Plain and Journal Bearings: Materials, Design and Lubrication: Worked Example

Worked Example: Full Journal Bearing Design

Given:

• Journal diameter: d = 2.3 inches
• Bearing length: L = 1.9 inches
• Load: W = 6,000 pounds
• Speed: N = 4,800 rpm
• Oil: SAE 30, supplied at 200°F through a single oil hole at 30 psi

Step 1–2: d = 2.3 in, L = 1.9 in (given)

Step 3: Bearing pressure

$$p_b = \frac{6{,}000}{1 \times 1.9 \times 2.3} = 1{,}372 \text{ psi}$$

Step 4–5: c_d = 0.003 in (from clearance chart); m = 0.003/2.3 = 0.0013; l/d = 1.9/2.3 = 0.83

Step 6: Assume Δt_a = 20°F → t_b = 200 + 20 = 220°F

Step 7: Z = 7.7 centipoises (from SAE 30 viscosity curve at 220°F)

Step 8: P′ = (6.9 × 1000 × 0.0013² × 1,372) / (7.7 × 4,800) = 0.43

Step 9: From charts: 1/(1 − ε) = 6.8 → ε = 0.85

Step 10: T′ = 1.46 (from torque parameter chart)

Step 11: T_f = (1.46 × 1.15² × 7.7 × 4,800) / (6,900 × 1.3) = 7.96 in-lb/in

Step 12: P_f = (1 × 7.96 × 4,800 × 1.9) / 63,000 = 1.15 hp

Step 13: X = 12 (from table at ~200°F)

Step 14: Q_R = (12 × 1.15) / 20 = 0.69 gpm

Step 15: C_n = 0.83² / (60 × 0.43) = 0.027

Step 16: q = 1.43 (from flow factor chart)

Step 17: Q_1 = (4,800 × 1.9 × 0.003 × 1.43 × 2.3) / 294 = 0.306 gpm

Step 18: Q_2 = (1.64 × 10⁵ × 30 × 0.003³ × 2.3 × (1 + 1.5 × 0.85²)) / (7.7 × 1.9) = 0.044 gpm

Step 19: Q = 0.306 + 0.044 = 0.350 gpm

Step 20: Δt = (12 × 1.15) / 0.350 = 39.4°F

Step 21: Δt_a (20°F) and Δt (39.4°F) differ by more than 5°F. New Δt_a = (20 + 39.4)/2 = 30°F.

Repeat with Δt_a = 30°F:

• t_b = 200 + 30 = 230°F → Z = 6.8 cP
• P′ = 0.49 → 1/(1 − ε) = 7.4 → ε = 0.86
• T′ = 1.53 → T_f = 7.36 → P_f = 1.07 hp
• X ≈ 11.9 → Q_R = 0.42 gpm
• C_n = 0.023 → q = 1.48
• Q_1 = 0.317, Q_2 = 0.050, Q = 0.367 gpm
• Δt = 34.4°F — within 5°F of Δt_a (30°F) ✓

Step 22: Minimum film thickness

$$h_o = \frac{0.003}{2} \times (1 - 0.86) = 0.00021 \text{ inch}$$

Step 23: Repeat for additional values of c_d and plot the complete lubrication study to find the optimum clearance range.

Use of Lubrication Analysis: Optimizing the Design

Once you've plotted minimum film thickness, temperature rise, friction horsepower, and oil flow versus diametral clearance, follow these steps to find the optimum design:

Step 1: Determine Acceptable Clearance Range from Film Thickness

• Bearings under 1 inch diameter: Minimum h_o of 200 × 10⁻⁶ inches
• Bearings 1 to 4 inches diameter: Minimum h_o of 500 × 10⁻⁶ inches
• Larger bearings: Minimum h_o of 750 × 10⁻⁶ inches

Conservative designs should increase these minimums.

Step 2: Check Temperature Rise

Determine the minimum acceptable c_d based on a maximum Δt of 40°F.

Step 3: Define the Clearance Band

If there are no requirements for maintaining low friction horsepower and oil flow, the acceptable range of c_d is now defined by Steps 1 and 2.

Step 4: Place Manufacturing Tolerances

Set tolerances within the band to optimize h_o.

Step 5: Adjust for Power Loss and Flow

If oil flow and power loss are a consideration, shift tolerances within the range permitted by h_o and Δt requirements.

