Steel-Back PTFE Composite Bearings and Water-Lubricated Bushings
Manufacture and application of bronze rings, particularly in machinery and equipment.
Steel-Back PTFE Composite Bearings and Water-Lubricated Bushings Bearing Solutions
The steel-back PTFE composite solves a problem solid bronze cannot: running dry, from day one, at useful loads, on a soft shaft. A thin PTFE-rich layer — often lead-free formulations today — bonded to a sintered bronze interlayer on a steel backing gives a bearing that is strong where it must be and slippery where it touches. This page covers construction, the dry and water-lubricated operating envelopes, and the boundary conditions where a composite wins or loses against bronze.
Construction: the three layers
| Layer | Typical thickness | Function |
|---|---|---|
| PTFE-rich sliding layer (with fillers) | ~0.02–0.05 mm | Low friction, self-lubricating film formation |
| Sintered bronze interlayer (CuSn10 type) | ~0.2–0.4 mm | Keying of the liner, heat spreading into the backing |
| Steel backing (low-carbon or stainless) | ~0.5–2.5 mm | Strength, press-fit integrity, geometry |
The division of labour explains the behaviour: the steel carries mechanical load and holds press-fit, the bronze interlayer anchors the liner and drains friction heat, and the thin PTFE layer does the sliding. The liner is sacrificial — wear life is a function of liner thickness and duty severity, not of bearing “strength”.
Liner variants: PTFE and POM
| Liner | Lubrication | PV, MPa·m/s (dry) | Character |
|---|---|---|---|
| PTFE + fillers (Pb-free formulations standard today) | None required | ~1.4–1.8 (continuous) | The classic dry-running maintenance-free bushing |
| Boundary-lubricated PTFE (indentations) | Grease at assembly / periodic | ~2.4–3.6 | Indents store grease; higher PV with marginal lubrication |
| POM (acetal) lined, indentations | Grease at assembly | ~1.8–2.2 | Lower friction on softer shafts, good shock-load behaviour |
Values are indicative continuous-duty figures from typical supplier engineering data; excursion limits (short peaks) run several times higher. Confirm per catalogue and duty cycle.
Operating envelopes, dry and wet
| Condition | Load, MPa | Speed, m/s | PV, MPa·m/s | Temperature |
|---|---|---|---|---|
| Dry, continuous rotation | up to ~25–30 | ≤1–2 | ~1.4–1.8 | −200 to +280 °C (PTFE base) |
| Dry, oscillating / short strokes | up to ~100+ | very low | PV rarely limiting | Stick-slip is the design risk |
| Water or process fluid lubricated | per fluid and design | higher speeds attainable | fluid-dependent | Check liner and steel-back chemistry |
Two properties stand out against bronze: the static load ceiling (well above 100 MPa briefly, since the steel back carries it) and the low-temperature range, where PTFE keeps working at temperatures that would embrittle oils and greases. The corresponding weakness is speed — dry running at high speed overheats the thin liner, and PV, not load, is the limiting quantity.
Water-lubricated applications
Composites are widely used where the process fluid is the lubricant: water pumps, offshore and marine equipment, food-wash-down zones. The design considerations differ from dry running:
- Fluid compatibility. PTFE and POM tolerate water and most aqueous process fluids; check the steel backing grade for corrosion (stainless backing where wash-down or seawater splash applies).
- Abrasive fluids. Silt-laden water attacks the thin liner quickly; upstream filtration or harder-faced designs protect life.
- Hydrodynamic potential. At sufficient speed, water film pressure can lift the load — water-lubricated composite bearings in pumps and stern tubes exploit this, and their load capacity at speed can exceed the dry figures substantially.
Boundary conditions: when bronze is the better answer
- Continuous high speed, dry. The liner cannot shed heat fast enough; a graphite-plugged bronze or a greased bronze bushing wins above roughly 1–2 m/s dry.
- Heavy impact plus dirt. The thin liner has no embeddability to spare; a leaded tin bronze or graphite-plugged bronze absorbs contamination better.
- Machined-in-place geometry. Composites are calibrated products; oversized bores that would be finish-bored in bronze need a different product family or an oversize composite.
- Very high temperature. PTFE tops out near 280 °C; beyond that, graphite-based systems take over (MT02).
Fitting and machining practice
- Housing bore tolerance follows the wrapped-bushing standards (ISO 3547 / DIN 1494 H7 typical); the bushing’s own outside diameter is calibrated for the standard press-fit range.
- Do not machine the liner. Cutting into a PTFE composite bore removes the functional layer and exposes the bronze interlayer at uneven depth. Select the bushing size so the as-fitted bore meets tolerance; if a special size is unavoidable, order it machined by the supplier.
- Split line and calipering. Wrapped composites have a split; expect and measure the fitted bore, not the free-state diameter.
- Shaft finish. Ground shafts around Ra 0.2–0.8 µm extend liner life; hardness requirements are far lower than for bronze — composites run acceptably on unhardened steel.
FAQ
- Can a PTFE composite bushing run completely dry?
- Yes — that is its design purpose. The PTFE layer forms a transfer film on the shaft during running-in and then runs on it. Continuous dry PV is roughly 1.4–1.8 MPa·m/s, so check duty: dry running at high speed, not high load, is the usual failure route.
- Why does the bushing have a sintered bronze layer if PTFE does the sliding?
