What Is Total Productive Maintenance?
What Is TPM?
TPM stands for Total Productive Maintenance. But what is TPM?
The concept of Total Productive Maintenance or TPM can be traced back to Nippondenso of the Toyota group. Maintenance is focused on preventing shutdowns and defects due to equipment failure. Preventative maintenance in TPM is seen as an investment, since it extends equipment’s operational life and reduces the risk of unplanned maintenance due to repairs.
Types of Maintenance Under TPM
According to “Smart Process Plants” by Bagajewicz, failure can be broken down into two basic types; sudden, major malfunctions and the slow deterioration due to wear and tear.
Maintenance can be broken down into corrective and preventative maintenance. Corrective maintenance is when something broken is repaired. Preventative maintenance includes tasks that should reduce the risk of corrective maintenance. Preventative maintenance encompasses tasks like changing the oil, replacing worn belts, checking and calibrating equipment, replacing filters and adjusting equipment.
In an ideal world, preventative maintenance prevents sudden failures until equipment is replaced when it wears out. A lack of preventative maintenance increases the risk of corrective maintenance. For example, motors running on oil that is becoming non-viscous and full of metal shavings may seize up and burn out. Or worn belts break, fly off machinery and damage other components.
Maintenance can also be divided into reactive and proactive maintenance. Reactive maintenance refers to fixing that which is broken or damaged. Proactive maintenance is maintenance taken before something breaks. Proactive maintenance may refer to preventative maintenance or catching problems early and fixing them, such as repairing a gear with one broken tooth before others break and affect the entire gear box.
How Is Maintenance Scheduled in TPM?
TPM uses predictive models, maintenance logs, employee reports of problems, system monitoring and scheduled work to plan repairs and maintenance due to wear and tear and minimize the risk of sudden, major malfunctions.
TPM seeks to minimize both the number of repairs and cost of maintenance. Unfortunately, in the real world, there are many constraints on maintenance. Maintenance staff must balance the cost of maintenance personnel with the money they save by preventing shutdowns and sudden failures. Maintenance schedules need to include slack for unexpected repairs. Not all repairs and maintenance are done on schedule, and tasks may slide to a later date.
TPM does not eliminate system downtime. Businesses do have to shut down equipment periodically for maintenance and planned repairs. However, the benefit of TPM is a serious reduction if not elimination in unplanned downtime, which is far more costly than planned outages.
Proactively replacing worn parts may necessitate a plant shutdown, but it is better to do so when there are not major production quotas due and employees get the day off than shutting down a plant while everyone stands around and then waits while the necessary parts are ordered.
TPM Tools
Reliability centered maintenance uses risk analysis to plan maintenance to support TPM. Those pieces of equipment whose shutdown stops the plant get priority. Or the equipment whose function is essential to the safety of workers gets priority. Maintenance models estimate the necessary frequency for maintenance; models can assume that maintenance is more likely as equipment ages or the number of hours it runs goes up.
Perfect maintenance returns equipment to as good as new condition. However, we don’t live in a perfect world. This means that maintenance affects the cumulative failure probability curve used to predict equipment failure and plan preventative maintenance. In the real world, maintained equipment has a cumulative failure probability curve below that of new equipment but better than that of unmaintained equipment. The preventative maintenance interval should be a fraction of the mean time between failures estimate.
FMEA is commonly used in reliability centered maintenance, as is fault tree analysis. HAZOP is used for hazard evaluations, to determine which equipment gets higher priority due to the hazards created when it fails. Total Productive Maintenance is more easily implemented with Computerized Maintenance Management Software or CMMS software. CMMS software is also called Equipment Management Information Systems or EMIS.
Sensors can play a part in Total Productive Maintenance. Vibration sensors record abnormal or excessive vibrations that indicate something is wearing out faster than expected or that a problem has occurred. Thermal sensors may reveal equipment that is overheating before it catches fire. Chemical analysis of oil can show unusual chemicals that may reveal leaks in the production equipment, carbonization of plastic before something catches fire or that the wrong oil has been used. Sonics may reveal pitting and wear before it is visible to the naked eye or internal cracks before metal assemblies break.
Relationship between TPM and Continuous Process Improvement Methodologies
Total Productive Maintenance seeks to use continuous process improvement methodologies like Six Sigma to improve processes and reduce downtime. Quality personnel monitoring the production line should know the variability caused by each operation and report increasing variability to maintenance staff so that they can check out the equipment and perhaps repair it before out of spec product is made.
Lean Management principles can be used to optimize the inventory of spare parts and maintenance supplies; the cost of these items needs to be kept down, but there must be at least a 95% chance that necessary items for maintenance and repair are on hand when the maintenance is planned or repairs are necessary. Or six sigma quality levels can be demanded of the inventory management system, so that maintenance personnel are never without necessary supplies.
Maintenance work orders should be designed to capture the details of break downs and observations documented at the time of repair. What is breaking down? When did it fail, and why? How long did it take to repair? This information can be used for fault tree analysis to identify the root causes. And the information on the labor and supplies required to perform repairs can be used to better accurately predict and plan future maintenance.
References
1. “Smart Process Plants” by Miguel Bagajewicz
2. “Certified Six Sigma Black Belt Handbook, Second Edition” by T.M. Kubiak and Donald Benbow
3. “Total Productive Maintenance” by Terry Wireman
