Oilless Bushings For Rubber & Plastic Machinery
We provide custom manufacturing of finished bronze parts and self-lubricating bronze, precisely based on our clients’ drawings.
Oilless Bushings for Rubber and Plastic Machinery: Heat, Load and Limits
Rubber and plastics processing combines three things a plain bearing dislikes: heat, abrasive filler, and a product that a stray drop of grease will scrap. Internal mixers, two-roll mills, calenders, extruders, injection moulding machines and pelletizers all have pivots and neck bearings where relubrication is impractical and contamination is expensive. This page sets out where oilless bushings are used on that equipment, what temperature and PV they can be specified to, how carbon black and glass fibre change the wear calculation, and the cases where a lubricated bearing is still the right answer.
Where oilless bushings sit on the machine
| Machine | Bushing locations | Typical temperature at the bearing | Load and motion |
|---|---|---|---|
| Internal mixer (Banbury type) | Rotor neck bearings, dust stop and seal rings, discharge door pivots, ram guides | Neck area commonly 60–120 °C (140–250 °F); higher near the discharge | High radial load with slow, uneven rotation and heavy shock at charge |
| Two-roll mill and open mill | Roll neck bearings, nip adjustment screws and slides, roll end thrust faces | Neck housing commonly 60–120 °C (140–250 °F) depending on cooling | Very high steady radial load, continuous slow rotation |
| Calender | Roll neck bearings, roll bending and cross-axis slides, take-off pivots | Roll surface often 60–120 °C (140–250 °F) | High steady load, slow rotation, small adjustment movements |
| Extruder | Screen changer slide plates, die head pivots, feed throat and hopper slides | Slide plates can see 180–260 °C (355–500 °F) on some polymers | Intermittent sliding under high load at low speed |
| Injection moulding machine | Toggle clamp pivots, tie bar nuts and guides, ejector guide bushings | Clamp area commonly 40–80 °C (105–175 °F) | High shock at clamp closure, oscillation over a fixed arc |
| Granulator and pelletizer | Feed roll pivots, screen cradle pivots, cutter adjustment slides | Ambient to about 60 °C (140 °F) | Shock and vibration, intermittent motion |
Why conventional lubrication struggles here
| Problem | Mechanism | What it costs |
|---|---|---|
| Heat | Above roughly 120–150 °C (250–300 °F) many greases oxidise, carbonise or lose their base oil by evaporation; relubrication intervals shorten sharply | Dry contact, scored journals, unplanned strip-down of a hot machine |
| Product contamination | Escaped grease or oil mist reaches the compound or the moulding | Rejected batches, and a difficult conversation where medical, food-contact or optical parts are involved |
| Abrasive filler | Carbon black, silica, glass fibre and mineral filler migrate into the clearance and mix with grease into a lapping paste | Wear rate several times the clean-running figure |
| Chemical attack on the lubricant | Process oils, plasticisers and some monomers dissolve or wash grease out of the joint | Lubricant disappears regardless of the relubrication interval |
| Access | Toggle pivots, roll necks and mixer necks require substantial disassembly to reach | Labour and downtime dominate the cost of the bearing itself |
Material options compared
| Type | Construction | Typical dry PV limit | Typical temperature range | Where it fits |
|---|---|---|---|---|
| Graphite-plugged bronze | Cast bronze body with graphite plug compound in drilled cavities; plugs typically cover 20–30% of the sliding area | Commonly quoted around 1.0–1.6 MPa·m/s continuous; surface speed usually below about 0.5 m/s (100 ft/min) dry | Alloy-limited rather than graphite-limited; bronze body usable well above 200 °C (390 °F) where the alloy retains strength | Mixer necks, roll necks, toggle pivots — high load, slow or oscillating motion. See JDB-type oilless bearing construction |
| High-tensile brass / manganese bronze with plugs | CuZn25Al6Fe3Mn3 or C86300 matrix with graphite plugs | Similar order to graphite-plugged bronze; higher load capacity from the stronger matrix | Check the alloy’s strength retention at temperature before specifying | Large roll neck and calender bearings where load per unit area is high |
| MoS₂-impregnated bronze | Bronze with molybdenum disulphide in the surface layer | Depends on the impregnation depth and binder | Limited by the binder and by oxidation of MoS₂ in air at elevated temperature | Dry, low-humidity positions and small-amplitude movements where graphite transfers poorly |
| PTFE-lined composite (steel backing, sintered bronze, PTFE lining) | Thin-wall wrapped bushing | Commonly quoted around 1.6 MPa·m/s continuous, higher intermittent | Roughly −200 °C to 250–280 °C (−330 °F to 480–535 °F) depending on the lining | Lighter loads, higher speeds, small oscillation angles — ejector guides, linkages, adjustment slides |
| Graphite-plugged cast iron or steel | Ferrous body with plugs | Lower than bronze; used where cost or size dominates | Wide | Large slow guides and wear plates, not precision bores |
Plug layout for the bronze options is not a detail to leave to the pattern maker. Hole diameter, pitch and coverage are derived from the load and the sliding distance per cycle; the method is set out in calculating plug size for graphite-plugged bronze bearings, and the family of designs is described under graphite bushings and bearings.
