Educating yourself on this material handling tool can help keep operations running smoothly and employees safe
November 2, 2009
Most fabricating operations have an overhead crane, but just what do these companies know about these material handling tools? This feature provides the answers to their questions.
Overhead traveling bridge cranes are very familiar sights in metal fabricating shops. These cranes come in single- and double-girder bridge designs. Photo courtesy of North American Industries Inc.
Plenty of metal fabricating operations have overhead cranes, but do plenty of those metal fabricating operations use the cranes to keep down overhead?
The simple point is that many of these companies may not be using the overhead cranes because they don't know what they are fully capable of. In many instances, they may have come with the building and are used to pick up the occasional heavy load when a lift truck is unavailable.
Actually, the lift truck is the de facto means of moving steel material around a metal fabricating facility. Most shops have them, and training drivers takes only a few hours.
Lift trucks aren't welcome everywhere, however. Space is tight in many shops, and many other operations might be looking at their wide aisles as a potential home for a new machine tool or staging for work-in-process.
That's where an overhead crane can make a difference as the main means of material movement. Imagine a tool and die shop that's served by one overhead crane that's able to carry a load the length and width of the building and drop it off at a machining center that's served by its own, lighter-duty crane. The operator can control the loading and unloading of the tooling, depending on where it is in the manufacturing process.
"Floor space is always crucial," said Ryan Marks, new equipment and parts manager, Uesco Industries Inc., Worth, Ill. "At the same time, you can have one operator running the crane, while another operator is doing something else. You don't have to dedicate a person to operate the crane. Usually with a forklift, you have someone driving around all day just delivering material."
Obviously, it's not logical to think that metal fabricators are going to start clamoring for a "Cash for Clunkers" program for their old lift trucks. On the other hand, it is logical for them to look at their overhead cranes as an underutilized resource. With that in mind, let's get educated on the world of overhead cranes.
The most common type of overhead crane used in manufacturing environments is the overhead bridge crane (see Figure 1). This crane runs on elevated tracks and is a popular choice because it offers hook motion along three axes—up and down, back and forth, and side to side. These cranes can run either on top of girders or be underhung.
The gantry crane operates in a similar fashion to the overhead bridge crane with the main difference being that it is supported on the floor, either connected to a set of tracks or on four wheels (see Figure 2). A pair of steel legs support the bridge from which the hoist is connected.
These cranes make a lot of sense when a load has to be transported between buildings. In other instances, they are the right fit when a manufacturer doesn't have long-term plans to stay in one spot.
"It's a smaller purchase. It doesn't impinge on your building. It's better for you in those circumstances where you have a leased building or something, so you want to make an investment that is not going to stay with your building," said Larry Dunville, president, Dearborn Overhead Crane, Mishawaka, Ind.
The jib crane is another common sight in shops. This cranes features a pivoting head and a boom assembly from which the hoist is attached. The base of the crane can either be floor-mounted, which provides 360-degree boom rotation, or attached to a building column, which limits rotation of the boom to 180 degrees. It's important to remember that the hook arrangements on jib cranes don't follow a traditional X-Y arrangement, but rather a radial one.
Craig M. Pelkola, director of technical services, North American Industries Inc., Woburn, Mass., stressed that monorail overhead lifting systems can be very effective when properly designed into a facility layout. Of course, such a crane has only two directions of hook travel—up/down and along the axis of the monorail beam—but that works perfectly for continuous-production environments, such as a painting operation.
Now with the general discussion of industrial cranes out of the way, a briefing on the different classifications of those cranes is necessary. The Crane Manufacturers Association of America (CMAA) classifies cranes according to starts per hour and the average percentage of full capacity being lifted:
Class A typically covers installation and maintenance cranes, material handling equipment that operates at slow speeds, has long idle periods between lifts, and sometimes requires maximum- capacity lifts. On the other end of the spectrum is Class F, which covers top-riding bridge and gantry-type, multiple-girder, electric overhead, traveling cranes. Class F dictates that the equipment must be capable of handling loads that approach rated capacity for a majority of its working life. It's the classification the requires the highest degree of reliability.
