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Duty Calls

The duty cycle of cranes is often misunderstood and some suppliers use that to their advantage, says Tad Dunville, director of corporate development at Ace World Companies.

This blog sets out to help those buying overhead cranes to understand duty cycle so they can make informed purchasing decisions that lead to economic and safe operation of equipment over a long lifetime of ownership.

There are two ways of rating a piece of equipment—capacity and duty cycle. It’s clearly important to observe the capacity of a crane and select a lifting technology that fits with the weight of load it is installed to lift. It’s also generally accepted that a crane should be chosen to fit its environment, the shape of load, and distance or height it is to be lifted. Crane buyers will also frequently consider if a load is to be suspended or turned by a crane during a production process. It’s all part of selecting the right tool for the job.

But duty cycle, one of the most important considerations of all, is often overlooked. Take two identical 15 ton capacity cranes, for example. Both have the capacity written in black letters on the yellow girder. One lifts 12 tons of weight once a month, very slowly, only a short distance. The other lifts 12 tons a hundred times a day, transports it the length of a workshop, lowers it and returns to the start, where another load is already waiting to be rigged. It’s obvious that these very different scenarios should be using cranes of equally different duty ratings. It’s not always the case.

The Crane Manufacturers Association of America (CMAA), an independent incorporated trade association affiliated with Material Handling Industry (MHI), rates the duty of cranes from A to F. Class A covers cranes on standby or used for infrequent service, where precise handling of equipment at slow speeds with long idle periods between lifts are required. At the other end of the scale, Class F covers continuous severe service cranes that must be capable of handling loads approaching rated capacity continuously under severe service conditions throughout its life. I won’t repeat information that CMAA makes readily available; one can imagine how the scale works between these two classes.

But how should a facilities manager looking to make a crane purchase apply that documentation? Chances are, they might not have bought a crane before—many only place orders for two or three in a career—and the process can be a daunting one, especially when there are companies out there happy to seize on any ignorance that may exist, either to sell the wrong type of crane or imply one is more capable than it actually is. CMAA’s classification scale is great, but I want to put it in even simpler terms.

Cut to the point

Two analogies help. First, take a lawn mower. One can go to a local store and buy a mower with a single blade or call an agricultural specialist and order a top of the range tractor with a six-blade attachment that spins like the wind. Essentially they are both as effective at cutting, say, 20 square meters of grass (ok, one would do it faster), but you’d only want to use one if you had to cut a golf course by lunchtime. Knowing which to use, and reading the incremental scale of cutting devices in between, is based on an understanding of duty.

Second, take a pick-up truck range. A hobbyist who uses his vehicle on a Sunday to go fishing would require a different model to a guy or gal carrying multiple passengers and heavy loads, five days a week across rough terrain. Again, the purchasing decision is based on duty. (As an aside, while we’re on the subject of road-going vehicles, we can use a car as another example of varied duty. Mileage, weight of passengers, terrain, style of driving, etc. are all going to impact on the wear of the car. In other words, the duty will be greater or less.)

In all these examples, increased duty should be met by tougher equipment assembled of more robust components. This is the same with cranes and it’s often the area where deception is greatest. A buyer can spend $50k or $1.5m on an overhead crane. I’m not suggesting it’s the case, but one price might be for a Class A crane and the other Class F. Using the lower duty cycle crane in a steel mill, for example, will quickly result in blown bearings, a shredded motor and a cooked gearbox. Putting a Class F crane in a small workshop that rarely uses it, extreme example though it is, would be a waste of money.

An investment will be made based on trust in the seller that the crane is fit for the required duty. However, there are gray areas related to components that crane sellers can exploit and not all cranes sold as ‘Class D’ are robust enough to perform at that level of duty. There isn’t really a go-to chart that buyers can use that says a certain duty cycle should have specific components, classified to a defined standard. Buyers need to check that corners are not cut and electrification, bearings, deflection, hoists, and more, are actually equipped to withstand the demands of the application.

The real world

The peculiarity with the crane market is that there are no tire kickers. One might visit the Porsche website a hundred times before even having the money to think about buying one. Similarly, one might dream of having a super yacht one day and subscribe to a manufacturer’s newsletter. People tend not to feel that way about cranes! If someone gives off buying signals, they probably want a crane. If they come to a manufacturer’s website and start searching for options, there’s a good chance they need one pretty soon. It means the buying fraternity sometimes lacks experience, and that is preyed upon, often mercilessly.

If this blog helps even one buyer during their next crane purchasing process it would have been worth writing.

Thank you for reading. Follow us on Twitter at @AceWorldCompany

Tad Dunville
Director of Corporate Development, Ace World Companies