There’s an old adage that you need to know the rules before you break the rules. That applies to everything from playing jazz to running a business.
I got to thinking about that idea after reading Industry Week’s cover story on OEE (Overall Equipment Effectiveness). The IW story discusses the benefits of OEE, and even gives a definition of OEE:
OEE tells users the percentage of time that equipment, when running or required for production, is producing good-quality products at an acceptable rate. It is the product of three ratios, or submetrics: machine availability rate, performance or run rate, and the quality rate. It is calculated by multiplying availability rate by production rate by first-pass quality rate.
This is a good textbook definition of OEE, but it leaves some gaps. This post and others in this series will fill some of those gaps. Once we fill in the gaps in the text book definition we’ll be in a better place to consider if we want to break any of the rules.
Mr. Sanders, my 5th grade teacher, was forever having us convert a story problem into an equation. Here is what that paragraph looks like as an equation:
OEE = Availability * Performance * Quality
Future posts will go into more detail on Performance and Quality. Today let’s look at Availability in more detail.
The formula for Availability is simple, but it relies on some operational definitions. Here are all the components for calculating Availability:
Availability = Available Time / Scheduled Time
(All Time components are typically measured in minutes.)
Available Time = Scheduled Time – Down Time
Scheduled Time – sometimes called “Planned Production Time” – is the time any piece of equipment is scheduled for operation. It does not include any planned shutdown time such as planned maintenance, employee breaks when the machine is not running, etc.
Down Time is a measure of unscheduled production stops, such as machine setup and changeover, or losses due to equipment breakdowns and material shortages.
We do not include planned shutdown time in this calculation. For example, if a machine is scheduled to run from 7:00 AM until 3:00 PM on a specific day, then the Scheduled Time is 8 hours, or 480 minutes. If a machine normally runs while an employee is at lunch, then the employee’s lunch time is included in the Scheduled Time. If the machine is scheduled to be shut down while the employee is taking a break, then the break is not included in Scheduled Time.
For Down Time, consider establishing a threshold for what constitutes a trackable event. For example, you may wish to track stoppages of more than 5 minutes so that you don’t bother with every little hiccup in the system.
Availability, like the other components of OEE, is reported as a percentage, so after we calculate the ratio we just multiply it by 100. Here is a typical calculation:
|Total time in shift||480 minutes (8 hours)|
|Lunch break||30 minutes|
|Machine setup||95 minutes|
|Other Down Time||17 minutes|
Scheduled Time = 480 minutes in shift – 30 minutes planned for lunch break = 450 minutes
Down Time = 95 minutes of machine setup + 17 minutes of other unscheduled down time = 112 minutes
Available Time = 450 minutes Scheduled Time – 112 minutes Down Time = 338 minutes
Availability = 338 minutes Available Time / 450 minutes Scheduled Time = 0.751 or 75.1%
This is the textbook definition of Availability. How do you calculate it at your facility?