In between those extremes fall Class C and Class D cranes, which cover a majority of the equipment in the metals processing field. Class C covers cranes that handle loads averaging 50 percent of the rated capacity with 5 to 10 lifts per hour averaging 15 feet. At this classification level, not more than 50 percent of the loads should be at rated capacity. Class D cranes are for loads approaching 50 percent of the equipment's rated capacity and when those loads are handled constantly during the workday. Users of this type of equipment usually require about 10 to 20 lifts per hour averaging 15 ft. For Class D cranes, not more than 65 percent of the lifts should be at rated capacity.
The CMAA describes both Class C and Class D as suitable for "machine shop" environments. While Class C would be appropriate for many shops, Pelkola suggested that a Class D crane for Class C applications could prove more cost-effective in the long term because the larger and more durable components, such as axles, gears, gearboxes, motors, and wheels, can extend the crane's operational life and minimize maintenance requirements.
Dunville noted that with the increase in duty cycle comes an increase in price. Based on his company's installations, Dunville said the cost difference between a Class B maintenance crane used for the occasional pick and a Class F crane that works 24 hours per day—both of which have the same capacity and same spans—is about 10-fold.
Marks offered up some very logical advice: "Never, ever, ever bring a load over someone's head."
That golden rule is part of a larger concern: Most companies don't spend the time and money to train crane operators properly. The thought is that the lowest-paid person on the shop floor—the material handler—doesn't need the benefit of training because the job function is a no-brainer. That piece of advice might work for the front office, but the people on the shop floor might have another opinion.
Ask a person operating a crane what the rated capacity is. Follow that up with a question about how much he is lifting at the moment. Correct answers for both reveal a very engaged employee. In all likelihood, the operator probably doesn't know the exact answers. Training can help them understand the importance of knowing what loads are safe and what loads are accidents waiting to happen.
Overload limit devices on a crane can kick in when an operator's bad judgment threatens co-workers, but one large loophole remains in crane safety.
"There is no requirement for overload protection on hoists," Dunville said. "They can throw the whole OSHA spec away and in one line do more for safety than what they are doing now with 137 pages."
To counteract that oversight, Dearborn Overhead Crane installs an overload circuit on large wire rope hoists. Dunville said it is good insurance against the possible legal problem of people overloading hoists. It's also a good safety move.
Without question, a company needs to consider just what the crane will be moving, how often it will be used, and how it will be supported. But Pelkola posed some other important questions:
What type of voltage rating is required?
Those questions become particularly important when a company begins to look at possible future needs. A crane may fit the bill for today, but come up short in the future (see Figure 3).
"Inevitably you are picking up something down the road that ends up being bigger than you anticipated," Dunville said.
That's why companies need to keep future load requirements in mind. Once a crane is installed and too little clearance exists between the bridge and the ceiling, a metal fabricator doesn't have much choice but to raise the roof if it wants that crane to be modified to pick up larger loads than it was originally designed for.
One thing is sure about older overhead cranes: The manufacturers didn't skimp on the steel. The bones that were suitable for the job 50 years ago are likely good enough for today.
The components, on the other hand, need to be replaced—end trucks, wheels, drives, motors, hoists, and electrical controls. A good modernization effort also will cover repainting, rewiring, and any needed repairs.
Modernization also provides an opportunity to upgrade the crane operations with new technological advancements.
The addition of variable-frequency drives for better speed control of the crane is a popular upgrade. The modern drive technology allows the operator to control acceleration and deceleration, rather than just getting one speed, with the single push of a button. Pelkola said that this reduces the need for crane maintenance because components are not exposed to severe impact normally seen with outdated controls. Also, because the variable-frequency drives do not allow the operator to reverse plug or jog the crane, parts such as brakes, bearings, and drive trains last longer.
Marks said radio remote controls are also a wise upgrade. An operator does not have to be close to the crane to operate it, with some control systems able to function within a range of a couple hundred feet from the crane.
Hopefully you have picked up some knowledge about the overhead crane in your shop or the one you might be thinking about purchasing. This information doesn't make you an expert, but it can help you become more informed on the subject, which is always important, particularly on large jobs.
Dunville said Dearborn Overhead Crane once had a customer that placed an order for two 500-ton cranes, a $1 million order. One might figure the order was very detailed, as this manufacturer of large stamping presses was working with some huge metal components. Not so. Dunville said the spec for the order was just one paragraph long.
After reading this, you shouldn't have any problem being verbose about your own material handling goals as they relate to cranes.