The BK and BF Ball Screw Support Blocks are essential components in a ball screw drive system, as their primary function is to axially fix and radially support the ends of the ball screw shaft.
They are specialized bearing units designed to handle the high axial forces, maintain precision, and accommodate the rotational speed of the screw shaft.
I. Functional Overview
The primary function of BK/BF support blocks is to provide the necessary support conditions that enable the ball screw to function correctly throughout its lifecycle.
1. Axial Fixity (The Main Role)
- Function: The BK/BF units lock the ball screw shaft in the axial direction (the direction of travel).
- Mechanism: They house a combination of angular contact thrust bearings (usually in a preloaded back-to-back or tandem arrangement) that are highly rigid and specifically designed to resist forces acting along the length of the shaft.
2. Radial Support
- Function: They hold the screw shaft concentric and stable, preventing radial runout or whipping during rotation.
- Mechanism: The bearings provide radial stiffness, ensuring the screw rotates around its true axis, which is crucial for achieving high rotational speeds (Critical Speed).
II. Key Differences: BK (Fixed) vs. BF (Supported/Floating)
A ball screw system requires at least one Fixed End to handle axial forces, and often a Supported End to handle high speeds. This is where BK and BF blocks come into play.
1. BK Support Block (The Fixed End)
- Design: Contains a high-rigidity bearing assembly (e.g., two or three angular contact bearings) that is preloaded to eliminate axial play.
- Function: To Bear All Axial Thrust Loads. All forces (thrust and tension) from acceleration, deceleration, and the load are transferred through this block. It is also responsible for maintaining the system's axial rigidity.
- Mounting: The shaft end is typically threaded to accept a locknut that secures the shaft against the bearing inner ring, eliminating all axial play.
2. BF Support Block (The Supported/Floating End)
- Design: Contains a less complex bearing assembly (often a single deep groove ball bearing).
- Function: To ONLY Provide Radial Support. This block keeps the far end of the screw concentric but allows the shaft to float slightly in the axial direction. This floating capability is essential to:
- Accommodate Thermal Expansion: As the screw heats up during operation, it lengthens. The BF end allows this expansion to occur without putting excessive stress on the bearings or warping the screw.
- Reduce Assembly Stress: Simplifies installation by not requiring micrometer-perfect axial spacing between the two support blocks.