Definition and Importance
The contact angle in bearings is a critical parameter that defines the angle between the direction of load on the bearing and the nominal line of action of the resultant forces transmitted by a bearing raceway member to a rolling element. This angle plays a significant role in determining the bearing’s load capacity, friction, rigidity, and ability to withstand mounting errors.
ISO 14278 Standard
According to ISO 14278 standard for linear motion rolling bearings, the nominal contact angle is defined as:
The angle between the direction of load on the linear bearing and the nominal line of action of the resultant of the forces transmitted by a bearing raceway member to a rolling element.
Types of Contact Angles
45 Degree Contact Angles for Equal Load Capacity
- Bearings with Gothic arch geometry, including miniature profiled rails and most roller bearing guides, typically have four points of contact between the ball and raceway. This results in a contact angle of 45 degrees.
- The benefit of this 45-degree angle is that it provides equal load capacity in all four primary directions: radial (downward and lift-off) and lateral (side) loading. This means that such guides can be used in any orientation without needing to de-rate their load capacity.
Higher Contact Angles for Better Radial Load Capacity
- Bearings using circular arc or offset Gothic arch geometry can have varying contact angles designed to produce higher load ratings in one direction at the expense of other directions.
- For instance, one design uses a 90-degree contact angle on top rows of balls with a smaller 30-degree angle on lower rows. This configuration offers extremely high load capacity for radial (downward) loads and high rigidity when radial loads are applied but sacrifices load capacity and rigidity in reverse radial and lateral directions.
- Another design based on offset Gothic arch geometry employs a 50-degree contact angle for all four rows of balls, providing higher load capacities in both radial and reverse radial directions but lower lateral load capacity.
Impact on Bearing Performance
- Load Capacity: The contact angle directly influences how much load a bearing can support. A higher contact angle generally increases radial load capacity but may reduce axial or lateral capacities.
- Rigidity: Bearings with specific contact angles can offer greater rigidity (lower deflection) under certain loads, which is crucial for applications requiring precise movement.
- Friction: The geometry affecting the contact angle also impacts friction levels within the bearing system.
- Mounting Errors: Properly chosen contact angles help bearings withstand mounting inaccuracies without significant performance degradation.
Understanding these aspects allows engineers to select bearings that best meet their application’s demands regarding load distribution, rigidity, friction management, and tolerance to mounting errors.
