Preventive Maintenance Fundamentals

Preventive maintenance (PM) is planned work done on equipment before it fails: replacing wear parts, adding lubricant, tightening connections, and fixing small problems on a schedule. The main goal is to reduce unplanned downtime by dealing with issues before they cause a breakdown. A good PM program also makes the workplace safer, because many injuries and fires start with equipment that was left to fail.

What You Will Be Able to Do

• Tell preventive, predictive, and reliability-centered maintenance apart.
• Rank equipment by criticality and build a PM schedule from real sources.
• Inspect belts, couplings, bearings, hydraulic and pneumatic systems, and coolant.
• Track PM in a CMMS and write a work order record that is useful later.
• Name the inspections OSHA requires and work safely with E-stops, lockout, and stored energy.

Why PM Pays Off

Reactive maintenance, also called run to failure, means fixing equipment after it breaks. It looks cheaper because nobody spends time on scheduled tasks, but the costs show up elsewhere:

• Unplanned downtime. A breakdown stops work with no warning, and everything downstream waits.
• Emergency repairs. Rush parts and overtime cost more than planned work.
• Collateral damage. A failed bearing can take the shaft, housing, and seal with it.
• Safety risk. Failures can cause fires, chemical releases, and mechanical injuries.

An often-quoted industry estimate says every $1 spent on PM saves $4 to $5 in avoided breakdowns, lost production, and quality defects. Treat that as a rule of thumb, not a measured law. Your real return depends on your equipment and how much a stoppage costs.

Maintenance Strategies

The difference between the middle two matters. Preventive work is scheduled by time or usage, whether or not the part needs it yet. Predictive work is based on condition data such as vibration, temperature, or oil analysis, so you act when the data says to.

Reliability-Centered Maintenance (RCM) goes further than basic PM. Basic PM applies a schedule to a machine. RCM analyzes how each component can fail, what each failure would do, and then chooses the right strategy for that component. Sometimes the right answer is run to failure. Most plants start with PM and add the rest as they mature. The goal is not more maintenance. It is the right maintenance at the right time.

Ranking Equipment by Criticality

You will never have the people and hours to treat every asset the same. A criticality ranking sets priorities based on what happens if the equipment fails. It decides what gets the best spare parts, the tightest inspection, and the first response.

Many programs use ABC analysis:

Worked example: score each item from 1 to 5 for safety, production, quality, and repair difficulty, then add them up.

Set your own cutoffs, for example 14 and up for A. Many sites add one override rule: anything with a top safety score is class A, whatever its total.

Building a PM Schedule

A PM schedule says what to inspect, clean, lubricate, and replace on each machine, and how often. Build it from these sources:

1. The equipment manual. The manufacturer's recommendations are your starting point.
2. Breakdown history. If a part fails around 8 months, schedule its replacement at 6.
3. Operator input. Operators know what rattles, drifts, and leaks.
4. Industry standards. NFPA 70B, for example, covers electrical maintenance.
5. Regulations. Some inspections are required by law, as covered below.

Then adjust with data. A component that is always in perfect condition at its PM may be able to go longer, with approval. One that fails between PMs needs a shorter interval or a better method. When a machine is modified or runs at a new speed or load, review its PM tasks.

The daily walkdown is the cheapest inspection you have. Listen as you walk. Grinding, squealing, or knocking are early signs of mechanical wear, often long before anything shows on a gauge.

Inspecting Common Components

Lock out the machine before you touch drives, guards, or moving parts.

Belts

V-belts should be tight enough not to slip under peak load and no tighter. A common target is a deflection of 1/64 inch per inch of span, pressed at the center of the span. A 40-inch span should deflect 40 ÷ 64 = 5/8 inch. Use a tension gauge with the force value from the belt maker, because deflection alone says nothing about how hard you pressed.

Tension is only half the check. Look at the belt itself. Cracks on the inner surface mean the rubber has aged, so replace the belt even if the tension is perfect. Glazing, fraying, and missing chunks also mean replacement. Replace matched belts as a set and check that the sheaves line up.

Couplings and Alignment

Uneven wear on a rubber coupling element, or rubber dust under the coupling, points to misalignment. Replacing the element alone only restarts the clock. Realign the shafts with dial indicators or a laser alignment tool, and check for a soft foot, where a machine foot does not sit flat on its base.

Bearings

Warning signs are rising vibration, heat, and noise. If a bearing shows early wear and is not getting worse quickly, plan the replacement for the next scheduled downtime. Order the part, schedule the labor, and keep watching it. An emergency shutdown is for a bearing that is failing fast or creating a hazard.

Bearings are also the most common failure point in electric motors. Published motor surveys rank them ahead of windings, usually because of poor lubrication or contamination.

Hydraulic Systems

Check fluid condition, not only the level.

Warning: Never feel for a hydraulic leak with your hand. A pinhole stream can inject fluid through your skin, and that is a medical emergency. Use a piece of cardboard, and keep your body clear.

Pneumatic Systems

Shop air commonly runs at 80 to 120 psi. Check that regulators hold their setting, filter bowls are clean, and lubricators are filled where used. Drain water from filters, traps, and receivers, because water ruins valves and cylinders. Listen for leaks, which waste energy all day. Air receivers need a working drain and a tested safety valve.

Coolant Concentration

Machines using water-soluble metalworking coolant need a concentration check with a refractometer. A typical range is 5 to 10 percent, but the product data sheet sets the target. The reading is not always the concentration. Multiply it by the product's refractometer factor. A reading of 3.0 with a factor of 2.5 is 3.0 x 2.5 = 7.5 percent.

