7.3 Lubrication

Why This Matters

Module 15208, Lubrication, is a 15-hour Level 3 module scored inside the Maintenance and Troubleshooting domain — not the Bearings and Fasteners domain — and that placement is deliberate. NCCER treats lubrication as a maintenance discipline in its own right: the majority of bearing, gear, and pump-seal failures a millwright investigates trace back to a lubrication problem (wrong product, contamination, or over- or under-application) rather than a manufacturing defect. Knowing how to select, apply, and safely handle lubricants is therefore tested as heavily as knowing bearing types themselves.

Grease vs. Oil: The First Decision

The exam expects you to match the lubricant type to the application, not just define each one. A slow-turning conveyor pillow-block bearing in a dusty environment favors grease, which resists contamination ingress. A high-speed turbine or a large gear reducer favors circulating oil, which can be filtered and cooled continuously.

Viscosity: The ISO VG Scale

Viscosity is a fluid's resistance to flow, and for industrial oils it is standardized by the ISO Viscosity Grade (ISO VG) system (defined in ASTM D2422). Each grade number is the oil's kinematic viscosity, in centistokes (cSt), measured at 40°C, with the actual product falling within about 10% of that number. Common industrial grades run ISO VG 32, 46, 68, 100, 150, 220, 320, and 460 — the higher the number, the thicker the oil. A light hydraulic system might specify ISO VG 32, while a heavily loaded, slow-turning gear reducer might call for ISO VG 460. Installing an oil with the wrong viscosity grade is one of the most common lubrication-related failures: too thin and the film breaks down under load; too thick and the oil cannot circulate or dissipate heat properly, especially in cold startup conditions.

Consistency: The NLGI Scale

Grease consistency is standardized by the National Lubricating Grease Institute (NLGI) using a worked-penetration test (ASTM D217): a weighted cone is allowed to sink into the grease for five seconds, and the depth in tenths of a millimeter sets the grade.

NLGI Grade 2 is the default answer for a general-purpose industrial bearing grease question unless the scenario specifies a reason to deviate (extreme cold, vertical shaft slump, or a centralized delivery system).

Lubrication Delivery Methods

• Manual grease gun — hand or pneumatic gun forces grease through a zerk (grease) fitting; simplest method, most prone to human error (missed points, over-greasing).
• Splash lubrication — a dipper or gear tooth on a rotating shaft splashes oil from a sump onto components; simple, used in small gearboxes.
• Oil ring (collar) lubrication — a loose ring rides loosely on the shaft, dips into an oil reservoir below, and carries a film up to a sleeve bearing as it rotates with the shaft.
• Bath lubrication — components (such as gear teeth) are partially submerged directly in an oil sump.
• Force-feed / circulating systems — a pump continuously delivers filtered, cooled oil to bearings or gears and returns it to a reservoir; used on large, high-speed, or heavily loaded machines such as turbines and compressors.
• Oil mist systems — compressed air atomizes oil into a fine mist delivered through tubing to bearing housings, both lubricating and maintaining a slight positive pressure that helps keep contaminants out.
• Automatic single-point lubricators — a battery- or gas-powered canister mounted directly on a zerk fitting dispenses small, timed grease shots, reducing missed or over-applied manual rounds.
• Centralized (single-line or dual-line) systems — one central pump and reservoir feed metering injectors that deliver precise shots of grease or oil to many points across a machine, common on long conveyor systems with dozens of bearings.

Selection, Storage, and Safety

Always match the lubricant's type, viscosity, and NLGI consistency to the equipment manufacturer's specification — "close enough" is not close enough, since an incorrect viscosity or thickener can shorten bearing life dramatically. Different grease thickener types (lithium, polyurea, calcium-sulfonate) are not always chemically compatible; mixing them can break down consistency and ruin the lubricant's protective properties. Store lubricants in sealed, clearly labeled containers, since dirt and water contamination are leading causes of premature bearing failure. From a safety standpoint, high-pressure grease guns can cause serious injection injuries through skin contact, and lubricants must never be assumed compatible with the seal or elastomer materials they will contact.

Over- and Under-Lubrication

Over-greasing is nearly as damaging as under-greasing. Packing a bearing housing completely full leaves no room for grease to churn and circulate normally, causing it to overheat and potentially blow past seals under pressure; the correct fill is typically only one-third to one-half of the housing's free space. Under-lubrication allows metal-to-metal contact, rapid wear, elevated running temperature, and eventual seizure.

Exam Scenario

A conveyor pillow-block bearing runs hot within days of a routine relubrication, even though the correct NLGI Grade 2 grease was used. Inspection finds the housing packed completely full of grease. The corrective action is not to switch lubricants — it is to remove excess grease down to roughly one-third to one-half fill and allow the bearing to run.

Key Takeaways

• Grease suits moderate-speed, hard-to-seal applications; oil suits high-speed, high-temperature, and heavily loaded machinery that needs active cooling.
• ISO VG grades (32–460 and beyond) rate oil viscosity in cSt at 40°C; NLGI grades (000–6) rate grease consistency by worked penetration, with Grade 2 as the general-purpose default.
• Delivery methods range from manual grease guns and splash/ring oiling to force-feed circulating systems, oil mist, and centralized single- or dual-line systems.
• Never mix incompatible grease thickener types, and always match lubricant specs to the OEM recommendation rather than "close enough."
• Fill bearing housings to one-third to one-half of free space — overfilling causes overheating just as reliably as under-lubrication.