Overview
C932 leaded tin bronze — universally known by its SAE designation “SAE 660” — is the workhorse bearing bronze for industrial machinery. The chemistry “83-7-7-3” (83% Cu, 7% Sn, 7% Pb, 3% Zn) is the result of a century of bearing-design evolution, optimizing four properties simultaneously:
- Compressive strength for load capacity (Sn alloying contributes)
- Embeddability for dirt tolerance (Pb particles provide soft inclusions that capture wear debris)
- Conformability to compensate shaft misalignment (Pb provides local cold-flow)
- Anti-galling behavior against steel shafts (Pb acts as solid lubricant at boundary lubrication conditions)
The result is a bearing material with somewhat lower load capacity than tin-only bronze or aluminum bronze, but vastly better forgiveness of imperfect lubrication, debris contamination, and shaft alignment. For the typical industrial sleeve bearing — pillow block, pivot pin, crane sheave, slide bearing — C932 is the default material globally.
Bearing bronze ecosystem
Different bearing bronzes optimize for different service conditions:
| Alloy | Composition | UTS (MPa) | Notes |
|---|---|---|---|
| C932 (SAE 660) | 83-7-7-3 | 240 | Standard general-purpose — this entry |
| C937 | 80-10-10-trace | 240 | Higher tin and lead — softer, more conformable |
| C941 | 79-10-10-1 | 207 | High-lead variant for severe boundary lube |
| C932 (SAE 660) — most common everyday | |||
| C863 (Manganese Bronze) | 63-25-Mn-Fe-Al | 825 | High-load, abrasive service |
| C954 (Aluminum Bronze) | 85-11Al-4Fe | 586 | Marine, high-temp, high-load |
| C955 (Nickel Al Bronze) | 81-9Al-4Fe-5Ni | 620 | Severe marine, aerospace |
| C90700 (Gear Bronze) | 88-Sn11-balance | 305 | High-tin for worm gears |
| C90800 (88-Sn8) | 88-Sn8-balance | 240 | Lead-free version of similar tin level |
Selection logic for bearings:
- Industrial general-purpose, modest load, modest speed: C932 (this entry)
- Heavier load, severe service: C863 manganese bronze or C954 aluminum bronze
- Marine immersion, aggressive corrosion: C954 aluminum bronze
- Worm gear teeth: C90700 / C90800 high-tin bronzes
- Lead-free regulatory compliance: C89320 / C89510 bismuth-tin bronzes, or aluminum bronze
- Oil-impregnated PM bronze: sintered Cu-Sn with oil retention, for low-load light-duty service (small motors, fans)
The C932 niche is the workhorse general bearing — moderately loaded, moderately fast, properly lubricated industrial sleeve bushings.
How lead works in bearing bronze
The 6–8% lead in C932 doesn’t dissolve in the copper-tin matrix. Lead has essentially zero solubility in alpha-bronze; on solidification, Pb separates into discrete soft globules distributed throughout the matrix. Microscopic examination of a polished C932 cross-section shows the lead as small (~10–50 μm) gray-black inclusions in the bronze-colored matrix.
Under bearing service, these lead particles do three things:
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Embed debris. Hard wear particles (steel chips, sand, contamination from the lubricant) embed into the soft Pb at the bearing surface rather than scoring the steel shaft. The bushing becomes a “sacrificial filter” — the bushing surface looks contaminated with embedded debris, but the shaft remains intact. This is the bearing-bronze design philosophy: the soft bushing protects the hard expensive shaft.
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Conform to surface irregularities. Minor shaft misalignment or surface waviness causes local high-contact-pressure regions. Pb particles at these regions cold-flow under load, redistributing contact and reducing peak pressures. The bushing “wears in” to match the shaft during initial run-in.
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Boundary lubrication. Under conditions where the lubricant film is too thin to fully separate the surfaces (low speed, high load, startup conditions), the lead at the bearing surface smears across the contact zone, providing a solid-lubricant film that prevents metal-to-metal welding and scoring.
The trade-offs:
- Pb melts at 327°C — limits high-temperature service to ~230°C continuous
- Pb is regulated — excludes C932 from water, food, electronics, children’s product applications
- Pb adds weight (high density) without contributing structural strength
Hydrodynamic vs. boundary lubrication
C932 bushings can operate in two distinct lubrication regimes:
Hydrodynamic lubrication — high speed, low/moderate load, adequate oil supply. The shaft rotates fast enough to drag a wedge of oil into the bearing clearance, generating hydrodynamic pressure that lifts the shaft off the bushing surface. Metal-to-metal contact is zero; wear is essentially zero. Theoretical service life is infinite in this regime.
Boundary lubrication — low speed, high load, oil starvation, startup conditions. The oil film is too thin to separate the surfaces; asperities on the shaft contact asperities on the bushing surface; wear occurs at every contact. C932’s lead inclusions are critical here — they prevent galling and welding that would occur in non-leaded bronze.
Real-world bushings spend time in both regimes — hydrodynamic during steady operation, boundary during startup, shutdown, and any oil-starvation transient. Bushing design provides for both:
- Oil grooves to ensure lubricant delivery to the bearing surface
- Adequate clearance for hydrodynamic film generation
- Soft bushing material for boundary tolerance
P-V (pressure × velocity) ratings for C932 sleeve bushings: ~75,000 psi-fpm for general industrial service, ~50,000 psi-fpm for continuous duty, ~120,000 psi-fpm short-term peak. These are guideline numbers; actual service depends on lubrication, environment, and shaft material.