Allowable Sleeve Bearing Pressures: Quick Reference

These pressures (in psi, based on projected area = length × diameter) are a general guide only. Actual allowable pressure depends on lubrication quality, bearing design, workmanship, velocity, and load nature.

Plain Bearing Materials: The Complete Selection Guide

the practitioner eventually rebuilt his expertise from the ground up. The most important lesson he learned? No single bearing material excels at everything. Selection is always a compromise.

The Seven Critical Properties

Every bearing material must be evaluated against these properties:

1. Fatigue resistance — Ability to withstand repeated stress without cracking or flaking
2. Embeddability — Ability to absorb foreign particles into the bearing surface, preventing shaft scoring
3. Compatibility — Resistance to galling or seizing under metal-to-metal contact at startup
4. Conformability — Ability to creep or flow slightly under load, compensating for misalignment
5. Thermal conductivity — Ability to absorb and carry away heat; most critical for preventing seizures from hot spots
6. Corrosion resistance — Resistance to organic acids that form in oils at operating temperatures
7. Load capacity — Ability to withstand hydrodynamic pressures during operation

Bearing Material Properties Comparison

Bearing Characteristics Ratings (1 = Best, 5 = Worst)

Babbitt (White Metal) Alloys

The most widely known bearing materials. Available in tin-base and lead-base compositions.

Tin babbitt (80–90% tin, 3–8% copper, 4–14% antimony):

• Increasing copper or antimony → increased hardness and tensile strength, decreased ductility
• Excellent embeddability—minimal journal wear from trapped dirt
• Resistant to acid corrosion
• Not prone to oil-film failure
• Easily bonded and cast
• Weakness: Low fatigue resistance; hardness and strength drop at elevated temperatures

Lead babbitt (10–15% antimony, up to 10% tin):

• Similar advantages to tin babbitt
• Disadvantages vs. tin-base: Lower strength and susceptible to corrosion

Manufacturing note: For small bushings and automotive engine bearings, babbitt is used as a thin coating over flat steel strip. After forming grooves and drilling holes, the strip is cut, rolled, and shaped into finished bearings. These are mass-produced at very low cost.

For larger heavy-duty bearings, thicker babbitt (1/4 to 1/2 inch) is cast onto steel or cast iron backing.

Copper-Lead

Binary mixture of copper and lead (20–40% lead). Lead is practically insoluble in copper, creating lead pockets in a copper matrix. Usually steel-backed.

Applications: Heavy-duty main and connecting-rod bearings; turbines and electric motors at moderate loads and speeds.

Leaded Bronze and Tin-Bronze

Leaded bronze (up to 25% lead): Excellent casting and machining characteristics, low cost, good structural properties. Available in standard bar stock, sand or permanent molds, investment, centrifugal, or continuous casting. Generally a first choice at intermediate loads and speeds.

Tin-bronze (~10% tin): Higher hardness and load capacity but poorer compatibility than leaded bronze.

Aluminum

Cast solid, steel-backed, or with overlay. Usually alloyed with tin, silicon, cadmium, nickel, or copper.

Advantages: High load capacity, fatigue strength, thermal conductivity, excellent corrosion resistance, and low cost.

Caution: High thermal expansion requires large clearances, which can make the bearing noisy at startup.

Silver

Highest fatigue rating of any bearing material. The steel backing may show fatigue before the silver does. However, silver alone has none of the desirable bearing qualities except fatigue resistance and thermal conductivity. Overlays (lead, lead-tin, lead-indium) provide embeddability and anti-scoring properties.

Applications: Aircraft master rod bearings, diesel engine main bearings.

Cast Iron

Inexpensive material for light loads (≤ 150 psi) and low speeds (≤ 130 ft/min). Must be well lubricated with large clearance to avoid scoring. Journal hardness of 150–250 Bhn recommended.

Porous Metals (Sintered)

Made by sintering bronze, iron, or stainless steel to produce a sponge-like structure that absorbs 10–35% oil by volume. Self-lubricating—used where lubrication supply is difficult, inadequate, or infrequent.

Should be flooded periodically to resaturate.

Plastics

Increasingly used for their corrosion resistance, quiet operation, moldability, and excellent compatibility (minimizing or eliminating lubrication needs).

