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Preventive Maintenance
Maintenance and Reliability

Preventive Maintenance

Service the machine before it breaks. Pick the right interval.

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Definition

What is Preventive Maintenance?

Preventive maintenance is scheduled equipment care performed on a fixed time or usage interval, regardless of whether the machine is showing symptoms. The goal is to replace parts and service systems before they fail, on a calendar the shop controls rather than during a breakdown the shop does not. Preventive maintenance trades small planned downtime now for large unplanned downtime later.

Preventive maintenance is the oldest formal maintenance strategy and the first one a small shop should set up. The idea is simple: most equipment failures happen because a known wear part finally gave out, and most of those parts give out around the same interval if you know what to look for. Service the part before that interval and the failure does not happen. Skip the interval and the failure shows up at the worst possible time, usually mid run on a Friday afternoon.

"Five planned hours on a Wednesday beats five unplanned hours on a Friday."

How preventive maintenance works

A preventive maintenance program starts with a list. For each machine in the shop, write down what is on it: motors, bearings, belts, filters, hydraulic lines, electrical contactors, sensors, lubrication points. Then, for each of those parts, write down what wears, how often, and what the early warning signs are. The result is a master task list per machine.

Each task gets an interval. Some are calendar driven: change the air filter every quarter. Some are usage driven: replace the cutting fluid every 500 hours. Some are condition driven, which is where preventive maintenance starts to overlap with predictive maintenance. The interval starts as a best guess from the manufacturer's documentation and gets refined by what the shop actually finds. If the air filter is still clean every quarter, push the interval to six months. If the hydraulic lines are seeping at 1,500 hours, pull the inspection in.

The schedule lives somewhere everyone can see. In a small shop that is usually a whiteboard, a printed calendar, or a basic CMMS. The maintenance lead walks the schedule weekly, books the time, and pulls the operator in when their machine needs an hour of downtime. Findings get logged: what was inspected, what was replaced, what looked off. That log is the engine that tunes the program over the next year.

Where preventive maintenance fits on the shop floor

Picture a 15 person CNC job shop with eight machines, mostly mills and a couple of lathes. The shop has been running on a "fix it when it breaks" model. Breakdowns hit roughly once a week, and each one costs four to twelve hours of recovery plus an emergency parts order. The owner has been told to "set up PM" but does not know where to start.

A practical PM rollout starts with the two machines that have broken most often in the past year. Pull the manuals. Build a one page task list for each, with monthly, quarterly, and annual buckets. Book the first round during the next slow week. Do the work, log what was found, and use the findings to refine the next round. After two months on those machines, expand to the rest of the shop, one machine per week. By the end of the quarter, the shop has a calendar and a binder. By the end of the year, breakdowns are down to one a month and almost always on the parts the shop has not gotten to yet. That is the trade preventive maintenance offers.

Common mistakes with preventive maintenance

  • Copying the manufacturer's schedule and never adjusting it. The manufacturer wrote that schedule for the worst case. Your shop is not the worst case. Tune intervals based on what you find.
  • Doing PM tasks without a log. Without a log there is no learning, and the program decays into a checkbox exercise the third time the maintenance lead changes.
  • Booking PM only when the shop is slow. PM that only happens when there is nothing else to do is not a program. Schedule the time and protect it.
  • Mixing PM with breakdown response. The same person doing both will always prioritize the breakdown and never get to the PM. Carve PM time out separately.
  • Running PM without autonomous maintenance. Most of what PM catches, operators could catch first if they were trained to look. PM should be the deeper second layer, not the only layer.

Preventive maintenance and related Lean tools

Preventive maintenance is a subset of planned maintenance and is usually paired with autonomous maintenance so the daily inspection layer catches what the monthly PM schedule does not. Its data driven cousin is predictive maintenance, which uses sensor signals instead of a calendar. PM exists in opposition to breakdown maintenance, the "fix it when it dies" default that PM replaces.

Common questions

The questions we hear most about this term.

How does preventive maintenance work?
Preventive maintenance works on intervals. Each machine has a list of tasks tied to time, run hours, or cycle count: change the hydraulic filter every 2,000 hours, grease the linear rails every 90 days, replace the contactor every five years. The shop schedules the work into a quieter shift or a planned changeover window. The interval comes from the manufacturer's recommendation at first and gets adjusted based on what the shop actually sees over time. Done right, the work is boring, predictable, and the equipment is rarely down on a surprise.
How is preventive maintenance different from planned maintenance?
Planned maintenance is the broader category: any maintenance scheduled in advance, including overhauls, predictive work, and capital projects. Preventive maintenance is a subset, specifically the time or usage based work meant to stop a known failure mode. Every preventive task is planned, but not every planned task is preventive. The terms get used interchangeably in conversation, and that is fine as long as the schedule itself is real.
Is preventive maintenance the same as predictive maintenance?
No. The trigger is the difference. Preventive maintenance is triggered by time or usage: the calendar says it is time. Predictive maintenance is triggered by data: a vibration sensor or oil analysis says the part is starting to fail. Preventive is older, simpler, and works for most equipment in a small shop. Predictive requires sensors, data, and the analysis skill to use them. The two coexist in most mature programs, with predictive applied to the few critical machines where the data investment pays off.
What are common mistakes with preventive maintenance?
The biggest mistake is over servicing equipment that does not need it. A weekly inspection on a machine that runs four hours a week is waste. The second is under servicing equipment that does. The interval should match how hard the machine actually runs, not the manufacturer's generic spec. The third is doing the work without recording what was found. Preventive maintenance without a log is a one shot event; preventive maintenance with a log lets the shop tune the intervals over time.
What does preventive maintenance look like on the shop floor of a small contract shop?
A whiteboard with a calendar grid, a binder with one page per machine, and a person whose job it is to walk that calendar every Monday. On the binder pages: monthly tasks, quarterly tasks, annual tasks. On the calendar: which machine gets what attention this week, and which operator is assisting. A 20 person machine shop can run a competent PM program with two hours of paperwork a week. The discipline is doing the work on Monday whether the shop is busy or not.

Ditch the whiteboards and spreadsheets.

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