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Machine elements

Bearing lubrication and grease selection

A bearing lubricant separates rolling contacts, limits wear, carries heat and protects against corrosion. Selection must combine base-oil viscosity, thickener consistency, temperature range, water resistance, speed, load and the relubrication method.

DESIGNATION

Read the grease code without mixing its functions

Lubricating grease DIN 51825 – K3N–20 identifies grease for rolling and plain bearings (K), NLGI consistency grade 3, an upper-temperature class N and a lower working temperature of −20 °C.

K · 3 · N · −20 = application · consistency · upper class · lower limit

NLGI grade describes worked consistency, not the viscosity of the base oil. Both properties matter, but they answer different design questions.

MULTI-WIRE DUTY

Select for water exposure and maintainability

  • Base-oil viscosity: preserve an adequate lubricating film at operating temperature without excessive churning.
  • Water behaviour: prefer low change under water exposure; Fischer lists grades 0 to 3 from no change to large change.
  • Corrosion protection: account for wash-water ingress and shutdown periods, not only running conditions.
  • Relubrication: provide a clean inlet and an exit path for old grease and contaminants.
Selection inputEngineering check
Speed and temperatureCheck bearing speed factor and the lubricant's usable range; heat lowers oil viscosity.
Load and shockConfirm film strength and suitable anti-wear or load-carrying additives where required.
Grease consistencyMatch feedability and retention; a higher NLGI number means a firmer grease.
Quantity and intervalAvoid overfilling: excess grease increases churning, temperature and seal pressure.
SourcesSource basis: Ulrich Fischer et al., Mechanical and Metal Trades Handbook, lubricant codes and ISO viscosity grades, p. 271; grease designation, NLGI consistency, temperature classes and water behaviour, p. 272.
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