Blog : How V-Curve Theory helps OEE
V-Curve Theory & OEE
And is there a better way to control packaging line speeds?
Accumulation-based production lines operate with individual machines linked with large amounts of conveyor space and generally require a lot of factory space.
Typically, the machines in an accumulation line have the capability of running at variable speeds and will normally have one machine (or one block of machines) that runs the slowest. This machine is referred to as the bottleneck or ‘critical’ machine.
The OEE (overall equipment effectiveness) or performance of this line is determined by the amount of time that the critical machine running at its rated speed.
Contents
- OEE Basics
- Why you need a well-balanced line
- V-Curve Theory in brief
- How V-Curve works
- Pros and Cons of V-Curve Theory
- How do you know if a line is optimised?
- A dynamic alternative to V-Curve
- Optimising V-Curve for better results
- Rethinking the critical machine
- Achieving a balanced production line
OEE Basics
As a reshfresher, OEE is a key performance indicator (KPI) that manufacturers can improve upon once they understand its source.
It has been used since the 1960s to simplify the performance of complex processes into a single metric.
The simple formula is:
OEE = Availability x Performance x Quality
Calculating accurate OEE comes with its own challenges, as is improving your OEE score.
However, there are three key reasons to track your OEE:
- You’ll be able to track the inefficiencies in your production line
- It will help you justify CAPEX, ensuring maximum ROI
- Operators and staff can visibly monitor their performance, keeping them motivated.
How a well-balanced line supports OEE
When a production line is well-balanced and effectively controlled, accumulation helps reduce the impact of minor stops on all machines except the critical machine.
Traditionally, machines are managed by sensors placed on the conveyors, with limited direct interaction between them. Line control, or line philosophy, refers to the method by which machines are stopped or started in response to changing conditions on the line. This includes stopping due to build back or backup of materials when a downstream machine halts or stopping due to a lack of materials when an upstream machine comes to a stop.
In the world of accumulation packaging lines, the V-Curve method is a widely recognised technique for setting production line speed.
But what exactly is the V-Curve, and is there a more effective approach?
V-Curve Theory in brief
The V-Curve theory is a strategic approach to balancing machine speeds in a production line.
It’s designed to manage the flow of products and optimise the line’s overall efficiency by coordinating the speeds of various machines based on the "critical," machine.
Accumulation production lines tend to be structured around the critical machine, which is typically the slowest and often the costliest piece of equipment on the line, usually the filler or blow-fill machine. The performance of this critical machine gauges the overall performance of the line.
When the critical machine is operational, the entire line functions smoothly; if it slows down, the entire line also slows down.
The other machines in the line are designed to run at speeds higher than the adjacent machines moving toward the critical machine. Accumulation systems are integrated into the line to prevent immediate upstream stoppages from affecting the rest of the line.
How the V-Curve Works
The V-Curve setup is relatively straightforward:
- Identify the Critical Machine: The process begins by determining the slowest machine on your line—this machine is considered the bottleneck or critical machine.
- Set Baseline Speeds: The speed of the critical machine becomes the baseline for all other machines in normal operating conditions.
- Adjust Neighbouring Machines: The two machines immediately before and after the critical machine are set to run 10% faster during recovery phases. Recovery occurs when the line needs to compensate for a build-back (downstream congestion) or lack (upstream slowdown).
- Incremental Speed Adjustments: Machines further along the line are adjusted to run incrementally faster—typically by 10%—in their recovery speeds.
- Utilise Sensors: Sensors on the infeed and outfeed of machines help dynamically manage flow, detecting and responding to changes in the production line, such as buildbacks or shortages.
Visually, this configuration resembles a 'V' shape, with the critical machine at the bottom and the other machines on either side ramping up in speed.
In the diagram above—let’s say it represents a beverage line—a drop in production of the critical machine (the filler), will result in a total line production drop.
However, line production will not slow if there is a drop in production of:
- the depalletiser or labeller (of up to 3,000 bottles per minute), or
- the case packer (of up to 6,000 cases per minute), or
- the palletiser (by as much as 10,000 bpm),
thanks to the power of accumulation.
Pros and Cons of the V-Curve Method
Advantages:
- Simplified Line Balancing: The V-Curve makes it easier to balance the production line, allowing for quick and straightforward setup.
- Intelligent Flow Management: Dynamic accumulation triggers within the system enable more responsive and intelligent management of product flow.
Disadvantages:
- Inconsistent Speeds: The only machine that maintains a consistent speed is the critical machine. Other machines frequently start and stop due to the variability in lack or buildback conditions, which can disrupt the line's overall efficiency.
- Increased Wear and Tear: Frequent speed changes across the line lead to higher inertia, causing minor stops, reducing machine operating windows, and increasing wear and tear. This can result in more breakdowns and a greater risk of quality issues.
- Suboptimal Conveyor Use: The V-Curve can lead to inefficient use of conveyor space, as the line may not be running at its full potential.
Line Optimisation
How do you know if a line is optimised?
An optimised line will:
- Run at its rated speed, steadily and without hunting for product or idling
- Operate automatically with minimum operator input
- Alert operators to issues with clear instructions
- Indicate stoppages clearly, identifying the root-cause (through built-in OEE reporting)
- Be more resilient to equipment breakdowns
- Restart faster (potentially instantly) after a breakdown
- Recover to normal production conditions quickly
When in lack (low prime), the minimum amount of product should be on the line.
When in build-back, the maximum amount of product is on the line.
A Dynamic Alternative to the V-Curve
While the V-Curve is a proven method, technological advancements, such as enhanced PLC (Programmable Logic Controller) communication, offer more precise and dynamic line control.
This alternative approach allows for a more flexible and responsive system that can better handle variations in production.
Dynamic Accumulation provides additional empty conveyor space, creating a larger buffer zone that can absorb the impact of stoppages or breakdowns.
While the critical machine continues to run at a steady speed, the other machines can adapt more effectively to changes in the line, reducing the frequency of stops and starts.
Optimising the V-Curve for Better Results
To maximise the benefits of the V-Curve, consider the following enhancements:
- Ensure Critical Machine Efficiency: The critical machine must operate at its rated speed for the V-Curve to function effectively. If it isn’t, this can lead to significant inefficiencies.
- Maximise Recovery Speed: Evaluate whether your recovery speeds are optimised. Consider the ‘ramp-up’ time—the period it takes for machines to reach their recovery speed—since this can dramatically affect line performance.
Rethinking the Critical Machine
Traditionally, the V-Curve designates the slowest machine as the critical one.
However, this may not always be the best approach. Instead, consider identifying the machine that has the most significant impact on your product quality and overall operations. This could provide a more accurate basis for optimising your line.
Achieving a Balanced Production Line
If your production line is running on a traditional V-Curve, with conveyors filled to capacity, frequent start-up delays, and regular operator intervention, there’s scope for improvement.
By setting appropriate recovery speeds, ensuring machines run at nameplate (their designed capacity), and accurately identifying your critical machine, you can achieve a more balanced and efficient production line.
Incorporating these strategies can help you realise the full potential of your production line, ensuring it operates smoothly, efficiently, and with minimal disruptions.
Choose an OEE Partner
Foodmach has been balancing production lines and helping customers improve their Overall Equipment Effectiveness for over 50 years.
Whether you need:
- a single machine that is optimised for OEE reporting,
- a series of new machines that integrate perfectly into your line to improve your OEE score, or
- an entire turn-key operation that will transform your business with Industry 4.0,
you can rest easy knowing that you’re dealing with the OEE experts.
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