How to Select Ball Screw Support Units BK/BF, FK/FF, and EK/EF: Fixed End and Supported End Explained
Understanding Fixed End and Supported End
Ball screw support assemblies are generally composed of two sides:
Fixed end support
Supported end support
The fixed end is responsible for axial positioning of the ball screw. It usually uses angular contact bearing combinations or other bearing structures capable of handling axial loads. A lock nut is commonly used to control axial positioning and preload.
The supported end mainly provides radial support while allowing the ball screw to accommodate a certain amount of axial expansion caused by temperature changes.
The exact bearing arrangement, preload method, and allowable load should always be confirmed according to the specific support unit drawing and manufacturer specifications.
BK and BF: Common Square-Type Support Units
BK usually represents the fixed end, while BF represents the matching supported end.
This series typically features a square or block-shaped housing design with clear mounting reference surfaces. It is widely used in:
General automation equipment
CNC machines
Industrial positioning systems
Medium and large-size ball screws
Different size series correspond to different:
Shaft diameters
Mounting hole spacing
Center heights
Bearing capacities
The model number should not be selected simply by matching the ball screw nominal diameter. The complete dimensional specification must be checked.
FK and FF: Round Flange-Type Support Units
FK is the fixed end and FF is the supported end.
This series generally uses a circular flange design and is suitable for applications where:
Installation from the machine frame side is preferred
Flange mounting is required
Space layout requires a round mounting interface
Functionally, FK/FF provides the same fixed-end and supported-end concept as BK/BF, but differences exist in:
Mounting direction
Flange dimensions
Positioning method
Center alignment structure
FK/FF should not be directly substituted for BK/BF without checking machine frame compatibility.
EK and EF: Compact Square-Type Series
EK is typically the fixed end and EF is the supported end.
This series is often selected for:
Compact equipment
Smaller ball screw applications
Limited installation space
Compared with BK/BF, EK/EF may differ in:
Housing dimensions
Center height
Bearing structure
Allowable axial load
Selection should always be based on the manufacturer's dimensional drawings rather than assuming that EK/EF is simply a smaller version of BK/BF.
Support Units Must Match Ball Screw End Machining
The ball screw shaft-end design must match the support unit.
Important dimensions include:
Fixed-end journal diameter
Bearing shoulder dimensions
Thread specifications
Lock nut size
Supported-end journal diameter
Shaft-end length
If the shaft end is too short:
The lock nut may not fully engage
Proper bearing positioning cannot be achieved
If it is too long:
Interference with covers or couplings may occur
When changing between different support unit series, the complete shaft-end drawing should be reviewed rather than modifying only the mounting holes.
Common Installation Mistakes
Typical installation problems include:
Installing the fixed end and supported end in the wrong positions
Misalignment between the two support units
Incorrect tightening of the lock nut
Ball screw axis not parallel with the nut mounting surface
Restricting thermal expansion at the supported end
Recommended installation sequence:
Establish the fixed-end reference position.
Align the supported end with the ball screw axis.
Connect the nut housing.
Check running torque throughout the full stroke.
Smooth full-stroke movement is a key indicator of correct installation.
When Should Fixed-Fixed Support Be Considered?
For:
Long stroke applications
High-speed operation
Systems requiring higher axial stiffness
a fixed-fixed support arrangement or pre-tensioned design may be considered.
Advantages include:
Increased critical speed
Improved axial rigidity
Better thermal deformation control
However, this configuration requires:
Correct bearing selection
Controlled preload
Higher installation accuracy
Two standard fixed-end supports should not simply be installed at both ends as a replacement for a designed fixed-fixed system.
Information Required for Quotation
To select the correct ball screw support units, provide:
Ball screw diameter and lead
Shaft-end dimensions
Thread specifications
Shoulder dimensions
Overall shaft length
Support unit mounting method
Center height
Mounting hole spacing
Load requirements
Operating speed
Coupling position
For replacement applications, clear photos of:
Front view
Side view
Bottom mounting surface
Nameplate marking
can significantly improve matching accuracy.
Tianjin OuNaiDa Ball Screw Support Solutions
Tianjin OuNaiDa can assist customers in selecting suitable:
BK/BF support units
FK/FF support units
EK/EF support units
based on ball screw shaft-end drawings and application requirements.
Confirming the shaft-end design and installation reference before ordering helps prevent assembly problems and reduces unnecessary modification costs.
Precision Motion Components | Empowering Global Intelligent Manufacturing
