Cast Bronze Bearings

Legacy context

For over a century, the name Cannon has been associated with precision metalwork in the Illinois region. Beginning in the late 1890s with A. P. Cannon’s innovative bronze oiler—a pump can designed to reach inaccessible spots—the enterprise laid a foundation of craftsmanship. By the mid-1940s, local businessmen reorganized the company, shifting focus toward bronze manufacturing.

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From 1946 onward, this facility became dedicated to producing functional bronze components, including bearings, bushings, and thrust washers. The site evolved into a vertically integrated operation, emphasizing efficiency through in-house casting and machining.

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This page serves as an educational reference on the historical legacy of cast bronze bearings and continuous casting methods. It is not affiliated with any currently operating business. The information presented here is drawn from preserved archival excerpts and is offered for historical context only.

Cast Bronze Bearings: A Practical Clearance Chart for Home Workshop and Small-Shop Appli

When you are selecting a cast bronze bearing for a home DIY project or a small B2B repair shop, the single most misunderstood variable is running clearance. Unlike off-the-shelf sintered bronze (Oilite) bushings, which have a standardized press-fit and ream-to-size protocol, cast bronze bearings are often machined from solid stock or purchased as semi-finished blanks. This means you, the builder, are responsible for setting the final bore diameter. Getting that diameter wrong by even 0.001 inch (25.4 microns) can lead to seizure, excessive vibration, or rapid wear. This guide provides a verifiable, educational framework for determining clearance, based on established mechanical engineering principles and bearing design handbooks, not on proprietary formulas.

Understanding the Physics: Why Clearance Is Not "Slop"

Running clearance is the intentional radial gap between the bearing bore and the shaft journal. It serves three critical functions: (1) it allows a hydrodynamic oil film to form, (2) it accommodates thermal expansion of the shaft and bearing, and (3) it provides space for foreign particles to pass without embedding. For cast bronze (typically C93200, C95400, or C86300), the material is relatively rigid compared to babbitt or polymer bearings. Therefore, the clearance must be generous enough to prevent metal-to-metal contact during startup, but tight enough to maintain oil pressure at operating speed.

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The classic engineering rule for bronze-on-steel is that clearance should be approximately 0.001 inch per inch of shaft diameter for general-purpose applications, but this is a starting point, not a universal truth. For high-speed or high-temperature applications, you must increase that figure. For oscillating or slow-speed, high-load applications, you may reduce it slightly. The chart below is a consolidated reference based on standard machinery handbooks (e.g., Machinery’s Handbook, bearing manufacturer design guides) and is suitable for shafts from 0.25 inch to 4.00 inches.

Compact Clearance Chart for Cast Bronze Bearings (Shaft Diameter vs. Recommended Diametr

Use the following values for diametral clearance (the difference between the bearing bore ID and the shaft OD). All values are in inches.

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*Note: These are diametral clearances. If you are measuring radial clearance (half the gap), divide these numbers by two. For metric shafts, multiply the shaft diameter in millimeters by 0.001 to 0.002 for general purpose, then convert to millimeters.*

Decision Criteria: How to Choose the Right Row and Column

Do not simply pick the middle value. Use the following decision tree to select your clearance:

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1. Identify the shaft material and hardness. If the shaft is hardened steel (Rc 40+), you can run tighter clearances because the shaft will not wear as quickly. If the shaft is soft mild steel or stainless steel, use the higher end of the range to prevent galling.
2. Determine the lubrication method. For oil bath or forced oil, use the lower half of the range. For grease lubrication or drip oil, use the upper half. Grease does not circulate, so you need more space for the grease to move and for thermal expansion.
3. Assess the duty cycle. Continuous operation generates heat. If the bearing will run for more than 2 hours at a time, add 0.001 inch to the selected clearance. Intermittent use (e.g., a manual press) can use the tightest values.
4. Check the housing fit. Cast bronze bearings are often pressed into a steel or cast iron housing. The press fit will reduce the bore ID by approximately 80% of the interference. For example, if you press a bearing with 0.002 inch interference into a housing, the bore will shrink by about 0.0016 inch. You must machine the bore after pressing, or pre-compensate by boring 0.0016 inch larger before pressing. This is the most common cause of seized bearings.

