Lubrication Failure (Adhesive Wear)

What it is. Unlike contamination, here the problem is not what's in the oil/grease, but the absence or insufficiency of the film itself. It happens from excess speed/temperature relative to the chosen lubricant's viscosity, wrong grease quantity (too little or — surprisingly — too much, which generates heat from the grease's own internal friction), an over-long regreasing interval, or sudden loss (jammed grease pump, ruptured seal). Without the protective film, metal surfaces contact each other directly — not particle scratching as in contamination, but pure metal-to-metal friction, which generates heat, "welds" microscopically, and tears away tiny bits of material (so-called smearing, or scoring).

How to recognize it in the field. Abnormally high temperature is the most direct and earliest signal — often before any perceptible vibration. At an advanced stage, rising noise and, in the worst case, bearing seizure from cold-welding between the surfaces.

Why it matters. It is one of the most preventable and cheapest-to-prevent failure modes — the fix is essentially a maintenance-plan audit, not an expensive physical intervention. And it is treacherous: excess grease is as dangerous as too little, which makes "regreasing out of caution, without following the manufacturer's table" a counterproductive practice.

Mechanism — adhesive wear (ISO 15243 §5.2.3)

When the film ratio $\lambda$ (see Rolling Bearing) drops below ~1, surface asperities contact directly. Under Hertzian contact pressure, microwelds form instantly between the asperities and break on the following movement, tearing material from one surface and transferring it to the other (smearing, galling). The visual result is a surface with a "smeared"/hazy appearance, distinct from the dull polish of abrasive wear (which removes material by cutting, not by adhesive transfer).

The two paths to insufficient film

1. Insufficient viscosity for the actual condition — the correct lubricant on paper can become wrong if operating temperature rises (viscosity drops exponentially with temperature) or actual speed diverges from what the design assumed.
2. Wrong grease quantity — too little grease doesn't fill the spaces needed to renew the film; excess grease creates resistance to the rolling elements' motion, which in turn generates heat from internal friction — the excess itself destroys local viscosity through the heating it causes (an effect known as churning/thermal churning).

Framework A — diagnosis

Mixed/Complex: when the trigger is a sudden, complete loss (jammed grease pump, ruptured seal), behavior is close to Random — damage evolves fast and there is no "characteristic age"; when it's chronic, progressive under-greasing (over-long interval, marginal viscosity), wear accumulates more predictably, with β>1. IT-MNT-001 treats this as a spectrum, not a single category — the correct reading depends on auditing the cause before assuming the pattern.

Framework B — prescription

CBM, with two complementary layers: the documentary regreasing-plan audit attacks the root cause (prevents the problem from starting), while thermography/broadband vibration monitors the condition (detects marginal lubrication already present, before it progresses to visible wear). Note this is one of the few maintenance tasks that requires no specialized sensor — comparing the plan against the manufacturer catalog is engineering audit, not field measurement.

$\kappa$, $\lambda$ and the safe operating window

The viscosity ratio $\kappa$ (actual viscosity at operating temperature ÷ the minimum viscosity required by the geometry/speed) is the design parameter the Reliability discipline should audit whenever the operating condition changes — a load swap, a speed increase (via VFD), or operation in a hotter environment than the original design assumed. $\kappa < 1$ already signals significant adhesive-wear risk even with the correct grease quantity, because the problem is not how much lubricant exists, but whether it has enough viscosity at the actual temperature to form the EHL film.

Why excess grease heats up faster than a shortage, at first

Thermal churning occurs because excess grease in the gaps between moving components must be continuously sheared with every revolution — that shearing work converts to heat, which in turn reduces the remaining grease's effective viscosity, creating a cycle that can push operating temperature well above design before any "shortage" symptom appears. That's why the regreasing quantity is always a fraction of the free-cavity volume (typically 1/3 to 1/2), never a full fill.

Relation to other failure modes

A marginal film ($\lambda$ low) that hasn't yet crossed into visible adhesive wear already accelerates the nucleation of contact fatigue — the boundary between "marginal lubrication accelerating fatigue" and "full adhesive wear" is a matter of degree, not a distinct mechanism, and a post-mortem sometimes finds both patterns coexisting on the same raceway.