Plain and Journal Bearings: Materials, Design and Lubrication: Mistake 1

Mistake 1: Ignoring the Startup Phase

Every journal bearing starts in boundary lubrication. Every single time. If your application involves frequent start-stop cycles (like a reciprocating compressor), the bearing experiences boundary lubrication hundreds of times per day. Factor this into your life expectancy calculations.

Mistake 2: Using the Wrong Area in Pressure Calculations

The bearing pressure formula uses projected area (d × L), not the cylindrical surface area (π × d × L). Using cylindrical area will underestimate your pressure by a factor of π, giving you dangerously optimistic results.

Mistake 3: Forgetting Temperature Effects on Viscosity

The bearing modulus M depends on viscosity. Viscosity depends on temperature. Temperature depends on friction. Friction depends on whether you're in thick-film or boundary lubrication. It's a coupled system. Always use the viscosity at the expected operating temperature, not at room temperature.

Mistake 4: Assuming SAE Numbers Are Viscosity Values

SAE 20 oil does NOT have a viscosity of 20 centipoise at all temperatures. The viscosity varies significantly with temperature. At 65°C, it's approximately 20 cp. At room temperature, it's much higher. At 100°C, it's much lower.

Mistake 5: Neglecting the Housing

the practitioner's bakelite housing couldn't dissipate heat. Always consider the thermal conductivity of the entire assembly — not just the bearing.

Mistake 6: Over-Tightening the Press Fit

Pressing a bearing into a housing with too much interference can close the internal pores of the porous bronze, reducing its oil storage capacity by up to 50%. Use the manufacturer's recommended interference fit.

Mistake 7: Not Providing an Oil Path for Auxiliary Lubrication

Adding a felt washer is pointless if there's no path for oil to reach the bearing surface. The bearing often needs a small groove or chamfer to allow oil migration from the auxiliary source to the running surface.

the practitioner's Final Design Report — A Template You Can Steal

Here's how the practitioner documented her bearing selection. Use this format for your own projects:

JOURNAL BEARING SELECTION REPORT

Project: Conveyor Drive Unit — Line 4 Date: [Current Date] Engineer: P. Patel

Design Input Data

Bearing Selection

Selected Bearing

Notes

1. Auxiliary lubrication via oil-soaked felt washer with steel retainer, re-oil every 2,000 hours.
2. Shaft to be ground to Ra ≤ 0.8 μm.
3. Housing bores to be machined concentric to within 0.02 mm TIR.

Engineering takeaway

the practitioner eventually rebuilt his production line with properly selected bearings. He framed the seized bearing and hung it on his office wall with a label that read:

"Check the modulus."

Here's what you should take away:

For beginners: A journal bearing is the simplest, cheapest way to support a rotating shaft — but "simple" doesn't mean "thoughtless." The selection process has clear, calculable steps. Follow them.

For experienced engineers: The bearing modulus (M ≥ 75) and the p × v factor (≤ 0.53 for self-lubrication) are your two non-negotiable design checks. Everything else is optimization.

For anyone specifying equipment: Ask your supplier for the bearing selection calculations. If they can't provide them — or if the bearing modulus is below 75 — push back. A few currency units in bearing cost versus thousands in downtime is not a difficult calculation.

Your Next Step

Pull out a bearing application from your current work. Calculate the three critical numbers:

1. Bearing pressure (p)
2. Surface velocity (v)
3. Bearing modulus (M)

If M < 75 — you now know exactly what to do.

What's the most surprising bearing failure you've encountered? Share it — because every seized bearing has a lesson buried inside it.

This guide is based on data from the Sintalite Porous Bronze Bearing Catalogue, converted to metric units. All formulas use SI-compatible units unless otherwise noted. For pressure-lubricated (hydrodynamic) bearing design, which involves significantly more complex analysis, consult a specialized tribology reference.

Context and scope

They don't spin. They don't roll. They just... sit there. And that's exactly why they're brilliant.

What Exactly Is a Journal Bearing? (And Why Should You Care?)

A journal bearing — also called a bush or plain bearing — is one of the simplest mechanical components ever designed. There are no rolling elements. No balls. No rollers. No cages.

It's essentially a sleeve (the bearing) wrapped around a shaft (the journal), sitting inside a housing. That's it.

