Grease, oil, viscosity, bearing lubrication, and contamination explained for the Red Seal Millwright exam — with failure analysis and real maintenance scenarios.
Most mechanical failures trace back to lubrication — wrong lubricant, wrong amount, wrong interval, or contamination. The Red Seal exam tests lubrication knowledge because it's one of the clearest indicators of whether a millwright understands preventive maintenance or just reacts to breakdowns.
Lubrication questions appear across multiple NOA blocks: bearings, gearboxes, couplings, hydraulics, and general maintenance procedures all have lubrication components. This guide covers what the exam expects you to know.
A lubricant does five things. The exam won't ask you to list these — but understanding them tells you why lubrication failures cause the failures they do.
Reduces friction
Separates moving surfaces to minimize metal-to-metal contact.
Reduces wear
Protects surfaces during start-up and boundary conditions.
Dissipates heat
Carries heat away from the contact zone.
Prevents corrosion
Forms a protective film on metal surfaces.
Excludes contaminants
Grease in particular acts as a seal against dirt and moisture.
Oil is a liquid lubricant — petroleum-based (mineral oil) or synthetic. It flows, circulates, and can be filtered and cooled, which makes it ideal for high-speed applications and enclosed systems like gearboxes.
Mineral Oil
Refined from crude oil. Cost-effective, widely used. Degrades faster than synthetics at high temperature.
Synthetic Oil
Engineered base stocks (PAO, ester, etc.). Better performance at temperature extremes, longer service life, higher cost.
Key oil properties:
Grease is oil held in a thickener matrix — typically a metallic soap (lithium, calcium, sodium) or non-soap thickener (polyurea, bentone). The thickener holds the oil in place and releases it under pressure and heat.
Components of grease:
Grease consistency (NLGI grade):
| NLGI Grade | Consistency | Typical Application |
|---|---|---|
| 000, 00 | Semi-fluid | Enclosed gear drives |
| 0, 1 | Soft | Centralized lubrication systems |
| 2 | Medium | Most bearing applications (standard) |
| 3 | Firm | High-temperature, high-speed |
| 4, 5, 6 | Very firm/hard | Specialty, open gears |
EXAM TIP
NLGI 2 is the default. If the exam doesn't specify, NLGI 2 is the correct general-purpose bearing grease grade.
NLGI Grease Grade Selector
Tap a grade to see consistency and application
NLGI 2
MediumNLGI 2 is the most common grade for general-purpose bearing lubrication. Lower grades flow better in centralized systems; higher grades stay in place at high temperatures.
Viscosity is a lubricant's resistance to flow. Too low — the oil film breaks down under load. Too high — excessive heat from fluid friction, poor circulation, hard starting.
ISO Viscosity Grade (ISO VG)
Standard for industrial oils. The number is the kinematic viscosity in centistokes (cSt) at 40°C.
Common grades: ISO VG 32, 46, 68, 100, 150, 220, 320, 460, 680
Higher number
Thicker oil — better for high load, low speed
Lower number
Thinner oil — better for high speed, light load
SAE grades
Used for engine and gear oils (SAE 30, SAE 90, etc.). Not directly interchangeable with ISO VG — know they are different systems.
EXAM TIP
Viscosity selection follows the application. A high-speed spindle bearing runs a light oil. A slow-speed heavily loaded gear drive runs a heavy oil. The exam will give you the application and ask for the correct viscosity direction — heavier or lighter.
Bearings are the most frequently tested lubrication topic on the Red Seal exam.
Oil vs. Grease for Bearings
| Favour Oil | Favour Grease | |
|---|---|---|
| Speed | High speed | Low to moderate speed |
| Temperature | Heat removal needed | Moderate temperature |
| Sealing | Enclosed housing | Exposed or sealed bearing |
| Maintenance | Continuous circulation systems | Periodic regreasing |
| Contamination | Clean environment | Dirty/wet environment |
Grease Quantity: The Most Common Lubrication Error
Over-greasing is one of the leading causes of bearing failure. Excess grease churns inside the bearing housing, generates heat, and degrades the lubricant rapidly. The grease has nowhere to go — it's forced through seals or builds pressure that damages the bearing.
General rule: Fill bearing cavity one-third to one-half full. Never pack a bearing housing completely full with grease.
EXAM TIP
If a bearing is running hot after regreasing, the first question is whether it was over-greased.
Regreasing Intervals and Procedure
Sealed vs. Shielded Bearings
Sealed Bearings (2RS)
Rubber seal. Factory-lubricated for life, cannot be regreased.
Shielded Bearings (ZZ)
Metal shield. Shields reduce contamination but are not seals; some designs can be regreased.
EXAM POINT
Never attempt to regrease a sealed-for-life bearing. It damages the seal and does not improve lubrication.
Enclosed gear drives use oil — either splash lubrication (gears dip into an oil sump) or forced circulation (pump, filter, cooler).
