How to Measure Ball Screw Support Bearing Axial Play

Ball screw support bearing axial play is the measurable axial movement of the screw shaft relative to the fixed bearing housing. Excessive movement at the fixed end can appear as backlash, reversal error, vibration or unstable positioning in CNC machines and automation equipment.

The correct measurement uses a dial indicator aligned with the screw axis and a controlled push-pull force on the shaft. The most important part is isolating the fixed-end support bearing so that ball nut clearance, table movement, coupling looseness and machine-frame deflection are not mistakenly included in the result.

First Identify What You Are Measuring

If the indicator is placed on the machine table without isolating individual components, the result is total axis lost motion. It cannot be identified as support bearing clearance alone.

Fixed End vs. Supported End

A common ball screw installation uses one fixed end and one supported or floating end.

The fixed end is normally the correct position for checking axial positioning stability. A floating or supported end should not be judged by the same zero-play requirement because its job and bearing arrangement are different.

For more information about these structures, see fixed and supported ball screw end supports .

Tools Required

• Dial indicator or electronic indicator with suitable resolution
• Rigid magnetic base or bolted indicator stand
• Controlled push-pull fixture, spring scale or force gauge
• Suitable shaft-end attachment if the shaft cannot be pulled safely by hand
• Torque wrench for checking or reinstalling the locknut according to specification
• Cleaning cloth and approved cleaner
• Machine lockout equipment and the assembly drawing

A feeler gauge is normally not needed because this is not an exposed gap measurement. A hydraulic press should not be used unless the bearing manufacturer specifies a test load and suitable fixture. Excessive force adds elastic deformation and may damage the bearing or support assembly.

Step 1: Make the Machine Safe

Switch off and lock out the machine before working near the ball screw, coupling or moving table. Release stored mechanical energy and support any vertical axis so it cannot fall or back-drive.

If the motor or coupling can apply holding torque or axial force to the screw, disconnect it according to the machine maintenance procedure. The shaft must be accessible without creating an uncontrolled movement risk.

Step 2: Isolate the Fixed-End Bearing

The most accurate bearing-only measurement is performed during assembly, with the fixed bearing and screw shaft installed but before the ball nut is connected to the moving table.

On an installed machine, detach the ball nut flange from the table or use the machine manufacturer's specified isolation procedure so the ball nut does not become the reaction point for the test force.

Before attaching the indicator, check for loose support-unit bolts, a loose bearing locknut, an unsecured housing or visible movement between the support block and machine base. These faults can produce a large reading even if the bearing itself is not worn.

Step 3: Position the Dial Indicator

1. Clean the screw-shaft end and select a flat measurement point.
2. Fix the indicator base to the bearing housing or a rigid machine-frame reference.
3. Place the probe near the centre of the shaft end.
4. Align the probe as closely as possible with the screw axis.
5. Apply sufficient probe preload to maintain contact in both test directions.
6. Confirm that the stand and magnetic base cannot move under the test force.

If the probe is significantly angled relative to the screw axis, cosine error will make the indicated movement smaller than the actual axial displacement. Placing the probe away from the shaft centre can also introduce error from shaft-end face runout.

Step 4: Apply Equal Push and Pull Forces

Use the test force specified by the support-unit or machine manufacturer. The force should be high enough to take up free movement but low enough to avoid unnecessary elastic deflection.

1. Apply the specified axial force in the push direction.
2. Allow the indicator reading to stabilize and record it as R_push.
3. Release the force without impacting the shaft.
4. Apply an equal force in the pull direction.
5. Allow the reading to stabilize and record it as R_pull.
6. Repeat the complete cycle at least three times.

Apply force along the shaft centreline. Side loading or pulling on the shaft at an angle can introduce radial movement and distort the result.

Step 5: Check Repeatability

The repeated push-pull cycles should produce closely grouped readings. If the result changes substantially between cycles, check the indicator base, shaft attachment, support-unit mounting bolts and test-force consistency.

The shaft may be rotated and measured again to check whether shaft-end face runout affects the setup. Do not confuse this separate runout check with measuring axial clearance at several positions around the bearing.

Axial clearance is a movement along the shaft axis. It is not calculated by averaging feeler-gauge readings around the bearing circumference.

Step 6: Compare with the Correct Specification

There is no single acceptable axial-play value for every ball screw support bearing. The result depends on:

• Support-unit model and bearing arrangement
• Bearing accuracy and preload class
• Test force
• Fixed-free or fixed-fixed installation
• Machine positioning requirement
• Housing and shaft-end rigidity
• Whether the measurement includes elastic deflection

A preloaded angular contact bearing arrangement may have no measurable free play while still showing small elastic displacement under load. This elastic movement is bearing-system compliance and should not automatically be treated as internal clearance.

Always compare the result with the support-unit drawing, bearing specification or machine manufacturer's acceptance limit under the same test conditions.

What Can Cause an Excessive Reading?

Do Not Eliminate Play by Overtightening

If excessive axial movement is found, do not continue tightening the locknut until the indicator reaches zero. The locknut torque, spacer thickness and bearing preload must follow the specified assembly procedure.

Excessive preload can cause:

• High starting and running torque
• Bearing temperature rise
• Reduced maximum speed
• Premature bearing fatigue
• Shaft distortion or shoulder damage

After adjustment or bearing replacement, measure the axial movement again, rotate the shaft manually and perform a controlled operating-temperature check according to the machine procedure.

When the Reading Is Not a Bearing Problem

If the fixed-end bearing shows acceptable axial stability but the machine still has reversal error, inspect the ball nut, nut housing, coupling, motor connection and table movement.

A systematic diagnostic sequence is available in the ball screw inspection and maintenance guide .

DLY Ball Screw Support Units

DLY supplies BK/BF, FK/FF and EK/EF ball screw support units for different screw diameters and machine structures. The fixed and supported units can be supplied together with the ball screw, nut, coupling and matching end machining.

When ordering a replacement support, confirm the support-unit model, shaft-end diameter, bearing seat, locknut thread, mounting-hole pattern and required installation arrangement.

View the DLY BK/BF fixed and supported ball screw units .

Conclusion

To measure ball screw support bearing axial play, place a dial indicator against the screw-shaft end, reference it to the fixed bearing housing and apply equal controlled push and pull forces along the shaft axis.

For a true bearing-only result, isolate the ball nut and moving table from the test-force path. Otherwise, the reading represents combined installed-system displacement.

Compare the measured movement with the specification for the actual support unit and test load. Do not apply a universal clearance limit or increase preload by overtightening the shaft locknut.

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