Sliding Bearing Load and Pressure: How to Calculate and Select the Right Bearing
A sliding bearing may be rated for a certain load, but the actual pressure acting on the bearing surface depends on the projected bearing area. Using load alone to select a bearing can lead to excessive pressure, deformation, accelerated wear, or premature failure.
Bearing load and bearing pressure are related but different engineering parameters. Load is the force applied to the bearing, while pressure describes how that force is distributed over the bearing's projected area. The same load can produce very different pressures depending on bearing dimensions and load distribution.
For engineers designing or selecting a plain bearing, the practical question is therefore not simply, "How much load can the bearing carry?" The more useful question is whether the bearing can safely handle the actual load under the complete operating conditions.
This guide explains how to determine bearing load, calculate projected bearing area and bearing pressure, evaluate static, dynamic, and shock loads, distinguish radial from axial loading, and use these calculations as part of a bearing selection process.
Quick Answer
For a typical cylindrical radial sliding bearing, projected area is A = D × L, and average bearing pressure is P = F / A. However, allowable load cannot be determined from pressure alone; material, speed, temperature, shaft condition, clearance, load distribution, and manufacturer data must also be considered.
What Is Sliding Bearing Load?
Sliding bearing load is the force transmitted from the shaft or mating component to the bearing during operation. Determining the actual load acting on the bearing is the first step in evaluating whether a bearing design is suitable.
The rated load of a machine or actuator is not necessarily the same as the load carried by an individual bearing. The actual bearing load may depend on the mechanical arrangement, lever arms, external forces, acceleration, inertia, friction, and how the load is distributed between multiple bearings.
Radial Load
A radial load acts perpendicular to the shaft axis. In a cylindrical plain bearing, the shaft transfers this load through the bearing surface, making radial loading one of the most common loading conditions for sliding bearings.
For a simple radial application, the nominal projected area can be used to calculate an average bearing pressure.
Axial Load
An axial load acts parallel to the shaft axis. It is normally carried by a thrust surface, flange, thrust washer, or another dedicated bearing geometry rather than the cylindrical projected area used for a radial bearing.
Axial loading can also create uneven contact or edge loading when alignment or geometry is unfavorable.
Static, Dynamic, and Shock Loads
Load behavior is also important. A static load remains approximately constant over the relevant design period. A dynamic load varies with time, position, or operating cycle. A shock load is a short-duration load with a potentially high peak value.
These conditions should not be treated as interchangeable:
| Load Type | Characteristics | Design Consideration |
|---|---|---|
| Static | Remains approximately constant over the relevant design period | Check pressure and deformation |
| Dynamic | Varies with time, position, or operating cycle | Consider pressure, speed, wear, and duty cycle |
| Shock | Short-duration, high peak load | Check peak pressure and deformation |
Dynamic loading does not necessarily mean a higher load than static loading. The important issue is that the load may vary over time and may produce peak values that need separate evaluation. Peak load may occur only briefly but can still govern bearing selection.
How to Calculate Projected Bearing Area
The projected bearing area is the nominal area used to relate a radial load to an average bearing pressure.
For a typical cylindrical radial sliding bearing, the projected area is calculated as:
A = D × L
where:
A = projected bearing area, mm²
D = bearing inside diameter, mm
L = bearing length, mm
For example, consider a cylindrical bearing with an inside diameter of 40 mm and a length of 40 mm:
A = 40 × 40 = 1,600 mm²
The calculation is straightforward, but the geometry must be identified correctly before applying the formula.
| Parameter | Symbol | Unit |
|---|---|---|
| Bearing inside diameter | D | mm |
| Bearing length | L | mm |
| Projected area | A | mm² |
This calculation applies to a typical cylindrical radial bearing. Other bearing geometries require an appropriate contact or projected-area definition. Flanged bearings, thrust washers, spherical bearings, and other specialized designs should not automatically be evaluated using the same D × L relationship.
The projected area is also a nominal engineering area. It does not prove that the load is actually distributed uniformly across the entire bearing surface. Misalignment, shaft deflection, housing deformation, clearance variation, and installation errors can produce localized pressure that is higher than the simple average calculation suggests.
How to Calculate Bearing Pressure
Once the actual radial load and projected area are known, the nominal average bearing pressure can be calculated as:
P = F / A
For a cylindrical radial sliding bearing:
P = F / (D × L)
where:
P = average bearing pressure, N/mm² or MPa
F = radial load, N
D = bearing inside diameter, mm
L = bearing length, mm
Because 1 N/mm² equals 1 MPa, the result can be directly expressed in MPa.
For example:
Load: 8,000 N
Bearing inside diameter: 40 mm
Bearing length: 40 mm
Projected area: 1,600 mm²
Therefore:
P = 8,000 / 1,600 = 5 MPa
Quick Calculation
| Parameter | Value |
|---|---|
| Load | 8,000 N |
| Diameter | 40 mm |
| Length | 40 mm |
| Projected area | 1,600 mm² |
| Bearing pressure | 5 MPa |
This result represents the nominal average pressure, not necessarily the maximum local contact pressure.
