The operating envelope for mining bronze bushings and how to read it

Load, sliding speed and PV are not three independent checks. They are one set, and quoting one without the other two is the most common way a specification goes wrong. Unit load tells you whether the material will deform or extrude. Sliding speed tells you how fast frictional heat is being generated at the contact. PV — unit load multiplied by sliding speed — is the proxy for the rate of that heat generation, and heat is what degrades both the graphite transfer film and the metal matrix.

So: work out the projected bearing pressure from force divided by diameter times bearing length, calculate the mean sliding speed from the swing angle and cycle time, multiply them, and compare all three figures against the envelope below. Record alongside them whether the duty is continuous or intermittent, because an intermittent peak and a continuous load of the same value are different design cases.

These figures apply to dry running. Where a PV or load value is quoted, the lubrication regime — dry, boundary or full-fluid — and whether the duty is continuous or intermittent must be stated alongside it. Plug coverage and diameter are set out under plug layout below. The same envelope applies across all bronze bushing applications; what separates one path from another is the operating condition that governs, not the numbers themselves.

Plug layout

Coverage of 20–30 % of the sliding surface is the working range, with some published standards specifying 25–35 %; plug diameters run from 3 to 10 mm. Plugs are laid out in staggered rows rather than a single line, so that every point on the counterface passes over a plug during each stroke — a straight axial row leaves unlubricated tracks. Two other rules: keep enough edge distance that the plugs do not break out at the chamfer or the end face, and keep enough wall thickness that the plug does not bottom out against the housing bore.

Friction and breakaway

The 0.08–0.20 dry coefficient is a running figure. Breakaway is higher, and on a cylinder-driven pivot or a manual adjustment mechanism that difference matters: it shows up as the initial stiction on the first movement after a dwell. It is one of the reasons graphite-plugged bronze is not the answer for precision positioning.

Temperature comes from the matrix

The temperature ceiling is set by the metal, not by the lubricant. Lead melts at 327 °C. Published maximum mechanical service temperatures are approximately 140 °C for C93700-class leaded tin bronze, approximately 250 °C for CuSn12 and approximately 260 °C for C86300; the incipient melting point of C93800 is 316 °C. Above approximately 250 °C the answer is an aluminium bronze, because only a change of matrix raises the ceiling; a tin bronze with embedded solid lubricant answers a different question — it is for a position whose temperature already sits inside the tin bronze ceiling but where no grease or oil supply can be maintained. Embedded graphite removes the lubrication limit, not the temperature limit. Be careful with the wide −40 to +400 °C band that appears in some suppliers’ catalogues for an entire product family: that describes what the solid-lubricant system and a short-term envelope will tolerate, not the strength limit of the metal matrix holding the plugs.

Selecting a bronze by duty for mining

Four grades cover most mining bushing work. The table pairs each duty condition with the grade that fits it and the ceiling that goes with it.

Rm = tensile strength, Rp0.2 = 0.2 % proof strength, A5 = elongation after fracture. Values marked “min.” are specification floors bound to the test-bar or casting section size named in the governing standard; values marked “typical” are representative production figures and are not specification minimums. Cross-standard relationships are comparable to, or similar but not identical to, the UNS designations shown, and require independent verification before substitution. Machinability on the CDA scale (free-cutting brass = 100) is 30 for C90500, 70 for C93200 and 50 for C95800. Data compiled from published EN, DIN, ASTM, SAE and BS sources, cited only as sources of published material property data.

Two points belong with every figure in that table. Section size effect: published values are bound to the test-bar or casting section size given in the governing specification; large sections generally show lower strength and hardness, particularly in yield; and the outer skin of a casting is harder than its core, so a part machined heavily from one face no longer retains the as-cast surface values. “min.” is not an expected value: these are specification floors rather than production targets, and no design should be built on the minimum figure.

