CNC Backlash: Why the Ball Screw Is Often Not the Real Problem

What Preload Really Does

Inside a ball screw, the balls travel between the screw raceway and the nut raceway.

If there is clearance between these contact surfaces, a load reversal can cause a small amount of free movement before the axis responds. This is what many people call backlash.

Preload creates a controlled internal force that keeps the balls in contact with the raceways.

In practice, preload can provide:

• Less internal clearance

• Higher axial rigidity

• More consistent motion during direction changes

• Better resistance to small load changes

That sounds like a simple improvement.

But it is not free.

Preload also raises the contact force between balls and raceways. The result can be higher friction, higher running torque, and more heat. A ball nut should not be selected with the highest possible preload just because the machine needs low backlash.

The better question is: how much preload can this application actually use?

Lead Accuracy and Backlash Are Not the Same Thing

This point is often missed when choosing a ball screw.

Lead accuracy describes how closely the screw’s actual travel matches the commanded travel over a given distance.

Backlash describes unwanted movement when the load direction reverses.

Both affect positioning, but they are different errors.

A C5 ball screw can still have a backlash problem.

A C7 ball screw with a properly preloaded nut can have very little internal clearance.

So when someone says, “My machine has backlash, so I need a C5 screw,” the first question should be: how was that backlash measured, and where is it coming from?

If the real issue is a loose bearing support or a coupling slipping on the shaft, a higher accuracy grade will not solve it.

Where Lost Motion Can Hide

This is where troubleshooting becomes more useful than reading a catalog.

Imagine the machine reverses direction, and the table moves 0.05 mm before the indicator starts to move. That 0.05 mm could come from several places.

Ball nut

The nut may have too much internal clearance, incorrect preload, or wear. This is the first place many people check, but it is not always the guilty part.

Bearing support

If the screw can move axially inside the bearing arrangement, that movement will appear at the axis. A good ball screw with a poor support design will still produce positioning problems.

Coupling

Check the coupling before ordering a new screw. A loose clamp, wrong bore size, damaged flexible element, or poor shaft engagement can create measurable lost motion.

Nut housing

The nut itself may be fine while the housing moves relative to the machine. This is easy to miss on machines with bolted or fabricated structures.

Linear guides and machine structure

If the gantry or table deflects under changing cutting forces, the indicator will show movement even when the ball screw has very little internal backlash.

The machine does not know where the error came from. It only knows that the axis moved.

A Simple Way to Check a Backlash Complaint

If someone tells me an axis has 0.05 mm backlash, I want to know how that number was obtained.

A useful test is simple.

Mount a dial indicator against the moving axis. Move the axis in one direction and set a reference. Then reverse the direction by a small amount.

If the motor turns but the machine does not respond immediately, there is mechanical clearance somewhere in the transmission.

Then you have to find it.

Check the coupling.

Check the bearing support.

Check the nut mounting.

Check the ball nut.

Check the guide system.

Do not assume the first suspect is automatically the problem.

Also pay attention to whether the backlash changes along the travel. If it is different at different positions, that is a useful clue. If it is fairly constant, the source may be elsewhere in the transmission.

Preload Is Always a Trade-Off

More preload can increase rigidity and reduce internal clearance.

But more preload also means more friction.

That can lead to:

• Higher motor torque

• Higher operating temperature

• More heat in the nut

• Faster wear

• More difficult high-speed operation

For a precision machining center, the extra rigidity may be worth it.

For a light CNC router, it may not be.

Consider two machines.

The first is a small CNC router cutting wood and plastic. The moving mass is low, cutting forces are moderate, and micron-level positioning is not required.

The second is a precision machining center where reversal accuracy and structural rigidity are critical.

It makes little sense to specify the same preload just because both machines use a 20 mm ball screw.

The screw diameter may be the same.

The application is not.

Do Not Use Preload to Fix Alignment Problems

This is a common mistake.

Suppose the screw and guide rail are not properly aligned.

The ball nut may feel tight at one end of travel and much freer at the other.

Increasing preload will not correct that.

It may make it worse.

Misalignment can increase:

• Running torque

• Friction

• Heat

• Nut wear

• Motor load

If the axis feels different depending on its position, check alignment before changing preload.

Run the axis slowly.

Feel for tight spots.

Check the mounting surfaces.

Check the relationship between the screw and the linear guides.

A ball screw must be supported and aligned properly before fine preload adjustments matter.

Preload Changes Motor Load Too

This matters more than many people expect.

A preloaded nut does not run with the same resistance as a lightly loaded or non-preloaded arrangement.

Actual running torque depends on several factors:

• Screw lead

• Screw diameter

• Preload

• External load

• Lubrication

• Speed

• Nut design

If you replace a low-preload nut with a heavily preloaded one, the machine may still have the same theoretical travel and speed, but the motor now has less torque margin.

That can be a problem on a small stepper-driven CNC.

It can also matter on a vertical Z-axis, where the motor is already working against gravity.

When changing preload, check motor torque margin, not just backlash.

Single Nut or Double Nut?

A double-nut ball screw is often used when high rigidity and very low backlash are required.

The two nuts can be adjusted relative to each other to create the required preload.

That can produce a very rigid assembly, but it also adds cost, length, and adjustment requirements.

A double nut is not automatically the right answer for every CNC machine.

For many general-purpose applications, a properly selected preloaded single nut is enough.

The choice depends on required rigidity, external load, positioning performance, and operating conditions.

