Bronze Wrapped Bushings, Bronze Alloys and PTFE Composite Bearings: Material Selection, Sizing and Application Matrix

Plain bearing terms: sleeve, slide, bush and dry bearing

In everyday machine language sleeve bushing, slide bushing, plain bushing and journal bearing describe the same family: a cylindrical component that supports a rotating or oscillating shaft on a low-clearance sliding surface. The word bushing usually implies a self-contained insert pressed into a housing; slide bearing is a broader German-influenced term that also covers thrust washers, slide plates and liners.

A dry bearing is a functional claim, not a geometry. It means the bearing runs without an external oil or grease supply for its design life or between scheduled maintenance, because a solid lubricant is built into the construction. Understanding which constructions legitimately qualify as dry-running prevents the most common selection error in this field.

Five constructions that are routinely confused under the name “bronze bushing”

Self-lubricating and dry-running are not synonyms. Solid bronze is neither. Bronze with solid-lubricant plugs and PTFE composite bearings are true dry-running constructions; oil-impregnated sintered bronze is self-lubricating but depends on an internal oil charge.

PTFE composite bearings: body material selection and envelope

How the construction works, from first principles

A PTFE composite bearing is a thin laminate. A load-carrying metal strip backing gives the component strength, dimensional stability and the ability to be pressed into a standard housing. Onto that backing a PTFE-based sliding layer, often reinforced with glass fibre or other fillers, is bonded either directly or over an optional porous sintered-bronze interlayer that anchors the polymer and conducts heat away from the interface.

Three physical facts drive everything that follows:

• Transfer film. During running-in, a thin layer of PTFE transfers onto the shaft. The bearing then slides against its own material, which is why dry friction is low and stick-slip is small.
• Creep, not wear, sets the pressure limit. PTFE creeps (cold-flows) under sustained load, so the maximum static pressure of a composite bearing is set by creep resistance, not by melting. The metal backing suppresses thermal expansion: the in-plane thermal expansion of a steel-backed composite is close to steel, not to bulk PTFE.
• PV is a heat balance. Interface heat is proportional to friction coefficient times pressure times velocity (mu x P x V). If PV goes above the published envelope, the polymer layer softens and wears quickly, or the bond to the backing degrades.

Because there is no oil to flush contamination, composite bearings suit fairly clean interfaces. The shaft is part of the bearing: it should be hard and smooth so it does not cut the PTFE and so the transfer film is stable.

Metal strip backing options (body material selection)

For corrosive or high-temperature machinery, the backing metal is chosen independently of the sliding layer. Published product data for this bearing class lists the following body alloys with their maximum recommended bearing load (values are representative of one published product line):

The load figures above are maximum static bearing loads published for one product family and are engineering reference values only. Nickel alloys named by family here are often sold under commercial trade names; specify by UNS number and governing material standard instead. Not every backing metal is available in every size.

Published operating envelope (reference)

Reference values taken from a published product spotlight for a fibre-reinforced PTFE composite bearing and are typical of this product class, not fixed limits for every supplier. Additional design points often quoted for this class include: high impact strength, high resistance to wear, low tendency to brinell or fret the counterface, dimensional stability, availability of custom sizes, and no lubrication requirement.

Bronze wrapped bushings: alloy, ISO 3547 sizing and fits

What a wrapped bushing is

A wrapped (rolled) bronze bushing starts as a flat strip of wrought phosphor-bronze. The strip is formed into a cylinder with a longitudinal seam, then sized and calibrated so that, after press-fitting, it takes the shape of the housing bore. The sliding surface is usually embossed with diamond indentations, or drilled with lubrication holes and grooves, that act as grease reservoirs. Wall thickness is small (roughly 0.75 to 2.5 mm), which saves material and conducts heat well, but the thin wall also means the bushing has no ability to carry load until it is fully seated in the housing.

The governing geometry standard is the ISO 3547 family for wrapped bushes (the former German DIN 1494 series has been superseded by it), supported by ISO 4382-2 for wrought bearing copper alloys and ISO 4384-2 for hardness testing. Lubrication features such as holes, grooves and indentations are covered by ISO 3547-3.

Strip alloys used for wrapped bushings

Typical mechanical and physical values of the strip

Values are representative for cold-rolled strip in a medium (R420-R450) temper, compiled from European copper-mill data sheets. Higher tempers raise strength and hardness but reduce elongation. Machinability ratings follow the Copper Development Association scale where free-cutting brass equals 100; phosphor bronze is deliberately hard to machine, which is why wrapped bushes are formed rather than cut.

ISO 3547 dimensioning and size selection

ISO 3547-1 lists preferred wall thicknesses of 0.75, 1.0, 1.5, 2.0 and 2.5 mm across a bore range from a few millimetres to roughly 300 mm. The width (length) of the bushing is a design choice; length tolerance bands are classified in the standard as fine, medium or coarse. Because a wrapped bushing is supplied slightly open and closes when pressed, ISO 3547-2, -5 and -6 define how to check the outside diameter, the wall thickness and the fitted inside diameter.

