Bottlenecks / Capacity Constraints

The term “bottleneck” (capacity constraint) comes from the area at the top of the bottle that limits the flow coming out. It doesn’t matter how big the rest of the bottle is—liquid will only flow out as fast as the size of the neck will allow.

A system is only as fast as its bottleneck process.

That is stating the obvious, but the concept holds true in any production environment, whether in the office, or on the manufacturing floor. There is one process, station, step, etc. that is the limiting factor that will prevent greater throughput. This is the rate limiting step that determines your capacity.

The power of knowing your bottleneck is considerable. It lets you increase your flow by improving one process, rather than your whole value stream. The reverse is also true. If you have a bottleneck, nothing you do anywhere else in the value stream will improve your throughput.

Keep in mind that the bottleneck may be unknown if demand is less than the capability of the slowest process.

Bottlenecks come in a great many forms. It may be the long cooking time in a bread oven. It may be the limited capacity on a truck that transports product around a corporate campus. It may be the approval process for a capital purchase.

Regardless of where your bottleneck it, if you want to increase the output of your whole system, you have to increase the capacity of your bottleneck.

This is an area where continuous improvement efforts shine. A machine that acts as a bottleneck may have a variety of different approaches that can resolve the issue.

1. Six Sigma can help resolve quality problems. Perhaps you have a low yield (percentage of good parts) coming off your machine that is contributing to making it a bottleneck. Imagine you need 80 units per day from a machine, that has an 80% yield and the capacity to make 100 total parts per day. It will produce 80 good parts and 20 defects. If demand rises to 90 per day, the machine becomes a bottleneck. You can reach your goal by buying another machine or improving yield to 90%. Which sounds less expensive?
2. Setup reduction can make the machine available for a greater portion of the day.
3. Lean efforts can improve the efficiency of that machine or process. Or it can improve an upstream process that feeds it.
4. Total Productive Maintenance can help resolve maintenance issues that slow production.

Here’s the frustrating thing about bottlenecks–once you resolve the issues around a bottleneck, and raise your production rates to meet demand, guess what happens: you will strain the next slowest process, creating a brand new bottleneck!

Even if your new bottleneck can meet the higher demand, you will ALWAYS have a bottleneck. There is always going to be a slowest step. Your demand may be below the capacity of the slowest process, but if demand rises, the bottleneck will be exposed.

On occasion, a bottleneck will jump around if the capacities of a few processes are similar. Normal variations in processes can make a bottleneck appear to jump back and forth between your slowest procedures. These situations tend to be a little tricky to resolve, as there is no clear culprit that is slowing down the works. Just go about the procedures below on the machines you most suspect of being bottlenecks.

Just remember: If something is not really a bottleneck, creating more capacity will not speed up your whole system. That point deserves some more attention.

If you speed up a non-bottleneck, the speed of your whole system will remain unchanged!

Think of this like a team of horses pulling a stagecoach. You will go at the speed of the slowest horse. If you make one of the fastest horses quicker, you will still travel at the speed of the slowest horse!

Here are some basic rules when dealing with bottlenecks:

1. NEVER starve a bottleneck. Do whatever you can to make sure bottlenecks do not stop production. The best way is to improve the upstream process to make it more reliable. A not-so-great way is to add a buffer in front of the bottleneck. While this violates Lean principles, it can be an effective short-term measure. Just make sure you don’t let it become long-term. (Side note: I have rarely seen a short-term measure that did not become permanent.) Use buffers with caution.
2. NEVER let a bottleneck wait for repairs. Pull your maintenance team off other projects if a bottleneck process goes down for maintenance issues. Bottlenecks don’t have the catch-up speed of other processes.
3. Bottlenecks should be on your kaizen list. You should be spending a disproportionate amount of your productivity or delivery improvement effort on bottlenecks. Remember, though, quality should trump cost and delivery when selecting projects.
4. Minimize setup time on bottlenecks. Keep in mind that this means to reduce setup time, not to do bigger batches of each product.
5. Pace your line to the bottleneck. Use your bottleneck to determine if your operation can meet your customer demand.
6. Don’t confuse external problems with a step being a bottleneck. Any process that is starved of raw materials will shut down. Make sure you are judging processes under normal production conditions.

From a job satisfaction standpoint, bottlenecks frequently cause rifts on a team. One station on an assembly line may be a bottleneck, and constantly fall behind, causing the whole team to stay late. Make sure you assess this problem and explain that the problem is the process and not the people.

‘The Goal’ by Eliyahu M. Goldratt gives a great deal more information on this subject. He refers to this as the Theory of Constraints. His teachings are told in a narrative that describes the efforts of a plant manager to turn his plant around, making it a very easy and enjoyable read.

Words of Warning About Bottlenecks

• Never let a bottleneck starve. Because it is your slowest step in a process, it doesn’t have catch-up speed to get your system back on track.
• Be careful about moving bottlenecks. With excessive variation and intermittent problems, bottlenecks may shift between various machines or processes.
• Don’t guess at your bottlenecks. Use facts and data to determine where it actually is.

If you are working at a bottleneck operation, expect a higher level of scrutiny. That’s because the pace of the whole system is dependent upon you keeping your bottleneck operating. You are essentially operating without a safety net. The whole value stream will have no way to get back on track if you fall behind.

Pay special attention to maintenance issues that can cause you to lose production time. Also make sure that any signals you have to upstream operations are sent properly so you don’t starve your bottleneck because of a communication issue.

If you want to improve the output of the system, you can only do so by getting more through your bottlenecks.

Your first task is to identify your constraints. When your demand is well below the capacity of your bottleneck, it might be hard to find. Most people uncover their bottlenecks, when demand rises past the point where the weak link can keep up.

NOTE: You will have only one bottleneck per product line. Even if multiple processes can’t keep up with demand, there is still one slowest process. You can, however, have multiple bottlenecks if you are managing several different product lines.

A disproportionate amount of your management effort should be spent on your bottlenecks. Any improvements you make on a non-constraint will do nothing for your total output. Most managers can easily recognize that the system can be improved by increasing the capacity of the slow machine or process.

Seasoned leaders, though, will see that variation on upstream processes can be just as harmful. That’s because fluctuations in the supply of components and raw materials shuts down a bottleneck just as quickly as a malfunction does.