Intermediate Roller Bearings and Shaft Movement:
Here’s a breakdown of how intermediate roller bearings of the roller type accommodate:
(a) Angular Misalignment:
While ideal shaft alignment is always strived for, some degree of angular misalignment (mismatch between shaft centerlines) can occur due to factors like hull flexing or thermal expansion. Roller bearings can accommodate some limited angular misalignment through the following features:
- Self-aligning design: Some roller bearings, particularly spherical roller bearings, incorporate a spherical inner race. This allows the inner race to slightly tilt within the outer race, accommodating small angular misalignment between the shaft and the bearing housing.
- Line contact bearings: Roller bearings with a line contact between the rollers and races (like cylindrical rollers) can tolerate a small amount of misalignment due to the inherent line contact geometry.
Limitations:
It’s important to note that exceeding the design limits for angular misalignment can lead to increased bearing wear, reduced bearing life, and potential damage. Regular shaft alignment checks and adjustments are crucial for optimal bearing performance.
(b) Longitudinal Movement of the Shaft:
Longitudinal movement (axial movement) of the shaft is necessary in a propulsion system for two main reasons:
- Thermal Expansion: As the engine operates, the propeller shaft and other components will experience thermal expansion. Longitudinal movement allows the shaft to expand and contract without binding or putting excessive stress on the bearings and couplings.
- Propeller Thrust: The propeller generates thrust as it rotates, pushing the vessel forward. This thrust creates a small axial force on the shaft. Longitudinal movement allows the shaft to move slightly to accommodate this force.
How Roller Bearings Accommodate Movement:
Roller bearings can accommodate longitudinal movement in a couple of ways:
- Internal Clearance: Roller bearings have a small amount of internal clearance between the rollers and the races. This allows for some axial movement of the shaft within the bearing without the rollers binding.
- Splayed Bearings (Optional): In some cases, double row roller bearings with a specific internal design (splayed bearings) might be employed. These bearings have an angled inner or outer race that allows for a controlled amount of axial movement while maintaining good radial load capacity.
Important Considerations:
- Excessive longitudinal movement can lead to problems like whipping of the shaft or propeller walk (propeller causing the vessel to steer off course). The design of the shaft line and bearings should allow for some controlled axial movement within acceptable limits.
- Roller bearings are generally not designed for large axial loads. The primary function of the roller bearing is to support the radial load from the shaft weight and propeller forces. Additional components like thrust bearings might be required to handle the axial thrust load.
By understanding how roller bearings accommodate some angular misalignment and longitudinal movement, we can appreciate their role in maintaining a functional and reliable propulsion system.