Beyond “capacity”, there is one additional factor to consider, “Duty Cycle”. It would be much like taking your 21 inch Toro and mowing the golf course. Yes it can successfully mow Augusta National, but after a couple of rounds, your new Toro would be ready for the scrap heap. The same goes for a “standard duty” bridge crane when required to make full capacity lifts 24/7! Yes…your 5 ton crane can lift 5 tons, but not in a continuous fashion. And that’s the catch…”continuous”.
What Does Duty Cycle Really Mean?
Can My Heavy Duty Toro 21” Lawn Mower,
Mow Augusta National golf course?
In the crane world the terms “heavy” and “heavy duty” are often confused. A crane is designed to lift, what it’s designed to lift, therefore it’s not “heavy”. Even if a crane is a 500 ton crane, if it only makes one pick per week, it’s cannot possibly be considered “heavy duty”. Heavy duty usage is a factor measured by the amount of work performed, and one pick a week is not much work.
As a result, the CMAA (Crane Manufacturers Association of America) has established a duty cycle rating system that ranges from a Class “A”-Standby Service to a Class “F”-Continuous Duty, to measure the amount of work to be performed by the crane. It consist of a somewhat confusing combination of primarily two factors,
- average percentage of full capacity lifted and
- the number of starts/hour
The equation below is the actual CMAA method of calculating duty cycle.
Don’t worry, unless you really want to, you will never have to see this equation again. I just wanted to show the numbers behind the Toro/Augusta Theory. I have put together a product selection matrix that will help you through the maze without calculating any numbers. It will be in the next post, with a download link for a free PDF copy.