Practical starting points only; the standard does not mandate a wall for a given bore. Longer bushings carry more load but become sensitive to misalignment, so keep the length-to-diameter ratio in the ranges given in section 5.

Fits, press-fit convergence and installation

• Housing: typically H7 for steel and cast housings; lightweight housings may need a larger interference or a locking feature because the shell follows the housing bore.
• Shaft: typically e7 to f8 for a wrapped bronze bushing, with a surface finish around Ra 0.4 to 0.8 micrometres [16 to 32 microinches].
• Running clearance: diametral clearance is typically 0.02 to 0.08 mm [0.0008 to 0.003 in], rising with diameter and with thermal expansion of the shaft.
• Press-fit convergence: after pressing, the bore shrinks by approximately twice the wall thickness (housing bore minus two walls), assuming the housing does not expand. Check the fitted bore with a plug gauge or three-point measurement and, if the bushing carries a machining allowance, ream or size it after fitting.
• Chamfers and lead-in: chamfer the housing mouth and the shaft end so the strip edge is not peeled during assembly; a 10 to 15 degree lead is common.
• Lubrication: fill the indentations and grooves with grease before assembly and schedule regreasing according to duty. An unlubricated wrapped bushing is a temporary condition, not a design mode.

Solid bronze sleeve bushings: casting alloy selection

When the bore is large, the load severe, or the part machined rather than rolled, the bushing is made as a solid bronze sleeve from a continuous-cast, centrifugal-cast or sand-cast bar. Solid bushes accept oil grooves, flanges and oil holes machined after casting, and they hold up where a thin shell would collapse. Alloy families below follow EN 1982 designations, with UNS / ASTM parallels shown as comparable, not identical.

Tin bronzes and leaded tin bronzes (general and hydraulic duty)

Minimum values in the table follow EN 1982 for continuous or centrifugal casting; sand casting gives lower values. Hardness is on the Brinell scale. Machinability is on the CDA scale (free-cutting brass = 100).

High-strength aluminium bronzes and manganese bronze (heavy pivots)

Minimum mechanical values follow ASTM B505 for continuous cast material. EN 1982 parallels (for example CC762S for the manganese bronze family) state similar but not identical limits; verify against the governing standard.

Selecting the bronze family

• Leaded tin bronze (CC493K / C93200) is the default: it is machinable, tolerant of misalignment, and embeds dirt. Choose it unless the load or environment says otherwise.
• High-lead bronze (CC496K / C93800) suits oil-starved, dirty, or water-lubricated service where embeddability matters more than strength, and where temperatures stay moderate.
• Plain tin bronze (CC480K / CC483K) suits clean, well-lubricated, high-pressure oil-film bearings and parts that also face wear or shock.
• Aluminium bronze (C95400 / C95500) and manganese bronze (C86300) are for slow heavy pivots, shock, and aggressive wear. They need a hard ground shaft, reliable lubrication, and machining with sharp carbide tooling.
• Where the position cannot be greased, move from solid bronze to bronze with graphite or PTFE plugs (a self-lubricating construction) instead of running solid bronze dry.

Sizing from first principles: pressure, velocity, PV and L/D

The three governing equations

The projected area is bore times length, not the wrapped surface area. Pressure is therefore a mean value; real peaks at the edge of the contact are higher, which is why conformability and chamfers matter.

Worked example

Unit conversions worth keeping on the drawing board

Length-to-diameter ratio and wall thickness

• For radial bushes, an L/D between 0.5 and 1.5 is the working band. Below 0.5 the bearing is very sensitive to edge loading; above about 2.0 misalignment and heat build-up dominate.
• If the frame deflects or the shaft deflects under load, shorten the bearing or move to a self-aligning or more conformable construction instead of extending it.
• Thin wrapped shells (1.0 to 2.5 mm) are economical and cool well; thick or solid bushes are chosen for strength, reaming, flange features and heavy shock.

Fit summary by construction

Fits are conventional engineering practice for each class; always confirm with the component maker and the ISO 3547 / ISO 4379 verification rules for the fitted bore.

Decision order that avoids rework

1. Describe the motion: continuous rotation, oscillation, or intermittent positioning. Oscillation rarely builds an oil film, so dry-running or plug-type constructions move up the list.
2. Fix the load and geometry, compute P, V and PV, and keep a 2x to 3x margin against nominal limits.
3. Ask whether the position can be lubricated and reached for maintenance. No access means a true dry-bearing construction.
4. Scan the environment: dust, water, chemicals, temperature, washdown. This decides the body metal and whether the sliding layer needs protection.
5. Set the shaft specification (hardness, finish) together with the bearing, then check the fitted clearance after press-in.

Application-position matrix by machine and duty

The matrix below is an OEM and field-practice summary: for each machine family it lists the typical bearing positions, the character of the duty, and the construction that normally fits. Treat it as a shortlist, not a substitute for the calculations in section 5.

Field practice summary compiled from plain-bearing engineering literature. For regulated sectors, follow the specific rules of the responsible design authority (see the disclaimer below).

Failure modes and how design avoids them

Frequently asked questions