The core design of sliding bearings enables them to support two basic types of load at the same time. The bearing mechanism is closely related to the physical properties of lubricating oil film. Here's a look at how these two loads are supported:
1.Radial Load: Support force perpendicular to the axis
When the shaft neck rotates, lubricant is squeezed into a wedge gap between the bearing shell and the shaft neck, forming a fluid kinetic oil film. The oil film produces pressure that supports the weight of the shaft neck and external radial forces (such as gear meshing and belt tension). For example, in an automotive engine crankshaft bearing of an automobile engine, the oil film must be able to withstand the explosive impact of the piston's downward movement while preventing the shaft neck from coming into direct contact with the bearing shell.
Key features:
Oil Oil Film Thickness: The stability of the oil film is controlled by adjusting the clearance between the shaft neck and bearing shell typically 0.001 to 0.002 times the shaft diameter).
Eccentricity Adaptation: when radial load increases, the axle neck is slightly deflected and the oil film thickness automatically adjusted to maintain lubrication. Material elasticity: Bearing pads are usually made of soft metals such as babbitt alloy, and whose elastic deformation compensates for manufacturing errors and prevents local overload.
2.Axial Load: thrust parallel to the shaft
Sliding bearings can withstand axial forces of a thrust pads or an end oil film. This is usually used in applications where axial displacement must be limited (e.g. compressors and turbines). Thrust bearing pads are usually designed with helical grooves or beveled surfaces, and the dynamic pressure effect of rotation is used to form an oil film against axial thrust or tension. For example, thrust bearings in marine propulsion shafting must be able to withstand the enormous axial thrust generated by the propeller and prevent shaft from moving.
Key features:
Cushion angle: The contact surface between the thrust pad and the shaft shoulder is designed at a slight angle (usually 5 to 15°) to balance oil film pressure and leakage.
Segment Floating: Large thrust bearings are constructed with multiple cushions, each of which floats independently to accommodate thermal expansion and load fluctuations.
Cooling Design: Frictional heat generated by axial loads must be dissipated through oil circulation or external cooling system to prevent oil film rupture. Synergistic Effects of Two Loads
Under actual operating conditions, sliding bearings often need to deal with radial and axial loads simultaneously. For example:
Helical gear transmission: Gear meshing forces are broken down into a radial component (carried by cylindrical bearings) and an axial component (carried by thrust bearings).
Turbine: The combined bearing of Steam turbine adopts an integrated radial-thrust design to simplify structure and improve reliability.
Propeller propulsion: Aft bearing must withstand both the hydrodynamic radial load and the thrust axial load of propeller. Adopting Conical bearings to realize bidirectional load bearing.