Axles, Shafts, Bearings, and Couplings

Axles

Axles are intended to support rotating bodies without turning the shaft itself. They are primarily subject to bending loads.

Shafts

Shafts are machine parts that always rotate with the components they support. They transmit power or torque and are subject to both torsion and bending loads. Shafts can be solid or hollow. When a keyway is present, consideration must be given to the shaft diameter due to stress concentration.

Bearings and Supports

Both axles and shafts require support points to bear their weight, facilitate rotation, and prevent displacement. Bearings are the components that provide these supports for shafts and axles.

Bearing Types

Friction Bearings

In friction bearings, the shaft or axle slides within the bearing surface. The most common types are horizontal and vertical radial bearings. Axial friction bearings are typically used only for small loads on vertical shafts.

• Radial Friction Bearings

  These support loads perpendicular to the shaft axis and come in various designs.

• Axial Friction Bearings

  Used to support the axial load (weight) of a vertical shaft. The bearing often consists of hardened steel pads (sometimes called 'tiles') resting on a pivot.

• Bearing journals and pivots are the parts of shafts that interface with radial and axial bearings.
• Retaining rings (or 'situation rings') are used to prevent axial displacement of the shaft or bearing.

Rolling Element Bearings

These bearings use rolling elements (balls or rollers) between the moving and stationary parts to reduce friction.

• Ball Bearings

  Typically used for supporting smaller loads and higher speeds.

• Roller Bearings

  Generally used for supporting larger loads.

Mixed (Radial-Thrust) Bearings

These bearings are designed to support both axial thrust and radial loads simultaneously. Two such bearings can be used to handle thrust in opposing directions.

Bearing Load Capacity

Bearings are selected and calculated based on their capacity to handle loads, categorized as static or dynamic.

Dynamic Load Capacity

Applies when the bearing is rotating under load. It relates to the bearing's fatigue life.

Static Load Capacity

Applies when the bearing is at rest under load. It relates to the maximum load the bearing can withstand without permanent deformation.

Lubrication and Maintenance

Bearings require lubrication, typically with grease or oil, to reduce friction, dissipate heat, and prevent wear and corrosion. Regular maintenance is essential for bearing longevity.

Bearing Fit

Connecting a bearing involves machining the shaft and housing to achieve a specific fit (e.g., interference fit) to ensure proper load distribution and prevent movement between components.

Bearing Mounting

Bearing supports can vary. Flat supports are often used on non-machined surfaces. If shafts are not perfectly aligned, self-aligning ball or roller bearings may be used to accommodate the misalignment.

Couplings

Couplings are mechanical devices used to connect two shafts together at their ends for the purpose of transmitting power or torque. They can also accommodate misalignment or provide for disconnection.

Rigid Couplings

Used to permanently join shafts that are precisely aligned. They do not tolerate misalignment.

• Sleeve (Muff) Coupling

  A simple type consisting of a sleeve fitted over the shaft ends.

• Clamp Coupling

  Consists of two halves clamped onto the shafts, transmitting torque through friction.

• Flange Coupling

  Consists of flanges keyed to the shaft ends and bolted together.

Flexible Couplings

Used to connect shafts that may have slight misalignment (angular, parallel, or axial). They absorb shocks and vibrations.

Disconnectable Couplings

These couplings allow for the connection and disconnection of shafts.

• Clutch

  A type of disconnectable coupling that can engage or disengage power transmission between shafts at will, often while one or both shafts are rotating.