Ball Screw Support Bearings for CNC Machines
Why the support bearing decides CNC accuracy
A CNC machine's ball screw turns motor rotation into precise linear movement. The ball screw gets the attention, but the bearings at each end are what hold it in place. If the support bearing lets the screw move even a few microns along its axis, that movement shows up directly in the part as backlash, poor surface finish or size errors.
Ball screw support bearings are built for one main job: holding the screw rigidly against heavy axial loads in both directions, at high speed, without play. Ordinary bearings are not designed for this.
This guide explains what support bearings do, the main types and support units, the standard mounting layouts, how to select one, and how to keep it running.
What ball screw support bearings do
A ball screw normally has two ends, and each one needs a different kind of support.
Fixed end:Â usually the motor end. This bearing locks the screw axially in both directions and carries all of the cutting and acceleration thrust. It needs high axial stiffness and preload, so it is usually a pair or set of angular contact thrust bearings.
Supported (floating) end: the far end. This bearing holds the screw radially and keeps it from whipping, but lets it slide a little along its axis as it grows with heat. A deep groove ball bearing is the usual choice here.
Why axial stiffness and preload matter
Every bearing deflects slightly under load. On a CNC axis, that deflection becomes positioning error. Support bearings are preloaded, meaning they are pressed together during assembly so there is no internal clearance left. Preload removes play and raises stiffness, so the screw stays put when the cutter loads it or the axis reverses.
Too little preload causes backlash and chatter. Too much causes heat, extra drag on the motor and shorter bearing life. That is why support bearings are sold in matched sets with a fixed, factory-set preload.
Types of ball screw support bearings
Most fixed-end support bearings are angular contact thrust ball bearings with a 60° contact angle (NSK). Standard angular contact bearings use 15° to 40°. The steeper angle trades some radial capacity for far more axial stiffness, which is exactly what a ball screw needs.
Bearing types
Type | What it is | Best for |
Single-row 60° angular contact thrust bearing | Separate bearings, matched and preloaded as a set | Machine builders who design their own housing; high-load axes |
Double-row integrated support bearing | Two rows of balls in one bearing, often with a bolt-on flange, sealed and greased for life | Quick fitting and compact designs; replaces a matched pair |
Standard angular contact pair (for example 7000 series) | General angular contact bearings in a back-to-back pair | Light-duty axes, routers and DIY CNC builds |
Deep groove ball bearing | Single-row radial bearing | The supported (floating) end only |
Matched arrangements
Back-to-back (DB):Â resists tilting best and gives the stiffest mounting. The most common choice for the fixed end.
Face-to-face (DF):Â more forgiving of small misalignment between housing and screw.
Tandem (DT):Â two bearings share load in one direction. Used inside triple and quad sets for heavy one-way thrust.
Triple and quad sets (for example DBD, DBB):Â three or four bearings for large machining centres or vertical axes that carry gravity load in one direction.
Ready-made support units
Many machines use complete support units: a housing with the bearings, preload, lock nut and seals already fitted.
Unit | Shape | Which end | Inside |
BK | Square block | Fixed | Preloaded angular contact pair |
BF | Square block | Supported | Deep groove ball bearing |
FK | Round flange | Fixed | Preloaded angular contact pair |
FF | Round flange | Supported | Deep groove ball bearing |
EK / EF | Small square block | Fixed / supported | For small screws |
The number after the letters gives the shaft size the unit fits, for example BK12 for a 12 mm journal. On the fixed side, these units typically use a JIS Class 5 angular bearing with a factory-set preload (THK).
Mounting arrangements
How the two ends are supported decides how fast the screw can spin before it whips, and how much push it can take before it buckles.
Layout | Fixed end | Other end | Speed and stiffness | Typical use |
Fixed-free | Angular contact set | Nothing | Lowest; only for short screws | Short, light axes; small tables |
Fixed-supported | Angular contact set | Deep groove bearing (floating) | Good; lets the screw grow with heat | Most CNC machine axes |
Fixed-fixed | Angular contact set | Angular contact set | Highest; can be pre-tensioned against heat growth | Long or high-speed axes, large machining centres |
Fixed-supported is the standard choice because it balances stiffness, speed and cost, and lets the screw expand without stressing the bearings.
Fixed-fixed gives the highest critical speed and stiffness. To control heat growth, the screw is often pre-stretched slightly during assembly, so warming it only relaxes the tension rather than pushing the ends apart. That makes alignment and assembly more demanding.
The linear guides on the same axis need the same care; see linear ball bearings for CNC machines and HIWIN linear guideways.
