Q: How do I select lubricant viscosity?

Viscosity selection balances film thickness against viscous power loss. Low viscosity thins the film but reduces churning losses; high viscosity builds thicker films but increases heat generation. Typical mineral oils: ISO VG 32 (η≈0.03 Pa·s) for high-speed light loads; VG 100 (η≈0.09 Pa·s) for low-speed heavy loads. Use the Stribeck calculator to find the viscosity that achieves Λ ≥ 3 at your operating conditions.

Question 1

What is the Stribeck curve?

Accepted Answer

The Stribeck curve is a plot of friction coefficient μ versus the Stribeck parameter (ηN/P), dividing lubrication into three regimes: boundary (low ηN/P, high friction, metal contact), mixed (transition, partial film), and hydrodynamic (high ηN/P, full film, very low friction). Optimal designs target the transition between mixed and hydrodynamic regimes.

Question 2

What is the Sommerfeld number?

Accepted Answer

The Sommerfeld number S = (η·N/P)·(D/2c)² is a dimensionless parameter characterizing hydrodynamic journal bearing performance. Here η is dynamic viscosity [Pa·s], N is speed [rev/s], P = W/(D·L) is unit load [Pa], D is shaft diameter [m], and c is radial clearance [m]. A higher S indicates better film formation; S > 0.1 generally ensures hydrodynamic lubrication.

Question 3

What is the film thickness ratio Λ and what values are safe?

Accepted Answer

The film thickness ratio Λ (lambda ratio) equals the minimum film thickness hmin divided by the composite surface roughness σ = √(σ₁²+σ₂²). Guidelines: Λ < 1 → boundary lubrication (metal contact, high wear); 1 ≤ Λ < 3 → mixed lubrication (partial contact); Λ ≥ 3 → full film hydrodynamic lubrication (no contact). For ground surfaces (Ra≈0.4 μm), σ≈0.5 μm, so hmin ≥ 1.5 μm is needed.

Question 4

How do I select lubricant viscosity?

Accepted Answer

Viscosity selection balances film thickness against viscous power loss. Low viscosity thins the film but reduces churning losses; high viscosity builds thicker films but increases heat generation. Typical mineral oils: ISO VG 32 (η≈0.03 Pa·s) for high-speed light loads; VG 100 (η≈0.09 Pa·s) for low-speed heavy loads. Use the Stribeck calculator to find the viscosity that achieves Λ ≥ 3 at your operating conditions.

Lubrication Regime Calculator — Stribeck Curve

Theory