Turbomachinery Engineer Asked :

We have speed factor in rolling bearings , what do we have for speed in journal bearings linked to oil lubrication ?

Khash Replied:

In rolling bearings, you’re used to the speed factor (n·dm) as a practical limit tied to heat generation and lubrication regime.

For journal (hydrodynamic) bearings, the equivalent thinking is not a single “speed factor,” but a set of dimensionless parameters that directly govern oil film formation, load capacity, and lubrication regime stability.

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1) The Core Parameter → Sommerfeld Number (S)

This is the closest equivalent to a “speed factor” in journal bearings.

$S=\frac{\eta \cdot N}{P}{\left(\frac{r}{c}\right)}^2$

Where:

• η = dynamic viscosity of oil
• N = rotational speed
• P = bearing pressure (load per projected area)
• r = journal radius
• c = radial clearance

Interpretation (very important for you):

• Speed (N) directly increases S
• Higher S → thicker oil film → safer hydrodynamic regime
• Lower S → risk of mixed/boundary lubrication

👉 In simple words:
Speed in journal bearings is valuable only because it helps build the oil film.

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2) Practical Engineering Form → Hersey Number

Often used interchangeably in charts:$$\text{Hersey}=\frac{\eta \cdot N}{P}$$

This removes geometry and focuses on operating condition only.

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3) The Real “Speed Indicator” You Should Care About → Stribeck Curve

Speed in journal bearings is always interpreted through lubrication regime:

• Low speed → Boundary lubrication
• Medium speed → Mixed lubrication
• High speed → Full hydrodynamic film

This is captured in the Stribeck parameter:$$\frac{\eta \cdot N}{P}$$👉 Same physics again: viscosity × speed vs load.

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4) What Actually Replaces “Speed Factor” in Practice

In real turbomachinery work (your field), engineers don’t say “speed factor.”
They think in terms of:

a) Minimum Oil Film Thickness (h_min)

• Must be > combined surface roughness
• Strongly dependent on speed

b) Lambda Ratio (λ)

$$\lambda =\frac{h_{min}}{\sigma }$$

• λ>3 → full film (safe)
• λ<1 → metal contact

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c) DN Value? ❌ Not used

Unlike rolling bearings, DN has no meaning here.

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5) What Speed Means Physically in Journal Bearings

Speed drives:

• Hydrodynamic pressure generation
• Oil wedge formation
• Film thickness
• Heat generation (shear)

But also:

⚠️ Too high speed:

• Increases shear heating
• Reduces viscosity → can reduce film if not controlled

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6) Your Practical Field Translation (Very Important)

In turbine / compressor journal bearings:

• At startup / shutdown (low N)
  → highest risk of wear
• At rated speed
  → safest condition (if oil viscosity is correct)

👉 This is opposite to rolling bearings where high speed is often limiting.

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Final Engineering Summary

Concept	Rolling Bearings	Journal Bearings
Speed indicator	n·dm (speed factor)	Sommerfeld / Hersey
Role of speed	Heat limitation	Film generation
Risk zone	High speed	Low speed
Lubrication regime	Mostly elastohydrodynamic	Hydrodynamic

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Bottom Line

If you want a one-line equivalent:

👉 Speed factor in journal bearings = (η × N) / P → governing oil film formation, not just speed limit

He asked:

Practical calculation for a turbine bearing

Linking it to viscosity selection (ISO VG)

Or connecting it to varnish impact on film thickness (very relevant to your work)

Khash Replied:

Practical Calculation for a Turbine Journal Bearing: Speed, Viscosity, Oil Film and Varnish

For a rolling bearing, we often think about speed factor.

For a turbine journal bearing, the better question is:

“At this speed, load, viscosity, clearance and temperature, can the oil create enough hydrodynamic film thickness?”

The key practical parameter is:

$S=\frac{\eta \cdot N}{P}{\left(\frac{r}{c}\right)}^2$

Where higher $S$S generally means stronger hydrodynamic film formation.

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1. Example Turbine Journal Bearing Data

Assume a steam turbine journal bearing:

Parameter	Value
Shaft diameter	150 mm
Bearing length	100 mm
Rotational speed	3,000 rpm
Bearing load	30,000 N
Radial clearance	0.10 mm
Oil grade	ISO VG 46
Operating oil viscosity at bearing temperature	0.012 Pa·s

Projected bearing area:$$A=D\times L$$A=D×L$$A=0.15\times 0.10=0.015m^2$$A=0.15×0.10=0.015m2

Bearing pressure:$$P=\frac{W}{A}$$P=AW​$$P=\frac{30,000}{0.015}=2,000,000Pa=2MPa$$P=0.01530,000​=2,000,000Pa=2MPa

Radius:$$r=0.075m$$r=0.075m

Clearance:$$c=0.0001m$$c=0.0001m$$\frac{r}{c}=\frac{0.075}{0.0001}=750$$cr​=0.00010.075​=750

Speed in revolutions per second:$$N=\frac{3000}{60}=50s^{-1}$$N=603000​=50s−1

Now:$$S=\frac{0.012\times 50}{2,000,000}\times {750}^2$$S=2,000,0000.012×50​×7502$$S=0.16875$$S=0.16875

This is not a universal “pass/fail” number by itself, because bearing design charts, eccentricity ratio, L/D ratio and groove design matter. But it is very useful for comparison.

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2. What Happens if We Change ISO VG?

At operating temperature, ISO VG 32, 46 and 68 do not keep their nominal viscosity. The real viscosity depends on temperature and VI.