How to select a ball screw support bearing
Start from the screw. The bearing bore must match the screw's machined journal diameter, not its outer thread diameter. A 16 mm screw, for example, usually has a 12 mm journal and takes a BK12 / BF12 pair.
Work out the axial load. Add cutting thrust and the force needed to accelerate the table and workpiece. On vertical axes, add the weight carried by the screw. Choose a bearing whose axial load rating covers the peak with margin.
Check speed. Confirm the bearing's speed limit covers the screw's top speed: screw speed (rpm) = rapid feed (mm/min) ÷ screw lead (mm). Fast, long axes may need a fixed-fixed layout.
Choose the preload. Light preload suits high-speed axes, and medium or heavy preload suits heavy cutting. Follow the bearing maker's guidance; most supply standard classes.
Choose the accuracy grade. Precision machine tools typically use P4 / ABEC 7 or better support bearings; routers and general machines can use P5 / JIS Class 5 units.
Pick a layout and housing. Decide on fixed-supported or fixed-fixed, and whether to use a ready-made BK/BF or FK/FF unit or bearings in your own housing.
Specify sealing and lubrication. Sealed, greased-for-life bearings suit most machines. Use relubricable bearings on heavy-duty or high-speed axes.
When buying a replacement, bring the old bearing's part number, its bore, outer diameter and width, the arrangement marking (such as DB or DF), and the machine make and model.
Installation, maintenance and failure signs
Most support bearing problems start at installation, not in service.
Installation tips
Keep matched sets together and in order. The bearings in a set are made and marked to work as a group. Mixing bearings from different sets, or fitting one the wrong way round, changes the preload. Follow the V-mark or arrow on the outer rings.
Press only on the correct ring. Push on the ring you are fitting, never through the balls, or you will dent the raceways.
Tighten the lock nut to the specified torque. Undertightening leaves play; overtightening adds preload and heat. Lock it with its set screw.
Align the ends. The fixed end, supported end and ball nut must line up with the linear guides. Misalignment loads the bearings and the screw unevenly.
Never hammer. Shock loads leave small dents (brinelling) that later turn into noise and play.
Maintenance
Sealed, greased-for-life units need no routine greasing. Check the seals and listen for noise.
On relubricable bearings, use the grease and interval the maker specifies, and avoid overfilling, which causes heat.
Keep coolant and chips away from the support units; damaged seals let them in fast.
Check axial play at regular service intervals by pushing and pulling the table with a dial indicator on the screw end.
Warning signs
Backlash or lost motion when the axis reverses
Poor surface finish, chatter marks or size errors that come and go
Growling or clicking from the motor end of the axis
The bearing housing running noticeably hotter than usual
Higher motor load or following-error alarms on the axis
If a matched set is worn, replace the whole set, not just one bearing.
Getting the right support bearing
The support bearing sets the limit on how accurate and rigid a CNC axis can be. Use 60° angular contact thrust bearings or a matched BK/FK unit at the fixed end, a deep groove bearing at the floating end, and treat preload and installation as carefully as the bearing choice itself.
Before ordering, give your bearing supplier the screw's journal diameter, the axis load and speed, the mounting layout and the old part number. With those details they can match the right bearing set or support unit for your machine.
Frequently asked questions
What is a ball screw support bearing?
A ball screw support bearing holds the end of a ball screw so it can turn freely but cannot move along its axis. On the fixed end it is usually a preloaded set of 60° angular contact thrust bearings, which carry heavy axial loads in both directions with very little deflection.
What is the difference between BK and BF support units?
A BK unit is the fixed-side support. It contains a preloaded angular contact bearing pair that locks the screw axially. A BF unit is the supported or floating side. It contains a deep groove ball bearing that holds the screw radially but lets it move slightly as it expands with heat.
Why do ball screw support bearings use a 60° contact angle?
A 60° contact angle points the load path mostly along the screw's axis, so the bearing is much stiffer and stronger against axial thrust than a standard angular contact bearing with a 15° to 40° angle. That stiffness keeps the screw from shifting under cutting and acceleration loads.
Can I use a deep groove ball bearing on both ends of a ball screw?
Not on the fixed end. A deep groove bearing has internal clearance and low axial stiffness, so the screw would move under load and cause backlash. It works well on the floating end, where it only needs to carry radial load.
How do I know if my ball screw support bearing is worn?
Common signs are backlash when the axis reverses, chatter or poor finish, size errors, noise from the motor end, and a hot bearing housing. To confirm, measure axial play with a dial indicator on the screw end while pushing and pulling the table.
Should I replace one bearing or the whole set?
Replace the whole matched set. Bearings in a set are made together to give a specific preload, so mixing a new bearing with worn ones changes the preload and shortens the life of the new one.
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