To calculate OEE, you need to perform the main calculation as well as the calculations that go into each of the three elements of availability, performance, and quality. As a reference to these calculations, consider the following example.
A computer chip manufacturer is implementing TPM and wants to get a one-week baseline measurement of OEE before the initiative is undertaken. The Six Sigma team collects the following data:
Shift Data
Item
Data
Shift Length
8 hours = 480 minutes
Short Breaks
2 for 15 minutes each = 30 minutes
Meal Break
1 for 30 minutes = 30 minutes
Downtime
43 minutes
Ideal Run Rate
70 pieces per minute
Total Pieces
22,271 pieces
Reject Pieces
1,423 pieces
The theoretical maximum time that the machines are operational in one shift is 8 hours per day. Planned downtime includes items such as planned maintenance activities; regular, planned meetings (such as team briefs, trade union meetings, etc.); and other planned stoppages (such as public holidays). Unplanned downtime also has to be factored in to get a true measure of the duration of plant operations. Unplanned downtime includes all activities when the machine was not performing its intended function, including breakdowns, material shortages, personnel unavailability, and other unplanned downtime.
The planned production time is the shift length minus breaks, which is 480 - 60, or 420 minutes. The actual running time is the planned production time minus the downtime, which is 420 minutes - 43 minutes, or 377 minutes.
The calculation for availability is:
Availability = [Operating Time / Planned Production Time] =
377 minutes / 420 minutes = 0.898 (89.8%)
Next, the manufacturer considers the performance ratio. The ideal cycle time, or run rate, is the time that a process can be expected to produce output in the optimal situation. It is sometimes called design cycle time. The ideal run rate for this process is 70 pieces per minute.
The calculation is:
Performance = [Pieces Produced / (Operating Time x Target Productivity Rate)]
22,271 / (377 x 70) = 0.844 (84.4%)
The third factor is the quality ratio, which considers quality loss. First, the manufacturer determines the number of good pieces by subtracting the rejected pieces from the total produced.
Good Pieces = [Total Pieces - Reject Pieces] = [22,271 - 1,423] = 20,848 pieces
Now, the manufacturer calculates the quality ratio. The quality rate formula is:
Good pieces / (Total Pieces)
20,848 / 22,271 = 0.936 (93.6%)
Now, to complete the OEE calculation, you multiply the three ratios together to get a final score.
OEE = Availability x Performance x Quality
OEE = 89.8% X 84.4% X 93.6% = 70.9%