Get the proper lift from crane technology

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Don't take your overhead cranes' capabilities for granted There are clear limits to just how far you can push a crane's rated capacity

"I GOT REAL NERVOUS, WATCHING THAT MOTOR lift off the truck bed and swing out over the shop floor. The crane was labeled 15 tons-and we had a 45,000pound motor." That's how a field service engineer described the handling of a 3,000 hp, 360 rpm vertical pump motor coming into a service shop for inspection. Luckily, the lift was accomplished twice without incident. But should any hoist be expected to handle a 50% overload?

Although traveling or overhead bridge cranes are a standard fixture in almost any manufacturing or repair facility routinely handling heavy machinery or component materials, they get little attention. We tend to take these workhorses for granted. For example, the Standard Handbook of Plant Engineering, 1995 edition, devotes only two of its nearly 1,600 pages to overhead cranes, barely going beyond simple definitions. 

 Whether in a warehouse, a manufacturing plant, or an electrical repair shop, most machinery or other equipment in common use is right there on the floor in front of us. We see it every day. The crane, however, is not so readily noticed. We use it often enough but seldom see much of it, up there above us. And we may not realize how extensively it is covered by design and safety standards-to an extent, in fact, that's unique among process machinery.

The crane rides on a pair of supporting rails secured above the workplace (usually near the top of a building's walls). Generally, the rails are located indoors, though they sometimes extend into a storage yard or outdoor work area. (Some cranes ride on one rail, the other end being supported by a leg or "gantry" rolling on wheels at the floor level. This is "semi-gantry" design. When no walls are involved, and the crane travels on two legs, as with the container cranes used at dockside, it's called simply a gantry crane. We won't be concerned with either of those variations here.)

Lifting capacities of such cranes can reach several hundred tons. However, the typical industrial range is from 5 to 25 tons, with an occasional unit of 50- to 150-ton capacity. They provide three basic movements (Figure 3). The "bridge" or lengthwise travel moves the entire structure along the supporting rails. "Trolley" or crosswise travel takes only the hoisting mechanism back and forth between the rails. The "hoist," of course, lifts and lowers the load. Separate electric motors, controls, and gear drives power each of these three modes of operation. A typical crane assembly also includes the supporting structure, electrical circuitry, and controls.

In the largest sizes, crane movements are governed by an operator riding in a cab built onto the bridge structure (Figure 5). Most overhead cranes, however, are not used steadily enough to justify assigning a person to such duty. Instead, their controls are in a "pendant" box suspended from a cable at floor level. When the crane is to be used, anyone working on the building floor can become its operator. An alternative, offering the operator a more flexible vantage point, is a radio control requiring no hardwired connections to the crane.