Babbitt metal used for hydrodynamic bearings is traditionally a soft, conformable, anti-friction alloy. Which base type best describes classic Babbitt compositions?

Introduction / Context:
Babbitt metals provide excellent embeddability and conformability in plain bearings, protecting shafts under boundary and mixed lubrication. Recognizing the base composition guides selection for speed, load, and temperature.

Given Data / Assumptions:

• Classic Babbitt: soft matrix with hard intermetallics.
• Operating regime: hydrodynamic bearings in pumps, motors, turbines.

Concept / Approach:
Traditional Babbitt alloys are tin-base (Sn ~80–90%) with antimony and copper hard phases dispersed in a soft matrix, allowing seizure resistance and embeddability. Lead-base Babbitts also exist for lower-speed, lower-temperature service, but the archetypal “Babbitt metal” is tin-base.

Step-by-Step Solution:
1) Identify key performance: conformability, embeddability, compatibility with shaft materials.2) Map to composition families: tin-base Babbitts deliver superior fatigue resistance at higher speeds vs. lead-base variants.3) Select tin-base as the classic answer.

Verification / Alternative check:
Standards (e.g., ASTM B23) list several tin-base Babbitts widely used in critical bearings, confirming the canonical base type.

Why Other Options Are Wrong:
Lead-base: valid for some applications but not the classic/primary Babbitt archetype.
Aluminium- or copper-base: these are separate bearing bronze or Al–Si families with different tribological behavior.
Magnesium-base: not used for Babbitt bearings.

Common Pitfalls:
Equating all bearing alloys with “Babbitt”; bronze and Al–Si alloys serve different regimes with less embeddability.

Final Answer:
tin-base alloy (Sn–Sb–Cu family)