Formula for OEE calculation – diagnosing problems and fixing them
The formula for OEE is simple, but the ingredients that go into the calculation for OEE decide whether you get a proper number.
Availability = (Running time / Available time)
Performance = (Real production / Theoretical production)
Quality = (Good products / Real production)
Formula for OEE = A x P x Q
Availability is a measure of how much time the machine was made available for production, after downtime losses like machine breakdown, setup change, no raw material, etc.
Performance is a measure of the quantity that has been produced compared to what could have been produced.
Quality is a measure of rejections.
Parameters in formula for OEE, and their meaning
Formula for OEE – abnormalities and fixes
Causes of wrong OEE, and fixes
Availability is abnormally high
Performance is abnormally low
Performance is abnormally high
Quality is abnormally low
Quality is abnormally high
Checklist for parameters in formula for OEE
Before doing the calculation for OEE for any time period, ensure that all these have been considered properly:
From Available time, exclude any time in which you have decided beforehand that it must not run:
– Weekly off days
– Lunch break
– Tea break
– Downtime for preventive maintenance.
– Downtime for autonomous maintenance.
– Downtime for machine cleaning in every shift
– Downtime for daily scheduled meeting when the machine is stopped.
In Running time
In Running time in the calculation for OEE, include only the time when the machine is actually running and producing.
– Cycle time – time between start and end of a part’s production.
– Standard time for part unloading and loading.
Exclude any time that is a downtime, not part of the running time:
– Part load-unload time that is more than the standard load-unload time
– Setup time at part change
– Inspection time
– Rework time
Use the correct values of standard cycle time and standard part unload-load time, and the correct value of Running time in the formula for OEE.
Include ALL parts produced, including rejected parts.
Exclude reworked parts. i.e. count every part only once, the first time it is made. Do not count it aqain when it is reworked. Rework time is downtime.
This is the parts produced minus rejected parts.
Exclude reworked parts. i.e. count every part only once, the first time it is made.
Action points for correct formula for OEE
- Ensure that you assign the correct downtimes for Availability in the formula for OEE.
- Ensure that you use the correct standard times for Performance.
- Ensure that you use the correct good parts and total parts, and ignore rework in Quality.
Ensure that you are truthful about considering downtimes. Do not consider a downtime as a part of the process time when it is not warranted. For example, DO NOT:
1. Consider excess part load-unload time (beyond the standard time) as part of the cycle time.
2. Consider setup time as part of the operating time.
The blog post ‘OEE formula – bad idea to cook up‘ explains why you need to be truthful with the downtimes, and
The strangler fig – an interesting tree
Cycling through a forest recently, I came across a whole lot of strangler fig trees. Very interesting trees, these. A strangler fig (the lighter coloured tree in the picture) starts its life on top of another tree, when a seed is deposited on the top of the tree by a bird or by wind. It grows upwards there to reach the sunlight, and grows roots downwards towards the soil. This is an adaptation for growing in dark forests where a whole lot of trees are trying to grow upwards to reach sunlight. The roots growing downwards cause the ‘strangling’ of the original support tree.
The strangler fig is a hemiepiphyte, a parasitic plant that grows on another plant, like orchids and ferns that grow on trees. A hemiepiphyte spends half its life (hence the hemi, like in hemisphere) as an epiphyte feeding off the support tree, and then when its roots touch the ground it starts feeding on its own too. Strangler figs belong to the genus Ficus, that has 10-odd species, all of whom bear fig fruits, some edible and some not. The fig fruit that we eat is from the Ficus carica, while the banyan is the Ficus Benghalensis (named after Bengal in India). Ficus religiosa is the Peepul tree or the Bodhi tree, considered sacred in Hinduism and Buddhism (because the Buddha attained enlightenment under this tree).
The original support tree sometimes dies because the strangler robs it of sunlight and cuts off its nutrient flow from the soil, and the strangler fig becomes a tree with a hollow central core.