Back to Maintenance Engineering

Study notes

LearnExamsInterviews

Qwestrum Engineering360 · Mechanical Engineering · Maintenance Engineering

Preventive Maintenance

Preventive maintenance (PM) services equipment on a planned schedule (time- or usage-based) to prevent failures before they occur, trading planned downtime for fewer breakdowns, per maintenance-engineering texts.
Exam tip: keep SI units consistent end-to-end, write the governing relation symbolically before substituting, and sanity-check magnitude and sign.

Key formulas & points

Skim these first — then read the full notes below.

  • Time-based (TBM) vs usage-based maintenance
  • Lubrication, inspection, adjustment, replacement schedules
  • PM reduces unexpected breakdowns but may over-maintain

Topic details

Introduction

Preventive maintenance replaces reactive breakdown repair with planned servicing, reducing unexpected failures and extending equipment life. Indian maintenance-engineering courses contrast it with breakdown and predictive strategies.

Scope in B.Tech and GATE syllabus

PM tasks — lubrication, inspection, adjustment, and scheduled replacement of wear parts — are triggered by calendar time or usage (running hours, cycles). The interval is set from manufacturer data and failure history to catch wear before it causes failure.

Why this topic matters in practice

The trade-off is planned downtime and part cost against the higher cost and unpredictability of breakdowns. Optimising the PM interval to minimise total cost is the exam focus.

Key relations & formulas

Formulas (Indian textbook notation)

  • TBMinterval=f(manufacturerspec,operatinghours)TBM interval = f(manufacturer_{spec}, operating_{hours})

Formulas (Indian textbook notation)

  • Failurerateλ(t):bathtubcurve−early,useful,wear−outFailure rate \lambda(t): bathtub curve - early, useful, wear-out

Formulas (Indian textbook notation)

  • MTBF = total_{operating}_\frac{time}{number_{of}}_failures

Formulas (Indian textbook notation)

  • PM cost = Σ(scheduled_{task}_cost) per period

Notation and sign conventions

Relation 1 —
TBMinterval=fTBM interval = f

Formulas (Indian textbook notation)

  • TBMinterval=f(manufacturerspec,operatinghours)TBM interval = f(manufacturer_{spec}, operating_{hours})
Write this relation with symbols exactly as in Maintenance Engineering — SRK Rao before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 2 —
FailurerateλFailure rate \lambda

Formulas (Indian textbook notation)

  • Failurerateλ(t):bathtubcurve−early,useful,wear−outFailure rate \lambda(t): bathtub curve - early, useful, wear-out
Write this relation with symbols exactly as in Maintenance Engineering — SRK Rao before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 3 —
MTBF = total_{operating}_\frac{time}{number_{of}}_failures

Formulas (Indian textbook notation)

  • MTBF = total_{operating}_\frac{time}{number_{of}}_failures
Write this relation with symbols exactly as in Maintenance Engineering — SRK Rao before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 4 —
PMcost=ΣPM cost = Σ

Formulas (Indian textbook notation)

  • PM cost = Σ(scheduled_{task}_cost) per period
Write this relation with symbols exactly as in Maintenance Engineering — SRK Rao before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Fundamentals and definitions

Preventive maintenance is scheduled intervention independent of the equipment's actual condition: fixed-time or fixed-usage servicing that assumes wear accumulates predictably.

Governing relations in practice

The optimal PM interval balances two opposing costs: too-frequent PM wastes labour, parts, and uptime; too-infrequent PM risks costly breakdowns. Total cost per unit time is minimised where the marginal PM cost equals the marginal breakdown-risk saving.

Design and analysis considerations

PM works best where failures are age-related (wear-out region of the bathtub curve); it is ineffective or wasteful for random (constant-hazard) failures, where condition-based (predictive) maintenance is better.

Advanced theory and extensions

A PM programme includes task lists, schedules, spare-parts planning, and record-keeping that feeds interval optimisation. Its benefits — fewer breakdowns, safer operation, longer asset life, and plannable downtime — must be weighed against its recurring cost. Setting and optimising intervals is the practical skill.

Assumptions and validity limits

State assumptions explicitly before using any relation for preventive maintenance — steady state, uniform properties, linear elastic material, ideal gas, incompressible flow, etc., as applicable.
Wrong assumptions invalidate the entire solution even when the formula is correct. In Maintenance Engineering viva and GATE descriptive questions, listing valid assumptions often earns separate marks.

Step-by-step problem approach

1. Read the question and list given data with SI units (common in Maintenance Engineering papers).
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to preventive maintenance.
4. Use equation 1:
TBMinterval=fTBM interval = f
.
5. Use equation 2:
FailurerateλFailure rate \lambda
.
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.

Applications & exam relevance

Preventive Maintenance appears in process plants and utilities. In Indian mechanical curricula this topic is tested because it connects theory to reliability and upkeep of plant equipment.
GATE and semester exams often combine preventive maintenance with earlier units — revise prerequisites before attempting mixed problems.
Industry interview panels sometimes ask: "Where did you use preventive maintenance?" — answer with a lab, mini-project, or plant visit example if possible.

