Never use averages for crane bottleneck detection in melt shops

“If the crane is 80% utilized, it will be the bottleneck!” How many times have I heard this? Yet every time, I find myself following up with a longer explanation about the dynamics of cranes and bottlenecks in a melt shop.

Momentary vs. long-term bottlenecks

On a macro level, the major bottleneck of a melt shop can often be easily spotted. However, when assessed on a heat-by-heat basis, bottlenecks shift significantly between equipment depending on the steel grade recipe, product mix, and overall plant utilization. In reality, there is rarely ever one static bottleneck.

Momentary bottleneck: The machine that restricts production temporarily. Recognized by its continuous activity over a given period — processing one heat after the other without idle time.

Long-term bottleneck: Identified by tallying which machine is the momentary bottleneck most frequently. The one most often acting as the momentary bottleneck emerges as the predominant long-term bottleneck, defining the maximum productivity of the melt shop for a given production scenario.

How this applies to cranes

It is rare for the crane to be a long-term bottleneck, but cranes can easily become momentary bottlenecks with significant influence on the overall production outcome.

For a crane to be identified as a momentary bottleneck, look for periods of consecutive transport tasks without any idle time — for example, five transport tasks in a row. In this period, the crane is 100% utilized, but this doesn’t necessarily imply negative consequences.

However, if the 4th or 5th transport in this sequence was already overdue, the ladle transport will arrive late to the next station. This can lead to:

• Best case: No major consequence if there are sufficient margins on transport time
• Moderate consequence: Delays in starting the next machine, resulting in changes to tundish weight, reduced casting speeds, or strand shut-off — leading to productivity and quality losses
• Worst case: Sequence interruptions causing major productivity loss, compromised quality, and significantly increased operating costs

The takeaway

A crane utilized 70% or 80% throughout the day doesn’t necessarily indicate a problem. It just means the crane is very busy. But if, in one situation, the crane causes a slowdown or sequence break, then the cranes are indeed limiting production.

How to check

1. Look for periods of uninterrupted crane work
2. Check if those transports caused a delay to the subsequent machine by comparing planned and actual start times
3. If there is no delay, move on — this high utilization period did not cause a direct problem
4. If it does delay production, check whether the situation could have been avoided by re-sequencing crane tasks
5. If everything was executed optimally and the delay was inevitable with severe consequences — you’ve identified the cranes as the bottleneck. Whether they were utilized at 50% or 80% on average is irrelevant.