Key plastic bearing materials:

• Laminated phenolics — Cotton fabric or asbestos bonded with phenolic resin. Excellent compatibility with various fluids. Strength and shock resistance are good. Low thermal conductivity requires attention to cooling.
• Nylon — Widest use for small, lightly loaded applications. Low friction, no lubrication required.
• PTFE (Teflon) — Exceptional low coefficient of friction, self-lubricating, chemical resistance, wide temperature range. High cost and low load capacity cause it to be used mostly in modified form.

All three are unaffected by dilute acids and alkalies.

Carbon-Graphite

Used where regular maintenance and lubrication cannot be provided. Dimensionally stable over wide temperature ranges, unaffected by chemicals.

Operating limits:

• In air: Up to 700–750°F
• Non-oxidizing atmosphere: Up to 1,200°F
• Without lubrication: Maximum 20 psi
• With metal alloy addition: Up to 350 psi (dry) or 600 psi (lubricated)

Running clearances (steel shafts, < 200°F):

Sintered Metal Bearings: ASTM Standards and Tolerances

For copper-base and iron-base sintered bearings per ASTM B438-83a and B439-83:

Flange and Thrust Bearing Tolerances

Parallelism on Faces (Maximum)

Cost note: Special tolerances require additional or secondary operations and are significantly costlier. Do not specify them unless truly required.

Improvement method and result

After the seizure incident, the practitioner spent six months studying bearing fundamentals. He didn't just read—he ran lubrication analyses on every major bearing in the plant. He built spreadsheets that calculated operating clearance, film thickness, temperature rise, and friction horsepower for different oil grades and clearance values.

When the No. 4 dryer bearing was replaced, the practitioner didn't just swap in a new bushing. He:

1. Verified the shaft diameter and surface finish — The scored shaft was ground and chrome-plated to restore hardness
2. Calculated the actual operating clearance — Accounting for thermal expansion at operating temperature
3. Ran a full lubrication analysis — Confirming that the selected oil viscosity at operating temperature would maintain adequate film thickness
4. Installed a temperature monitoring system — With alarm set at 200°F and shutdown at 230°F
5. Changed the lubrication delivery — From oil ring to pressure-fed, increasing the safe load capacity by a factor of four
6. Documented everything — Creating a bearing specification sheet that traveled with the equipment

The No. 4 dryer ran for three more years without a single bearing issue. the practitioner became the plant's go-to expert on rotating equipment reliability.

Your Next Step: The One Thing to Do Before Your Next Bearing Selection

The difference between an engineer who replaces bearings and one who designs with them comes down to one question:

Have you run the lubrication analysis?

Not a rule-of-thumb estimate. Not a "this worked last time." A real, step-by-step analysis that tells you:

• What your minimum film thickness is
• Whether your oil viscosity at operating temperature can maintain that film
• What your actual temperature rise will be
• How much lubricant flow you need
• What clearance range gives you the best combination of film thickness, temperature, power loss, and flow

Here's your challenge: Pick one critical plain bearing in your plant, your design, or your project. Pull the shaft diameter, speed, load, oil type, and inlet temperature. Run through the 23-step procedure in this guide. Plot the results for at least three different diametral clearances.

You will learn more about that bearing in two hours than you've learned in the last two years of operating it.

That's the difference between replacing bearings and engineering with them.

This guide covers plain bearing classes, journal bearing types, hydrostatic bearings, modes of bearing operation, retention methods, sealing methods, hardness and surface finish, machining, lubrication methods, lubricant selection, greases and solid lubricants, journal bearing components, grooving and oil feeding, heat radiating capacity, the complete 23-step journal bearing lubrication analysis with worked example, sleeve bearing pressure allowances, use of lubrication analysis for design optimization, and the full spectrum of plain bearing materials including babbitt, copper-lead, bronze, aluminum, silver, cast iron, porous metals, plastics, sintered metals, and carbon-graphite.

How to Choose the Right Alloy Before Your Machine Pays the Price

A single bearing failure shut down the practitioner's entire bottling line for eleven days. Not because the replacement part was expensive — the babbitt sleeve bearing itself cost less than a decent dinner. The real damage? Lost production, emergency freight charges, a scrambled maintenance crew working overtime, and one furious operations director wondering why nobody caught it sooner.

The answer was painfully simple: the practitioner had the wrong bearing material for the job.

His team had spec'd a standard tin-base babbitt for a high-temperature, heavy-load pump application. The babbitt's fatigue strength crumbled under repeated thermal cycling. The bearing surface cracked, flaked, and eventually seized against the journal — all within eight months of installation.

