Ball screw parallelism describes how closely the screw axis follows the direction of travel established by the machine's linear guideways. Correct alignment also requires the fixed-end bearing, floating-end bearing, and ball nut housing to share a compatible centerline.
If these components are forced into different positions during installation, the ball screw may still rotate, but the assembly can develop uneven torque, heat, vibration, noise, positioning instability, and premature wear.
What Must Be Aligned in a Ball Screw System?
A ball screw does not define the movement direction of a machine axis by itself. In most systems, the linear guideways constrain the table or carriage and establish its path of travel. The ball screw must be installed to follow that path without forcing the nut or support bearings sideways.
The installation therefore involves several related conditions:
- Parallelism between the screw axis and linear guideway travel
- Coaxiality between the fixed- and floating-end bearing seats
- Correct height and lateral position of the ball nut housing
- Squareness of the support-unit mounting surfaces
- Concentricity of the machined screw journals
- Alignment between the motor shaft, coupling, and screw shaft
These conditions should be evaluated as one assembly. Correcting one point while forcing another component out of position does not create a properly aligned axis.
Parallelism, Coaxiality and Runout Are Different
These terms are often confused during ball screw installation.
Parallelism
Parallelism describes whether the ball screw axis remains in the same vertical and horizontal relationship to the linear guideway travel over the full stroke.
Coaxiality
Coaxiality describes whether the fixed support, floating support, screw journals, and ball nut are positioned around a compatible common axis.
Radial runout
Radial runout is the change measured at a surface as the screw rotates. It may be caused by a bent shaft, journal machining error, bearing installation, contamination on a locating surface, or incorrect assembly.
Placing a dial indicator against the shaft and rotating it primarily measures runout. It does not by itself confirm that the screw is parallel to the linear guideway travel.
Why Ball Screw Alignment Matters
A ball screw is designed primarily to transmit axial load. Misalignment introduces unwanted radial forces and bending moments into the screw, ball nut, support bearings, and mounting structure.
Possible consequences include:
- Uneven running torque
- Abnormal temperature rise
- Noise and vibration
- Accelerated raceway and ball wear
- Support-bearing overload
- Premature coupling failure
- Reduced positioning consistency
- Servo overload or excessive motor current
Higher preload does not correct misalignment. A preloaded ball screw is usually more sensitive to installation error because it already contains internal load.
Tools Used to Check Ball Screw Parallelism
The exact equipment depends on the machine structure and required accuracy. Common inspection tools include:
- Dial test indicator and rigid magnetic or mechanical stand
- Precision straightedge
- Gauge blocks
- Precision level
- Alignment mandrel or reference bar
- Torque wrench
- Low-range torque measuring equipment
- Laser alignment equipment for higher-accuracy installations
- Laser interferometer for final positioning verification
A caliper or outside micrometer can verify component dimensions, but it cannot by itself establish the alignment of the complete ball screw axis.
Pre-Installation Checks
Inspect the ball screw assembly
Check the screw shaft, machined journals, ball nut, return components, seals, and support units for visible impact damage, corrosion, contamination, or deformation.
Do not remove the ball nut from the screw unless the correct transfer sleeve and reassembly procedure are available. Removing it directly may allow the balls to fall from their circulation circuits.
Inspect the mounting surfaces
Clean the bearing-housing seats, nut-bracket surface, locating shoulders, and threaded holes. Burrs, paint, chips, dents, and trapped contamination can shift a support unit or distort the nut housing when the bolts are tightened.
Check that the mounting surfaces meet the flatness, squareness, height, and position requirements of the machine drawing.
Complete the linear guideway installation first
Because the linear guideways normally establish the carriage path, they should be installed, aligned, and checked before final ball screw alignment. Attempting to use the ball screw to pull a misaligned table into position will place unwanted radial load on the nut.
Confirm the support arrangement
Identify the fixed end and floating end before assembly. The fixed end establishes the axial position of the screw, while the floating end normally supports the shaft radially and allows axial movement caused by thermal expansion.
Ball Screw Installation and Alignment Procedure
The following procedure describes the general logic of a fixed–floating ball screw installation. The manufacturer's drawing and tightening requirements should take priority where they differ.
Step 1: Install the fixed-end support
Mount the fixed-side support unit on its prepared reference surface. Keep the fasteners snug enough to locate the unit while still allowing controlled alignment if the design requires adjustment.
Confirm that the support-unit locating surface is seated correctly and that no burr or contamination is trapped underneath it. Tighten the bearing locknut and support-unit fasteners according to the specified procedure.
Step 2: Fit the screw into the support units
Handle the screw carefully and avoid applying impact force through the bearing raceways. The machined journal should fit the specified bearing arrangement; a feeler gauge between the shaft journal and bearing is not the correct method for setting alignment.
Install the floating-end support without unintentionally fixing the screw axially. At this stage, leave the housing fasteners sufficiently loose if adjustment is required.
Step 3: Position the ball nut housing
Attach the ball nut to its housing or bracket according to the specified mounting orientation. Support the screw so that the nut is not required to carry the weight of the shaft during installation.
Connect the nut housing to the moving table with the bolts initially loose or lightly snug. This allows the nut bracket to settle into a position compatible with the screw axis and guideway travel.
Do not pull the ball nut into alignment by fully tightening the mounting bolts against a visible gap. If a gap exists, inspect the mounting height, bracket geometry, support-unit position, and reference surfaces.
Step 4: Move the table through the stroke
Move the carriage slowly by hand or at the lowest safe drive speed. Observe whether resistance changes at different positions.
