility walls, under-hung cranes are meant for loads of 15 tons or less. What's more, because the hoist runs on the bottom of the I-beam, an under-hung crane can't lift the load as high as a top-running crane system.

Distance is no impediment when working with cranes. “We've done 1,000-foot-long bays, and we've done spans of up to 120 wide with one crane,” Kennedy says.

Control systems

While cranes are a mature technology, control systems continue to evolve. “We've seen improvements like variable speed drives that keep loads from swinging and provide operator control down to very tight tolerances,” says Gorbel's Butwid. “That's where the biggest developments have come from.”

While cranes were traditionally controlled by operators working in cabs above the factory or DC floor, that too is changing.

The most common and reliable way to control a crane outside of the cab is with a pendant, or wired, control. Standard on most cranes, a pendant control is hardwired by a cable to the motors and control units on the crane. The operator holds the control unit and follows the crane as it moves through the facility. While a pendant control is a cost-effective solution, functionality is typically limited to two speeds and to basic lifting and travel motions. While several cranes can be “festooned,” or wired together, so that one pendant control can manage several pieces of equipment, this is often an expensive way to manage machinery.

“For that reason, pendant controls are best used in light-duty applications where one crane is used a few times a day or week,” says Aaron Kureck, controls product manager for Magnatek (800-288-8178, www.electromotive.com).

Instead, new cranes are increasingly operated with radio remote control systems. These systems include a radio receiver that is mounted on the crane and wired into the PLCs and motors that are going to be turned on and off by the system. They also include a handheld transmitter that the operator uses to send a signal to the receiver. The control may come with buttons, paddles, joy switches or rotating dials that the operator uses to control the crane's movements. What's more, one remote control unit can be programmed to operate multiple cranes.

Safety and efficiency are the primary reasons users are turning to wireless controls. “An operator on a pendant is going to be near the load as it moves,” says Jeremy Pearson, North American sales director for Cattron Group International (724-962-3571, www.cattron.com). “With a wireless unit, the operator can operate the crane at a safe distance.”

What's the next evolution in crane technology? Using electronics to more closely monitor the performance of the motors, brakes and sensors of the crane. These solutions integrate a crane monitoring and maintenance system with the equipment's controlled area network, or CAN. “This allows maintenance personnel to monitor the system remotely or to plug in a laptop and do preventative maintenance,” says Pearson. “It's the next step, and it's going to simplify the operation of crane systems.”