How to Install Bearings for a Large Lead Screw: Fixed and Floating Support Guide

Installing bearings for a large lead screw requires more than selecting a bearing that fits the machined shaft end. The bearing arrangement must control the screw axially, support it radially, maintain alignment with the nut and guide system, and still allow the assembly to accommodate thermal expansion where required.

For many large ball screw assemblies, the usual arrangement is a fixed side with preloaded angular-contact bearings and a floating or supported side with a radial bearing. The fixed side establishes the axial position of the screw, while the opposite side supports the shaft without unintentionally locking both ends against thermal movement.

First Confirm the Bearing Support Arrangement

Do not begin installation until the fixed side, floating side, bearing orientation, shaft-end dimensions, and axial locating method are clearly identified.

Common ball screw support combinations include BK/BF, EK/EF, and FK/FF. In these pairs, BK, EK, and FK are normally used on the fixed side, while BF, EF, and FF are used on the floating side.

The support model must match the bearing-seat diameter, shaft shoulder, thread size, locknut position, housing dimensions, and complete end-machining drawing. View the DLY ball screw support unit range for common fixed- and floating-side structures.

Tools and Measurements Required

Large screws require suitable lifting and measuring equipment. Trying to handle a long or heavy shaft using only a normal workbench can bend the shaft, damage the thread, or contaminate the bearing seats before installation begins.

Step 1: Inspect the Screw Shaft Ends and Support Units

Before fitting a bearing, compare the actual components with the assembly drawing. A bearing can appear to fit while the shoulder position, thread length, spacer thickness, or housing depth is incorrect.

Confirm the following items:

• Fixed-side and floating-side shaft-end dimensions
• Bearing model, internal arrangement, and installation direction
• Bearing-seat diameter and fit
• Shaft shoulder width, height, squareness, and surface condition
• Thread size, locknut model, and locking method
• Spacer, collar, seal, and retaining-ring positions
• Support-unit mounting height and hole pattern

Inspect the journals for burrs, impact marks, corrosion, and raised edges. The shaft shoulder and bearing seat must be clean because even a small particle trapped behind the bearing can create axial runout or prevent complete shoulder contact.

Do not polish an incorrectly sized bearing seat until the bearing fits. Measure the journal first and compare it with the drawing and bearing-fit requirement.

Step 2: Support the Large Screw During Installation

A large lead screw should be supported at several positions during bearing installation. Allowing a long shaft to hang from one end can introduce bending stress and make the bearing appear difficult to install even when the component dimensions are correct.

• Use padded supports that cannot scratch the threaded surface.
• Keep lifting straps away from seals, bearing seats, and finished journals.
• Do not lift the complete assembly by the ball nut or nut flange.
• Keep the screw horizontal unless the assembly procedure specifically requires another position.
• Prevent the ball nut from moving uncontrolled along the shaft.
• Cover exposed threads and journals when work is paused.

For a ball screw, do not remove the nut from the screw shaft unless the correct transfer sleeve and reassembly procedure are available. Losing balls or changing the recirculation condition can damage the nut assembly.

Step 3: Install the Fixed-Side Bearing Assembly

The fixed side determines the axial reference of the screw. Errors at this end directly affect axial play, stiffness, runout, coupling alignment, and positioning stability.

Using a Preassembled Support Unit

When using a preassembled BK, EK, FK, or similar fixed-side support unit, do not dismantle the bearing stack unless the support-unit manufacturer specifically permits it. The angular-contact bearings may already have a defined orientation and adjusted preload.

1. Confirm that the shaft journal, thread, and shoulder match the support unit.
2. Carefully insert the machined shaft end through the seal and bearing bore.
3. Protect the seal lip from turning outward or being cut by the shaft shoulder or thread.
4. Seat the bearing inner ring or spacer completely against the shaft shoulder.
5. Install the locknut and tighten it according to the support-unit or machine drawing.
6. Secure the locknut using the specified set piece, setscrew, tab washer, or other locking device.

Using Separate Bearings and a Custom Housing

If the fixed side uses separate angular-contact bearings, preserve the specified bearing arrangement. A face-to-face, back-to-back, or tandem pair cannot be changed simply because the bearings have the same dimensions.

When the bearing inner ring has an interference fit on the shaft, apply installation force to the inner ring. Do not push the bearing onto the shaft by pressing on the outer ring, because the mounting force would pass through the rolling elements and may indent the raceways.

For a large bearing, controlled thermal mounting or a suitable hydraulic method may be safer than heavy mechanical force. Do not heat a precision bearing with an open flame, and do not exceed the bearing manufacturer's permitted temperature.

There is no universal locknut torque for every large screw. The correct value depends on the bearing model, thread size, bearing arrangement, required preload, spacer design, and lubrication condition. Use the drawing or bearing-support specification rather than copying a torque value from another screw size.

Step 4: Install the Floating-Side Bearing

The floating side supports the opposite end of the screw radially. In a fixed-floating arrangement, it must not unintentionally become a second fixed side.