Do Not Confuse Backlash With Repeatability

Here is a simple example.

Suppose you command the axis to 100.000 mm several times, and it repeatedly stops around 100.020 mm.

The axis is not very accurate, but it may be quite repeatable.

Now approach the same position from opposite directions and get:

• 100.020 mm

• 99.980 mm

That difference tells you something about reversal behavior.

But even here, do not automatically call the entire difference “ball screw backlash.”

Servo tuning, friction, thermal effects, guide preload, and structural movement can all affect the result.

For a useful diagnosis, you need to know whether the error is:

• Position-dependent

• Direction-dependent

• Temperature-dependent

• Load-dependent

That information is more valuable than a single backlash number.

What About C5 vs C7?

This is where preload and accuracy grade often get mixed together.

C5 and C7 describe the ball screw’s accuracy grade. Preload describes the internal loading condition of the nut.

They are different specifications.

For example, you can have a C5 ball screw with a preloaded nut.

You can also have a C7 ball screw with a preloaded nut.

If the machine has backlash, moving from C7 to C5 does not automatically solve the problem.

If the problem is lead accuracy, then the C5/C7 decision matters.

If the problem is internal clearance, preload and nut configuration matter more.

If the problem is a loose bearing support, neither C5 nor C7 will fix it.

This distinction can save a lot of money when specifying a replacement screw.

Wear Changes the Diagnosis

A newly installed ball screw with excessive backlash and a ball screw that develops backlash after years of operation should not be judged in the same way.

If the machine was originally stable and gradually developed more reversal movement, wear becomes a much stronger possibility.

The ball nut and screw raceways work under load every time the axis moves. Lubrication, contamination, load, speed, alignment, and duty cycle all affect service life.

Signs worth checking include:

• Increasing reversal error

• Increasing running noise

• Higher motor load

• Uneven movement

• Reduced rigidity

• Backlash that changes over time

If the machine has been running for years, do not assume an adjustment will bring the assembly back to its original condition.

Sometimes the nut has simply reached the end of its useful service life.

Thermal Expansion Can Look Like a Mechanical Problem

There is another issue that often gets mixed into the same discussion.

A machine may be accurate when cold and gradually shift after several hours.

That is not conventional backlash.

The ball screw, bearings, machine structure, and surrounding components all change temperature during operation.

A long steel ball screw can expand enough to create measurable positioning error.

If the error changes significantly between a cold machine and a fully warmed-up machine, investigate thermal behavior before changing preload.

This is especially important on:

• Long-travel CNC axes

• High-speed machines

• Precision machining equipment

• High-duty-cycle automation

Sometimes the machine needs thermal compensation or a different feedback strategy rather than a tighter ball nut.

So How Much Preload Do You Need?

There is no single answer.

For a general-purpose CNC router, excessive preload may add friction and heat without giving a useful performance benefit.

For a precision machining system, higher rigidity and low reversal error may justify a more heavily preloaded configuration.

A useful way to think about it is:

Low preload

Good when low friction, high speed, and low motor load are important.

Moderate preload

A practical choice for many CNC and automation applications where rigidity and running efficiency both matter.

Higher preload

More appropriate when rigidity and reversal performance are important enough to justify the additional friction and heat.

The actual preload value should come from the ball screw manufacturer’s specification, not from a generic rule.

If Your CNC Has Backlash, Check These Before Replacing the Screw

If I were standing beside the machine, this is roughly the order I would follow:

1. Measure it first.
   Do not diagnose backlash by feel.

2. Check the coupling.
   Make sure the motor and screw are actually locked together.

3. Check the bearing support.
   Look for axial movement.

4. Check the nut mounting.
   The nut housing should not move relative to the machine.

5. Check alignment.
   Look for tight spots or changing resistance along the travel.

6. Check the preload specification.
   Make sure the installed nut is actually the configuration the machine requires.

7. Then check the ball screw for wear.
   Especially if the machine has accumulated significant operating hours.

This approach is usually more useful than immediately ordering a replacement ball screw.

What Information Should You Give a Ball Screw Supplier?

If you are ordering a new screw, do not just send:

“SFU2005, 800 mm, please quote.”

That gives the supplier dimensions, but not much about the application.

A more useful request would include:

• Screw diameter: 20 mm

• Lead: 5 mm

• Travel: 800 mm

• Moving load: 40 kg

• Maximum speed: 10 m/min

• Installation: Horizontal

• Required positioning accuracy: ±0.02 mm

• Motor: 750 W servo

• Duty cycle: 8 hours/day

• Required backlash or reversal performance

If this is a replacement, add information from the existing machine:

• Current backlash

• Operating hours

• Existing preload

• Running temperature

• Noise or vibration

• Whether the error changes along the travel

Now the supplier can help diagnose the problem instead of simply quoting another screw.

The Goal Is Not to Make the Ball Screw as Tight as Possible

Preload is useful because it reduces internal clearance and increases rigidity.

Too little preload can leave unwanted movement in the machine.

Too much preload can create friction, heat, motor load, and unnecessary wear.

And if the backlash is actually coming from a coupling, bearing support, or machine structure, changing the ball nut will not solve it.

So when a CNC axis develops backlash, do not start with:

“Which ball screw should I buy?”

Start with:

“Where is the movement actually coming from?”

Once you know that, the solution is usually much clearer.

Sometimes it is a ball nut.

Sometimes it is a bearing.

Sometimes it is a coupling that needs five minutes with a hex key.

And sometimes the ball screw was never the problem in the first place.