Plain Bearing Specific Load Calculator
Calculate the projected specific load p of a radial plain bearing from load, journal diameter and bearing width – a nominal value, not the oil-film pressure.
Formula
p = F / (d * b)
Specific load is referred to the projected area, i.e. the rectangle formed by diameter d and width b -- not the curved cylindrical surface area pi*d*b. This is the conventional, simple comparison figure historically used for early plain-bearing sizing steps.
What this model does not show
p is a nominal figure averaged over the projected area. It is neither the actual hydrodynamic oil-film pressure distribution (which in operation is highly non-uniform and locally much higher than p), nor a Hertzian contact pressure, nor the real pressure distribution in the bearing. There is also no universal permissible value for p -- a comparison figure always comes from an actual datasheet for the specific material pairing.
Worked example
A plain bearing carries F = 5,000 N with a journal diameter d = 50 mm and bearing width b = 40 mm. Projected area: d*b = 50 mm x 40 mm = 2,000 mm2. Specific load: p = 5,000 N / 2,000 mm2 = 2.5 N/mm2 = 2.5 MPa. If a datasheet value of p_perm = 10 MPa is available for the bearing-metal pairing used, utilization is 2.5/10 = 25%.
Common mistakes
- Using the cylindrical surface area pi*d*b instead of the projected area d*b -- this gives an incorrect, too-low p.
- Equating p with the actual hydrodynamic peak oil-film pressure, which can be many times higher.
- Using a 'generic' permissible p value found online instead of a datasheet value for the actual material pairing and application.
Frequently asked questions
Why is the projected area d*b used instead of pi*d*b?
This is the historically established definition of nominal specific load in plain-bearing design (e.g. Roloff/Matek) and gives a simple, long-used comparison figure -- regardless of the actual contact surface being curved.
Is p the same as the oil pressure in the film?
No. The hydrodynamic oil pressure in the wedge-shaped film is unevenly distributed and can be locally much higher than the nominal specific load p; p is only an average over the projected area.
Is there a universal permissible limit for p?
No, there is no universal value. Permissible specific loads depend heavily on the material pairing (e.g. white metal, bronze, sintered bearing), lubrication and duty type, and must be taken from an actual manufacturer datasheet.
What's the difference to a Hertzian pressure?
Hertzian pressure describes the local, highly concentrated contact pressure between two touching, curved elastic bodies (e.g. rolling contact). Plain-bearing specific load p is instead an area-averaged nominal value for a lubricated plain bearing, not a contact stress.
How does a wider bearing affect p?
A larger width b at the same load F and diameter d lowers p proportionally (p = F/(d*b)), but also changes the width-to-diameter ratio b/d, which matters for a full hydrodynamic design (Sommerfeld number, eccentricity).
What comes after calculating p?
With p you can determine the PV value using the Plain bearing sliding speed and Plain bearing PV value calculators; p is also needed in the Plain bearing Sommerfeld number calculator for a full hydrodynamic assessment.