Babbitt metal
A soft metal that lets a shaft embed grit rather than be scored by it — bearing alloy, designed to be the part that wears.
Babbitt is a soft matrix of tin or lead holding hard crystals of antimony and copper compounds. The structure is the design: the hard particles carry the load and the soft matrix wears away slightly around them, leaving channels that hold oil.
Its real virtue is embeddability. A hard bearing surface faced with a hard shaft turns any grit that gets between them into an abrasive that scores both. A babbitt bearing lets the particle bed into the soft metal and disappear, and the shaft — the expensive part — survives. The bearing is deliberately the sacrificial component.
It also conforms: it deflects slightly under load to spread the pressure over a misaligned or deflecting shaft, which a rigid material cannot. Both properties are reasons to choose a weak material on purpose, and it is one of the clearest cases in engineering of a specification that is not 'stronger is better'.
Processing
Cast or centrifugally lined onto a steel or bronze backing shell, because babbitt has no useful strength of its own — the layer is often less than a millimetre thick and the shell carries the load. Bonding to the shell is the critical step and the usual failure.
A worn bearing can be melted out and re-lined, which was routine for a century and is still done for large marine and turbine bearings where a replacement is a custom part.
Uses
Plain journal bearings in steam and gas turbines, large marine engines, generators, compressors and industrial machinery. Historically in every railway axlebox and every automotive engine.
Rolling-element bearings displaced it from small machinery entirely, and it survives at the large end because a hydrodynamic plain bearing on a full oil film has effectively unlimited life and no equivalent exists in a rolling bearing at those loads and speeds.
History
Isaac Babbitt patented his alloy in 1839, and its importance is that it made high-speed rotating machinery practical: before it, bearings were bronze and shafts were scored and machinery seized.
The lead-based grades are the ones being displaced, by aluminium-tin and polymer-lined bearings, and it is regulation rather than performance driving the change.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
contains
- Tin — element · 0–91% · the matrix of the tin-based grades, which are the ones modern specifications call for
- Antimony — element · 4–15% · forms the hard cuboid crystals that carry the load in the soft matrix
- Copper — element · 2–8% · forms hard needles that keep the antimony crystals from segregating during casting
- Lead — element · 0–85% · the matrix of the lead-based grades, which are being displaced by regulation rather than by performance
is an input to
- Casting — process · lined onto a steel or bronze shell, because it has no useful strength of its own
is used in
- Automotive manufacture — industry · historically in every engine bearing, and displaced by aluminium-tin and polymer-lined shells
is an alternative to
- Bronze — alloy · the bearing choice before rolling-element bearings existed: bronze carries more load, babbitt embeds grit and spares the shaft
was succeeded by
- Duralumin — alloy · in engine bearings, by way of aluminium-tin — the soft-matrix principle survived and the tin and lead did not
is produced by
- Alloying and melting — process · tin, antimony and copper, arranged so hard crystals sit in a soft matrix that wears away around them and holds the oil
Sources
- Material WorldOur own writing
- Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)