A ball screw should normally move with relatively consistent resistance throughout its usable stroke. If the nut moves smoothly over most of the screw but becomes noticeably tight, difficult to rotate or almost stuck at one particular position, this usually indicates a localized mechanical or installation problem.
The cause may come from the screw shaft, ball nut, support bearings, mounting surfaces or even contamination inside the ball circulation system. Identifying the exact location of the resistance is therefore more useful than simply adding more lubricant or increasing motor torque.
Below are the most common reasons why a ball screw becomes tight only at certain positions and how to check them.
1. The Ball Screw Shaft Is Slightly Bent
A bent screw shaft is one of the most common reasons for resistance that changes according to position. When the bent section passes through the ball nut or support bearings, the screw may experience additional radial load and become harder to rotate.
This situation is more common on long ball screws, especially when the screw has been damaged during transportation, stored incorrectly or subjected to an external impact.
Typical signs include:
- The screw rotates normally in some areas but becomes tight at a repeatable position.
- The shaft appears to move sideways while rotating.
- Vibration increases as rotational speed rises.
- The problem remains even after lubrication.
A dial indicator can be used to measure screw runout. For longer screws, checking the shaft at several positions is important because bending may occur only in one section.
2. The Screw and Nut Are Not Properly Aligned
Ball screws are designed mainly to carry axial load. If the nut housing, screw shaft and support bearings are not installed on the same centerline, additional radial force can be introduced into the system.
In mild cases, the screw may still rotate normally through most of the stroke. However, as the nut approaches a position where the alignment error becomes larger, resistance can increase significantly.
This is especially common when the ball screw is mounted on a machine base that is not perfectly straight or when the nut housing is tightened before alignment is completed.
One simple test is to slightly loosen the ball nut housing bolts and move the axis manually. If the tight spot becomes noticeably smoother, installation alignment should be checked carefully.
3. The Mounting Surface Is Uneven
Even when the ball screw itself is straight, an uneven mounting surface can distort the support units, nut housing or machine structure.
For example, if the fixed-side support unit and floating-side support unit are installed at slightly different heights, the screw may be forced to bend after the bolts are tightened.
The same problem can occur when burrs, dirt or paint remain between the support unit and mounting surface. A very small mounting error can become significant on a long axis.
Check the following:
- Cleanliness of the mounting surfaces.
- Flatness of the machine base.
- Height alignment between the fixed and supported ends.
- Ball nut housing alignment.
- Whether resistance changes after mounting bolts are loosened slightly.
4. Contamination Has Entered the Ball Nut
Dust, metal chips and other particles can enter the ball nut if the sealing system is damaged or the operating environment is heavily contaminated.
Small particles may become trapped between the balls and raceway. When the contaminated area reaches the loaded contact zone, the nut may suddenly feel tighter.
Simply forcing the screw to rotate through the tight position is not recommended because hard particles can damage the precision raceway surfaces.
If contamination is suspected, inspect the seals, lubrication condition and exposed screw surface first. Clean the external screw surface before adding fresh lubricant.
5. Local Raceway Damage or Wear
Damage does not always occur evenly along the entire ball screw. In many machines, the axis repeatedly operates within the same working range. This means one section of the screw may experience considerably more load cycles than another.
Raceway pitting, indentation or localized wear can change the contact condition between the balls and raceways, producing abnormal resistance at a specific position.
Inspect the screw surface carefully under good lighting. Visible marks, rust spots, dents or unusual polishing patterns can indicate localized damage.
6. The Ball Nut or Return System Has a Problem
A ball nut contains a circulation system that continuously returns the balls as the nut moves along the screw. If a return tube, deflector or internal circulation component is damaged, ball circulation may become uneven.
Incorrect ball size, missing balls or damaged internal components can also produce abnormal resistance. In these cases, the tightness may appear periodically as the nut moves.
If the ball nut has been disassembled previously, special attention should be paid to whether the correct balls were reinstalled and whether the circulation components are positioned correctly.
7. Excessive Preload
Some ball screws use preload to reduce axial backlash and improve positioning rigidity. However, excessive preload increases internal contact force.
Normally, excessive preload produces relatively consistent resistance rather than only one tight spot. However, when excessive preload is combined with a slightly bent screw or installation misalignment, the resistance may become much more noticeable at certain positions.
High motor current, abnormal temperature rise and difficult manual rotation may all indicate excessive internal load.
How to Find the Cause Step by Step
When a ball screw becomes tight at a specific position, replacing the complete assembly immediately is usually unnecessary. A systematic inspection can help separate installation problems from actual component damage.
| Check | What to Look For |
|---|---|
| Manual movement | Confirm whether the tight position is repeatable. |
| Lubrication | Check for dry grease, contamination or insufficient lubrication. |
| Mounting alignment | Check nut housing and support unit alignment. |
| Screw runout | Use a dial indicator to identify shaft bending. |
| Raceway surface | Look for dents, rust, pitting or abnormal wear. |
| Ball nut | Listen for abnormal noise and check circulation condition. |
Should You Continue Using a Ball Screw With a Tight Spot?
If the resistance is caused only by installation alignment, correcting the installation may restore normal operation without replacing the ball screw.
However, continuing to operate a screw with severe localized resistance can increase load on the motor, bearings, coupling and ball nut. If the problem is caused by raceway damage or shaft deformation, continued operation may accelerate wear.
For this reason, the cause should be identified before increasing motor torque or forcing the machine to continue running.
How to Prevent Position-Related Ball Screw Tightness
- Use proper alignment procedures during installation.
- Keep the screw shaft clean during assembly and operation.
- Lubricate the ball screw according to the working environment and operating frequency.
- Avoid impact or improper handling during transportation.
- Check screw runout before installing long ball screws.
- Inspect seals and protective covers regularly in contaminated environments.
Choosing a Replacement Ball Screw
If inspection confirms that the screw shaft or ball nut is damaged and replacement is necessary, the first step is to identify the basic specifications of the existing ball screw.
For standard industrial applications, information such as screw diameter, lead, overall length, nut type and accuracy requirement is usually enough to begin matching a replacement.
DLY supplies standard cold-rolled and precision ball screw assemblies for CNC machines, automation equipment and other linear motion systems. You can view our ball screw products for commonly used specifications.
Need Help Identifying the Problem?
If your ball screw becomes tight at a repeatable position, send us the screw diameter, lead, total length, ball nut model and photos or a short operating video. These details can help determine whether the problem is related to installation, shaft deformation or the ball screw assembly itself.
Contact DLY for ball screw selection, replacement and standard linear motion solutions.
