Ball Screw Inspection and Maintenance Guide

Start Here: Match Your Symptom to a Cause

Before disassembling anything, identify what the machine is actually doing. The same component - a worn support bearing, for example - can produce different symptoms depending on how it has failed. Working from the symptom backward is faster and more reliable than checking every component in sequence.

Use this table as your entry point. Sections 1 through 5 below each address one area in detail.

Section 1: Support Bearing Axial Play

Support bearings hold the screw shaft in fixed axial position at each end. When they develop play - due to loose lock nuts, worn races, or incorrect preload - the screw shaft itself moves axially under load. This axial movement passes directly into positioning error and is one of the most common causes of backlash in CNC machines.

How to measure axial play correctly

The measurement must be taken on the screw shaft directly, not on the nut or the machine table. Measuring at the table combines multiple error sources and makes it impossible to isolate the bearing.

Equipment needed: Magnetic-base dial indicator, 0.001mm resolution or better.

Procedure:

1. Mount the dial indicator on a fixed reference surface - the machine frame or a heavy fixture bolted to the table. The probe tip should contact the end face of the screw shaft, centered on the shaft axis.
2. Lock the nut so it cannot travel along the shaft during the test. Do not allow the nut to provide the axial reaction force - this would load the nut rather than the bearing.
3. Apply a steady axial push force to the shaft by hand, approximately 10–20N. Note the indicator reading.
4. Apply a steady axial pull force in the opposite direction with the same force. Note the reading.
5. The total indicator movement between push and pull is the axial play of the support bearing assembly.

Interpretation:

• Less than 0.005mm: Acceptable for most CNC applications.
• 0.005–0.015mm: Borderline. Recheck preload washer condition and lock nut torque before deciding on replacement.
• More than 0.015mm: Bearing play is contributing to positioning error. Investigate the cause before reassembling.

What the measurement tells you:

If play is present but the bearing races feel smooth when rotated by hand, the most likely cause is a loose lock nut or a preload washer that has compressed over time. Retighten the lock nut to the correct torque and recheck. If play returns within a short operating period, the washer has fatigued and needs replacement.

If play is present and the bearing feels rough, notchy, or has detectable radial wobble when rotated, the bearing itself is worn and needs replacement. Adjusting preload on a worn bearing provides only temporary improvement.

Correct preload for support bearings

Support bearings on ball screws are typically angular contact bearings installed in a fixed-fixed or fixed-free configuration. The correct installation condition is a slight axial preload - enough to eliminate internal clearance and prevent the bearing from running loose under reversing loads, but not so much that it generates excessive heat or reduces bearing life.

A correctly preloaded support bearing should show no detectable axial movement under a 10–20N hand force applied to the shaft end. The shaft should rotate smoothly without notchiness or resistance variation. After running at operating speed for 15–30 minutes, the bearing housing should be warm to the touch (30–50°C above ambient) but not hot.

If a replacement bearing is being installed, use the same bearing specification as the original. Angular contact bearings for ball screw support are typically available in 15°, 25°, or 60° contact angle configurations - the original contact angle must be maintained, as different angles have different axial load capacity and stiffness.

Section 2: Nut Clearance Inspection and Repair

Axial clearance in the nut is wear-related, not adjustment-related in the same way as bearing preload. The balls and raceways in the nut accumulate micro-wear over time, which gradually increases the internal clearance between ball and raceway. When this clearance becomes large enough, it shows up as backlash at the machine table - most visibly as a position error that appears only when the axis reverses direction.

Measuring nut clearance

Before measuring nut clearance, verify that support bearing axial play is within specification (Section 1) and that all flange connection screws are tight (Section 3). Either of these factors, if present, will contaminate the nut clearance measurement.

Procedure:

1. Mount the dial indicator on the machine table (not on the nut or shaft), with the probe contacting a fixed reference on the machine frame.
2. Drive the axis to the midpoint of its travel and stop.
3. Apply a light axial load to the table in the positive direction (push) and zero the indicator.
4. Apply a light axial load in the negative direction (pull) with the same force.
5. The indicator reading is the total axial clearance in the nut, assuming bearing play has already been confirmed to be within specification.

Acceptable clearance depends on the application. For general automation with C7 precision, up to 0.05mm is often acceptable. For CNC machining with C5 precision requirements, nut clearance should typically be kept below 0.01–0.02mm.

Double nut (DFU series): clearance adjustment

Double-nut designs - such as the DFU series - use a precision spacer between the two nut halves to generate internal preload. When clearance develops, the spacer can be replaced with a slightly thicker one to restore preload and eliminate the clearance.

The spacer thickness increase required depends on the measured clearance and the ball circuit geometry. As a starting point, a spacer increase of 1.5–2 times the measured clearance will approximately restore zero clearance. Fine adjustment is done by measuring clearance after each spacer change until the target is reached.

