Exploring Availability
Key takeaways:
- Availability measures equipment readiness for production.
- It is calculated as actual operating time over planned production time.
- Downtime losses include equipment failure and setup/adjustment time.
- Modern OEE accounts for planned downtime like maintenance.
Nakajima’s OEE calculation
In the image below, you can see a complete breakdown of how Nakajima calculates OEE in his Introduction to TPM: Total Productive Maintenance book.
The illustration effectively shows how the six big losses of OEE relate to the three components of OEE: Availability, Performance, and Quality.
Originally, Nakajimi focused on planned production time and did not encourage the inclusion of factors like planned maintenance and planned changeovers. Since then, manufacturers and lean specialists have put forth new OEE calculations that capture more losses and are better suited modern manufacturing processes.
So, before we dig in to availability according to Nakajima’s original calculation, let's briefly look at how OEE has evolved over the years.
The evolution of OEE
There are multiple ways of calculating OEE, with adjustments made to the definition of losses and the allocation of stop-causes depending on industry, use case, and individual preference.
Over the years, people have tailored OEE according to specific requirements, leading to various adjustments to Nakajima’s original concept.
Additionally, some have developed completely new metrics that build on the principles of the original OEE.
In general, manufacturers and lean specialists have put forth new OEE calculations that capture more losses and are better suited to modern manufacturing processes.
Here's a list of some of the models based on OEE found in literature between 2002-2019:
Understanding equipment availability
Availability is a critical component of Overall Equipment Effectiveness (OEE). It measures the amount of time that a piece of equipment is ready and available for production, relative to the planned production time.
This metric is vital in manufacturing as it directly impacts the total output a facility can produce. High availability means equipment is consistently ready for operation, thus maximizing production time and output.
How to calculate Availability
Here’s how to calculate the Availability rate in OEE according to Nakajima:
Availability rate =
(Operation time / Loading time) x 100
Loading time is often called the Planned production time, or the scheduled time that the equipment is supposed to operate.
So, in simpler terms, you can say that Availability is calculated as the ratio of the Actual Operating Time to the Planned Production Time.
Availability rate =
(Actual Operating Time / Planned Production Time) x 100
It’s important to note that planned production downtime (such as cleaning and team standup meetings) is excluded from Operation time.
Here, Operation time is:
Operation time =
Loading time (aka Planned Production Time) – Downtime
Here's the OEE Availability calculation put together:
Factors affecting availability
Several factors can affect Availability in a manufacturing environment.
According to Nakajima, Availability is affected by two downtime losses:
Equipment failure – This is an unexpected event, such as a machine breaking down due to a belt snapping.
Setup and adjustment – Time lost due to the changes in operating conditions. For example, production or shift starts, and equipment changeovers for different products.
It’s important to say that in this early calculation of OEE a lot of information is being left out. More modern OEE calculations take into account planned downtime, for example.
Planned downtime includes activities like scheduled maintenance. While necessary for long-term efficiency, planned downtime reduces the available time for production and might be able to be reduced or scheduled at better times.