The ball screw should rotate smoothly without binding. If the nut housing shifts as the table moves, its original position was not aligned with the travel path.
Move the carriage near the fixed end, middle, and floating end while gradually locating the nut housing and support units. Avoid final tightening at only one end of the stroke without checking the remaining travel.
Step 5: Check horizontal and vertical alignment
Where the machine structure permits, mount a dial indicator on the moving carriage and measure against a suitable reference surface or alignment mandrel associated with the screw axis. Move the carriage along the linear guideways without rotating the reference component.
Record the change in both the horizontal and vertical directions. This evaluates how the screw reference axis changes relative to the guideway travel.
A threaded raceway is not an ideal indicator surface. Use a machined reference surface, alignment bar, support-unit reference, or another method defined by the machine design.
Step 6: Check screw runout separately
Place the dial indicator against the specified machined journal or reference diameter and slowly rotate the screw. This test checks radial runout rather than parallelism.
If excessive runout is measured, inspect the shaft straightness, journal machining, bearing seating, locknut condition, and contamination on locating shoulders.
Step 7: Adjust with appropriate methods
If the screw axis is not aligned with the guideway travel, correct the responsible mounting position. Depending on the machine design, adjustment may involve:
- Repositioning the support-unit housing
- Correcting the lateral position of the nut bracket
- Using accurately prepared shims under approved mounting surfaces
- Correcting the height of the nut housing
- Removing burrs or contamination
- Remachining an out-of-tolerance mounting surface
Do not use random stacks of soft or uneven shims. Shims should provide stable support across the required contact area and should not distort the housing when tightened.
Step 8: Tighten components progressively
Once alignment is acceptable, tighten the nut housing and support-unit fasteners progressively in the specified sequence. Recheck alignment and running resistance after each stage because tightening can shift or distort the components.
Do not assume that the position remains unchanged after final torque is applied.
How to Confirm Alignment Without a Direct Parallelism Measurement
Some machine structures do not provide access for direct indicator measurement along the screw axis. In these cases, alignment can also be evaluated through several related observations.
Running torque over the full stroke
Measure or monitor the torque required to move the assembly at a low, constant speed. A large increase near one end or at a repeated position can indicate misalignment, local interference, contamination, or shaft deformation.
Nut-housing movement
With the mounting bolts safely loosened according to the installation procedure, observe whether the nut housing tries to move sideways or vertically as the table travels. Movement indicates that the housing position is not compatible with the screw axis.
Motor current
After mechanical installation is complete, monitor motor current at a low, constant speed in both directions. Significant current changes along the stroke may indicate varying mechanical resistance.
Motor current is a supporting indicator, not a substitute for mechanical measurement. Guideway friction, seals, load distribution, and drive settings can also affect the reading.
What Parallelism Tolerance Should Be Used?
There is no single ball screw parallelism tolerance that applies to every machine. The acceptable value depends on:
- Screw diameter and length
- Accuracy grade
- Nut preload
- Support-bearing arrangement
- Machine travel
- Operating speed
- Required positioning accuracy
- Rigidity of the nut bracket and support housings
Use the tolerance stated on the machine drawing, ball screw drawing, or manufacturer's installation instructions. A general value taken from another machine may be unsuitable for the current assembly.
Post-Installation Checks
Check smooth movement
Move the axis through the complete stroke at low speed. Check for binding, abnormal noise, repeated torque peaks, and sudden motor-current changes.
Check temperature
Run the axis gradually under controlled conditions and monitor the ball nut, fixed-end support, floating-end support, motor, and coupling. Rapid or localized heating may indicate excessive preload, lubrication problems, bearing error, or misalignment.
Check coupling alignment
The coupling should compensate only for the limited misalignment specified by its design. It should not be used to conceal major offset between the motor shaft and screw shaft.
Verify positioning accuracy
After the mechanical installation is stable, verify positioning accuracy and repeatability using equipment appropriate to the machine's requirements. High-precision axes may require a laser interferometer or linear-scale comparison.
Positioning compensation should be performed only after looseness, misalignment, and other mechanical errors have been corrected.
Common Ball Screw Installation Mistakes
- Using the ball screw to pull misaligned guideways into position
- Fully tightening the nut housing before checking the entire stroke
- Forcing the ball nut bracket against a mounting-height error
- Confusing screw runout with parallelism
- Using a feeler gauge to set the shaft-to-bearing fit
- Clamping the floating end axially
- Using the coupling to compensate for major shaft offset
- Ignoring burrs and contamination on mounting surfaces
- Rotating the screw at high speed before completing low-speed checks
- Removing the ball nut without a transfer sleeve
Signs of Ball Screw Misalignment
Recheck the installation if the system develops:
- Uneven torque along the stroke
- Binding near one end
- Increasing motor current
- Abnormal noise after tightening the nut housing
- Rapid heating at the nut or support bearings
- Repeated coupling damage
- Vibration at operating speed
- Premature seal, ball, raceway, or bearing wear
If resistance decreases when the nut housing or support-unit fasteners are loosened, the mounting position or surface accuracy should be inspected before the machine returns to operation.
DLY Ball Screw Installation Support
DLY supplies rolled and ground ball screws with corresponding shaft-end machining and support-unit options. Common fixed- and floating-side combinations include BK/BF, FK/FF, and EK/EF.
Explore DLY's ball screw range and ball screw support units.
For a new installation, provide the ball screw model, diameter, lead, length, accuracy grade, preload requirement, support arrangement, and shaft-end drawing. The installation height and interfaces of the ball nut, support units, and coupling should be reviewed together.
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Send your ball screw specifications and installation drawing to DLY for technical review.
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