Approximate dynamic viscosity at operating bearing temperature:

Oil Grade	Approx. dynamic viscosity at hot bearing condition	Relative film effect
ISO VG 32	0.008 Pa·s	Lower film
ISO VG 46	0.012 Pa·s	Baseline
ISO VG 68	0.017 Pa·s	Higher film

Because $S$S is directly proportional to viscosity:

Oil Grade	Sommerfeld Number
ISO VG 32	0.112
ISO VG 46	0.169
ISO VG 68	0.239

So, increasing viscosity increases film strength.

But this is the trap:

Higher viscosity does not automatically mean better bearing reliability.

Because higher viscosity can also cause:

• Higher shear heating
• Higher oil temperature
• Higher power loss
• Reduced cooling efficiency
• Poorer heat removal from bearing metal
• Possible instability in some bearing designs

So ISO VG selection is not “choose thicker oil.”
It is choose the correct viscosity at the actual bearing operating temperature.

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3. Practical Viscosity Selection Logic for Turbine Bearings

For turbine journal bearings, viscosity selection must balance two opposite needs:

Need 1: Enough viscosity

To generate hydrodynamic film thickness and avoid mixed lubrication.

Need 2: Not too much viscosity

To avoid excessive heat generation and oil churning/shear losses.

The correct oil grade depends on:

• Shaft speed
• Load
• Bearing diameter
• Bearing clearance
• Bearing length
• Oil supply temperature
• Bearing metal temperature
• Oil flow rate
• Bearing design: plain, elliptical, tilting pad
• OEM requirement

This is why many steam and gas turbine systems commonly use ISO VG 32 or ISO VG 46 turbine oils, while some larger or slower machines may require higher viscosity grades.

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4. The Film Thickness Connection

The simplified reliability question is:$$\lambda =\frac{h_{min}}{\sigma }$$Where:

• hmin​ = minimum oil film thickness
• σ = combined surface roughness

Practical interpretation:

Lambda Ratio	Condition
λ > 3	Full hydrodynamic separation
λ = 1–3	Mixed lubrication risk
λ < 1	Surface contact likely

So the real goal is not just a good viscosity number.

The real goal is:

Keep minimum oil film thickness safely above shaft and bearing surface roughness under all operating conditions.

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5. Where Varnish Becomes Very Important

This is where turbine oil chemistry enters the mechanical calculation.

A bearing may be designed correctly.
The ISO VG may be correct.
The oil pressure may be acceptable.
The speed may be normal.

But varnish can still reduce the real lubrication margin.

How?

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6. Varnish Impact on Film Thickness

Varnish does not need to be thick to become dangerous.

In journal bearings, oil film thickness can be in the range of only a few microns to tens of microns, depending on the machine and condition.

So even a thin varnish layer can change the effective geometry.

Example:

Item	Value
Designed radial clearance	100 µm
Varnish deposit on bearing surface	5 µm
Varnish deposit on journal/bearing effective area	5 µm
Effective clearance reduction	~10 µm
New effective clearance	90 µm

That looks small.

But in the Sommerfeld relationship:$$S\propto {\left(\frac{r}{c}\right)}^2$$So clearance has a squared effect.

Reducing clearance may increase calculated Sommerfeld number, but that does not mean safer operation. Why?

Because varnish is not a precision-machined bearing surface.

It creates:

• Local high spots
• Distorted oil wedge geometry
• Restricted oil flow
• Higher local temperature
• Poor heat transfer
• More shear stress
• Surface roughness increase
• Mixed lubrication zones

So varnish can make the bearing look “tighter,” but actually make it less stable and less predictable.

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7. Varnish Changes the Bearing from Designed Geometry to Dirty Geometry

A clean journal bearing has designed:

• Clearance
• Surface finish
• Groove geometry
• Oil wedge formation
• Heat removal path

A varnished journal bearing has disturbed:

• Clearance profile
• Oil flow distribution
• Thermal balance
• Surface energy
• Friction behavior
• Minimum film location

This means the bearing may still run, but with reduced reliability margin.

This is why varnish is dangerous:

It does not always cause immediate failure. It silently reduces the safety margin.

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8. Practical Example: Film Margin Loss

Assume:

Parameter	Clean Bearing	Varnished Bearing
Minimum oil film thickness	15 µm	10 µm
Combined roughness	3 µm	4 µm
Lambda ratio	5.0	2.5

Clean bearing:$$\lambda =\frac{15}{3}=5$$Full film lubrication.

Varnished bearing:$$\lambda =\frac{10}{4}=2.5$$Mixed lubrication risk.

This is the critical point:

The oil grade did not change. The speed did not change. The load did not change. But varnish reduced the lubrication regime.

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9. Practical Field Message

For turbine journal bearings, speed is not only a speed number.

Speed is part of a lubrication system:

$$\text{Film Reliability}=f(\text{Speed, Viscosity, Load, Clearance, Temperature, Surface Condition, Oil Chemistry})$$

And varnish attacks two parts at the same time:

1. Mechanical geometry
   By changing clearance, surface condition and oil wedge formation.
2. Oil chemistry
   By increasing polarity, oxidation products, deposits, acidity and antioxidant stress.

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10. Final Khash Summary

In rolling bearings, high speed can be the enemy.

In journal bearings, low speed is often the dangerous zone because the oil wedge is weak.

But in turbine journal bearings, even correct speed and correct ISO VG oil are not enough if the oil chemistry is dirty.

Because:

Speed builds the oil film.
Viscosity supports the oil film.
Clearance shapes the oil film.
Varnish destroys the confidence in the oil film.

That is why turbine oil reliability is not only oil analysis.

It is the protection of the invisible micron-level oil film that keeps the rotor alive.