A resource for effective troubleshooting and Overall Equipment Effectiveness (OEE) from the leaders in simulation training.
Greetings once again, Troubleshooters! Thanks for joining us again here on Troubleshooting Thursdays. Today is Part 2 of our short series on overall equipment effectiveness (OEE), a critical measure of plant reliability. Last week in Part 1 we looked at how to calculate your plant’s Overall Equipment Effectiveness (or the OEE of a piece of equipment). The OEE score is important because it tells you how efficiently your plant is running.
Just as a refresher, here’s the formula:
OEE = Quality (%) x Performance (%) x Time Availability (%)
Today we’re going to talk about how to get those numbers for quality, performance, and time availability so you can plug them into the formula above. Just before we get started, though, in case you’re interested, there’s another, simpler formula that does not take into account the three loss-related factors, availability, performance, and quality:
OEE = (Good Count x Ideal Cycle Time) / Planned Production Time
The first formula is preferred because the measures of quality, performance, and availability offer insights into the underlying causes of inefficiency in the plant. Okay, let’s get started.
In order to determine your OEE quality, you need to know your Good Count. Good Count is basically the flip side of Quality Loss, the number of parts or units that are rejected because they don’t meet quality standards (including those that need rework).
Virtually all manufacturing plants have quality-control checks in place, and regularly measure the number of parts or units that are rejected or need to be reworked. So you should have access to this number.
The remainder of your output—that is, the parts or units that are made right the first time without any need of reworking—is considered Good Parts, and the total number of them is the Good Count. (The number of all your units—rejects, reworks and all—is the Total Count.)
So, the formula for calculating your plant’s production quality is:
Quality = Good Count / Total Count
It’s a simple percentage of good units relative to total units. In a perfect world, it would be 100%.
Performance (or speed)
The performance measure considers the factors that cause the production line (or piece of equipment) to run at less than maximum speed, thereby slowing down your production. You need to know your Ideal Cycle Time, which is the theoretical shortest time it should take to produce one unit under ideal circumstances. You also need to know your Total Count (used in the quality formula above), which is simply the entire number of units produced in one cycle, including rejects and reworks.
Finally, you need to know your Run Time, which is the time your production line is scheduled to be running and is actually running (so it’s your planned production time minus downtime).
The formula for determining what percentage of the ideal you’re operating at is:
Performance = (Ideal Cycle Time x Total Count) / Run Time
Because the Run Time number has had downtime subtracted, it doesn’t take into account the problems downtime can cause. It’s just a measure of how efficiently things are going when the machines are up and running.
Which brings us to availability. This one is pretty simple. Availability focuses on significant stoppages in planned production, that is, downtime.
Availability = Run Time / Planned Production Time
Remember, Run Time is the time the line is actually up and running as opposed to what was planned (so you subtract any downtime from the planned production time). So the formula could also look like this:
Availability = (Planned Production Time – Downtime) / Planned Production Time
This formula gives you the percentage of the planned time that the production line or the piece of equipment is actually available to produce.
It’s important to note that low availability can be a huge problem. In some industries, downtime can cost as much as $22,000 per minute due to lost production.
Why do these measures matter?
When you’re looking at your OEE, these three sub-measures of quality, performance, and availability are important to break down. Why? Because they each point to different potential problems.
For example, say you’ve calculated your OEE using the simple formula at the beginning of this post (OEE = (Good Count x Ideal Cycle Time) / Planned Production Time) and it comes out to 50%. That’s a pretty bad number. You know something is wrong, but you don’t know what’s causing it.
On the other hand, suppose you used the A-P-Q formula and found that your quality is 100% and your performance is 100% but your availability is only 50%. You would still have the same score of 50%, but you would have the advantage of knowing that the cause of the problem is too much unplanned downtime, and you could take steps to fix that.
Paul Zepf, in his article How to Calculate Overall Equipment Effectiveness: A Practical Guide gives another great example. Picture a factory that can make 60 widgets a minute. In an 8-hour (480-minute) shift, the factory should be producing 28,000 widgets (480 x 60). But say there are only 14,400 widgets on the pallet at the end of the shift. That’s only half what it should be, and the OEE is 50%.
Availability, performance and quality show the underlying cause
But—does that number mean the machinery was stamping out good quality widgets the whole time, but just very slowly, so that the quality was 100%, but the quantity was only 14,400 instead of 28,800?
Or, does it mean that the machines were running at full-tilt, but quality control personnel had to throw out every second widget?
Or, does it mean that the widgets all met the quality standard and the line was churning them out at top speed, but that one machine broke down and brought the whole line to a grinding halt for 4 hours until it was repaired?
Plant managers will only know the answer to these questions—and be able to improve the OEE—if they know what the underlying cause of the problem is.
When availability is the problem
If your problem is downtime, pump up your availability with the Simutech Training System, Simutech Multimedia’s comprehensive training solution that teaches a systematic method for quickly, safely, and efficiently diagnosing and repairing electrical faults in manufacturing equipment, helping you slash costly downtime.
If you get a chance, check out this awesome production time calculator. Using the measures we’ve talked about in this post, you can calculate production time for a given order.
Okay, Troubleshooters, that’s it for this week. Join us on TST next Thursday as we look at incorporating 3D into your training program for a totally immersive experience!
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