On the grades themselves: CuZn25Al5 is a high-tensile brass rather than a true bronze, and its typical tensile strength of around 780 MPa is why it is the starting point for the highest static loads and impact duty. The nickel aluminium bronzes give up some of that strength but hold it in the presence of water and slurry, which is why they are the choice wherever wash-down or wet screening is involved. CuSn12-C and CuSn12Ni2-C are the sliding grades, with the nickel-bearing variant giving a higher proof strength at the same tensile floor. CuSn7Zn4Pb7 is the general-purpose grade and machines at 70 on the CDA scale, which matters when you are cutting hundreds of identical bushes — but check the temperature ceiling before using it near a hot process.

Pin, housing and fit specification for mining bushings

Published guidance for graphite-plugged bronze is a counterface of at least 40–45 HRC at Ra 0.8 µm or finer. That is not a nicety. The graphite transfer film needs a surface smooth enough to retain it and hard enough not to be abraded by it. A pin at 200 HB will wear, and once it has worn it stops supporting the film, so bushing life collapses even though the bushing itself was correctly specified.

The following is a composite description of representative duty, not a measured installation. Where a conveyor pivot fleet shows a history of short bushing life, the counterface specification is the first thing to check: pins brought up to the mid-40s HRC and ground to Ra 0.8 µm typically outlast several bushing changes, and they stop being scrapped alongside each one. In that pattern the bushing was never the weak link.

Clearance on large pins

Large-diameter pivots need more clearance than small ones, for four reasons that compound: thermal growth of both parts, elastic deflection of the pin under load, the tolerance stack-up across a long bearing, and the reality that a large pin cannot be held to the same geometric tolerance per unit length as a small one. As a working rule, start from a diametral clearance around 0.1–0.2 % of the pin diameter with a practical minimum, and increase it for hot duty, for long bearings and where deflection is significant. Guidance for the standard fits is set out under fits and clearances for bronze bushings.

Retention and bore closure

A press fit alone is not enough where the load reverses or the machine vibrates — both of which describe a screening plant. Add mechanical retention: a flanged form, dowels, or a split design. Remember that pressing a bushing into a housing closes the bore, and the reduction scales with both the interference and the wall-thickness-to-outside-diameter ratio; measure after installation and finish-machine where the tolerance requires it.

Failure appearance and what it means for mining bushings

A removed bushing tells you why it came out, if you read it in the right order.

• Polished flats in the load zone. The joint is carrying more load or more PV than the envelope allows, or the swing angle is so small that the load never redistributes around the bore. Look at the length: if the flat is shorter than the bearing, the deflection is real.
• A dark graphite film smeared on the pin. This is the system working. A uniform grey film on the counterface is the transfer film, not a fault, and it should not be cleaned off during reassembly.
• Wear concentrated at one end of the bore. Edge loading from misalignment, from pin deflection, or from a housing bore that is not square to the axis. No material change fixes this; the geometry does.
• Scored or galled pin. The counterface was too soft, too rough, or abrasive got in. Expect the replacement bushing to fail early if the pin is reused in this condition.
• Plugs standing proud, broken out, or missing. Insufficient wall thickness behind the plug, inadequate edge distance, or damage during installation rather than a wear failure.
• Uniform wear all round the bore. Normal end of life. Note the measurement against the original clearance so the next interval can be set from data instead of guesswork.

When graphite-plugged bronze is the wrong answer

Five situations, and in each of them the joint will be better served by something else:

• Continuous rotation above roughly 0.5 m/s. Heat is generated faster than the film and the matrix can carry it away. Use an oil-lubricated bearing or a rolling bearing.
• A reliable, sealed oil supply is already there. A fluid film does the job properly; the plugs add cost without adding life.
• Precision positioning or servo duty. Breakaway friction and stick-slip behaviour govern motion quality, and that is not what this product family is for.
• A duty a rolling bearing was designed for: continuous rotation, moderate load, predictable rating life.
• Where the pin cannot be brought up to at least 40–45 HRC at Ra 0.8 µm or finer. In that case either upgrade the pin or stay with a grease-lubricated bushing that tolerates a softer counterface.