Ball Screw Support Units

What is a ball screw support unit?

A ball screw support unit is a ready-to-mount bearing housing that holds the end of a ball screw shaft in position. A ball screw turns rotary motion from a motor into precise linear motion, and to do this accurately, the screw must be held firmly at its ends so it can rotate smoothly without moving out of line. Support units do exactly that, carrying the radial loads and axial thrust created as the ball nut drives a table, carriage, or axis back and forth.

Each support unit combines a precision bearing, or a set of bearings, with a machined housing, seals, and locking parts, so it can be bolted straight onto the machine. Ball screws are usually supported at both ends, with a fixed side unit at one end and a support side unit at the other.

A typical ball screw support unit has:

• Housing - a precision-machined block, usually square or flanged, with mounting holes and a reference surface for accurate alignment
• Bearings - angular contact ball bearings on the fixed side to carry axial thrust in both directions, or a deep groove ball bearing on the support side to carry radial loads
• Seals - keep lubricant in and dirt out of the bearings
• Lock nut and collar - secure the bearings to the end of the ball screw shaft and hold the preload, on fixed side units
• Snap ring - holds the bearing on the shaft end, on support side units

Key features:

• Accurate shaft support - holds the ball screw precisely in line with the ball nut and linear guides
• Axial thrust capacity - fixed side units absorb the thrust created as the ball nut pushes and pulls the load
• Preloaded bearings - fixed side units often use preloaded angular contact bearings for high rigidity and no axial play
• Thermal expansion allowance - support side units allow the screw to grow and shrink with temperature without stressing the bearings
• Ready-to-mount design - no need to machine housings or select and fit bearings separately
• Sealed and lubricated - supplied greased and sealed for low maintenance
• Standard sizes - made to suit common ball screw diameters and end journal sizes

In simple terms, a ball screw support unit is the bearing block that holds each end of a ball screw, keeping it accurate, rigid, and smooth-running.

Why are ball screw support units essential?

Ball screw support units are essential because a ball screw can only be as accurate as the way it is supported. If the screw is not held firmly, it can move axially, whip at high speed, bend, or run out of line, which leads to positioning errors, vibration, noise, and early wear of the screw and nut. Correctly chosen and installed support units keep the screw stable and the machine precise.

Their main benefits are:

• Accurate positioning - rigid, preloaded fixed side units remove axial play, so the axis moves exactly as commanded
• High rigidity - firm support resists deflection under cutting forces and heavy loads
• Smooth, quiet running - precision bearings and accurate alignment reduce vibration and noise
• Protection of the ball screw - correct support prevents bending, whipping, and uneven loading that would damage the screw and nut
• Thermal compensation - support side units let the screw expand with heat without building up stress
• Faster assembly - ready-made units save time compared with designing and machining custom bearing housings
• Easy replacement - standard units make repairs and upgrades straightforward
• Long service life - sealed, lubricated bearings run reliably with little maintenance

These qualities make ball screw support units important in a wide range of equipment, including:

• CNC machine tools - milling machines, lathes, routers, machining centres, and grinders
• 3D printers and laser cutters - precise linear axes for printing, cutting, and engraving
• Automation and robotics - linear actuators, pick-and-place systems, and positioning stages
• Packaging machinery - precise linear movement in filling, sealing, and handling equipment
• Semiconductor and electronics equipment - high-precision positioning and inspection systems
• Medical and laboratory equipment - accurate positioning in scanners, analysers, and test equipment
• Injection moulding and presses - drives for clamping and positioning axes
• DIY and hobby CNC builds - small routers, mills, and engraving machines

The main types of ball screw support units

Ball screw support units are usually grouped by which end of the screw they support, their housing shape, and their bearing arrangement.

By position:

• Fixed side units - fitted at the drive end of the screw, usually next to the motor. They use a pair of angular contact ball bearings, often preloaded, to locate the screw axially and carry thrust in both directions as well as radial loads
• Support side units - fitted at the free end of the screw. They use a single deep groove ball bearing to carry radial loads only, and allow the screw to move axially as it expands and contracts with temperature

By housing shape:

• Square or block type - a rectangular housing that bolts down onto a flat surface, with a mounting height matched to the ball screw centre line. These are common on machine beds and CNC frames, and often have names such as BK for fixed side and BF for support side
• Round flange type - a cylindrical housing with a mounting flange that bolts onto a vertical face or end plate, often with names such as FK for fixed side and FF for support side
• Compact and small types - smaller units for miniature ball screws in compact machines, instruments, and 3D printers, often with names such as EK for fixed side and EF for support side