Increasing the projected area generally reduces average pressure for a given load. However, increasing bearing dimensions is not a universal solution. Available space, shaft diameter, housing design, clearance, speed, thermal conditions, load distribution, and manufacturing requirements can all limit the practical dimensions.
Therefore, bearing pressure is a calculation used for engineering evaluation, not a standalone bearing selection criterion.
How Load Conditions Affect Bearing Evaluation
Different load conditions require different engineering checks. The pressure calculation is the same, but the way the result is used depends on how the load behaves over time.
| Load Condition | Evaluation Focus |
|---|---|
| Static load | Check nominal pressure and deformation against allowable limits. |
| Dynamic load | Evaluate pressure, speed, wear, and duty cycle over time. |
| Shock load | Check peak pressure and possible local deformation even if duration is short. |
The distinction between average load and peak load is particularly important. An application may have a relatively moderate average load but experience short-duration peaks during acceleration, reversal, starting, stopping, or impact.
Shock loading requires additional attention because a high instantaneous load can produce local deformation even when the average operating load appears acceptable.
When evaluating a real application, consider:
Peak rather than only average load
Acceleration and inertia forces
Load distribution between multiple bearings
Thermal expansion and resulting additional forces
Changes in load throughout the operating cycle
The appropriate evaluation therefore depends not only on how large the load is, but also on when, where, and how the load is applied.
Radial Load vs Axial Load
Radial and axial loads should be evaluated according to the bearing geometry and load path.
A radial load acts perpendicular to the shaft axis and is commonly supported by the cylindrical surface of a plain bearing. For a conventional cylindrical radial bearing, the nominal pressure relationship is:
P = F / (D × L)
An axial load, by contrast, acts parallel to the shaft axis. It normally requires a thrust surface or a bearing geometry specifically designed to support axial forces, such as a flanged bearing, thrust washer, or combination bearing.
For this reason, an axial force should not simply be substituted for F in the radial bearing pressure formula.
Applications involving both radial and axial loads should identify the two load components separately and evaluate each according to the relevant bearing geometry and contact area.
This distinction becomes especially important in compact assemblies, where a radial bearing may be exposed to unintended axial forces because of misalignment, thermal expansion, mounting conditions, or adjacent component loads.
Bearing Load Capacity vs PV Capacity
Bearing load capacity and PV capacity describe different aspects of sliding bearing performance.
Allowable load describes how much force a bearing can support under specified conditions. The applicable value depends on the bearing material, geometry, temperature, lubrication, shaft condition, and other operating factors. Specific product data may be based on defined test conditions and should not be assumed to represent the allowable load for every application.
PV capacity considers the combined effect of bearing pressure and sliding speed. A bearing may satisfy a pressure requirement but still operate beyond its suitable PV range at a high sliding speed. Conversely, an application may fall within a material's PV capability while the pressure itself is too high.
Therefore, bearing selection should consider at least:
Allowable pressure or load under specified conditions
Sliding speed
PV value
Operating temperature
Shaft material and surface condition
Running clearance
Lubrication and environment
Load capacity alone is not sufficient for bearing selection. PV and other application conditions must also be evaluated.
Bearing load ratings are condition-dependent and should not be treated as universal material constants. Allowable load under specified conditions should always be confirmed with manufacturer data.
For the detailed calculation of pressure and sliding speed as a combined PV value, see the related guide on How to Calculate Bearing PV Value.
How Load Distribution Affects Bearing Performance
The simple pressure calculation assumes a nominal load distribution, but actual contact conditions may be less uniform.
A shaft can deflect under load. A housing can deform. Installation errors can introduce angular misalignment, while variations in clearance can change the contact pattern. These effects may concentrate the load near one side or at the bearing edge.
This edge loading or localized contact can produce a local pressure substantially different from the nominal average value calculated from the projected area. Edge loading may not be visible in the average pressure calculation.
Potential consequences include:
Localized high pressure
Accelerated wear
Increased friction
Local deformation
Premature bearing failure
Good installation accuracy and appropriate mechanical alignment can help reduce these risks. Depending on the application, engineers may also consider self-aligning structures, suitable running clearance, shaft stiffness, housing stiffness, and a bearing material appropriate for the expected contact conditions.
This is why the calculated average bearing pressure should be treated as an engineering screening value rather than a complete description of the actual contact stress.
How to Select a Bearing Based on Load
Bearing selection should follow a sequence rather than relying on load capacity alone.
Step 1 — Determine the Actual Load
Identify the radial and axial components separately. Establish whether the load is static, continuously varying, cyclic, or subject to shock or peak events.
Step 2 — Determine the Projected Area
For a typical cylindrical radial bearing:
A = D × L
For other bearing geometries, use the appropriate contact or projected-area definition.
Step 3 — Calculate Bearing Pressure
For a cylindrical radial bearing:
P = F / (D × L)
Use the relevant operating load and, where appropriate, separately evaluate peak loading.
