In this guide
Your gearbox runs beautifully for the first hour of the demo. Then it gets warm. Then it gets loud. Then it seizes — in front of everyone. The gears and bearings were sized correctly, the alignment was fine, and the entire failure was a ₹200 decision: no one lubricated it, or lubricated it wrong.
Lubrication is the cheapest reliability engineering that exists. A correct lubricant in the right amount extends bearing life tenfold; the wrong one (or none) destroys the same bearing in days. This guide covers what lubrication actually does, how to choose oil vs grease and which grade, and the maintenance practices that keep student-built machines alive.
What the lubricant is doing in there
Two metal surfaces in contact would weld microscopically and tear — that's adhesive wear. A lubricant keeps them apart with a film:
- Hydrodynamic lubrication: a full fluid film separates the surfaces (journal bearings at speed, gear teeth at the contact). Film thickness depends on speed × viscosity ÷ load — faster and more viscous builds a thicker film.
- Elastohydrodynamic (EHL): in rolling bearings, the contact pressure elastically flattens the surfaces and the oil's viscosity spikes under pressure, forming a thin but tough film — often under a micron thick.
- Boundary lubrication: at start/stop or very high load, the film collapses and additives (anti-wear, extreme-pressure) form sacrificial chemical layers on the metal. This is why additive packages matter more than base oil in heavily loaded contacts.
- λ (lambda) ratio = film thickness ÷ composite surface roughness. λ > 3: full separation; 1 < λ < 3: mixed; λ < 1: boundary — wear regime. You don't calculate λ for a student project, but knowing it exists explains why smooth surfaces + right viscosity = long life.
Oil vs grease: the fundamental choice
| Oil | Grease | |
|---|---|---|
| What it is | Liquid base oil + additives | Base oil + thickener (soap) + additives — oil suspended in a sponge |
| Suits | High speeds, circulating systems, gearboxes with splash/pump feed, heat removal | Low–medium speeds, sealed-for-life, vertical shafts, places oil would leak out |
| Re-lubrication | Easy (drain/refill, circulation) | Harder (purge through nipple, or disassemble) |
| Cooling | Excellent — carries heat away | Poor — stays put |
| Contamination | Filters and drains handle it | Traps contaminants (good and bad) |
| Student verdict | Gearboxes, chain drives (drip/bath), high-speed spindles | Ball/roller bearings in project machines — sealed or shielded bearings pre-greased are the default choice |
For most student projects: buy sealed (2RS) or shielded (ZZ) bearings — they come pre-greased for life from the factory with the correct fill, and you never touch lubrication. Open bearings + manual greasing is for machines you're prepared to maintain.
Viscosity grades: reading the numbers
ISO VG grades (industrial oils): the number ≈ kinematic viscosity in cSt at 40°C. VG 32, 46, 68, 100, 150, 220… Higher number = thicker.
| Application | Typical ISO VG |
|---|---|
| High-speed spindles, light instruments | VG 10–32 |
| General gearboxes, hydraulics | VG 46–68 |
| Heavily loaded gearboxes, worm gears | VG 150–460+ |
| Slideways (way oil) | VG 68 (with tackiness additives) |
Rule of thumb: higher load and lower speed → higher viscosity; higher speed and lower load → lower viscosity. Too thin = metal contact; too thick = churning losses, heat, and (in cold starts) oil starvation.
Grease grades (NLGI): 000 (fluid) → 0 → 1 → 2 → 3 (stiff). NLGI #2 is the general-purpose standard — it's what "bearing grease" means unless specified otherwise. #1 for cold or centralized systems; #3 where leakage must be minimized.
Multigrade engine-style oils (SAE 10W-40 etc.) are for engines, not general machinery — don't substitute them for ISO VG industrial oils in gearboxes by guesswork.