After adjustment, run the axis through its full stroke at normal speed and check the nut temperature. A correctly preloaded double nut will run 5–10°C warmer than a clearance-adjusted single nut. If the nut runs significantly hotter than this after adjustment, the spacer increase was too large and preload is excessive - back down to the next smaller spacer size.

Note that spacer adjustment can only compensate for clearance caused by wear. If the raceways show visible pitting, flaking, or surface damage, spacer adjustment will not restore accuracy - the nut assembly needs replacement.

Single nut (SFU series): when to adjust and when to replace

Single-nut designs do not have a spacer adjustment mechanism. The internal clearance is fixed by the ball diameter and raceway geometry. When clearance develops due to wear, there are two options:

Option 1 - Replace with oversize balls

If wear is minor (measured clearance less than 0.05mm and no visible raceway damage), replacing the balls with a slightly larger diameter can restore internal preload and eliminate clearance. Standard ball replacement increments are +0.025mm and +0.050mm above the nominal ball diameter.

Before attempting this, inspect the raceways visually and by feel. Run your finger along the exposed screw shaft in the area that was covered by the nut. The raceway should feel smooth and uniform. If you can feel ridges, pitting, or roughness, ball replacement will not restore performance - the raceway geometry is no longer correct regardless of ball size.

When replacing balls, all balls in the circuit must be replaced with the same batch and the same oversize grade. Mixing balls of different diameters - even by 0.005mm - creates unequal load distribution and accelerates wear on the loaded balls. Order a complete replacement set rather than individual balls.

After fitting oversize balls, check running torque by rotating the shaft by hand. The resistance should be slightly higher than before (the preload has increased) but the rotation should be smooth throughout the full travel. If the shaft feels tight or irregular at any point, the oversize increment is too large for the actual wear condition - go back to the smaller increment.

Option 2 - Replace the nut or the complete screw-and-nut assembly

If raceway wear is visible, if the measured clearance exceeds 0.05mm, or if the accuracy requirement is C5 or tighter, the nut assembly should be replaced rather than adjusted. For the SFU series, replacement nuts are available as standard stock items and can be installed without replacing the screw shaft if the shaft itself is undamaged.

When ordering a replacement nut, specify the screw nominal diameter, lead, and nut type. If the original precision grade is not known, measure the screw shaft diameter and lead with a micrometer and pitch gauge - this is usually sufficient to identify the correct replacement.

Section 3: Flange Connection and Mechanical Looseness

Loose flange screws are one of the most frequently overlooked sources of apparent ball screw clearance. The nut is mounted to the machine carriage via a flange face, secured by four or more cap screws. Over time, vibration and reversing loads can gradually back these screws out, introducing clearance between the flange face and the carriage.

This type of looseness is easy to misdiagnose as nut clearance or bearing play, because the symptom - backlash at the table - is identical. The difference is that flange looseness has a characteristic: when you push and pull the table by hand while watching the nut body, the nut itself will move slightly relative to the carriage if the flange is loose. With nut clearance, the nut body stays fixed and the shaft moves inside it.

Inspection procedure:

1. With the machine stationary and servo drives enabled, push and pull the table by hand while watching the interface between the nut flange and the carriage surface.
2. Any visible gap opening or movement at this interface confirms flange looseness.
3. Use a torque wrench to check each flange screw against the specification for that screw size and grade. Do not rely on feel - flange screws that feel "snug" by hand are often undertorqued.
4. If any screw was loose, retighten all screws in a cross pattern to the specified torque, then recheck the table clearance measurement.

Prevention: Apply thread-locking compound  to flange screws during initial installation and after any reinstallation. This does not prevent removal with a standard wrench but prevents vibration-induced loosening.

Section 4: Lubrication - Diagnosing and Correcting Problems in Service

Lubrication failures in ball screws in service present differently from the problems addressed at installation. In a machine that has been running for months or years, the most common lubrication problems are lubricant depletion, lubricant degradation, and blocked delivery lines in centralized systems.

Recognizing lubrication problems

The earliest and most reliable indicator of a lubrication problem is a temperature increase at the nut. When lubricant film becomes insufficient, ball-raceway friction increases, and the nut runs hotter. A baseline temperature recorded at commissioning - or simply a consistent operating temperature observed over time - makes this trend detectable before it causes damage.

Other indicators:

• Noise increase, particularly a higher-frequency sound during motion (not the low-frequency resonance of a mechanical looseness problem)
• Increased drive current at constant speed, visible in the servo drive monitor
• Roughness or resistance variation when the axis is moved slowly by hand with drives disabled

If any of these are present, do not continue running the machine at normal load and speed. Add lubricant first and recheck.

Adding lubricant to a running installation

For a nut with a grease fitting, inject grease slowly through the fitting while the axis is stationary. Use the same lubricant type that was used at installation - do not mix grease types. A typical replenishment dose for an SFU 2005 nut is 0.3–0.5 grams; for a larger DFU 3210, approximately 1–1.5 grams.