    ┌─────────────────────┐
    │       Housing        │
    │   ┌─────────────┐   │
    │   │   Bearing    │   │
    │   │  ┌───────┐  │   │
    │   │  │ Shaft  │  │   │
    │   │  │(Journal│  │   │
    │   │  │   )    │  │   │
    │   │  └───────┘  │   │
    │   └─────────────┘   │
    └─────────────────────┘
      Typical Plain Bearing
       (Cross-Section View)

Two main types exist:

• Pressure-lubricated — lubricant is pumped into the bearing under pressure (think: motor vehicle engines). These require complex design and aren't "off-the-shelf."
• Non-pressure lubricated (self-lubricating) — the bearing material itself is porous and pre-soaked with oil. These are the ones you can select from a catalogue and drop straight into your design. This is where the real story gets interesting.

The most common self-lubricating type? Porous bronze bearings — manufactured using powder metallurgy from pure copper and tin powders, pre-impregnated with SAE 20 oil (up to 30% oil by volume). Under many operating conditions, they need zero additional lubrication.

You read that right. A bearing that lubricates itself.

Failure trigger and engineering context

Let's go back to the practitioner's factory.

Here's what she was dealing with:

• High shaft speed (the conveyor rollers spun at 2,000+ rev/min)
• Moderate radial loads (nothing extreme, but constant)
• Zero thrust loads (purely radial application)
• Tight radial space (compact housing, no room for bulky bearing assemblies)
• Cost pressure (management wanted the bill-of-materials slashed by 20%)
• Noise complaints (the production line ran near an office wall)

Rolling element bearings were technically working — but they were wrong for the job. They were over-engineered, expensive, noisy, and kept failing because of the constant vibration environment on that conveyor.

the practitioner needed something simpler, quieter, cheaper, and tougher in this specific application.

She needed a journal bearing.

. Surface Finish of the Shaft

The shaft (journal) must have a fine ground finish, preferably lapped. A rough shaft surface will tear through the bearing material and destroy the oil film. This isn't optional — it's foundational.

. Surface Hardness of the Shaft

The shaft should be steel with at least 0.35–0.45% carbon content (1040 grade minimum). For heavy-duty applications, harden the shaft. A soft shaft against a soft bearing = disaster.

. Grade of Lubricant

Higher viscosity lubricant → longer bearing life. But there's a catch: higher viscosity also means higher friction. So high-viscosity lubricants should only be used with high loads. For most applications using self-lubricating porous bronze, SAE 20 oil is standard.

Pro tip: In high-load applications, cut a grease groove into the bearing so grease can be pumped directly into the contact zone — extending life dramatically.

. Heat Dissipation

Friction generates heat. Heat reduces lubricant viscosity. Reduced viscosity → more wear → more heat. It's a death spiral if your housing can't dissipate heat.

Housing material matters:

If you're mounting a journal bearing in a plastic housing, you'd better have very low loads and speeds — or an external cooling method.

. Shock Loads

Porous bronze bearings handle moderate radial shock loads well thanks to their oil-cushioned operation. But excessive, prolonged shock loads cause metal-to-metal contact, which accelerates wear exponentially.

Watch out for: Large out-of-balance forces in rotating members — these effectively act as repeated shock loads and will destroy bearing life.

. Clearance (The 1/1000 Rule)

This is one of the most commonly botched aspects of journal bearing installation.

The bearing is press-fit into the housing. A shouldered tool and arbour press are standard. But between the journal (shaft) and the bush (bearing), there must be a running clearance.

The rule of thumb:

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

Where d is the journal diameter.

Too tight = seizure. Too loose = hammering, noise, and premature failure.

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

This ratio should fall in the 0.5 to 1.5 range.

A safe starting point for your first design iteration: assume L/d = 1 (meaning the bearing length equals the shaft diameter).

The Three States of Lubrication

Imagine a shaft sitting inside a journal bearing at rest. Gravity pulls the shaft down. Metal touches metal at the bottom. This is called boundary lubrication — and it's where wear happens.

Now start spinning the shaft slowly. Oil gets dragged around by the rotating surface. The shaft starts to "float" — but still occasionally touches the bearing wall. This is thin-film (transition) lubrication. Moderate wear can still occur.

Speed up more. The oil film thickens. The shaft lifts completely off the bearing surface. Zero contact. Zero wear. This is thick-film lubrication — the holy grail.