Common Mistakes in Setting Clearance

• Mistake 1: Using the "one-thou per inch" rule for small shafts. For a 0.25 inch shaft, 0.001 inch clearance is actually 0.004 inch per inch of diameter, which is excessive. Small shafts need proportionally tighter clearances because the oil film thickness is also smaller. Always use a chart, not a linear rule.
• Mistake 2: Ignoring thermal expansion of the shaft. A steel shaft running at 150°F (65°C) will expand about 0.0006 inch per inch of length, but more importantly, it expands radially. For a 2-inch shaft, that is about 0.0012 inch radial growth. If you set clearance at 0.003 inch diametral, you only have 0.0015 inch radial clearance left. That is dangerously low. Always add 0.001 inch to the clearance if operating temperature exceeds 120°F.
• Mistake 3: Reaming to size before pressing. As noted above, pressing a bronze bushing into a housing reduces the bore. If you ream to final size before pressing, the bore will be too small after pressing. You must either ream after pressing or oversize the bore by the expected shrink factor.
• Mistake 4: Confusing clearance with tolerance. Clearance is the gap. Tolerance is the allowable variation in machining. You should machine the bearing bore to a tolerance of ±0.0005 inch, and the shaft to ±0.0002 inch. If you cannot hold these tolerances, use the upper end of the clearance range to avoid seizure.
• Mistake 5: Using a clearance chart for sintered bronze. Sintered bronze (Oilite) is porous and self-lubricating; it requires a much tighter clearance (often 0.0005 inch per inch) because the oil is stored in the pores. Cast bronze is dense and requires external lubrication, so it needs more clearance. Do not mix these charts.

Verifiable Educational Detail: The Oil Film Wedge

To understand why clearance matters, consider the hydrodynamic wedge. When a shaft rotates, it drags oil into a converging wedge between the shaft and the bearing. This wedge generates pressure that lifts the shaft off the bearing surface. The minimum oil film thickness for a smooth, rigid bearing is typically 0.0002 to 0.0005 inch. If your clearance is too tight, the wedge cannot form because the oil is squeezed out. If your clearance is too loose, the oil pressure drops and the shaft wobbles, causing edge loading. The clearance chart above is designed to keep the ratio of clearance to shaft diameter (c/D ratio) between 0.001 and 0.003, which is the empirically verified range for stable hydrodynamic operation in bronze bearings.

Actionable Reference: A Step-by-Step Setup Procedure

1. Measure the actual shaft diameter at room temperature with a micrometer. Do not trust the nominal size.
2. Select your clearance from the chart based on speed and load. Write down the target diametral clearance.
3. Calculate the target bearing bore ID by adding the clearance to the shaft diameter. Example: Shaft = 1.0000 inch, General Purpose clearance = 0.0030 inch. Target bore ID = 1.0030 inches.
4. Press the bearing into the housing first, if it is a bushing. Measure the bore ID after pressing. It will be smaller than before.
5. Machine or ream the bore to the target ID from step 3. Use a boring bar or an adjustable reamer. Check with an inside micrometer at three depths and two angles.
6. Deburr the edges and create an oil groove or hole if needed. Do not chamfer the load-bearing area.
7. Assemble and test by hand. The shaft should rotate freely with a slight resistance, but no binding. If it binds, you have too little clearance. If it rattles, you have too much.

Final Note on Material Selection

The clearance chart applies to standard cast bronze alloys (C93200, C95400, C86300). If you are using a high-lead bronze (C93700) for heavy loads, use the upper end of the range. If you are using aluminum bronze (C95400) for high strength, use the lower end. Always verify your specific alloy’s thermal expansion coefficient, but for most DIY and small-shop work, the chart above will keep you out of trouble. Remember, no chart can guarantee performance; you must verify with a test run at low speed and check for heat buildup after 15 minutes of operation. If the bearing exceeds 180°F (82°C), stop and increase clearance.