Plain and Journal Bearings: Materials, Design and Lubrication: What Exactly Is a Journal Bearing?

What Exactly Is a Journal Bearing?

Here's the thing most people get wrong: they think all bearings have balls or rollers inside them. That's only one type.

A journal bearing (also called a bush or plain bearing) is the simplest bearing design in existence. No balls. No rollers. No cages. Just a cylindrical sleeve that wraps around a rotating shaft.

The Anatomy — Four Parts, Zero Complexity

Think of it like this: your fist (the journal) slides into a paper towel tube (the bearing). Now spin your fist. That's a journal bearing.

Key insight for you: The journal is not necessarily a different diameter than the shaft. In many designs, the journal IS the shaft — same diameter, same piece of metal.

Two Fundamental Types

Before you design anything, you need to understand there are two categories:

1. Non-Pressure Lubricated (what this guide covers) The bearing material itself stores or attracts lubricant. No external pump. No pressurized oil supply. These are "off-the-shelf" solutions — exactly what the practitioner should have been using.

2. Pressure Lubricated Lubricant is pumped into the bearing under pressure — like the crankshaft bearings in your car engine. These require complex design calculations and are not something you select from a catalogue.

This guide focuses entirely on non-pressure lubricated porous bronze bearings — the type you'll encounter in 80% of light-to-moderate industrial applications.

Why Would You Choose a Journal Bearing Over a Rolling Element Bearing?

Let's settle this debate right now. Here's an honest comparison:

Advantages of Journal Bearings

✅ Low cost — Often 5–10× cheaper than an equivalent rolling element bearing

✅ Silent operation — No balls or rollers means no clicking, no rumbling. Critical in applications like HVAC fans, office equipment, and medical devices.

✅ Compact radial footprint — A journal bearing can fit in spaces where a ball bearing simply won't. The wall thickness is minimal.

✅ High-speed capability — At very high RPMs, rolling element bearings struggle with centrifugal forces on the balls. Journal bearings? They actually get better at high speed (more on this in the lubrication section).

✅ Lubricant versatility — Oil, grease, water, or even air. Some porous bronze bearings are "lubed for life" with SAE 20 oil impregnated right into the metal.

Disadvantages — Be Honest With Yourself

❌ Lower radial load capacity — If your shaft carries massive radial loads, rolling element bearings win.

❌ Zero thrust load capability — Standard cylindrical journal bearings cannot handle axial (thrust) forces. You need a flanged version for that, with the shaft designed to match.

❌ Low misalignment tolerance — Self-aligning types exist in small sizes, but generally, if your shaft and housing aren't well-aligned, the bearing will wear unevenly and fail.

❌ Shaft quality is critical — The shaft surface needs a fine ground finish, preferably lapped. Rough shafts destroy journal bearings.

❌ Size limitations — Standard porous bronze bearings max out around 50 mm inside diameter. Above that, you're into custom territory.

The Decision Matrix

Bottom line: Journal bearings are most suitable for applications involving relatively high-speed shafts with moderate radial loads and low-to-zero thrust loads — particularly when cost, noise, and space are important considerations.

The Science of Lubrication — Why the practitioner's Bearing Seized

This is where most engineers' eyes glaze over. But this is also where the practitioner's story becomes a masterclass in what happens when you skip the fundamentals.

The Three Lubrication Regimes

When a shaft sits inside a journal bearing, the lubrication goes through three distinct phases as speed increases:

Phase 1: Boundary Lubrication (The Danger Zone)

What happens: At rest, or at very low speeds, the journal sits on the bottom of the bearing. Metal touches metal. The only thing preventing welding is a microscopic layer of lubricant molecules clinging to the surfaces.

The risk: This is where wear happens. This is where the practitioner's bearing lived for too long because the shaft speed was too low for the load it carried.

Phase 2: Thin-Film (Transition) Lubrication

What happens: As speed increases, oil gets dragged around by the spinning shaft. It starts to penetrate the gap between journal and bearing. The shaft begins to "lift off" — but it's not fully floating yet. Occasional metal-to-metal contact still occurs, especially under shock loads.

