A ball screw that is not moving smoothly may still complete its stroke, but the resistance may increase at certain positions, change between forward and reverse travel, or become noticeably higher after installation. These symptoms should not be ignored. Continued operation can increase motor load, damage the ball raceways and shorten the service life of the screw, nut and support bearings.
Rough movement does not always mean that the ball screw itself is defective. The actual cause may be insufficient lubrication, contamination, mounting misalignment, bearing preload, coupling stress, a damaged circulation component or impact during handling. The most effective approach is to identify when and where the resistance occurs before replacing any component.
What Does "Not Moving Smoothly" Mean?
Different movement patterns point to different causes. Before disassembling the assembly, observe whether the resistance is constant, periodic or limited to one part of the stroke.
| Observed symptom | Likely inspection direction |
|---|---|
| Resistance is high over the entire stroke | Lubrication, excessive preload, bearing setting or general misalignment |
| A tight spot appears at the same position | Local raceway damage, contamination, shaft bending or mounting distortion |
| Resistance changes after the nut housing is tightened | Nut housing alignment, mounting surface flatness or bolt-tightening sequence |
| Resistance changes after coupling installation | Motor-to-screw alignment or axial stress from the coupling |
| Movement became rough after a collision | Over-travel, damaged circulation parts, brinelling or a bent screw shaft |
1. Insufficient or Unsuitable Lubrication
Lubrication forms a protective film between the balls and raceways. If the lubricant quantity is insufficient, has deteriorated or cannot reach the loaded contact area, rolling resistance and temperature may increase. A ball screw stored for a long period may also require fresh lubricant before operation.
What to check:
- Dry or discoloured raceway surfaces
- Old grease mixed with particles or machining residue
- A blocked lubrication port or an unsuitable lubrication position
- A sudden increase in running temperature or drive torque
Clean the accessible screw surface and apply the lubricant specified for the operating speed, load, temperature and environment. After relubrication, move the assembly through several low-speed strokes so that the lubricant can distribute through the raceways. Do not assume that adding excessive grease will improve movement; excessive grease can temporarily increase agitation resistance.
2. Chips or Debris in the Raceway
Metal chips, abrasive dust, wood particles, coolant residue and dried grease can enter an exposed ball screw. These contaminants interrupt normal rolling contact and may create a tight spot that returns at the same position during every stroke.
Inspect the exposed grooves, nut seals and surrounding covers. Remove accessible contamination with a clean, lint-free cloth and then replenish the lubricant. Do not force hard particles through the nut, use abrasive tools on the raceway or direct high-pressure compressed air toward the nut opening. Equipment operating around chips, dust or coolant should use suitable wipers, bellows or telescopic covers.
For detailed protection measures, see how to prevent ball screw damage caused by debris ingress.
3. Misalignment Between the Screw, Nut and Supports
The screw shaft, nut housing and support bearings must operate on a consistent axis. If the nut housing is offset or tilted, the ball screw receives an unintended radial or moment load. The assembly may feel acceptable before tightening but become difficult to turn after the nut housing or bearing seat is fully secured.
Check the mounting surfaces for burrs, contamination and uneven seating. Verify the relative position of the fixed-side bearing seat, support-side bearing seat and nut housing with suitable measuring equipment. Tighten the mounting bolts progressively and observe whether the rotational resistance changes during the process.
High-preload assemblies are more sensitive to installation errors. Do not use the mounting bolts to force a visibly misaligned nut housing into position.
4. Support Bearing or Coupling Stress
Rough movement may originate outside the ball nut. Incorrect support-bearing assembly, excessive bearing preload, a loose locknut or an offset motor coupling can all increase the torque required to rotate the screw.
If permitted by the machine design and carried out safely, separate the drive system into stages and compare the rotational resistance before and after the coupling is connected. If resistance increases only after coupling installation, check motor shaft alignment, coupling insertion depth and unwanted axial compression. If it remains high with the coupling disconnected, inspect the support bearings and ball screw assembly.
5. Incorrect Nut Mounting
The ball nut flange must sit evenly against its mounting surface. Burrs, trapped particles, uneven bolt tightening or an incorrectly machined housing can tilt the nut and create uneven loading inside the raceways.
Before installation, clean both contact surfaces and confirm that the flange seats without rocking. Tighten the bolts gradually in a cross pattern where applicable. If the nut was removed from the screw shaft without a transfer sleeve, or if balls have fallen out, do not continue operating the assembly. Ball quantity, ball arrangement and preload may already have changed.
6. Damaged Ball Circulation Parts or Over-Travel
Return tubes, end caps and other circulation components guide the balls from one circuit to the next. An impact during installation or an over-travel event can deform these parts, interrupt ball circulation or create indentations in the raceways.
If rough movement began immediately after a limit-switch failure, collision or nut overrun, stop the machine and inspect the assembly before restarting it. Check the external circulation parts for cracks, deformation or looseness. Internal circulation damage normally requires professional evaluation; forcing the screw to rotate may spread the damage to additional raceway sections.
7. Bent Shaft, Raceway Damage or Excessive Internal Load
A ball screw can be damaged by dropping, bending, improper lifting or applying a load beyond the selected operating condition. A bent shaft often produces a repeating change in resistance as the screw rotates. Raceway dents, flaking or abnormal wear may create a tight spot at a specific travel position.
Check shaft runout and inspect the accessible raceway under adequate lighting. Also review whether the selected preload, axial load, speed and acceleration are suitable for the application. Excessive preload can improve rigidity but also raises starting torque, heat generation and sensitivity to mounting errors.
Do not straighten a precision ball screw by uncontrolled force or polish a damaged raceway with abrasive material. If there are visible dents, flaking, corrosion, missing balls or repeated hard spots, the assembly should be professionally inspected or replaced.
Troubleshooting Order
Checking the system in a controlled order helps avoid unnecessary nut disassembly and makes it easier to identify when the resistance was introduced.
- Record the symptom: Determine whether resistance is constant, periodic or limited to one position.
- Inspect the exposed screw: Look for chips, dried lubricant, corrosion, dents or visible deformation.
- Check lubrication: Confirm lubricant condition, supply route and distribution.
- Review recent events: Identify whether the problem followed installation, maintenance, a collision or long-term storage.
- Isolate external resistance: Check the motor, coupling and support bearings where safe and permitted.
- Verify alignment: Inspect the nut housing, bearing seats and mounting surfaces.
- Stop if internal damage is suspected: Do not force a nut with missing balls, damaged return parts or a severe hard spot.
When Should the Ball Screw Be Replaced?
Cleaning, correct lubrication and alignment adjustment may restore smooth movement when no permanent damage has occurred. Replacement or professional inspection should be considered when the assembly has visible raceway flaking, deep dents, corrosion, a bent screw shaft, missing balls, damaged circulation parts or persistent hard spots after the external installation factors have been eliminated.
When requesting a replacement, provide the screw diameter, lead, total length, thread length, nut model, accuracy grade, preload requirement and shaft-end drawing. Operating load, speed, mounting orientation and contamination conditions also help determine whether the original selection is suitable.
Keep Ball Screw Motion Stable
A ball screw that is not moving smoothly should be treated as a system-level problem. Begin with contamination and lubrication, then check mounting alignment, support bearings, coupling stress and nut seating. If the problem followed an impact or appears repeatedly at the same position, inspect for circulation damage, shaft bending and raceway defects before the machine returns to normal operation.
DLY supplies rolled and ground ball screws with optional shaft-end machining and matching support units. Selection should be based on the required load, speed, accuracy, installation arrangement and operating environment.