Machining notes: the easy bronze
C932 machines beautifully thanks to the lead content. Standard machining recipe for bushing production:
- Receive continuous-cast bar or tube near-net size (ASTM B505)
- Cut to length on saw or lathe
- Rough turn OD to size + 0.005–0.020 in stock
- Rough bore ID to size + 0.005–0.020 in stock
- Stress relief at 260°C for tight-tolerance parts (optional)
- Finish turn OD to size
- Finish bore ID to size
- Burnish or hone ID to bearing finish (8–32 Ra)
- Machine oil grooves, lubrication features as needed
- Final cleaning and inspection
Cutting parameters:
- Sharp carbide or HSS
- Speed 200–400 SFM
- Feed 0.005–0.025 in/rev
- Light coolant or dry cutting
- Standard tool geometry
Machinability rating 70 (versus C360 brass = 100) — among the best of any bearing-grade alloy. Tool life excellent, chip control clean, surface finish good without specialty technique.
Joining: don’t weld, mechanically retain
C932 is essentially not weldable in production. The lead content causes:
- Toxic Pb fume during welding (occupational hazard)
- Zn fume (~3% Zn content adds to brass-like fume issue)
- Hot shortness — lead segregates to grain boundaries during solidification, causing weld cracking
CDA rates all common welding processes “Not Recommended.” Brazing rated “Good” but must be done carefully (avoid strain during the hot-short cooling range).
The standard approach: mechanically retain C932 bushings:
- Press-fit into housing bore with controlled interference (typically 0.001–0.003 in interference on diameter, depending on size and material). Bushing OD designed for the press-fit; housing material absorbs the radial pressure.
- Set screws through housing wall into mating flat on bushing OD, for applications where high axial load could otherwise displace the bushing.
- Flange or shoulder integral to the bushing geometry, retained axially by the mating part.
- Adhesive bonding with anaerobic retainer compounds for some light-duty applications.
Repair welding of damaged bushings is feasible but should be left to specialty bronze welders — preheat 480°C, ERCuSn-A or ERCuSn-C filler, controlled technique. For production assembly, design for mechanical retention.
Lead-free alternatives
Bismuth-tin bronzes (C89320, C89510, C89520) substitute Bi for Pb, maintaining similar microstructure (soft inclusion particles in bronze matrix) without lead. These alloys are RoHS-compliant and acceptable for water service.
Trade-offs:
- Slightly higher cost (~20–40% premium)
- Slightly lower load capacity in some service profiles
- Acceptable mechanical and bearing properties for most general bushing applications
For lead-free bearing service in water-contact applications (potable water pump bushings), regulated electronic equipment, food processing equipment, and children’s products, bismuth-tin bronzes are the primary C932 substitute. For unregulated industrial service, C932 remains the cost-effective standard.
Applications by industry
- Industrial machinery (general) — pillow block bushings, pivot pins, hinge pins. The dominant application by volume.
- Construction and mining equipment — boom pin bushings, bucket pin bushings, articulation joint bushings. The C932 bushing is the near-universal solution for these high-load slow-rotation pin bearings on excavators, loaders, dump trucks, and mining haulers.
- Agricultural machinery — implement pivot points, hitch bushings, drive shaft U-joint bushings.
- Material handling — conveyor pulley bushings, crane sheave bushings, hoist drum bushings.
- Heavy truck and bus — suspension link bushings (though elastomeric bushings dominate modern designs), drivetrain bearing surfaces.
- Marine — propeller shaft cutless bearings (water-lubricated; the bronze provides backing for the rubber wear face), rudder bushings, deck hardware. For aggressive marine service, aluminum bronze C954 is the upgrade.
- Power generation — pump shaft bushings, fan and blower bushings, valve stem guides.
- Pulp and paper — paper machine roll bushings, dryer can bushings, calender stack bushings.
- Steel mills — rolling mill roll neck bearings (large centrifugal-cast tubes), guide roll bushings.
- Diesel engines — wrist pin bushings, connecting rod bushings, rocker arm bushings. Specialty engine-grade bronzes with tighter inclusion control and additional alloying are used in most modern engines, but C932 covers older designs and aftermarket replacement parts.
- General fabrication — any rotating or sliding mechanical joint in industrial machinery where a sleeve bearing is appropriate.
Failure modes worth designing around
Wiping from lubrication failure is the dominant in-service failure mode. Oil starvation, contaminated lubricant, broken oil delivery, overload conditions — any condition that breaks the boundary lubrication regime — causes rapid galling, surface smearing, and eventual seizure. Failure mode investigation typically traces to specific lubrication system fault. Design oil delivery systems with redundancy or fail-safe response.
Lead melt-out at elevated temperature — above ~230°C continuous, lead inclusions soften and migrate. Loss of lead degrades anti-galling behavior. For continuous high-temperature service (engine cylinder walls, hot process equipment), use aluminum bronze C954 or manganese bronze C863.
Fatigue spalling from repeated impact load. Continuous-cast and sand-cast structures have moderate fatigue strength. Hydrodynamic lubrication mitigates by reducing peak surface stresses.
Cold flow / creep under sustained heavy load. Soft bronze deforms over time. Design contact stress below ~3000 psi for long-life sleeve bushings.
Lead leaching in water service — regulatory exclusion. Use bismuth-tin bronzes or aluminum bronze for water-contact bearings.
Galvanic corrosion of mating metals — isolate from aluminum, zinc, magnesium, steel in moist mixed-metal assemblies.
Welding hazards — Pb fume + Zn fume + hot shortness. Don’t weld; use mechanical retention.
Mercury embrittlement — rare service but catastrophic. Don’t use in Hg-bearing environments.
Stress corrosion cracking in ammonia/amine environments — mitigated by stress relief and reduced sustained tension.