Plain bearing specific load

What does this quantity describe?

The specific bearing load pL = F/(b·dL) is the mean pressure referred to the projected bearing area (width times diameter). It must stay below the allowable value pL,allow for the bearing material, usually stated as Rp0.2/3 for steady load and Rp0.2/6 for unsteady load.

Think of the average pressure a piece of furniture exerts on a carpet: the same load spreads to a lower pressure over a larger footprint (a wider or larger bearing).

Formula and variables

pL = F / (b · dL)

• pL = F / (b · dL)
• F = pL · b · dL
• b = F / (pL · dL)

Choose the inputs correctly

F is the resultant radial force acting on the bearing, b the load-carrying axial bearing width, and dL the bearing bore diameter (equal to the nominal shaft diameter).

How to use the calculator

Enter F, b and dL to get pL. For preliminary sizing, instead enter pL as the allowable value and two of the other three quantities to find the third.

Worked example

F = 5,000 N, b = 40 mm and dL = 50 mm give pL = 5,000 N / (40 mm · 50 mm) = 2.5 MPa.

Understand the result and units

pL = 2.5 MPa is compared against the allowable value for the chosen bearing material; too high a specific load risks wear or seizing of the sliding surfaces.

F is usually given in N, b and dL in mm, and pL in MPa (N/mm²).

Why there are two allowable values for pL

For steady load (constant force direction and magnitude), pL,allow is usually taken as Rp0.2/3, because a continuously applied load needs a smaller safety margin against local yielding than a fluctuating one. For unsteady load (shocks, changing load direction, frequent start-stop), Rp0.2/6 is used instead to cover the additional dynamic stress on the sliding surfaces. Which case applies must be determined from the application's load spectrum before choosing pL,allow.

Typical applications

Specific bearing load is the first design step for any plain bearing, such as on crankshafts, gearbox shafts, pumps and turbines, before the hydrodynamic parameters (Sommerfeld number, film thickness) are checked.

Assumptions, limits and common mistakes

pL is only a mean value; it does not capture edge loading from shaft deflection, misalignment or uneven load distribution across the bearing width. For fast-running bearings, the Sommerfeld number and film thickness must also be checked.

Common mistake: A common mistake is using the bearing shell's outside diameter instead of the bore diameter dL, or confusing the load-carrying width b with the total bearing width including chamfers.