What is preload in a tilting-pad journal bearing?

Preload is a dimensionless geometric ratio comparing assembled bearing clearance with the clearance produced by the machined pad curvature. It is not an external force applied to the shaft.

How is journal bearing preload calculated?

Using one common convention, preload m equals 1 minus assembled diametral clearance Cb divided by pad diametral clearance Cp. The dimensions must use the same diametral or radial convention and the same reference condition.

What does zero preload mean?

Zero preload means assembled clearance equals pad clearance. At the centred position, the pad and assembled bore radii are equal under the ideal geometric definition.

Is more bearing preload always better?

No. Preload affects stiffness, damping, film shape, temperature, power loss and load sharing. The appropriate value depends on the complete bearing and rotor system, including hot clearances and manufacturing tolerances.

Does this preload formula apply to thrust bearings?

No. This guide addresses the geometric preload or preset used for profiled and tilting-pad journal bearings. Thrust-bearing pad loading and mechanical or spring preload require different definitions.

Finished tilting-pad journal bearing assemblies used to explain bearing preload

Journal Bearing Preload Explained

What preload means, how it is calculated and why pad curvature and assembled clearance must be considered together.

Understand the Geometry Calculate Preload

Preload Is Geometry, Not an Applied Force

In a tilting-pad journal bearing, preload describes the relationship between the curvature machined into each pad and the clearance that remains when the pads are installed around the shaft. Positive preload moves the pads inward from the concentric position and establishes a converging film geometry even when external bearing load is low.

Rolling-element preload, thrust-bearing mechanical preload and journal-bearing geometric preload are different concepts. This page addresses journal-bearing geometry.

Core Definition

Two Clearances Define Preload

Use either diametral dimensions throughout or radial dimensions throughout. Mixing the two conventions produces the wrong result.

D_j

Assembled bore D_b Pad bore curvature D_p Shaft D_j

Conceptual geometry only – centres, clearances and pad movement are exaggerated.

m = 1 − (C_b / C_p)

C_p = D_p − D_jPad diametral clearance from the machined pad bore diameter and shaft diameter.

C_b = D_b − D_jAssembled or set diametral clearance from the installed bearing bore and shaft diameter.

m = preload factorA dimensionless ratio. A value of 0.30 is also described as 30% preload.

Some drawings and software use radial clearances or different symbols. The ratio is unchanged only when numerator and denominator use the same convention. State the convention with every value.

Geometry States

What the Preload Number Means

m < 0

Negative Preload

Assembled clearance is greater than pad clearance. This can substantially reduce support from lightly loaded pads and alter stiffness and damping. It may result from geometry, tolerance or thermal change and requires engineering review.

m = 0

Zero Preload

Assembled and pad clearances are equal. Under the ideal definition, pad and assembly radii coincide when centred. Near-zero preload can improve effective damping in some rotor systems, but it is not automatically optimum.

0 < m < 1

Positive Preload

Pad clearance is greater than assembled clearance. The pads are located inward relative to their machined curvature, producing a preset converging geometry and hydrodynamic forces at low external load.

A classic Texas A&M tilting-pad design tutorial describes 0.2–0.6 as typical examples, while Kingsbury publishes 0.30–0.35 for specific standard direct-lubricated platforms. These are product and application references, not a universal selection range.

Geometry Check

Preload Factor Calculator

Enter three diameters measured or specified in the same units and at the same reference condition. The calculator derives both clearances before applying the preload ratio.

1
Calculate pad clearance: C_p = D_p − D_j
2
Calculate assembled clearance: C_b = D_b − D_j
3
Calculate preload: m = 1 − C_b/C_p

Calculated geometry is not an approved operating value. Confirm tolerance, temperature, pad deflection, pivot movement and rotor-dynamic requirements.

Design Effects

Changing Preload Changes More Than Clearance

The direction and size of each effect depend on what other dimension is held constant and on the full bearing and rotor model. See how these effects are represented in the stiffness and damping coefficient guide.

Increasing Positive Preload

Usually increases bearing stiffness when achieved by reducing assembled clearance
Develops hydrodynamic support on pads at lower external bearing load
Changes pad load sharing and the shaft equilibrium position
Can reduce clearance margin for thermal growth and tolerance variation
Can alter minimum film thickness, temperature, oil flow and power loss

Reducing Preload

Moves pad and assembled clearances closer together
Can increase effective damping in some turbomachinery systems
May reduce direct stiffness and change critical-speed separation
Can unload upper or lightly loaded pads as preload approaches negative
Requires rotor-dynamic review rather than a bearing-only judgement

Cold Versus Operating Geometry

Why the Drawing Value May Not Be the Hot Value

Differential Expansion

The shaft, pads, lining and carrier do not necessarily heat or expand equally. Pad thermal crowning and bore growth can change both C_p and C_b during operation.

Pad and Pivot Deflection

Hydrodynamic pressure, pad bending and pivot contact deformation change effective running geometry. A rigid cold dimensional model cannot capture these effects alone.

Tolerance Stack

Pad curvature, pivot height, carrier bore, housing fit, shaft size and assembly condition all contribute. Calculate minimum and maximum preload, not only the nominal value.

Measurement and Specification

A Practical Preload Check

01
Confirm the Convention
Identify whether the drawing uses diametral or radial clearance, and whether dimensions are cold, hot, nominal or measured.
02
Verify Shaft Diameter
Use the applicable journal size and temperature condition. Include journal tolerance, wear and any coating or repair allowance.
03
Establish Pad Curvature
Determine the effective machined pad bore or radius from controlled measurement, manufacturing records or the approved drawing.
04
Measure the Assembly
Assemble pads in the correct carrier and housing condition. Account for pad count, load orientation and the specified clearance method.
05
Calculate the Range
Apply tolerances to C_p and C_b, then review nominal, minimum and maximum preload against the bearing analysis.

Common Errors

Preload Values That Cannot Be Compared

Mixing radial clearance in one term with diametral clearance in the other.

Calling assembled clearance alone the preload without knowing pad curvature.

Comparing cold drawing preload directly with predicted hot operating preload.

Changing preload without recalculating temperature, film thickness and rotor dynamics.

Ignoring manufacturing tolerances and reporting only a nominal ratio.

Applying journal-bearing preload terminology to thrust pads or rolling-element bearings.

Common Questions

Journal Bearing Preload FAQ

Is preload an applied force?

No. In this context it is a geometric ratio between assembled and machined-pad clearance. The resulting geometry influences the hydrodynamic forces generated in operation.

Is more preload always better?

No. It changes stiffness, damping, load sharing, film shape and thermal margin. The selected value must suit the entire rotor-bearing system.

Can preload be calculated from assembled clearance?

Not by itself. Shaft diameter and the pad's machined bore or radius are also required to establish pad clearance.

Does the same formula apply to thrust bearings?

No. This geometric preload definition is used for profiled and tilting-pad journal bearings. Thrust-bearing load equalisation and mechanical preload are separate subjects.

Technical Basis

The preload definition and design discussion were cross-checked against the Texas A&M Turbomachinery Laboratory papers Methods to Effectively Evaluate Modern Journal Bearing Performance, Fundamentals of Fluid Film Journal Bearing Operation and Modeling and Tilting Pad Bearing Design, together with published Kingsbury journal-bearing data. Values are explanatory examples, not Oiltech design limits.

Need to Confirm Bearing Preload?

Send the shaft, pad and carrier drawings with operating speed, load, lubricant and temperature data for an engineering review.

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