Calendar Based Maintenance: Last Due vs Last Done

Planned preventive maintenance (PPM), more commonly referred to as simply planned maintenance (PM) or scheduled maintenance, or any other acronym referring to Preventive Maintenance is any variety of scheduled tasks, routines, lubrications, inspections and adjustment on a set frequency. These tasks or instructions are generally based on a review of the Failure Modes and Effects Analysis of the Asset. The scheduled visit is typically done by a maintenance technician with a qualified skill set to complete the pre-defined list with the result being that the equipment, item on a piece of equipment or device attached to a process is operating within certain parameters. Thus ensuring that unscheduled breakdowns, shutdown for emergency repairs, or deviation from design throughput and quality is avoided.

SOMAX allows for these inspections to be done in multiple ways, with multiple results.

Calendar Based Last Due Scheduling

This is the most reliable and predictable way to schedule preventive maintenance through calendars ensuring the defined failure modes frequencies are complied with. Simply put, the user determines a frequency in days or months that the PM job should be performed. Once those numbers of days or months pass they can run the Work Order Generator to automatically create work orders identical to the PMs, meaning the technician, task, attachments, and other information will transfer.

• The advantage of this process is that a Preventive Maintenance schedule will NEVER be missed or allowed to lapse. Preventive Maintenance work orders will be generated on the Schedule that they were designed with, ensuring the defined failure modes frequencies are complied with.

• The disadvantage to this process is a duplicate work order may be generated if Maintenance Management is not proactive in monitoring Overdue Work Orders and allows Preventive Maintenance work orders to run past there Scheduled or Required Dates. 

Calendar Based Last Done Scheduling

This is an alternate way to schedule preventive maintenance through calendars. Similarly, the user determines a frequency in days or months that the PM job should be performed. However, when the work order generator runs after that number of days or months has passed, it will check to see if the last generated work order has been performed. If it has not, the system will stop and ignore this new work order until the last one that was due has been performed.

• The advantage of this process is that a Preventive Maintenance schedule will only generate once the current work order has been completed. Duplicate work orders will not be generated.

• The disadvantage to this process is that the failure modes that Preventive Maintenance work orders are meant to find on their pre-defined schedules may be missed. Maintenance Management MUST Closely monitor Preventive Maintenance Schedule and Required dates to insure that failures do not occur because work orders were not completed in their scheduled or required date ranges. This also allows for a greater chance of failure as the failure mode frequencies defined for the PM may not be observed.
  

Advantages of Both Processes

• Easier for planning as the system does all the scheduling, all attachments and other information from the PM job will transfer to the work order
• Costs are distributed evenly, and it is easy for management to determine labor requirements on a predefined basis. Preventive Maintenance labor leveling can be accomplished
• There are no initial startup costs for instrumentation, IoT devices, networking or PLC driven supervision of equipment.
• Follows predefined work based on Failure Mode and Effects analysis of your Assets in their current operating context.

Disadvantages of Both Processes

• May require skill levels not easily found
• Requires honesty from users who complete the work orders
• Less reliable than PLC fault reporting
• May create high cost maintenance due to excessive lubricant usage, parts change out and replacement of non-failed parts if the PM schedules are not reviewed properly to determine if they are correct in the current operating context.
• May create training and labor costs as Preventive maintenance should be done on a Failure Mode and Effects analysis. This ensures that your PMs are defined for work that needs to be done to prevent failures not just to replace parts.