Solid Lubricants in Bronze Bearings: Graphite, MoS₂ and PTFE
Manufacture and application of bronze rings, particularly in machinery and equipment.
Solid Lubricants in Bronze Bearings: Graphite, MoS₂ and PTFE
A solid lubricant carries load and shears at the same time. Embedded into a strong bronze base, it supplies low-friction films to contact zones that oil never reaches — furnace rollers, underwater gates, high-vacuum and clean-room equipment, and any position a grease gun cannot visit. This page compares the three working solids, explains plug patterns and coverage ratios, and marks the limits that decide between graphite, MoS₂ and PTFE.
How plugged bearings work
A plugged (or “graphite-plugged”, JDB-type) bearing is a two-phase composite assembled mechanically: holes drilled in a high-strength bronze base are filled with plugs of a solid lubricant. During running-in, the plugs transfer a thin solid film onto the shaft; from then on, the film shears instead of the surfaces. The bronze carries the load, the lubricant manages the friction.
Three design consequences follow:
- Running-in matters. The transfer film forms over the first revolutions; light loading during running-in gives a uniform film and longer life.
- Coverage sets capacity. Load is carried by the metal between plugs, so more plug area means less metal area — the coverage ratio is a strength-versus-friction dial (see below).
- Shaft finish is part of the design. A ground shaft of roughly Ra 0.4–1.6 µm holds the transfer film; a rough shaft scrapes it off faster than the plugs can renew it.
The three solid lubricants compared
| Lubricant | Usable temperature | Friction behaviour | Chemical character | Watch out for |
|---|---|---|---|---|
| Graphite | up to ~400–450 °C in air | Low, stable; needs some moisture or adsorbed vapour to work | Inert to most media, conductive | In very dry or vacuum environments, graphite friction rises — pair with MoS₂ there |
| MoS₂ (molybdenum disulfide) | up to ~350–400 °C in air (oxidises above) | Low; lowest friction when dry and in vacuum | Inert; sulfur-reactive with some metals at high temperature | Oxidation to MoO₃ above its range; not for hot oxidising service |
| PTFE | up to ~250–260 °C | Lowest friction of the three; low load strength | Chemically inert, non-conductive | Cold flow under high load; handled in composites (MT03), not as bare plugs |
Plug-type bronze bearings use graphite as the default and MoS₂ or mixed plugs where service is dry, hot in vacuum, or both. PTFE appears in the metal-polymer composite family rather than as a cast-in plug.
Plug patterns and coverage ratio
Plug area (coverage ratio) is the central specification of a plugged bearing. Typical values:
| Duty | Plug coverage | Rationale |
|---|---|---|
| Heavy load, slow motion, interrupted lubrication | ~15–25% | Load capacity of the bronze matrix dominates |
| Moderate load, continuous slow sliding | ~25–33% | Balance of film supply and matrix strength |
| Oscillating, short strokes, no lubrication access | ~30%+ | Film renewal is the bottleneck; accept lower load |
Plug diameter is chosen relative to wall thickness (commonly d ≈ 6–12 mm plugs, deeper than wide), arranged in staggered rows or helical patterns on the working surface. For very large plates, plug layout is engineered per drawing rather than taken from a catalogue.
Operating limits and failure modes
- Temperature ceiling. Above the lubricant’s working range (Table 1), the transfer film fails and friction climbs sharply — the base alloy usually survives, but the bearing function does not.
- PV ceiling. Plugged bearings handle high load at low speed, or moderate load at moderate speed; they do not combine both. Published PV ceilings depend on the base alloy and plug layout — treat catalogue values as indicative and confirm with the supplier per duty cycle.
- Water and steam. Graphite works in water — one reason plugged bearings dominate hydro gate and turbine duty — but watch for base-alloy corrosion (see MF03/MF04).
- Plug pull-out. Under vibration with wrong fit or undersized plugs, plugs can migrate. Proper interference fit and plug geometry are manufacturing details worth auditing on large custom plates.
Maintenance-free is a duty statement, not a product class
“Maintenance-free” only holds within the PV and temperature envelope the design was sized for, and after correct running-in. A plugged bearing outside its envelope will fail exactly like an unlubricated plain one — just later. Size from duty data, not from the label.
What to specify in an RFQ
- Load (N or MPa on projected area), speed, motion type (rotary / oscillating / linear, stroke)
- Lubrication state: none, initial greasing, or water — and whether “no maintenance” is a hard requirement
- Temperature range, including short excursions
- Shaft material, hardness and surface finish
- Preferred base alloy or the corrosion environment, so the base can be chosen
- Plug coverage ratio preference, or leave it to the supplier with the load data above
FAQ
- Why does graphite fail in a vacuum?
- Graphite’s low friction depends on adsorbed water vapour and other surface films. In high vacuum or bone-dry gas those films desorb, and friction and wear rise. MoS₂, which lubricates by its own lamellar structure without needing vapour, is the plug material for vacuum and very dry service.
- What coverage ratio should my wear plate have?
- Follow load, not habit. Around 25–33% coverage suits most continuous slow-sliding duty; heavier loads want lower coverage to keep metal area up, and short-stroke oscillation wants higher coverage to keep the film renewed. Give the supplier the load and motion data and let the layout follow it.
- Can a plugged bearing also be greased?
- Yes — many designs allow initial greasing or occasional topping-up, and grease complements the solid film. But the design promise is that the bearing survives without it; if grease supply is reliable, a conventional bronze may be more economical.
- Graphite plug or MoS₂ plug for a furnace roller bearing at 350 °C?
- At 350 °C in air both are near their ceiling: graphite is the usual choice (stable to ~400–450 °C), MoS₂ is oxidising out of range. Above graphite’s ceiling the duty needs a different material class entirely.
- Do plugged bearings need running-in?
- Yes. The transfer film that does the lubricating forms during the first operating hours. Light loading and clean conditions during running-in set the film pattern for the bearing’s whole life.
Customization Bronze rings
The rings are manufactured to meet exact client specifications, with precise measurements and tailored designs.
Bronze rings Specific Uses
Solid Lubricants in Bronze Bearings Material
graphite- and solid-lubricant-plugged cast bronze bearings, and solid-lubricant powders in composites.
Choosing the base alloy
| Base | Strength position | Typical use |
|---|---|---|
| CuZn25Al5-type high-tensile brass | Highest static strength of the common bases | Heavy-duty JDB-type bushings and plates |
| CuSn12 / leaded tin bronze | Lower strength, better conformability | Moderate loads, smoother running |
| CuAl10Ni5Fe4 (NAB) | High strength, seawater capable | Marine, hydro gates and turbines |
| Cast iron (special cases) | High stiffness, low cost | Large slides where corrosion is not a factor |
For corrosion-critical duties, check the base alloy family pages: aluminium bronzes for seawater, high-tensile brass for strength, tin bronzes for worm-gear-compatible smoothness.
Manufacturing Capabilities, Graphite, MoS₂ and PTFE bearing
Customization and Support
Manufacturers often provide bespoke solutions, working closely with clients to design and produce slip rings that meet specific operational needs. Technical teams can advise on material selection, design optimization, and performance enhancements.
By advanced CNC machining and high-quality bronze alloys, manufacturers can produce slip rings that deliver superior performance, reliability, and longevity in even the most challenging conditions.