Common mistakes in exams

• Applying time-based PM to random (constant-hazard) failures where it does not help
• Setting intervals from calendar time when usage (hours/cycles) is the real driver
• Ignoring the cost trade-off and over-maintaining equipment
• Confusing preventive (scheduled) with predictive (condition-based) maintenance

Quick revision checklist

Before attempting preventive maintenance problems, confirm you can:
1. Time-based (TBM) vs usage-based maintenance
2. Lubrication, inspection, adjustment, replacement schedules
3. PM reduces unexpected breakdowns but may over-maintain
Revise the solved examples in Maintenance Engineering — SRK Rao and one previous-year GATE or university paper for this unit.

Worked examples

Try the problem first — open the solution when you are ready to check.

PM interval cost trade-off

Problem

Explain why halving the PM interval does not necessarily reduce total maintenance cost.

Solution

More frequent PM cuts breakdown risk but adds labour, parts, and downtime cost; below the optimum interval the extra PM cost outweighs the breakdown saving, so total cost rises.

Conceptual check — Preventive Maintenance

Problem

In a Maintenance Engineering semester or GATE paper you are asked: "State the main assumption, the governing relation, and one practical consequence of preventive maintenance." What should a complete answer include?

Practice questions

Most-asked interview and GATE questions for this topic — expand any item for a model answer.

  1. 1
    What is Preventive Maintenance, and why does it appear in B.Tech / GATE syllabi?

    Model answer

    Preventive maintenance (PM) services equipment on a planned schedule (time- or usage-based) to prevent failures before they occur, trading planned downtime for fewer breakdowns, per maintenance-engineering texts.
  2. 2
    State the relation TBM interval = f and name each symbol.

    Model answer

    The governing relation is TBMinterval=fTBM interval = f. Write every symbol with SI units before substituting numbers.
  3. 3
    State the relation Failure rate λ and name each symbol.

    Model answer

    The governing relation is FailurerateλFailure rate \lambda. Write every symbol with SI units before substituting numbers.
  4. 4
    State the relation MTBF = total_operating_time/number_of_failures and name each symbol.

    Model answer

    The governing relation is MTBF = total_{operating}_\frac{time}{number_{of}}_failures. Write every symbol with SI units before substituting numbers.
  5. 5
    State the relation PM cost = Σ and name each symbol.

    Model answer

    The governing relation is PMcost=ΣPM cost = Σ. Write every symbol with SI units before substituting numbers.
  6. 6
    Explain: Time-based (TBM) vs usage-based maintenance

    Model answer

    Time-based (TBM) vs usage-based maintenance — state the assumption range and one exam trap linked to this point.
  7. 7
    Explain: Lubrication, inspection, adjustment, replacement schedules

    Model answer

    Lubrication, inspection, adjustment, replacement schedules — state the assumption range and one exam trap linked to this point.
  8. 8
    Explain: PM reduces unexpected breakdowns but may over-maintain

    Model answer

    PM reduces unexpected breakdowns but may over-maintain — state the assumption range and one exam trap linked to this point.
  9. 9
    How would you correct this error in a viva: Applying time-based PM to random (constant-hazard) failures where it does not help?

    Model answer

    Identify the wrong assumption or unit mix-up, rewrite the correct relation, and recompute with a one-line sanity check.
  10. 10
    How would you correct this error in a viva: Setting intervals from calendar time when usage (hours/cycles) is the real driver?

    Model answer

    Identify the wrong assumption or unit mix-up, rewrite the correct relation, and recompute with a one-line sanity check.
  11. 11
    How would you correct this error in a viva: Ignoring the cost trade-off and over-maintaining equipment?

    Model answer

    Identify the wrong assumption or unit mix-up, rewrite the correct relation, and recompute with a one-line sanity check.
  12. 12
    How would you correct this error in a viva: Confusing preventive (scheduled) with predictive (condition-based) maintenance?

    Model answer

    Identify the wrong assumption or unit mix-up, rewrite the correct relation, and recompute with a one-line sanity check.

Exams & GATE

  • 1
    SRK Maintenance Engineering — PM schedule from equipment criticality.
  • 2
    Avoid: Applying time-based PM to random (constant-hazard) failures where it does not help
  • 3
    Avoid: Setting intervals from calendar time when usage (hours/cycles) is the real driver
  • 4
    Avoid: Ignoring the cost trade-off and over-maintaining equipment

📖 Standard books (India)

  • Maintenance Engineering — SRK Rao

    Read: Syllabus unit

    Reliability, RCM, and maintenance planning

Explore related topics

See real mechanical engineering careers

After exams and interviews, see how engineers actually built careers — milestones and decisions from people in the field.

Browse Mechanical Engineering journeysPlan your path