EP (Extreme Pressure) Additives
Gear oils for worm drives, hypoid gears, and high-load applications use EP additives that chemically bond to metal surfaces under extreme contact pressure. Do not use EP-additive gear oil in a system with yellow metal (bronze, brass, copper) components — EP additives are corrosive to non-ferrous metals.
EXAM TIP
EP gear oil and bronze worm gears are an incompatibility the exam tests directly.
Mixing incompatible lubricants is a common — and serious — maintenance error. Grease thickeners do not always mix safely. Mixing incompatible greases can cause softening (grease flows out of the bearing), hardening (grease blocks flow in centralized systems), or loss of EP performance.
Common incompatibilities:
| Thickener | Compatible With | Incompatible With |
|---|---|---|
| Lithium | Lithium complex | Calcium, sodium, polyurea |
| Lithium complex | Lithium | Calcium sulfonate, polyurea |
| Polyurea | Generally isolated | Most others — flush before switching |
| Calcium sulfonate | Itself | Polyurea, lithium complex |
Oil Compatibility
Different base oils (mineral vs. synthetic vs. semi-synthetic) are generally miscible, but mixing can dilute additive packages and reduce performance. When changing oil type, drain and flush the system completely.
EXAM TIP
When switching greases of uncertain compatibility, flush the bearing housing before repacking. When in doubt, check the manufacturer's compatibility data.
Contamination is the primary cause of lubricant degradation and a major source of bearing failure.
Particle Contamination
Dirt, metal wear particles, and debris act as abrasives. Even particles below 10 microns cause significant wear in rolling element bearings. Filtered oil systems should be maintained at target cleanliness levels (ISO 4406 cleanliness codes).
Water Contamination
Water in oil causes rust and corrosion, additive depletion, oil emulsification (milky appearance), and microbiological growth in some systems.
Detect water by: Milky or creamy oil appearance, crackle test (oil sample heated — water causes crackling/popping).
Thermal Degradation
Heat oxidizes oil, increasing viscosity, forming varnish and sludge deposits. Darkening oil colour and acidic smell indicate oxidation.
EXAM TIP
Oil analysis is the diagnostic tool for contamination and degradation. The exam may ask what a water-contaminated oil sample looks like (milky/emulsified) or what particle contamination causes (abrasive wear).
Manual greasing (grease gun)
Standard for most bearing regreasing. Lever guns, pistol grip, or pneumatic. Battery-powered guns can over-pressurize — use with care on small bearings.
Oil can / hand oiling
Used for lightly loaded, accessible points. Not reliable for critical equipment.
Oil bath / splash
Gearboxes, enclosed drives. Level is critical.
Wick / pad oilers
Absorb oil and maintain contact with a surface (used on some slow-speed journal bearings).
Centralized grease systems
Single-point or multi-point injectors, progressive divider blocks, or spray systems. Used where multiple lubrication points must be serviced reliably or equipment is hard to access.
Oil mist / air-oil systems
Very small quantities of oil carried in an air stream. Used in high-speed spindle bearings and enclosed systems. Requires clean, dry air supply.
EXAM TIP
Centralized systems reduce human error and improve consistency. The exam may ask about divider block systems — know that a single blocked outlet can cause the entire circuit to stop delivering lubricant.
| Symptom | Most Likely Cause |
|---|---|
| Bearing running hot after regreasing | Over-greased |
| Bearing running hot, insufficient grease found on inspection | Under-lubricated, wrong interval |
| Spalled bearing races | Fatigue from overload or contamination |
| Corrosion pitting on bearing races | Water contamination |
| Abrasive wear on bearing elements | Particle contamination |
| Grease hardened/caked in housing | Oxidized grease, overheating |
| Grease soft/runny, flowed out of housing | Incompatible greases mixed, wrong NLGI grade |
| Milky oil in gearbox | Water ingress |
| Varnish deposits in gearbox | Oxidized oil — overdue oil change |
| EP gear oil damaging bronze components | Wrong oil specification for the application |
Work through these exam-style scenarios. Tap each to reveal the approach.
A ball bearing in a pillow block housing is running at elevated temperature. Upon inspection, excess grease is found purging from the seal. What is the most likely cause and what is the corrective action?
A gearbox manufacturer specifies EP 220 gear oil. The unit has a bronze worm gear. What concern does this raise and what should be done?
Two greases with different thickener types have been mixed in a bearing housing. The grease is found to be soft and has flowed out of the housing. What caused this?
What does a milky or creamy appearance in a gearbox oil sample indicate?
A centralized progressive divider system serves eight lubrication points. One point is blocked. What effect does this have on the other seven points?
Lubrication questions show up across the NOA — in bearings, gearboxes, couplings, and maintenance procedures. The more automatic this knowledge is, the more marks you pick up without burning exam time.
Related guides:
MW Red Seal Millwright Prep is built by a millwright, for millwrights. Content is aligned with the National Occupational Analysis (NOA) for Industrial Mechanic (Millwright) — the same document that structures the Red Seal exam.