How Incorrect Greasing Can Trigger Bearing Failure

In a rolling‑element bearing, fresh grease is essential for reducing friction, yet it can also precipitate failure if not applied correctly. The key problem arises when new grease is introduced into a bearing that has long accumulated hard, crusty deposits from prior lubrications.

During long intervals between relubrications, a soap‑based thickener and oil slowly bleed out, forming a protective film that stays on the bearing’s inlet port and along the cavity walls. Over time, heat, vibration, centrifugal forces and contamination harden this film into a solid wall that can trap wear particles, rust and manufacturing debris.

When a grease gun forces a fresh batch into the cavity, it must break through this “rock” to reach the bearing’s core. Hydrostatic pressure from the gun can exceed 15,000 psi (103,421 kPa), which is enough to dislodge solid fragments and send them into the raceway. If the new grease fails to penetrate, it may exit via the shaft seal or vent port, leaving the bearing starved of lubricant and prone to sudden, catastrophic failure.

Large electric‑motor bearings often rely on line‑extension fittings for greasing. Hardened thickener and solids can accumulate in these lines, and a relubrication can act like a plunger, pushing those solids deep into the bearing cavity.

To avoid this maintenance paradox—where both neglect and over‑lubrication cause failure—engineers must answer several critical questions before greasing:

• Is grease the appropriate lubricant, or should oil (mist, bath, circulating) be used instead?
• Which grease formulation is required—complex soap, non‑soap, high‑temperature, synthetic, high‑viscosity, or a grease with a specified National Lubricating Grease Institute (NLGI) grade?
• What quantity of grease is necessary, and can feedback tools such as vibration analysis, thermal imaging, or acoustic sensors guide the amount?
• What is the optimal relubrication interval based on operating temperature, environment, bearing type, orientation and vibration level?
• Which delivery method—grease gun, single‑point automatic lubricator, centralized system—offers the best control and consistency?
• If greasing manually, what procedure ensures complete coverage without creating a hard wall?
• Should the bearing be replaced with a sealed or shielded design to reduce maintenance?
• Is periodic analysis of used grease advisable to monitor wear and contamination?

Mastering these variables requires rigorous training and continual education. Only with precision and skill can a new batch of grease truly protect a rolling‑element bearing and extend its service life.

Jim Fitch, President & Co‑Founder, Noria Corporation