Step 4 — Compare with Manufacturer Data
Compare the calculated pressure with the manufacturer's allowable load or pressure data for the specific bearing grade and specified operating conditions.
Do not assume that a material's published value represents a universal allowable load for every application.
Step 5 — Check the Operating Conditions
Evaluate the other factors that can affect bearing performance:
Sliding speed
PV value
Temperature
Shaft material and surface condition
Running clearance
Lubrication
Water, dust, chemicals, or other environmental conditions
Load distribution and alignment
The overall engineering flow can therefore be summarized as:
Applied Load → Projected Bearing Area → Bearing Pressure → Material Allowable Pressure → PV / Speed / Temperature / Shaft / Clearance → Bearing Selection
For PTFE composite applications, manufacturers may provide allowable pressure data for specific bearing grades such as MG-1. Such data should be evaluated together with the actual application conditions rather than used as an isolated material limit.
Common Mistakes When Calculating Bearing Load and Pressure
Several mistakes can make an apparently correct bearing calculation misleading.
Using machine-rated load instead of actual bearing load
The force applied to an individual bearing may differ from the machine's rated or actuator load.Ignoring peak or shock loads
Average operating load may not represent the highest pressure experienced during acceleration, reversal, impact, or stopping.Using the wrong projected area
The cylindrical radial formula should not automatically be applied to thrust or specialized bearing geometries.Applying radial formulas to axial loads
Axial loads require evaluation based on the actual thrust-bearing geometry and contact area.Assuming uniform load distribution
Misalignment, shaft deflection, housing deformation, and clearance variation can create localized high-pressure regions.Selecting by load alone
Speed, PV, temperature, shaft condition, clearance, lubrication, and environment can all affect whether a bearing is suitable.
Bearing Load and Pressure Calculation Checklist
Before selecting a sliding bearing, collect the following information:
| Check | Required Information |
|---|---|
| Radial load | N |
| Axial load | N |
| Static load | N |
| Dynamic load | N |
| Shock load | N |
| Bearing inside diameter | mm |
| Bearing length | mm |
| Projected area | mm² |
| Bearing pressure | MPa |
| Speed | rpm |
| PV | MPa·m/s |
| Temperature | °C |
| Shaft condition | Material / Hardness / Roughness |
| Clearance | mm |
| Environment | Dry / Lubricated / Water / Dust / Chemicals |
Having this information available makes it easier to determine whether a standard bearing is suitable or whether an engineering evaluation is required.
When Engineering Evaluation Is Recommended
Manufacturer or engineering support is particularly useful when the calculated load is close to the applicable allowable range or when the application contains conditions that make a simple pressure calculation insufficient.
An engineering evaluation is recommended for applications involving:
Significant dynamic or shock loading
Combined radial and axial loads
High temperatures or corrosive environments
Tight clearance requirements
Custom dimensions
New OEM designs
Long service-life requirements
Significant misalignment or unusual load distribution
When requesting an evaluation, provide the shaft diameter, bearing dimensions, radial and axial loads, speed, motion type, operating temperature, shaft material and surface condition, lubrication, environment, and expected service life.
Have a specific bearing application? Send us the shaft diameter, bearing dimensions, load, speed, motion type, operating temperature, and environment. Our engineers can help evaluate the appropriate bearing material and design.
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FAQ
How do you calculate sliding bearing pressure?
For a cylindrical radial bearing, calculate projected area A = D × L, then average pressure P = F / A. Result is in MPa when force is in N and dimensions in mm.
What is the difference between bearing load and pressure?
Bearing load is the applied force. Bearing pressure is that force divided by projected area. The same load can produce different pressures when dimensions or load distribution change.
What is projected bearing area?
For a cylindrical radial bearing, projected area is A = D × L. Other geometries require their own area calculation.
Can a sliding bearing handle both radial and axial loads?
Some designs support both, but radial and axial loads should be evaluated separately. Flanged bearings, thrust washers, or combination designs may be needed.
Should peak load or average load be used for bearing selection?
Both are relevant. Average load describes normal operation; peak or shock load must be checked for high-pressure conditions and deformation.
What information is needed for a bearing load evaluation?
Bearing dimensions, shaft diameter, radial and axial loads, peak loads, speed, motion, temperature, shaft condition, clearance, lubrication, environment, and service life.
Conclusion
Calculating sliding bearing load and pressure is a fundamental step in bearing selection, but the calculation should not end with P = F / A.
The practical engineering sequence is:
Determine the actual load → Calculate the projected area → Determine bearing pressure → Compare with applicable material data → Check PV and operating conditions → Verify the bearing selection.
A bearing that satisfies a nominal load requirement may still be unsuitable because of excessive speed, temperature, poor shaft condition, insufficient clearance, shock loading, or uneven load distribution.
For this reason, bearing load capacity should always be evaluated in the context of the complete application rather than treated as a single universal number.
