1. The "Fixed-Supported" Configuration Strategy
The fundamental design principle of the BK/BF combination is to create a Fixed-Supported mounting arrangement.
- BK (Fixed): Anchors the screw to the motor, absorbing all axial (thrust) loads.
- BF (Supported): Supports the radial weight of the screw but allows the shaft to "float" axially.
Why not Fix both ends?
During high-speed operation, friction generates heat, causing the steel ball screw to expand (elongate). If both ends were rigidly fixed, this thermal expansion would have nowhere to go, causing the screw to bow (buckle) or destroying the bearings. The BF unit is designed specifically to allow this thermal growth without losing radial stability.
2. BK Series: The "Fixed" Side (Motor End)
The BK unit is the critical component for positioning accuracy. Its internal design is focused on Axial Rigidity.
- Bearing Selection: Inside a standard BK block, you will not find standard ball bearings. Instead, it houses a pair of Angular Contact Ball Bearings (JIS Series 70xx).
- The "DF" Arrangement: These two bearings are mounted in a "Face-to-Face" (DF) configuration.
Why DF? This arrangement allows the unit to absorb axial thrust loads in both directions (push and pull). It also provides a slight self-aligning capability to compensate for minor machining errors in the mounting surface.
- Preload: The bearings are preloaded (tightened against each other) to eliminate internal clearance. This ensures that when the motor reverses direction, there is zero backlash at the support point.
- Sealing: Special seals are installed to retain the factory-packed lithium soap grease and prevent dust entry.
3. BF Series: The "Floated" Side (End)
The BF unit looks similar on the outside but serves a completely different mechanical function.
Bearing Selection: The BF unit typically houses a single Deep Groove Ball Bearing (Series 60xx).
The Floating Principle:
- The bearing is secured to the shaft with a snap ring, but it is not axially clamped tightly inside the housing in the same manner as the BK.
- This allows the bearing (and the screw shaft) to move slightly axially as the screw heats up and lengthens.
Load Handling: The BF unit is designed only to handle radial loads (supporting the weight of the screw and preventing whip). It takes zero thrust load.
4. Compact & Standardized Geometry
The BK/BF series was designed to solve the problem of custom housing manufacturing.
- Low Profile: The rectangular block design allows the support to be mounted directly to the machine base without requiring tall standoff pillars.
- Reference Edges: The base and sides of the blocks are ground to act as reference surfaces. This allows installers to use a dial indicator against the side of the BK block to ensure it is perfectly parallel to the linear guides.