This guide exists so you never make that mistake. Whether you're a junior engineer selecting your first bearing material or a veteran machinist who wants the full technical picture in one place, what follows is the most comprehensive reference you'll find on plain bearing materials — every alloy, every property, every trade-off, and the hard data you need to make the right call.

The Seven Properties That Make or Break Every Bearing

Before you look at a single alloy table, you need to understand what separates a bearing material that works from one that destroys itself. Every bearing material is a compromise. No single material excels at all seven critical properties. Your job is to find the best combination for your specific application.

Here are the seven properties that govern every plain bearing material selection:

. Fatigue Resistance

What it is: The ability of the bearing lining to withstand repeated stress and strain cycles without cracking, flaking, or disintegrating.

Why it matters: Every rotation of the shaft loads and unloads the bearing surface. In a connecting rod bearing spinning at thousands of RPM, that's millions of stress cycles per day. A material with poor fatigue resistance won't last — it'll develop surface cracks that propagate until chunks of bearing material break free.

. Embeddability

What it is: The ability of the bearing lining to absorb and trap small dirt particles circulating in the lubrication system.

Why it matters: No lubrication system is perfectly clean. Microscopic particles of grit, metal shavings, and contaminants circulate through even well-filtered systems. A bearing with good embeddability traps these particles by allowing them to sink into the softer bearing surface, pulling them out of the contact zone before they can score the shaft. Poor embeddability means those particles ride between bearing and journal, gouging both surfaces.

. Compatibility (Anti-Scoring)

What it is: The ability of the bearing and shaft to resist galling or seizing during metal-to-metal contact.

Why it matters: This is the property that saves you at startup. Before the hydrodynamic oil film builds up, the shaft and bearing are in direct contact. A compatible bearing material resists welding itself to the shaft during these critical moments. This is arguably the most "bearing-specific" property, because in a well-designed system, metal-to-metal contact should only occur at startup.

. Conformability

What it is: The ability of the bearing material to creep or flow slightly under load, allowing the shaft and bearing surfaces to match each other's contours.

Why it matters: No machined surface is geometrically perfect. Conformability compensates for minor misalignment, shaft deflection, and manufacturing tolerances during the initial run-in period. Soft materials like babbitt a desktop spreadsheet application here — harder materials like tin-bronze demand tighter manufacturing precision.

. Thermal Conductivity

What it is: The ability to absorb and dissipate heat generated in the bearing.

Why it matters: The critical function isn't removing bulk frictional heat from the oil film — it's preventing localized hot spots caused by asperity breakthroughs or embedded foreign particles. A hot spot can initiate seizure in milliseconds. Materials with high thermal conductivity spread that heat before it concentrates.

. Corrosion Resistance

What it is: The ability to resist chemical attack from organic acids that form in lubricating oils at operating temperatures.

Why it matters: Oil degrades over time, especially at elevated temperatures. The byproducts include organic acids that can attack bearing surfaces. Some materials — particularly lead-base alloys — are notably susceptible to this corrosion, while tin-base babbitts and aluminum alloys resist it well.

. Load Capacity

What it is: The ability to withstand the hydrodynamic pressures exerted during operation.

Why it matters: This is raw strength. Higher load capacity means the bearing can support greater forces without plastic deformation or structural failure. But here's the trade-off that defines bearing engineering: the materials with the highest load capacity typically have the worst embeddability and conformability.

The Bearing Material Trade-Off Matrix

Here's the truth that experienced engineers learn early: you cannot have it all. The table below is the single most important reference in bearing material selection. Study it carefully.

Bearing Alloy Properties Comparison

Babbitt (White Metal) Alloys — The Most Trusted Name in Bearings

The Origin Story

Many different bearing metal compositions carry the name "babbitt," but the original alloy — developed in the 19th century — was likely composed of approximately 89.3% tin, 3.6% copper, and 7.1% antimony. Today, both the Society of Automotive Engineers (SAE) and the American Society for Testing and Materials (ASTM) have formally classified white metal bearing alloys into standardized grades.

Tin and lead-base babbitts remain the best-known of all bearing materials. Their exceptional embeddability and compatibility under boundary lubrication have earned them a place in an enormous range of applications: household appliances, automobile and diesel engines, railroad cars, electric motors, generators, steam and gas turbines, and industrial and marine gear units.