By bearing arrangement:

• Angular contact bearing pairs - used in fixed side units, mounted back to back or face to face to give high axial rigidity
• Dedicated ball screw support bearings - high contact angle angular contact bearings used in high-precision and heavy-duty units for very high axial stiffness
• Deep groove ball bearings - used in support side units for radial support and free axial movement

By support arrangement on the machine:

• Fixed and support - the most common arrangement, with a fixed side unit at the motor end and a support side unit at the other end, balancing rigidity and thermal expansion
• Fixed and fixed - fixed side units at both ends, used for very high rigidity and high speeds, sometimes with the screw pretensioned to allow for heat
• Fixed and free - a fixed side unit at one end only, used for short screws, low speeds, and light loads
• Support and support - support side units at both ends, used only for light, low-precision applications

How to choose the right ball screw support unit?

Choosing the right ball screw support unit means matching it to the ball screw, the machine layout, and the precision and loads required. Work through these points in order:

• Ball screw diameter and end machining - match the unit to the screw diameter and to the machined journal on each end of the screw. Fixed and support side units usually have different bore sizes, and the screw ends must be machined to suit them
• Fixed or support side - choose a fixed side unit for the drive end and a support side unit for the free end, unless your design needs a different arrangement
• Housing type - choose a square block type for mounting on a flat bed or base, or a round flange type for mounting on a vertical face or end plate
• Centre height - make sure the shaft centre height of the unit matches the ball nut and linear guides, or plan to use spacers or shims
• Axial load and rigidity - check the axial load capacity and rigidity of the fixed side unit, and choose larger or higher-precision units for heavy cutting forces or high accelerations
• Speed - check that the bearings and support arrangement suit the maximum screw speed, and consider fixed and fixed support for long, fast screws
• Precision grade - choose a precision level that matches the ball screw and machine, from general automation to high-precision machining
• Screw length and thermal expansion - use a support side unit at the free end of long screws, so the screw can expand without stressing the bearings
• Motor connection - check that the fixed side unit leaves enough shaft length and space for the coupling and motor mount
• Environment - consider dust, swarf, coolant, and moisture, and make sure seals and covers give enough protection
• Matching pairs - choose fixed and support side units from the same series, so their centre heights and sizes match
• Quality and brand - choose units from reputable linear motion suppliers, as poor-quality bearings and housings can reduce accuracy and shorten screw life

FAQ - all you need to know about ball screw support units

What is the difference between fixed side and support side units?

A fixed side unit uses a pair of angular contact bearings to hold the screw axially and carry thrust in both directions. A support side unit uses a single deep groove ball bearing to support the screw radially while letting it move axially as it expands with temperature.

Which end should the fixed side unit go on?

The fixed side unit is usually fitted at the motor end of the ball screw, where it locates the screw and carries the axial thrust. The support side unit goes at the opposite, free end.

Do I need to machine the ends of my ball screw?

Yes. The ends of the ball screw must be machined to suit the bore, length, thread, and snap ring groove of the support units. Many ball screws are supplied with standard end machining to match common support units.

Why are fixed side and support side bores different sizes?

The fixed side end usually needs a larger journal for the bearing pair, lock nut, and motor coupling, while the support side end only needs a smaller journal for a single bearing. Always check the catalogue dimensions for both units.

What does preload mean in a fixed side unit?

Preload is a small, controlled axial force applied to the angular contact bearings. It removes internal play and increases rigidity, so the screw does not move axially when the direction of motion changes.

How do I align ball screw support units?

Mount the units so the ball screw is parallel to the linear guides and in line with the ball nut. Check the centre height and alignment with a dial gauge, and use shims if needed. Poor alignment causes stiffness, noise, and early wear.

Can I use support side units at both ends?

Only for light, low-precision applications. Without a fixed side unit, the screw has no firm axial location, so it cannot hold position accurately under load.

How tight should the lock nut be?

Tighten the lock nut to the manufacturer’s recommended torque, then secure it with its locking screw. Overtightening can damage the bearings or change the preload, while undertightening can let the screw move axially.

Do ball screw support units need lubrication?

Most units are supplied greased and sealed, so they need little maintenance. Some units can be relubricated, and they should be checked as part of regular machine maintenance, especially in dusty or wet conditions.

What are the signs of a worn support unit?

Signs include axial play in the screw, noise, roughness, vibration, heat at the bearing, loss of positioning accuracy, and visible damage to seals. Worn units should be replaced to protect the ball screw and maintain accuracy.