Preventive and Predictive Maintenance Predictive maintenance aims to predict the occurrence of problems so that action can be taken to avert downtime. The most common usage of predictive maintenance is with wear parts. Variations in the key dimensions of wear parts are measured and when they go below a predefined level, the part is replaced. In this way downtime and poor machine performance is averted. There are two stages to predictive maintenance: first identify a characteristic that varies as a component nears its end-of-life and second, define the level at which action should be taken. For example when a bulb is nearing its end of life it may gradually reduce its operating temperature before complete failure. By understanding the failure mechanism and the characteristic associated with its failure, the breakdown can be predicted. The temperature of the bulb at a defined distance is measured at regular intervals and logged. And temperature variations associated with failure are noted. A procedure is set up such that any recurrence of these temperature variations with future bulbs results in the earliest possible replacement of the bulb. There are 5 steps to effective predictive maintenance: Step 1: List the significant functions that constitute the machine: For example; " The loading section " The machine transportation system " The Labelling system " The tooling section " The unloading section Step2: Define the acceptable performance criteria for these functions: For the transportation system, the product must move at a linear speed and no jerky motion is acceptable (it is currently not possible to quantify acceptable tolerances). Step 3: List the consequences of the function failure: Are there safety or economic implications or hidden elements where the consequence of failure may not be immediately detected? For example failure of the transportation system leads to product being scrapped resulting in economic loss. Step 4: List the causes of each functional failure: Elongation of any or the drive belts, wear out within the gear boxes or bearings may cause deterioration in the performance of the transportation system. Step 5: Identify what can be done to prevent failure For the drive chains: " Weekly lubrication extends their life " Weekly inspection of chains for tight spots " Replace drive chains every 6 months For the gear box " Inspect the quality of the lubrication oil every month " Inspect gear box for smooth action " Change the oil at least every 3 months " Replace the gearbox every 2 years " Etc. For the bearings " Replace bearings when there is evidence of damage. Depending on the answers to the analysis at least one of the following four options is taken 1. On-condition task maintenance 2. Scheduled restoration 3. Scheduled discard 4. Default task On condition maintenance means inspecting the equipment components at regular intervals to find and correct potential faults. In the example, on-condition tasks include inspecting the drive chains and the gear box Scheduled restoration involves reworking the equipment components at specified intervals to reduce the potential for failure. The example refers to applying lubrication to the chains and changing the oil in the gearbox at defined intervals. Scheduled discard involves replacing the components at regular intervals in anticipation of failure. Returning to the earlier example, the chains are to be replaced every 6 months and the gearbox, every two years. Default tasks involve either redesigning the machine function if none of the previous tasks can suitably control exposure to failure or alternatively not performing any scheduled maintenance where the cost of preventive maintenance is higher than the cost of rectifying the failure and its consequences. In the above example, bearing are only replaced when there is evidence that they are not working correctly. In each case, the team considers the failure mechanism and the associated reliability data before deciding on appropriate preventive measures. Historical records of machine performance, advice from vendors and feedback from technical staff are recommended to determine the appropriateness of each preventive maintenance task. Predictive and preventive maintenance means intensive analysis of equipment to establish the tasks that will minimise breakdowns. Initially designing preventive maintenance systems is time consuming. The www.passtechnique.com website may help to speed up the process by providing the maintenance team with tools for solving problems. The www.managemaintenance.com website discuses ways of implementing preventive maintenance plans. |
