The point of any preventive maintenance program is to identify equipment failures in their early stages so you, as a maintenance activity can coordinate the man hours and logistics required to prevent breakdowns. Figuring out the best scheduling intervals for different PM tasks can be difficult if you don't have OEM manuals to get you started. Luckily, there have been studies on this topic and I will share with you what seems to be the best way to start you pm program from scratch.
Step 1: Set a PM to CM Ratio For Your PM Program
The most efficient maintenance activity ever studied, maintained a 6:1 PM to CM ratio when defining PM schedules, meaning one correctable failure was discovered for each PM tasks performed. Here is a good starting point.
To determine preventive maintenance frequency using the 6:1 ratio, take a look at past equipment failures to see if there is a pattern and/or calculate the mean time between failure (MTBF). For example, If it looks like a motor bearing is failing due to brinneling on average every three months, start with a bi-weekly pm schedule for physical inspections or vibration analysis to see if a new bearing or higher quality bearing is needed.
Step 2: Create Your Scheduled Tasks and Procedures (hopefully in a maintenance software program)
At this point, brainstorm a running list of PM tasks that are required for the equipment you are trying to maintain based on your experience with that equipment. Create procedures (here is a form to help you do this) for your tasks to help standardize the process. Then group together like tasks that have the same or similar inspection frequencies (weekly, monthly, yearly etc.) using the 6:1 ratio as a rule of thumb.
Step 3: Test Your Handiwork
Make sure technicians are aware of their daily work loads and have the time to complete PM tasks according to the procedures you have created for them. Staggering PM's will help manage the work load and makes it so your team does not get overwhelmed with monthly PM's on the first day of every month.
Step 4: Analyze the Results.
If you are using maintenance software to manage your preventive maintenance program, analyzing the work histories of you equipment should be easy. Look at the type and frequency of failures that have occurred during the first round of inspections.
If there were no correctable faults detected during the first six PM inspections for any task, it could be because of one or both of these reasons:
- PM procedures are not being followed or the fault detection/assessment abilities of technicians need improvement (start with this).
- PM inspections are scheduled too frequently and need to be extended in order for the onset of a failure to form.
If multiple correctable faults are found within the first six inspections for any task, it could be because of one or both of these reasons:
- PM procedures are lacking the appropriate tasks needed to prevent the fault (revise your procedures based on the root cause of failures).
- PM tasks are too infrequent and their scheduled intervals need to be shortened in order to detect the onset of failure sooner.
Step 5: Tweak The Program.
A good metric to measure the effectiveness of your current ratio is the annual maintenance cost per replacement value of an asset.
It looks like this:
Ideally, you want this percentage to be as low as possible. Lower cost of maintenance means your PM program is working as it should i.e. preventing expensive catastrophic equipment meltdowns. Try different ratios to see which one will give you the lowest annual maintenance costs.