The risk: Moderate wear. The journal may occasionally contact the bearing surface during vibration or load spikes.

Phase 3: Thick-Film (Hydrodynamic) Lubrication — The Goal

What happens: At high enough speed, the oil forms a complete wedge-shaped film between the journal and the bearing. The shaft is literally floating on a cushion of oil. Zero contact. Zero wear.

This is where you want your bearing to operate.

Visualizing the Transition

Imagine a friction vs. speed curve:

Friction Torque ▲ │╲ │ ╲ ← Boundary lubrication (HIGH friction, HIGH wear) │ ╲ │ ╲ │ ╲___ ← Transition zone │ ╲ │ ╲__ ← Thick-film (LOW friction, ZERO wear) │ (friction rises slowly with speed │ due to fluid viscosity) └──────────────────────────────────▶ Shaft Speed

The most desirable operating point is right at the onset of thick-film lubrication. Below this point: wear and high friction. Above this point: friction slowly rises due to fluid shear, but no wear occurs.

The Bearing Modulus — Your Single Most Important Number

Here's where the math saves the machine. There's one formula that tells you whether your bearing will operate in the safe thick-film zone or the destructive boundary zone.

The Formula

$$M = \frac{\mu \times v}{p}$$

Where:

Critical note: 1 centipoise = 1000 Pa·s. The formula uses centipoise and MPa specifically to produce the modulus value.

The Magic Threshold

If M > 75 → Thick-film lubrication occurs ✅

If M < 75 → You're in boundary or transition territory ⚠️

This is the number the practitioner never calculated.

What Each Variable Tells You

If M is too low, you have three levers to pull:

1. Increase μ (viscosity) — Use a heavier oil. But beware: higher viscosity = higher friction once thick-film is achieved. It's a tradeoff.
2. Increase v (speed) — Run the shaft faster. Obviously, this isn't always an option.
3. Decrease p (pressure) — Reduce the load OR increase the bearing dimensions (bigger diameter or longer length).

If M is much greater than 75: Thick-film lubrication is assured, but friction might be unnecessarily high. Consider reducing lubricant viscosity to lower operating temperature.

The Complete Selection Procedure — Step in the supplied reference's walk through this exactly as a professional engineer would. We'll follow an illustrative engineering practitioner, a junior mechanical designer tasked with selecting bearings for a new conveyor drive shaft.

the practitioner's Design Brief

• Shaft diameter: 30 mm
• Shaft speed: 1,450 rev/min
• Total radial load: 500 N (distributed between two bearings)
• Operating condition: Continuous
• Lubricant: SAE 20 oil (standard supply with Sintalite bearings)

Step 1: Determine Load Per Bearing

The load is shared between two support bearings:

$$F = \frac{500}{2} = 250 \text{ N}$$

Your takeaway: Always confirm how load distributes. A shaft with two bearings and a central load splits it evenly. An overhung load? That's a different story entirely.

Step 2: Choose Initial Bearing Length

For a first trial, use the rule of thumb:

$$\frac{L}{d} = 1$$

This means bearing length (L) = shaft diameter (d).

So: L = 30 mm

The L/d ratio is your design lever for bearing proportions:

Why 0.5 to 1.5? If L/d is too small, bearing pressure will be excessively high and lubricant retention becomes difficult. If L/d is too large, friction increases, alignment becomes critical, and the bearing may develop metal-to-metal contact at the edges.

Step 3: Calculate Bearing Pressure

$$p = \frac{F}{A} = \frac{F}{d \times L}$$

$$p = \frac{250}{30 \times 30} = \frac{250}{900} = 0.278 \text{ MPa}$$

Note: The area used is the projected area (diameter × length), NOT the curved surface area. This is a common mistake.

Step 4: Calculate Surface Velocity

$$v = r \omega = \frac{d}{2000} \times \frac{2\pi N}{60}$$

Where N = rotational speed in rev/min, d = diameter in mm:

$$v = \frac{30}{2000} \times \frac{2\pi \times 1450}{60} = 0.015 \times 151.84 = 2.278 \text{ m/s}$$

Step 5: Check Maximum Allowable Pressure

Because the velocity is greater than 1.0 m/s, you can't use the simple table below — you need to use the pressure-velocity chart.

