The length-to-diameter ratio, L/D, is a quick way to describe the proportions of a journal bearing. It is dimensionless: divide the bearing length by the journal diameter, using the same units for both measurements. The table summarizes three regions discussed in a Penn State mechanical-engineering reference for hydrodynamic journal bearings.

Measure the right dimensions

Use the axial bearing length and the journal diameter. Do not substitute the housing outside diameter or the journal radius. If the bearing is split, use the assembled bearing’s effective axial length. For example, a 40 mm long bearing supporting a 50 mm journal has L/D = 40/50 = 0.8, which falls below 1 and is within the short-bearing region described by the reference.

The source describes ratios from about 1 for a “square” bearing to about 2 in practical arrangements. It calls a bearing with L/D below 1 short or narrow and discusses a short-bearing analysis for cases where that model is appropriate. These descriptions are useful orientation points, not a rule that every bearing should be sized to a particular ratio.

Why changing the ratio changes the design

Increasing length enlarges the projected area and can affect load distribution, lubricant flow, friction, and temperature. It may also make a bearing more sensitive to shaft bending or misalignment. Shorter bearings can better accommodate some deflection and edge-loading conditions, but their capacity and oil-film behavior still depend on load, speed, viscosity, clearance, and geometry. There is no ratio that is best for every duty.

Hydrodynamic bearings rely on relative motion to generate a supporting lubricant film. The source’s short-bearing approximation is tied to that model; it is not automatically valid for dry-running polymer bushings, rolling-element bearings, hydrostatic bearings, or lightly lubricated contacts. Do not transfer the ranges to another bearing type just because it also has a cylindrical shape.

Use L/D as a check, not a capacity rating

After choosing a trial ratio, calculate projected pressure using the intended load and the relevant projected area, then check the full bearing model for the operating regime. Consider steady and peak loads, journal speed, lubricant viscosity at operating temperature, diametral clearance, supply conditions, heat rejection, surface finish, shaft material, and allowable misalignment. For rotating equipment, vibration and transient start-stop behavior may also affect the design.

Misalignment and shaft deflection can concentrate load near the bearing edge. A long bearing is not automatically safer because its apparent area is larger. Conversely, a short bearing may require a different lubrication or film-thickness check. Use the manufacturer’s limits for the selected material and follow the governing machine or design standard.

For a drawing review, state how L and D were measured and write the computed ratio next to the bearing designation. Check that the chosen length fits the housing, lubrication grooves, shoulders, seals, and assembly method. Then document the load case and operating conditions used in the bearing calculation. If these inputs are unknown, the ratio alone is not enough to approve the design.

Source: Penn State, Hydrodynamic Bearings, section 7.7.1. The cited reference discusses values for hydrodynamic journal bearings; its approximate range should not be treated as a universal plain-bearing recommendation.

As a second arithmetic check, a bearing that is 60 mm long around a 40 mm journal has L/D = 1.5. That value sits inside the source’s approximate practical range, but that fact alone does not show that the bearing can carry its load or maintain a stable film. If the bearing length is interrupted by grooves or relief features, confirm which effective length the calculation model requires rather than blindly using the overall component length.