Heat is the variable that governs the design
Two effects have to be checked at working temperature rather than at room temperature.
- Strength falls as the bushing gets hot. PV limits quoted in catalogues are usually room-temperature figures. The allowable load has to be reduced as the bushing temperature rises, and aluminum bronze retains useful strength to higher temperatures than tin bronze does. Where a neck bearing runs above about 150 °C (300 °F), ask for the strength figures at that temperature instead of assuming the ambient-temperature number.
- The clearance changes when the journal is hot. Taking about 12 × 10⁻⁶/K for a steel roll neck and 18 × 10⁻⁶/K for bronze, the arithmetic change in geometric clearance over an 80 K rise on a 200 mm (7.9 in) neck is of the order of 0.1 mm (0.004 in). The steel housing constrains the bushing, so the real figure is smaller — treat 0.1 mm as an upper bound and confirm by measurement. Rolls that are water-cooled internally and heated externally can also run out of round, so measure the neck at working temperature, not cold.
Abrasive filler: exclusion beats hardness
Carbon black, precipitated silica, glass fibre and mineral fillers are harder than any copper alloy. Silica and glass sit around 6–7 on the Mohs scale; bronze does not. Choosing a harder bushing does not solve the problem, and a hard bushing will score an unhardened journal faster than a softer one wears.
- Keep the filler out. Seals, dust lips and purge grooves at the mixer dust stop matter more than alloy selection. Where the dust stop no longer seals, no bushing alloy will give a long life.
- Give the debris somewhere to go. Grooves and relief areas let abrasive particles leave the loaded zone instead of being rolled through it.
- Harden the counterface. Journals at about 300 HB and above, finished to Ra 0.4–0.8 µm (16–32 µin), resist the scoring that starts the wear cycle.
- Plan for measured wear. Set a clearance limit and check it at fixed intervals, rather than replacing on a calendar.
Selection and installation checklist
| Item | Typical starting value | Note |
|---|---|---|
| Running clearance | 0.10–0.20% of journal diameter, plus allowance for hot running | On a 200 mm (7.9 in) neck that is 0.20–0.40 mm (0.008–0.016 in) before the temperature allowance |
| Journal hardness and finish | About 300 HB, Ra 0.8 µm (32 µin) or better | Softer journals suit leaded tin bronze, which conforms rather than scores |
| Housing fit | Housing bore H7, bushing outside diameter r6 or s6 | Thin-wall composite bushings follow their own fit tables |
| Plug coverage | Roughly 20–30% of the sliding area | Distributed so every point of the journal sweeps plugs in normal motion |
| Installation | Press with a mandrel or arbor, never by hammering on the flange | Check the bore after pressing; a thin-wall bushing will close down to the housing fit error |
| Run-in | Reduced load and speed for the first period of operation | The transfer film needs sliding distance to establish; friction is higher until it does |
| Measurement | Bore and journal at 20 °C (68 °F), recorded on the inspection report | See reading and measuring bearing tolerance |
Where an oilless bushing is the wrong choice
- Continuous rotation at meaningful surface speed under load. Above the dry PV figure, heat has nowhere to go. Extruder and gearbox shafts running at speed belong on rolling or hydrodynamic bearings.