Friction
Torque
   ▲
   │ \
   │  \        Boundary          Transition       Thick-Film
   │   \       Lubrication       (Thin-Film)      Lubrication
   │    \         ↓                  ↓                 ↓
   │     \        │                  │                 │
   │      ╲       │                  │                ╱
   │       ╲──────┤──────────────────┤──────────────╱
   │              │                  │            ╱
   │              │                  │          ╱
   │              │                  │        ╱
   └──────────────┴──────────────────┴──────────────────▶
                                                    Shaft Speed

The key insight: Friction is highest at boundary conditions (low speed), drops through the transition zone, then rises again gently in thick-film mode due to fluid friction. The sweet spot is right at the onset of thick-film lubrication.

The Bearing Modulus: Your Design Compass

How do you know if thick-film lubrication will occur? Calculate the bearing modulus M:

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

Where:

1 centipoise (cp) = 1000 Pascal-seconds (Pa·s)

The critical threshold: M ≥ 75

• M > 75 → Thick-film lubrication is assured. Minimal wear. Long life.
• M < 75 → You're operating in the transition or boundary zone. Consider increasing lubricant viscosity, increasing shaft speed, or reducing bearing pressure (by increasing bearing length).
• M >> 75 → Thick-film lubrication is guaranteed, but friction will be higher. Consider reducing lubricant viscosity.

This single number tells you whether your bearing will last months or decades.

The Complete Selection Procedure: Your 8-Step Checklist

Here's the exact process the practitioner followed — and the one you can follow for any porous bronze journal bearing application. Grab a calculator.

Step 1: Gather Your Data

You need three numbers:

• Shaft (journal) diameter d (mm)
• Rotational speed N (rev/min)
• Radial load F (N) — if load is shared between two bearings, divide by 2

Step 2: Choose a Bearing Length

Look up standard bearing sizes for your journal diameter from the manufacturer's catalogue. Start with L/d = 1 as a first trial (bearing length = shaft diameter).

Note: Standard metric sizes should be used wherever possible. Non-standard sizes can be custom ordered, but require large quantities to be economical.

Step 3: Calculate Bearing Pressure

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

Where:

• F = radial bearing load (N)
• d = diameter (mm)
• L = bearing length (mm)
• p = bearing pressure (MPa)

Remember: 1 MPa = 1 N/mm²

Step 4: Calculate Surface Velocity

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

Simplified:

$$v = \frac{\pi \times d \times N}{60{,}000} \quad \text{(m/s)}$$

Step 5: Check Against Maximum Allowable Pressure

Compare your calculated pressure p against the maximum allowable pressure for your velocity:

If your bearing pressure exceeds the maximum → try a longer bearing. If even the longest available length doesn't work → journal bearings aren't suitable for this application. Consider a pressure-lubricated plain bearing or a rolling element bearing instead.

Step 6: Calculate the p·v Factor

$$p \times v$$

If this product exceeds 0.53 → auxiliary lubrication is needed (felt washer, oil reservoir, wick feed, or grease cap).

If the p·v factor exceeds 0.53 by only a small margin, you may be able to reduce bearing pressure by increasing bearing length to bring the factor below 0.53 — avoiding the need for auxiliary lubrication entirely.

Step 7: Check the Bearing Modulus

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

Using SAE 20 oil at approximately 65°C operating temperature: μ ≈ 20 cp.

• M > 75 → Thick-film lubrication assured. You're good.
• M < 75 → Consider increasing lubricant viscosity or modifying design parameters.
• M >> 75 → Lubrication will be thick-film, but friction (and therefore heat) will be high. Consider reducing lubricant viscosity.

Step 8: Select the Catalogue Number

The bearing designation encodes the dimensions. For example:

SMC 303830 → Inside diameter 30 mm, Outside diameter 38 mm, Length 30 mm

Record the catalogue number and all relevant design data for your bill of materials.

Worked Example: the practitioner's Conveyor Bearing

Let's walk through the exact calculation the practitioner did.

Given:

• Shaft diameter: d = 30 mm
• Speed: N = 1,450 rev/min
• Total radial load: F = 500 N (mid-way between two bearings)

Step 1: Load per bearing = 500 / 2 = 250 N

Step 2: Choose L/d = 1 → L = 30 mm

Step 3: Bearing pressure:

$$p = \frac{F}{d \times L} = \frac{250}{30 \times 30} = 0.278 \text{ MPa}$$

Step 4: Surface velocity:

$$v = \frac{\pi \times 30 \times 1450}{60{,}000} = 2.278 \text{ m/s}$$

Step 5: Since v > 1.0 m/s, check the manufacturer's pressure-speed chart. For d = 30 mm at 1,450 rev/min → p max = 0.47 MPa.