Too lean, and you get rust, poor tool life, and bacteria. Too rich, and you get foam, residue, and skin irritation. Top up with premixed coolant, not plain water.

Motor Insulation

An annual insulation test with a megohmmeter, or megger, measures how well a motor's windings are insulated from its frame. Only qualified people should run this test, with the motor locked out, because the meter applies high voltage. The winding must be discharged afterward.

The classic rule of thumb is a minimum of 1 megohm per 1,000 volts of rating, plus 1 megohm. A 460-volt motor works out to 0.46 + 1 = about 1.5 megohms. Treat that as a floor. Later editions of IEEE 43 set higher minimums for many machines, such as 5 megohms for most windings under 1,000 volts and 100 megohms for most modern form-wound stators. Follow your plant's procedure if it is stricter.

The trend matters more than any one reading. A motor that fell from 1,200 to 60 megohms since last year has a problem, even though it clears the floor. Readings change with temperature and moisture, so compare them under similar conditions or use corrected values.

CMMS and PM Compliance

A CMMS, or Computerized Maintenance Management System, is the software that runs a PM program. It generates work orders from calendar time, run hours, or meter readings. It tracks what was done, by whom, and with which parts, and it stores equipment records and spare parts data.

Three measures come out of it:

• PM compliance is the percentage of PM work orders finished on time. If 188 of 200 were on time, compliance is 188 ÷ 200 = 94 percent. The target for a healthy program is 90 percent or higher. Below about 80 percent, the program is falling behind and reliability will suffer.
• MTBF, or Mean Time Between Failures, is run time divided by the number of failures. 720 hours with 3 failures gives 240 hours.
• MTTR, or Mean Time To Repair, is repair time divided by the number of repairs. 6 hours over 3 repairs gives 2 hours.

Documentation

The most useful parts of a work order are the measurements taken, the conditions found, and the corrective actions performed. Also record the date, who did the work, the parts used, and any follow-up needed.

"PM complete" tells the next person nothing. Compare it with this: "Belt tension 5/8 inch at 40-inch span, in spec. Hairline cracks on inner surface, replaced belt set. Drive sheaves aligned. New belts to be rechecked at 24 hours."

Good records prove compliance in audits, support changes to intervals, help diagnose repeat problems, and justify replacing equipment that costs too much to keep running. A finding with no follow-up work order is a known problem left to fail.

Regulatory Inspections

OSHA has no single PM standard, but several rules require inspection of specific equipment. These items get extra attention because failure can be catastrophic, causing deaths, injuries, or major property damage. State plans may add to these rules.

Boilers and other pressure vessels are also covered by state laws, often with inspections by a licensed inspector. NFPA 70B, the electrical maintenance standard, became a mandatory-language standard with its 2023 edition. It binds a site when a jurisdiction, contract, or insurer adopts it, so ask which edition your site follows.

Stop Systems and Lockout During PM

Lockout/tagout (LOTO) isolates every energy source before maintenance begins, so nobody can be hurt by a machine starting or releasing energy. Only an authorized employee, someone trained to apply locks for servicing that machine, does this work.

An E-stop is not isolation. Pressing an E-stop stops motion, but power is still present upstream, and it may not bring the machine to a safe state. OSHA's definition says push buttons, selector switches, and other control-circuit devices are not energy isolating devices. Shut down with the normal stop, then lock out at the disconnects and valves.

Test E-stops on a schedule. OSHA sets no single interval. A common practice is at least monthly, with some systems checked weekly or at the start of each shift. Press each button, confirm the machine stops, and confirm it will not restart until reset. If one fails, tag the machine out of service and report it.

Deal with stored energy. After locks go on, OSHA requires stored energy to be relieved, disconnected, restrained, or otherwise made safe. That covers springs, capacitors, compressed air, and hydraulic accumulators.

Warning: Gravity is stored energy. Closing the pump and bleeding the pressure is not enough on a hydraulic press or lift. A raised ram can drift down through leaking seals. Block or pin it mechanically before anyone reaches under it.

Common Mistakes

Check Your Understanding

1. Your area had 150 PM work orders due last month, and 126 were done on time. How are you doing?

126 ÷ 150 = 84 percent. That is above 80 percent but below the 90 percent target, so the program is starting to fall behind. Find out what is getting skipped and why.

2. A hydraulic lift table is raised for a seal change. The pump is locked out and the gauge reads zero. Can you reach under it?

No. Pressure is gone, but gravity is not. Lower it fully or block it mechanically first.

3. A 480-volt motor read 900 megohms last year and reads 45 megohms today. The rule-of-thumb floor is about 1.5 megohms. Is it fine?

No. It clears the floor, but a drop that large is a warning. Have it investigated for moisture, contamination, or insulation breakdown, using your plant's limits.

4. A filter has been nearly clean at every monthly check for 18 months. What should you do?

Use that record to propose a longer interval, then get it approved and update the schedule in the CMMS. Do not just start skipping it.

Key Takeaways

• PM reduces unplanned downtime by fixing issues before failure.
• Preventive work follows a schedule, and predictive work follows condition data. RCM picks the right strategy for each component.
• Rank equipment by the impact of failure, so class A items get attention first.
• Record measurements, conditions found, and actions taken, then follow up.
• Target 90 percent PM compliance or better.
• An E-stop is not isolation. Lock out, release stored energy, and block anything gravity can move.