- The bronze interlayer mechanically keys the thin PTFE liner to the steel backing and conducts friction heat away from the contact into the backing. Without it, the liner would peel and overheat locally.
- Can I bore a PTFE composite bushing oversize in place?
- No. Machining the bore cuts into the functional liner and leaves the bronze interlayer exposed at uneven depth. Composite bushings are used as calibrated; special sizes must be ordered pre-machined.
- PTFE composite or graphite-plugged bronze for a maintenance-free application?
- Decide by duty. Low-to-moderate load, low speed, soft shaft, wide temperature range — composite. Heavy load, shock, large size, high temperature, water immersion — graphite-plugged bronze. Where both fit, the composite is usually the lower-cost option in series production.
- Are PTFE composites available lead-free?
- Yes. Lead-containing PTFE formulations have largely been replaced by lead-free filler systems across the industry. For food-machinery or export-sensitive applications, request the liner composition declaration explicitly.
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Frequently asked questions
Which bushing positions wear first on a toggle clamp?
In practice the link pin joints in the centre of the toggle train, followed by the crosshead guides. The link pins carry the amplified clamp force through a rotation of only a few degrees, which is the amplitude at which grease is worked out of the contact rather than drawn into it. The crosshead guides carry the crosshead weight and the reaction from the toggle train, and they slide rather than rotate, so any loss of lubricant film shows up there quickly. Tie bar bushings on the moving platen usually wear more slowly, because the load is more evenly distributed and the alignment is better controlled. On a fast-cycling machine, expect the link pins and the crosshead guides to set the maintenance interval.
What is running-in, and why does it matter for tie bar life?
Running-in is a small, controlled amount of early wear that converts the initial line or point contact between a new bushing and its pin into a conforming area contact. It happens whether or not it is planned. Planned, it is carried out at reduced load and speed with adequate lubrication, and the bronze gives up the material rather than the pin. Unplanned, it happens at full clamp force on the first production shift, the contact pressure at the line of contact is greater than what the surface can carry, and the material that moves comes from whichever surface is softer or rougher. A tie bar is expensive, difficult to remove, and its surface condition governs the life of every bushing that runs on it.
Do toggle pins and tie bars need to be hardened?
For a bronze bushing to give its published service life, yes. The published counterface requirement for graphite-plugged bronze is 40–45 HRC or harder with a surface finish of Ra 0.8 µm or finer, and the same specification is the sensible default for a grease-lubricated bronze bushing on a toggle pin or a tie bar. Below that hardness, asperity contact during running-in and under shock load damages the pin rather than the bushing, and once a tie bar is scored it abrades every replacement bushing fitted to it. Hardening the pin or the bar costs a fraction of replacing a tie bar, and it belongs on the drawing rather than left to the shop floor.
Which grade is used for toggle pivots at high clamp force?
Start from the projected bearing pressure on the pin rather than from the clamp tonnage alone, because the toggle geometry amplifies the cylinder force by a factor that changes through the stroke. Where pressure is high and the section is compact, CuZn25Al5 (CC762S comparable, comparable to C86300) at 780 MPa typical and 210 HB typical is the usual starting point. Where the geometry allows more bearing area and sliding behaviour governs, CuSn12Ni2-C (CC484K, comparable to C91700) at 300 MPa minimum tensile and 180 MPa minimum yield gives good conformity and galling resistance. Confirm every figure against the material test certificate, and note that published minimum values are specification floors rather than expected production values.
Is heat from the mould a problem for clamp bushings?
It can be, and it is the input most often missing from the specification. The clamp unit sits close to a mould running at elevated temperature, and heat conducts into the platen and along the tie bars. The number that matters is the metal temperature at the bushing, not the mould temperature setting. Where the measured metal temperature at the bushing stays low, a leaded bearing bronze is appropriate; where it rises, the alloy choice has to change, because published maximum mechanical service temperatures for leaded tin bronze classes sit far below the melting point of lead at 327 °C. Measure the metal temperature at the bushing bore during steady production before committing to an alloy.
Can you switch from grease to oil-free bronze without changing the pins?
Only if the existing pins already meet the counterface requirement. A graphite-plugged bronze bushing is specified against a counterface of 40–45 HRC or harder at Ra 0.8 µm or finer, and against a dry-running allowable surface pressure of approximately 100 N/mm² with a sliding speed ceiling of 0.5 m/s. If the pins are soft, scored or rough, fitting a plugged bushing will not restore the surface — it will run against the damaged one. Measure hardness, diameter, roundness and surface finish first. Where the pins are sound, the change is straightforward; where they are not, the pin work is the job and the bushing is the smaller part of it.
How does bushing friction stability affect flash and part consistency?
Through clamp repeatability. Every cycle the clamp closes, builds tonnage and holds it while the cavity is filled and packed. If friction in the toggle joints varies from cycle to cycle — because the joint is in a stick-slip regime, or because the lubricant film breaks down intermittently — then the point at which the toggle reaches full extension, and therefore the tonnage actually applied, varies with it. Applied tonnage that drifts below what the cavity needs shows as flash at the parting line; tonnage that varies between shots shows as dimensional variation in the part. A bushing with a stable coefficient of friction is therefore a process-capability item rather than a maintenance item.
What are the wear limits and inspection intervals for platen and tie bar bushings?
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