Our calculated p = 0.278 MPa < 0.47 MPa ✅ Bearing pressure is OK.

Step 6: p × v factor:

$$0.278 \times 2.278 = 0.633$$

Since 0.633 > 0.53 → Auxiliary lubrication is needed.

(the practitioner chose a felt washer with oil reservoir — simple, reliable, and cheap.)

Step 7: Bearing modulus (using SAE 20 oil at 65°C, μ = 20 cp):

$$M = \frac{20 \times 2.278}{0.278} = 164$$

Since 164 >> 75 → Thick-film lubrication is assured. ✅

Step 8: Selected bearing: SMC 303830 (30 mm bore, 38 mm OD, 30 mm length), with a felt washer and oil reservoir for auxiliary lubrication.

Results Summary Table

Journal Bearings vs. Rolling Element Bearings: The Head-to-Head

So when should you reach for a journal bearing instead of a roller or ball bearing? Here's the honest comparison.

Where Journal Bearings Win

Where Journal Bearings Lose

The Bottom Line

Journal bearings are best for moderate radial loads, high speeds, zero or low thrust, compact spaces, and budget-conscious designs — especially where noise matters.

Auxiliary Lubrication Methods: When Self-Lubrication Isn't Enough

When the p·v factor exceeds 0.53, the bearing's built-in oil supply won't last. You need to feed it externally. Here are four proven methods:

1. Felt Washer Soaked in Oil — A felt washer sits against the bearing face, continuously wicking oil into the contact zone. Retained by a steel retainer ring. Simple and effective.

2. Felt Wick and Oil Well — A wick draws oil from a small reservoir in the housing up to the bearing surface. Works via capillary action. Good for continuous, slow-feed applications.

3. Oil Reservoir with Felt Washer or Wool — A larger oil supply chamber in the housing, with felt or wool to regulate flow. Best for longer service intervals.

4. Felt Pad and Spring with Grease Cap — A spring-loaded felt pad presses against the shaft, fed by a screw-cap grease fitting. Ideal for applications where oil isn't practical.

Which to choose? For most industrial applications, option 1 or 3 gives you the best balance of reliability and low maintenance.

Quick-Reference Formulas Card

Keep this handy. Print it. Tape it to your desk.

Critical thresholds to memorize:

• M ≥ 75 → Thick-film lubrication
• p·v ≤ 0.53 → Self-lubrication sufficient
• p·v > 0.53 → Auxiliary lubrication required

Engineering takeaway

the practitioner didn't invent anything new. She didn't use cutting-edge materials or exotic manufacturing processes. She used a bearing technology that's been around for centuries — and she applied it correctly.

The conveyor ran quieter. The maintenance costs dropped. The spare parts inventory shrank. Her supervisor stopped crossing his arms and started asking her opinion on other projects.

The lesson for you:

The best engineering solution isn't always the most complex one. Sometimes the answer is a simple bronze sleeve, some oil, and the discipline to run the numbers properly.

Journal bearings won't solve every problem. They have real limitations — load capacity, thrust capability, shaft finish requirements. But in the right application, they are unbeatable on cost, noise, and reliability.

Next time you're spec'ing a bearing for a moderate-load, high-speed, radial-only application — before you default to a ball bearing because that's what you've always done — run the eight steps. Calculate the bearing modulus. Check the p·v factor. You might be surprised.

The quiet hero might be exactly what your machine needs.

Your Next Move

If you're a student: Work through the example above with a different shaft diameter (try 20 mm or 50 mm). Change the speed. Change the load. See how the bearing modulus and p·v factor shift. This builds intuition faster than any textbook chapter.

If you're a practicing engineer: Pull up your current project's bearing schedule. Identify any application where the load is moderate, the speed is high, and the space is tight. Run the journal bearing selection procedure. You might find a cost-saving opportunity hiding in plain sight.

If you're a procurement or maintenance professional: Ask your engineering team whether any of your frequently-replaced rolling element bearings could be substituted with self-lubricating porous bronze bushes. The cost and downtime savings can be substantial.

Got a tricky bearing application you're unsure about? Drop it in the comments with the shaft diameter, speed, and load — and let's work through the selection together.

This post is part of a series breaking down the core chapters of mechanical design data into practical, applicable knowledge. If the bearing modulus just clicked for you in a way it never did before — share this with a colleague who needs it.