May 1, 2004
By Darren Dolcemascolo
Often an overlooked component of a lean manufacturing program, proper equipment maintenance is a key to lean manufacturing success. Continuous flow manufacturing will not allow for frequent, unplanned equipment down-time. TPM (Total Productive Maintenance) is an excellent method for meeting the demands continuous flow manufacturing places on equipment. TPM does the following:
It increases OEE (Overall Equipment Effectiveness) using improvement activities.
It establishes an autonomous maintenance program performed by equipment operators.
It establishes a planned maintenance system.
It requires training to improve operation and maintenance skills.
It institutes a system for MP (maintenance prevention) design and early equipment management.
Increasing Overall Equipment Effectiveness
TPM is aimed at eliminating the so-called “six big losses:”
Setup and Adjustment Losses
Idling and Minor stoppage losses
Quality defects and rework
Start-up/yield losses (reduced yield between machine start-up and stable production)
Overall equipment effectiveness (OEE) is the key metric in determining how well equipment is performing with regards to the big six losses. OEE measures equipment effectiveness in terms of availability, performance, and product quality. Availability tells us what percentage of time the equipment is actually running when we need it. It is calculated as follows:
Availability = Operating Time / Loading Time X 100%
Operating Time = Loading Time – Downtime
Loading Time: The daily or monthly time available for operation minus planned stops (breaks in the production schedule, stops for planned maintenance, meetings, etc.)
Downtime: total time for unscheduled stops (breakdowns, re-tools, adjustments)
Performance is based on two factors: Operating Speed Rate and Net Operating Time. The Operating Speed Rate tells us how fast a machine is running compared to its designed/ideal speed. The Net Operating Time is the time during which equipment is being operated at a constant speed within a specified period. Performance is calculated as follows:
Performance Rate = Net Operating Time X Operating Speed Rate
Net Operating Time = (units of output x actual cycle time per unit) / (loading time – downtime) X 100%
Operating Speed Rate = ideal cycle time / actual cycle time per unit X 100%
Quality rate is simply the rate of quality products or 100% - defect rate.
Finally, we calculate OEE:
Overall Equipment Effectiveness (OEE) = Availability X Performance Rate X Quality Rate
After the above metrics are calculated for each piece of equipment, equipment improvement project teams determine which losses have the greatest impact on equipment effectiveness, and then prioritize improvement efforts accordingly.
Autonomous Maintenance Program
An autonomous maintenance program stabilizes equipment and halts accelerated deterioration. The program makes operators responsible for cleaning and inspection, lubrication, precision checks, and other light maintenance tasks. In carrying out these activities, operators learn more about their equipment and become better equipped to detect problems early. To implement such autonomous maintenance, operators are systematically trained in a step-by-step program.
Planned Maintenance System
Planned maintenance improvement is led by the maintenance department. The maintenance department will handle all of the planned maintenance tasks that are beyond the scope of the autonomous maintenance program. These are tasks that require special skills, significant disassembly, special measuring techniques and tools, etc. As equipment operators improve their skills, the maintenance group will perform fewer and fewer planned maintenance activities and will focus their efforts on improvements designed to reduce the maintenance required on equipment.
Maintenance Prevention (MP) Design and Early Equipment Management
MP Design involves discovering weak points in currently used equipment and feeding back this information to equipment design engineers. Similar to design for manufacturability, MP design takes the following factors into consideration:
Ease of autonomous maintenance (operator maintenance)
Ease of operation
MP Design can be applied to develop criteria for selecting “off-the-shelf” equipment as well.
Early Equipment Management is a system for dealing with problems that surface during test-running, commissioning, and start-up of new equipment. During this period, production and maintenance engineering people must correct problems caused by poor selection of materials at the design stage, errors occurring during fabrication of the equipment, or installation errors. In an ideal world, Early Equipment Management should not be very complicated (particularly if MP Design is properly applied at the design stage.)
Each of the elements of TPM are significant parts of a foundation for lean manufacturing. Most importantly, they work together to increase Overall Equipment Effectiveness (OEE). Without sufficiently high OEE, lean success becomes much more difficult to achieve.
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