Tin-Base Babbitt

Composition: 80–90% tin, 3–8% copper, 4–14% antimony.

Key behaviors:

• Increasing copper or antimony raises hardness and tensile strength but reduces ductility
• Exceeding the standard composition ranges causes fatigue resistance to decrease — a critical warning for anyone tempted to "strengthen" the alloy
• Excellent dirt embedding ability — very little tendency to wear the journal
• Resists corrosive effects of acids in lubricating oil
• Not prone to oil-film failure
• Easily bonded and cast

The two critical drawbacks:

• Low fatigue resistance — rated 5 out of 5 (worst) on the characteristics scale
• Strength drops significantly at elevated temperatures — both hardness and tensile values decrease appreciably as temperature rises

Lead-Base Babbitt

Composition: Generally 10–15% antimony, up to 10% tin, balance lead.

Key behaviors:

• Similar embeddability to tin-base — traps dirt effectively, minimal journal wear
• Resists acid corrosion in oil
• Not prone to oil-film failure
• Easily bonded and cast

Chief disadvantages compared to tin-base:

• Lower strength overall
• Susceptible to corrosion — rated 3 versus tin-base's rating of 1

Manufacturing: How Babbitt Bearings Are Made

Small bearings (fractional-horsepower motors, automotive engines): Babbitt is applied as a thin coating over a flat steel strip. Oil distribution grooves are formed, holes drilled, and the strip is cut to size, then rolled and shaped into the finished bearing. Available for shaft diameters from 0.5 to 5 inches. These strip bearings are produced by the millions yearly in highly automated factories and offer an excellent combination of low cost with good bearing properties.

Larger bearings (heavy-duty equipment): A thicker babbitt layer is cast onto a rigid backing of steel or cast iron. The recommended process for sound bonding includes chemical and electrolytic cleaning of the bearing shell, thorough rinsing, tinning, and then centrifugal casting. After machining, the babbitt layer is typically ¼ to ½ inch thick.

The Trimetal Solution

Compared to other bearing materials, babbitts generally have lower load-carrying capacity, lower fatigue strength, somewhat higher cost, and more complicated design requirements. Their strength also decreases rapidly with increasing temperature.

The engineering solution? Composite or "trimetal" bearings. An intermediate layer of high-strength, fatigue-resistant material is placed between a steel backing and a thin babbitt surface layer. This gives you the babbitt's superb surface properties (compatibility, embeddability, conformability) while the intermediate layer handles the structural loads. These composite bearings frequently eliminate any need for using alternate materials with poorer bearing characteristics.

White Metal Bearing Alloy Specifications (ASTM B23-83)

The following table provides composition and mechanical properties for the standard ASTM white metal alloys. All compression test specimens were cylinders 1.5 inches long × 0.5 inch diameter, machined from chill castings. Brinell tests used a 10 mm ball at 500 kg load applied for 30 seconds.

Note: Alloys 7 and 15 also contain approximately 0.45% and 1.0% arsenic, respectively.

What the numbers reveal: Notice how dramatically properties drop between 68°F and 212°F. ASTM Alloy 1 loses nearly 40% of its compressive yield strength and more than half its Brinell hardness at 212°F. This temperature sensitivity is the Achilles' heel of babbitt alloys and the primary reason engineers reach for stronger materials in high-temperature applications.

SAE Bearing and Bushing Alloy Classifications

The SAE system classifies bearing alloys by base metal and application. Here is the complete breakdown:

Tin-Base Alloys

Lead-Base Alloys

Lead-Tin Overlay Alloys

Copper-Lead Alloys

Copper-Base Bushing Alloys

Cadmium-Base Alloys — The High-Temperature Specialist

Composition: 1–15% nickel, or 0.4–0.75% copper, and 0.5–2.0% silver, balance cadmium.

Cadmium alloy bearings offer greater fatigue resistance than babbitts and their prime attribute is high-temperature capability — operating up to 500°F compared to babbitt's 300°F ceiling.

The critical limitation: Very poor corrosion resistance (rated 5 out of 5 — worst). This single shortcoming severely limits their use. You'll only see cadmium-base bearings where temperature demands absolutely require them and corrosion can be controlled through lubricant selection and maintenance practices.

Shaft hardness requirement: 200–250 Brinell — significantly harder than what babbitt demands.