C93200 Bronze Bushing

Legacy context

For over a century, the name Cannon has been linked to precision metal manufacturing in Keithsburg, Illinois. Beginning in the late 1890s with the founder’s bronze pump oil can—a product designed to reach otherwise inaccessible points—the site’s industrial heritage was shaped by its railroad-adjacent location. By the mid-1940s, local businessmen reorganized the enterprise, establishing a focus on bronze manufacturing that has continued since 1946.

Key point 1

This educational reference explores that legacy, with particular attention to C93200 bronze bushing production. The material’s properties and the historical methods of continuous casting and custom machining are examined here as part of a broader technical narrative. The site is maintained as an independent, non-commercial resource for those researching bronze alloys and their applications. It does not represent an active business, nor does it offer products or services. All information is drawn from preserved historical records and presented for educational purposes only.

Understanding C93200 (SAE 660) Bronze

C93200, commonly known as SAE 660 or "bearing bronze," is a leaded tin bronze alloy. Its nominal composition is approximately 83% copper, 7% tin, 7% lead, and 3% zinc. The lead content is the defining feature: it acts as a solid lubricant, allowing the bushing to operate under boundary lubrication conditions where a full oil film may not exist. The tin provides strength and wear resistance, while the zinc acts as a deoxidizer during casting. This combination yields a material that is machinable, conformable (it can slightly deform to accommodate shaft misalignment), and resistant to galling against steel shafts. For a home DIY or small B2B operation, C93200 is often the default choice for general-purpose plain bearings, but it is not a universal solution. Understanding its limits is critical to avoiding premature failure.

Key Decision Criteria: Load, Speed, and Lubrication

The first decision criterion is the product of load and surface velocity, often expressed as the PV factor (pressure in psi multiplied by velocity in feet per minute). For C93200, a commonly cited safe operating limit is around 50,000 psi-ft/min for continuous operation with adequate lubrication, though this can vary with actual conditions. In practical terms, this means a bushing supporting a 1-inch shaft rotating at 500 RPM with a 1,000-pound radial load is near the edge of its continuous-duty envelope. For intermittent or slow oscillating motion, you can push higher loads, but for high-speed continuous rotation, you should consider a porous bronze (oil-impregnated) or a bimetal bearing. Always calculate your actual PV before purchasing. A second criterion is shaft hardness. C93200 is softer than the shaft, which is intentional—it wears preferentially. However, if your shaft is unhardened mild steel with a rough surface finish (above 32 microinches Ra), the abrasive action will rapidly wear the bushing. Use a hardened shaft (Rockwell C 45 or higher) or at least a smooth, ground shaft for acceptable life. Third, consider the lubrication method. C93200 works best with a continuous oil supply, grease fittings, or at least periodic manual lubrication. If your application is fully sealed and maintenance-free, this alloy is the wrong choice; you need a self-lubricating composite or a porous bronze with pre-oiled sintered structure.

Common Mistakes in Sizing and Installation

A frequent error is assuming that a thicker bushing wall provides more load capacity. In a plain bearing, load capacity is a function of projected area (bore diameter times length), not wall thickness. A 1-inch bore bushing that is 1 inch long has the same projected area whether the wall is 1/8 inch or 1/4 inch. The wall thickness only matters for structural rigidity and press-fit retention. Oversizing the wall for "strength" wastes material and can cause heat dissipation problems because the heat path from the bore to the housing is longer. Another mistake is ignoring the housing bore tolerance. C93200 bushings are typically press-fit into a housing with an interference fit of 0.001 to 0.002 inches per inch of bushing outer diameter. If you machine the housing pocket too loose, the bushing will spin in the housing, which destroys the oil feed holes and generates excessive heat. If too tight, you may collapse the bushing bore, reducing clearance to zero. Always measure the bushing OD and the housing ID after installation, not before. A third common error is using a bushing with a grease groove pattern that does not align with the load zone. The load should be applied to the solid portion of the bushing, not over a groove. If you cut a spiral groove for grease distribution, ensure it does not cross the loaded arc. For a simple radial load, a single axial groove at the unloaded top is often sufficient.

Clearance and Surface Finish: The Numbers That Matter

For C93200, the recommended running clearance (difference between shaft diameter and bushing bore after installation) is typically 0.001 to 0.002 inches per inch of shaft diameter. For a 1-inch shaft, that means a bore of 1.001 to 1.002 inches. This clearance is not a suggestion; it is a thermal and hydrodynamic requirement. Too little clearance causes the bushing to seize as the shaft expands from friction heat. Too much clearance causes vibration, loss of oil film, and edge loading. After pressing the bushing into the housing, the bore will shrink by roughly 80% of the interference fit. If you press a bushing with a 0.002-inch interference, expect the bore to close by about 0.0016 inches. Therefore, you must machine or ream the bushing bore to final size after installation, not before. Many DIY users skip this step and then wonder why the shaft binds. The shaft surface finish should be 16 microinches Ra or better for continuous duty. A rough shaft acts like a file, and the lead in the bronze will smear and transfer to the shaft, causing a rough, noisy bearing. If you cannot improve the shaft finish, consider a softer bushing material like C93700 (more lead) but accept a shorter life.

Lubrication and Maintenance Realities

C93200 is not a "maintenance-free" material. It requires a lubricant film to function. For a home DIY application like a trailer axle or a farm implement pivot, a grease fitting is mandatory. Use a lithium-based or molybdenum-disulfide grease, and pump until fresh grease exits the seal. For a small B2B machine that runs continuously, an oil bath or drip oiler is better. The lead in C93200 provides emergency dry-running capability for a few minutes, but do not rely on it. If you see the bushing turning blue or black, you have exceeded the temperature limit (around 300 degrees Fahrenheit for continuous operation). At that point, the lead will start to leach out, and the bushing will fail quickly. A simple rule: if you cannot touch the housing after 10 minutes of operation, you are overloading or under-lubricating. Also, beware of using C93200 in a dusty environment without seals. The abrasive particles will embed in the soft lead and turn the bushing into a grinding tool. Use felt seals or rubber lip seals on both ends.

Compact Actionable Reference for C93200 Bushing Selection

When to Avoid C93200 Entirely

If your application involves high-speed rotation (above 1,000 surface feet per minute), continuous operation without maintenance, or exposure to corrosive chemicals, C93200 is not the right material. For high-speed, use a bronze with a higher tin content or a steel-backed babbitt bearing. For maintenance-free, use a sintered bronze (oil-impregnated) or a PTFE-lined composite. For corrosive environments, use a bronze with higher nickel content or a plastic bearing. C93200 is a workhorse, but it is a workhorse for dirty, slow, and well-lubricated jobs. Knowing when to say no is as important as knowing how to install it. Always test your specific application under controlled conditions before committing to a production run. The data provided here is based on standard tribological principles and published material properties, but actual performance depends on your exact geometry, alignment, and duty cycle.

This independent educational reference summarizes general technical concepts. Verify current standards, dimensions, and manufacturer specifications before making a procurement or engineering decision.