For velocities ≤ 1.0 m/s, use this table:

From the pressure-vs-speed chart: For a 30 mm diameter shaft at 1,450 rev/min, the maximum allowable pressure is approximately 0.47 MPa.

the practitioner's calculated pressure is 0.278 MPa. Since 0.278 < 0.47, the bearing pressure is acceptable. ✅

Step 6: Check the p × v Factor

This is a quick sanity check on heat generation:

$$p \times v = 0.278 \times 2.278 = 0.633$$

The critical threshold: If p × v > 0.53, auxiliary lubrication should be provided.

Since 0.633 > 0.53, the practitioner needs to add auxiliary lubrication — such as a felt washer reservoir or an oil wick.

Pro tip for you: If the p × v factor only slightly exceeds 0.53, you may be able to bring it below the threshold by increasing the bearing length (which reduces pressure). For example, going from L = 30 mm to L = 35 mm would reduce p and potentially eliminate the need for auxiliary lubrication.

Step 7: Verify Thick-Film Lubrication (The Bearing Modulus)

Assuming continuous operation at approximately 65°C, the SAE 20 oil has a viscosity of about 20 centipoise:

$$M = \frac{\mu \times v}{p} = \frac{20 \times 2.278}{0.278} = \frac{45.56}{0.278} = 164$$

Since 164 >> 75, thick-film lubrication is solidly assured. ✅

If M had been less than 75: the practitioner would have needed to either increase viscosity (heavier oil), increase speed (not always possible), or decrease pressure (bigger bearing or lower load).

Step 8: Select the Catalogue Number

Based on the standard metric bearing table, a 30 mm bore cylindrical bearing has the catalogue number SMC 303830 (30 mm bore, 38 mm OD, 30 mm length).

Since auxiliary lubrication is needed, the practitioner specifies a felt washer reservoir arrangement.

The Complete Selection Workflow — Summary

START │ ▼ [1] Determine load per bearing (F) │ ▼ [2] Choose L/d ratio (start with 1.0) → get bearing length L │ ▼ [3] Calculate bearing pressure: p = F / (d × L) │ ▼ [4] Calculate surface velocity: v = (d/2000) × (2πN/60) │ ▼ [5] Check p < maximum allowable pressure for that speed │ → If NO → increase L (or use different bearing type) │ → If YES ↓ ▼ [6] Check p × v factor │ → If > 0.53 → plan auxiliary lubrication │ → If ≤ 0.53 → self-lubrication may suffice │ ▼ [7] Calculate M = μv/p │ → If M < 75 → change μ, v, or p until M > 75 │ → If M ≥ 75 → thick-film confirmed ✅ │ ▼ [8] Select catalogue bearing number → DONE

Standard Bearing Dimensions — Your Reference Tables

Standard Metric Cylindrical Bearings

Non-Standard Metric Cylindrical Bearings

For applications where standard bearings don't quite fit, non-standard sizes are available in smaller bore ranges:

Flanged Bearings (For Thrust Loads)

When you need to handle axial positioning or light thrust loads, flanged bearings add a lip on one end:

Catalogue number decoding: SMC = Standard Metric Cylindrical. SMF = Standard Metric Flanged. The digits encode inside diameter, outside diameter, and length. For example, SMC 303830 = 30 mm bore, 38 mm OD, 30 mm length.

Design Factors That Make or Break Your Bearing

the practitioner learned these the hard way during her first year. the practitioner never learned them at all.

. Surface Finish of the Shaft

The shaft (journal) surface must be fine ground and preferably lapped. Think mirror-smooth.

Why? A porous bronze bearing has an open, soft surface designed to embed foreign particles and retain lubricant. If the shaft is rough, it acts like sandpaper against the bearing — accelerating wear dramatically.

Minimum recommendation: Surface roughness Ra ≤ 0.8 μm (32 microinches) for standard applications.

. Shaft Hardness

The shaft should be steel with at least 0.35–0.45% carbon content (1040 grade or equivalent).