After adding grease, run the axis through several slow full-stroke cycles before returning to normal operation. Monitor nut temperature for 15–30 minutes after resuming normal speed. If temperature returns to baseline, the lubrication problem is resolved. If it remains elevated, the problem may be more advanced - inspect the nut for contamination or damage.

Blocked centralized lubrication lines

On machines with centralized oil lubrication systems, blockages in the distribution lines or metering valves are a common cause of localized lubrication failure - the system appears to be operating, but one or more ball screws are not receiving adequate oil.

Symptoms of a blocked line are localized: one axis runs hot or noisy while others are normal. To diagnose, disconnect the delivery line at the nut fitting and trigger a lubrication pulse manually from the machine control. Oil should flow from the disconnected line within a few seconds. No flow, or very slow flow, confirms a blockage.

Blockages most often occur at the metering valves or at the nut fitting itself, particularly if the lubricant was changed without flushing the system. Clear the blockage by disconnecting the affected line segment and blowing through with low-pressure compressed air, or by replacing the metering valve if it cannot be cleared.

After clearing, reconnect the line, trigger several manual lubrication pulses, and verify flow at the nut fitting before reconnecting.

Contamination inside the nut

If the machine operates in an environment with metal chips, coolant, or fine dust, contamination entering the nut will accelerate wear and can cause abrupt failures. Signs of contamination include gritty resistance when the axis is moved by hand, dark or discolored grease visible at the nut seals, and rapid recurrence of clearance after adjustment.

If contamination is confirmed, the nut should be removed and cleaned before relubrication. Do not simply add fresh grease on top of contaminated grease - the particles will continue to abrade the raceways regardless of lubricant quantity.

Nut seal condition should be checked whenever the nut is removed. Worn or damaged lip seals allow contamination to enter regardless of lubrication frequency. Replacement seals are available for most standard nut designs and should be replaced as a matter of course whenever the nut is off the machine.

Section 5: Servo Drive Parameter Issues

Not all ball screw problems are mechanical. Some vibration, crawling, and positioning instability symptoms have their root cause in servo drive parameter settings rather than in the mechanical components.

The key distinction is this: mechanical problems tend to be consistent and repeatable - the same symptom at the same position or speed, every cycle. Servo parameter problems tend to be variable - the symptom changes with speed, load, or temperature, or it appears and disappears without obvious mechanical cause.

Position loop gain

The position loop gain (Kp) determines how aggressively the servo responds to position error. If Kp is set too high for the mechanical stiffness of the axis - which can happen after a ball screw replacement, a change in load, or a change in axis orientation - the axis will oscillate or vibrate, particularly during acceleration and deceleration, and particularly at low speed.

If this pattern matches the symptom being observed, reduce Kp by 10–20% and retest. The correct Kp is the highest value that produces stable, smooth motion without oscillation throughout the speed and load range of the application.

Velocity loop gain and integral

The velocity loop gain (Kv) and integral time constant control how the drive responds to speed error. Mismatched velocity loop parameters produce crawling or hunting at low speed - the axis moves in a series of small jerks rather than smoothly.

This symptom is often confused with ball screw backlash because both produce jerky motion on reversal. The difference: backlash produces a consistent dead zone (a range of motor rotation with no axis movement), while a velocity loop problem produces irregular motion throughout the stroke, not just at reversal.

When to call in servo expertise

Servo parameter tuning on a modern drive is iterative and requires access to the drive's monitoring and auto-tuning functions. The description above provides a starting point for diagnosing whether the problem is parameter-related, but full optimization should be done by someone familiar with the specific drive system in use.

If servo parameters were unchanged but a problem has appeared after a mechanical repair (bearing replacement, nut replacement, or preload adjustment), the new mechanical configuration has different stiffness characteristics and the drive parameters will need to be re-optimized for the new condition.

Preventive Maintenance Schedule

The most cost-effective maintenance is the kind that prevents failures rather than responding to them. The schedule below is a starting framework - the correct intervals for a specific machine depend on its duty cycle, environment, and precision requirements, and should be adjusted based on actual experience with that machine.

When Inspection Shows Replacement Is Needed

Some conditions cannot be corrected by adjustment or relubrication. If inspection reveals any of the following, the affected component should be replaced rather than returned to service:

• Visible pitting, flaking, or spalling on the screw shaft raceway or inside the nut
• Ball discoloration (blue or brown tinting indicates heat damage from lubrication failure)
• Screw shaft with visible bow or damage to the thread profile
• Support bearings with rough or notchy rotation that does not improve after cleaning and relubrication
• Nut seals that are torn, hardened, or no longer contacting the shaft

For the SFU and DFU series, replacement nuts are stocked as standard items and can be installed on the existing screw shaft if the shaft is undamaged, which avoids the cost and downtime of replacing the complete assembly.