1. Install the specified bearing on the supported-side shaft journal.
2. Apply fitting force only through the ring being mounted with an interference fit.
3. Install the specified snap ring, retaining collar, or shaft-side locating component.
4. Insert the bearing into the BF, EF, FF, or custom housing.
5. Confirm that the housing arrangement retains the bearing radially without eliminating the designed axial movement.

A common mistake is tightening collars, covers, or housing components until the floating bearing is clamped between two axial shoulders. This may cause the screw to bind as its temperature changes.

In a fixed-fixed assembly, both ends may be axially controlled, but the preload and bearing spacing must follow the machine design. Do not convert a fixed-floating assembly into fixed-fixed support during installation.

Step 5: Temporarily Mount the Screw Assembly

The screw axis must match the path established by the linear guideways and moving table. The ball screw should transmit axial force; it should not be used to correct an inaccurate rail or mounting base.

A practical installation sequence is:

1. Align and secure the linear guideways and moving table.
2. Attach the ball nut to its nut bracket, but keep the final fasteners slightly loose.
3. Position the fixed-side support unit on its prepared reference surface.
4. Position the floating-side support unit without fully tightening it.
5. Move the table toward the fixed side and adjust the nut bracket and support-unit position.
6. Reciprocate the table several times through the complete stroke.
7. Confirm that movement remains smooth before final tightening.

Do not force the ball nut, nut bracket, or support housing sideways to make incorrectly positioned mounting holes fit. Stored installation stress may produce high motor current, uneven running torque, bearing heating, and premature nut wear.

For a more detailed alignment procedure, see Ball Screw Parallelism: How to Align a Ball Screw During Installation.

Step 6: Check Runout and Axial Clearance Before Final Tightening

Smooth hand rotation alone is not enough to approve the installation. Use a dial indicator to verify the shaft-end condition and axial location.

Compare all measurements with the actual ball screw, support-unit, or machine drawing. Large screw diameter alone does not define an acceptable universal runout value.

While monitoring the indicators, complete the final tightening progressively. A common sequence is:

1. Ball screw nut
2. Nut bracket
3. Fixed-side support unit
4. Floating-side support unit

Measure again after the bolts reach their specified final torque. Tightening can shift the housing or distort an inaccurate mounting surface.

Step 7: Align the Motor and Coupling

The coupling should connect two aligned shafts; it should not pull the motor and screw into alignment.

• Check motor-shaft and screw-journal coaxiality.
• Confirm the specified coupling insertion depth and shaft gap.
• Do not push the coupling against the bearing locknut or support housing.
• Tighten coupling screws using the specified sequence and torque.
• Rotate the system manually again after connecting the motor.

A flexible coupling can compensate for limited installation error, but it cannot correct major angular or parallel misalignment. Excessive coupling deflection increases load on the motor bearing and ball screw support bearing.

Step 8: Perform a Controlled Run-In Test

Do not run a newly installed large screw immediately at full speed and full load.

1. Rotate the screw manually through the full stroke.
2. Run the axis at low speed without external load.
3. Check noise, vibration, motor current, and bearing-housing temperature.
4. Increase speed gradually while monitoring the same conditions.
5. Apply working load progressively rather than immediately.
6. Recheck support-unit bolts, locknut security, and alignment after the initial run-in.

Common Bearing Installation Mistakes

• Treating every large threaded shaft as if it uses the same bearing arrangement
• Mixing the fixed-side and floating-side support units
• Disassembling a factory-adjusted support unit unnecessarily
• Changing the orientation of a matched angular-contact bearing pair
• Hammering directly on the bearing or hardened shaft end
• Transmitting mounting force through the rolling elements
• Using a universal locknut torque without checking the bearing design
• Axially clamping the floating-side bearing
• Forcing the nut bracket to compensate for support-unit misalignment
• Connecting the motor before checking shaft-end runout
• Accepting smooth hand rotation as the only inspection result
• Running immediately at full speed without a controlled run-in

What Information Should Be Confirmed Before Ordering?

For a large screw assembly, the screw shaft, nut, support bearings, housing, locknut, coupling, and end machining should be confirmed as one system.

Provide the following information:

• Screw type, diameter, lead, total length, and effective stroke
• Required accuracy grade
• Horizontal or vertical installation
• Maximum speed and axial load
• Fixed-floating, fixed-fixed, or other bearing arrangement
• Support-unit model or bearing specification
• Complete fixed-side and floating-side machining drawings
• Nut type, flange direction, and nut-bracket dimensions
• Motor and coupling information

DLY supplies cold-rolled and precision-ground ball screws, together with customized lengths, shaft-end machining, ball nuts, BK/BF, EK/EF and FK/FF support units, and related couplings. End machining should be confirmed from the support-unit and machine drawings before production.

Conclusion

Correct installation of bearings for a large lead screw begins with identifying the fixed and floating sides, verifying the shaft-end dimensions, and supporting the long shaft correctly during assembly.

The fixed-side bearing stack must seat correctly against its reference shoulder and be secured using the specified locknut procedure. The floating side must provide radial support without unintentionally preventing designed axial movement. Final approval should include shaft-end runout, axial movement, full-stroke resistance, coupling alignment, and a controlled run-in test.

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