For heavy-duty applications, the shaft should be hardened.

Why? A soft shaft will wear against the bearing, particularly during the boundary lubrication phase at startup. The harder the shaft, the longer both components last.

. Lubricant Grade

This is where the tradeoff gets interesting:

Standard supply: Sintalite porous bronze bearings come pre-impregnated with SAE 20 oil, which has a viscosity of approximately 20 centipoise at 65°C.

For high-load applications: Use a heavier oil. But only on the auxiliary lubrication supply — you can't easily change what's already impregnated in the bearing.

For extending bearing life: Cut a grease groove into the bearing and pump grease through it periodically.

. Heat Dissipation

Friction generates heat. Heat reduces oil viscosity. Reduced viscosity means thinner oil film. Thinner film means more friction. More friction means more heat.

This is a thermal runaway loop — and it's how bearings seize.

The housing material matters enormously:

. Shock Loads

Sintalite bearings handle moderate radial shock loads well because of their oil-cushioned operation. However, excessive prolonged radial shock loads will cause metal-to-metal contact, accelerating wear.

Large out-of-balance forces in rotating members will also reduce bearing life significantly.

. Clearance — The 1/1000 Rule

The running clearance between journal and bearing should be:

$$\text{Clearance} = \frac{d}{1000}$$

For a 25 mm journal:

$$\text{Clearance} = \frac{25}{1000} = 0.025 \text{ mm}$$

Installation method: The bearing is usually a light press fit in the housing (using an arbour press). The shouldered installation tool ensures the bearing seats squarely.

. Length-to-Diameter Ratio (L/d)

Keep this between 0.5 and 1.5.

The p × v Factor — Your Thermal Design Check

Beyond the bearing modulus (M), there's a second critical check: the pressure-velocity product.

The Formula

$$p \times v \text{ factor} = p \times v$$

Where p is in MPa and v is in m/s.

The Threshold

Why This Matters

The p × v factor is directly proportional to heat generation. Higher pressure × higher speed = more energy dissipated as heat at the bearing surface. If the self-lubricating capacity of the porous bronze isn't enough to handle that heat, the oil will degrade, viscosity will drop, and you're back to the practitioner's scenario.

What To Do If p × v > 0.53

Option A — Reduce p × v below 0.53: Increase bearing length to reduce pressure. This is often the simplest fix.

For example, if p × v = 0.60 and p = 0.30 MPa with L = 25 mm, increasing L to 30 mm reduces p to 0.25 MPa, giving p × v = 0.25 × 2.4 = 0.60. Still too high? Go to L = 35 mm.

Option B — Accept it and provide auxiliary lubrication: Four proven methods exist (see next chapter).

Auxiliary Lubrication Methods

When the p × v factor exceeds 0.53, or when you want extra insurance for long service life, use one of these four methods:

Method 1: Felt Washer with Oil Reservoir

A felt washer soaked in oil is placed against one end of the bearing, held in place by a steel retainer. As the bearing runs and temperature rises, oil wicks from the felt into the bearing.

Best for: Horizontal shafts, moderate speeds, applications where periodic re-oiling is feasible.

Method 2: Felt Wick and Oil Well

A felt wick extends from an oil reservoir (a cavity in the housing) to the bearing surface. Capillary action draws oil upward to the bearing.

Best for: Applications where gravity-fed lubrication isn't possible, vertical or angled shafts.

Method 3: Oil Reservoir with Felt Washer or Wool Packing

A larger reservoir is built into the housing, packed with felt or wool saturated in oil. This provides a longer autonomous operating period between service intervals.

Best for: Remote or hard-to-access installations.

Method 4: Grease Cap with Felt Pad and Spring

A screw cap filled with light grease presses against a felt pad via a spring. As grease is consumed, the spring maintains pressure to keep feeding the bearing.

Best for: Applications requiring the longest maintenance intervals, or where oil is inappropriate (dusty environments, food processing).

Auxiliary Lubrication Selection Guide

The Materials Behind the Magic

You've been reading about "porous bronze" bearings. Here's what that actually means — and why the material science matters.