Additives: the chemistry that matters
| Additive | What it does | Where you need it |
|---|---|---|
| Anti-wear (AW, e.g. ZDDP) | Protects under mixed/boundary film | General gears, hydraulics |
| Extreme pressure (EP) | Prevents welding under very high contact pressure | Worm gears, hypoid gears, heavily loaded contacts |
| Rust & oxidation inhibitors | Protects during storage/idle, extends oil life | Everything, especially humid workshops |
| Tackiness agents | Makes oil cling to vertical/sliding surfaces | Way oils, chain lubes, open gears |
| Anti-foam | Prevents aeration in circulating systems | Gearboxes, hydraulics |
Compatibility warning: greases with different thickeners (lithium vs polyurea vs calcium sulfonate) can be incompatible — mixing can soften the mixture into soup or harden it into wax. When re-greasing, know what's in there; when in doubt, purge thoroughly or stick to one product family.
How much: the quantity mistakes
- Bearings: the classic error is over-greasing. Fill only 30–50% of the free space in the bearing (less at high speed — 20–30%). A bearing packed solid churns the grease, overheats, and the grease oxidizes into abrasive crust. Sealed bearings are factory-filled correctly — leave them alone.
- Gearboxes: fill to the oil-level mark (usually the middle of the lowest gear's teeth dip into the oil — splash lubrication). Overfilling causes churning heat and seal leaks; underfilling starves the top bearings.
- Chains: lubricate the pins and bushings (the articulation points), not just the outside plates — drip or brush oil into the chain while slowly rotating; wipe excess.
- Lead screws: light oil or PTFE-based lubricant on the threads; keep grit off (a bellows cover is worth more than fancy oil).
Re-lubrication intervals (student-practical)
For intermittently-run project machines (not 24/7 production):
- Sealed bearings: never (that's the point of sealed).
- Open bearings with grease nipples: a few pump strokes every 3–6 months of use — stop when fresh grease just appears at the seal.
- Gearbox oil: check level monthly; change yearly or when dark/milky (milky = water contamination).
- Chains: oil when they look dry or sound dry — typically every few weeks of use.
- Slideways: a wipe of way oil before each use session.
Contamination control: wipe nipples before greasing (you're otherwise injecting workshop grit into the bearing), keep oil containers sealed, and never mix oils without checking compatibility.
Warning: Lubrication work on running machinery is how fingers get caught — nipples and oil cups are reachable precisely because they're near moving parts. Lubricate with the machine stopped and isolated unless the design specifically provides for running lubrication with guards in place. Dispose of used oil properly (collect it — never pour it into drains or soil); oily rags are a spontaneous-combustion and slip hazard — store in closed metal bins.
Diagnosing lubrication failures
| Symptom | Likely lubrication cause |
|---|---|
| Bearing runs hot (>70–80°C on the housing) | Over-greased, wrong viscosity, or lubricant degraded |
| Gearbox whine increasing over weeks | Oil level low, viscosity wrong, or water contamination |
| Grease black and crusty | Oxidized from overheating or age — clean out and repack |
| Milky oil | Water ingress — find the entry (condensation, washdown, seal) |
| Chain elongating fast ("stretch") | Pin/bushing wear from dry running — lubricate the articulations |
| Polished/worn slideways | Way oil absent or wrong — metal-on-metal polishing |
Common mistakes
- No lubrication at all — "it came with the bearings" (open bearings often ship with only preservative oil).
- Over-greasing — more is not better; packed bearings overheat.
- Wrong viscosity by guessing — VG 68 where VG 220 was needed, or vice versa.
- Mixing incompatible greases — the mixture fails worse than either alone.
- Lubricating the outside of a chain instead of the pins.
- WD-40 as a lubricant — it's a water displacer and penetrant, not a lasting lubricant. Fine for freeing rust, wrong for bearings.
- Ignoring contamination — greasing through a dirty nipple, open oil cans in a grinding shop.
Where to go from here
- Bearing Selection Basics — selecting the bearings you're lubricating.
- Belt vs Chain Drive Selection — chain drives live or die by lubrication.
- More machine-maintenance topics in the Mechanical branch hub.