- Where the oil is doing the cooling. On a hot roll neck the circulating oil removes heat as well as separating the surfaces. Removing it raises the contact temperature, so water-cooled housings or a different arrangement is needed before an oilless conversion makes sense.
- Small oscillation angles. Below roughly 2–3° of movement the plugs never sweep the loaded arc and fretting takes over. A PTFE-lined composite usually handles that duty better.
- Light load at high speed. Graphite needs contact pressure and sliding distance to transfer; a lightly loaded fast joint tends to fret instead. Use the composite or a bonded solid film.
- Corrosive fume from degraded polymer. Where thermal degradation can release acidic fume — PVC is the usual case — check the alloy against the actual fume composition before committing to bronze.
- Where a lubricated bearing is accessible, cool and already giving acceptable life. Converting adds cost without a measurable gain if contamination and access are not the problem.
Service checks
| Check | Typical interval | Renew when |
|---|---|---|
| Clearance at the neck or pivot | Every 1,000–2,000 operating hours, or each scheduled strip-down | Clearance roughly double the as-built value, or visible play without tools |
| Journal condition | At every bushing replacement | Scoring through the hard layer, or ovality beyond the machining allowance |
| Seal and dust stop condition | Every maintenance window | Filler is reaching the bearing surface |
| Plug condition | At every bushing replacement | Plugs worn flush over a large part of the loaded arc |
Intervals quoted are workshop practice, not a machine builder’s schedule. Where the equipment manual gives a figure, follow it.
What to send for a quotation. Drawing or dimensions (ID, OD, length, flange, groove or plug pattern), journal material and hardness, steady and peak load, speed or oscillation angle and cycle time, minimum and maximum operating temperature, polymer or compound being processed, and quantity.
Frequently asked questions
Can oilless bushings replace greased roll neck bearings on an existing mill?
Sometimes. The checks are load per unit area against the dry PV limit, the neck temperature at the bearing, the journal hardness and finish, and whether the housing can be line-bored. Where the neck runs hot or the load is high, a graphite-plugged high-tensile brass or manganese bronze is usually evaluated first. Send the neck drawing and the roll separating force if you have it.
What temperature can these bushings run at?
The bronze body is rarely the limit; the alloy’s strength retention, the plug compound and the housing fit are. Bronze plug bearings are used well above 200 °C (390 °F) in some positions. Ask for the allowable load at your operating temperature, because the room-temperature PV figure does not apply there. PTFE-lined composites are usually limited to around 250–280 °C (480–535 °F).
Graphite plugs, MoS₂ or a PTFE composite?
Graphite-plugged bronze for high load at low speed or oscillation, which covers most mixer, mill and toggle duty. MoS₂-impregnated bronze where the joint is dry and the movements are small. PTFE-lined composite for lighter loads and higher speeds. The comparison between the first two is set out in oilless bushes versus traditional lubricated bearings.
Do they need any lubrication at all?
Not in service. A thin film of assembly grease is normal to protect the journal during start-up before the transfer film forms. Some users add a small amount of grease at long intervals to flush debris out of the joint; that is fine, but the design should not depend on it.
Can you make split or half bushings?
Yes. Roll necks and large pivots are commonly supplied as half bushings so the bearing can be renewed without withdrawing the roll. We need the housing bore and the cap bolt arrangement to hold the halves in place.
Which positions on an injection moulding machine suit oilless bushings?
Toggle clamp pivots and tie bar guides are the usual candidates, because they oscillate under high load and any grease that escapes ends up near the mould. Ejector guides tend to be lighter loaded and faster, so a composite or a dedicated oilless ejector guide bearing is often a better match; more on that machine is on the plastic injection moulding machine page and injection moulding machine bronze bush page.
Send the bearing and the duty
Send the drawing or the worn part, the journal material and hardness, the load and motion, and the temperature range. We will confirm whether a self-lubricating bronze suits the position, recommend one or two options with numbers, and quote to your drawing.