Sintalite Porous Bronze

These bearings are manufactured using powder metallurgy:

1. Pure copper and tin powders are blended
2. The powder is compressed into a die at high pressure
3. The compact is sintered (heated below melting point) so particles bond
4. The resulting structure is about 25% porous by volume
5. The bearing is then vacuum-impregnated with SAE 20 oil — oil fills every pore

The result: a bearing that carries its own lubricant supply. As the bearing heats up during operation, oil seeps out of the pores to lubricate the journal. When it cools, oil gets drawn back in by capillary action.

This is self-healing lubrication. And under normal operating conditions, these bearings can run for thousands of hours without external lubrication.

Why Dissimilar Metals Matter

The bearing material MUST be different from the shaft material. Here's why:

Same metal + friction = welding. Two steel surfaces rubbing together under load will micro-weld at contact points, tearing material off both surfaces. This is called galling or seizure.

Dissimilar metals resist welding. A bronze bearing against a steel shaft creates a material pairing that naturally resists adhesion.

Material Pairing Guidelines

Beyond Bronze: Other Bearing Materials

the practitioner's Post-Mortem — What Actually Went Wrong

Let's go back to the practitioner's failed bearing and reconstruct the failure using everything you've learned.

The Failure Analysis

Given:

• Shaft diameter: 25 mm
• Running speed: 200 rev/min (slow!)
• Radial load: 1,800 N per bearing
• Bearing length: 25 mm (L/d = 1)
• No auxiliary lubrication
• Housing: plastic (bakelite)

Step 1: Bearing pressure

$$p = \frac{1800}{25 \times 25} = \frac{1800}{625} = 2.88 \text{ MPa}$$

Step 2: Surface velocity

$$v = \frac{25}{2000} \times \frac{2\pi \times 200}{60} = 0.0125 \times 20.94 = 0.262 \text{ m/s}$$

Step 3: Check maximum pressure

At v = 0.262 m/s (in the 0.25–0.50 range), maximum allowable pressure = 2.8 MPa.

the practitioner's pressure: 2.88 MPa. This EXCEEDS the maximum. ❌

The bearing was overloaded from day one.

Step 4: The p × v factor

$$p \times v = 2.88 \times 0.262 = 0.755$$

This is well above 0.53, yet no auxiliary lubrication was provided. ❌

Step 5: The bearing modulus

$$M = \frac{20 \times 0.262}{2.88} = \frac{5.24}{2.88} = 1.82$$

M = 1.82. The threshold is 75.

This bearing was operating at a bearing modulus of less than 2.5% of the minimum required for thick-film lubrication. It was running in pure boundary lubrication — metal on metal — from the moment it was installed.

Step 6: The housing

Bakelite housing = terrible heat dissipation. The little oil that was available heated up, lost viscosity, and offered even less protection.

Root Cause Summary

Five out of five checks failed. This wasn't bad luck. This was a completely inadequate design.

What the practitioner Should Have Done

Option 1: Use a larger bearing — 30 mm bore, 38 mm OD, 40 mm length. This would reduce pressure significantly.

Option 2: Switch to a rolling element bearing. At this speed and load, a ball bearing would have been more appropriate.

Option 3: If a journal bearing was required for noise or space reasons, add pressure lubrication and an aluminium housing, and consider a bearing material with higher load capacity.

Quick-Reference Formula Sheet

Here's every formula you need, in one place. Bookmark this section.

Core Formulas

Critical Thresholds

Unit Conversions You'll Need

Maximum Bearing Pressure Reference Chart

For velocities above 1.0 m/s, the maximum allowable bearing pressure depends on both shaft speed and shaft diameter. Here's the relationship:

Key Data Points (from the pressure-speed chart)

Reading the chart: As shaft diameter increases, the surface velocity at a given RPM increases, which reduces the allowable bearing pressure. This is why larger shafts at high speeds need proportionally larger bearings.

The trend: Maximum pressure is roughly inversely proportional to surface velocity. Double the speed → halve the allowable pressure.

Common Mistakes — and How to Avoid Every One

After working through this entire chapter, here are the mistakes that catch even experienced engineers: