What Preload Actually Does
Inside a ball screw, the balls run between the screw raceways and the nut raceways.
If there is clearance between the contact surfaces, changing the direction of the load can produce a small amount of free movement. That's what we normally associate with backlash.
Preload applies a controlled internal load to the ball screw so the contact surfaces remain engaged.
The practical result is:
- less internal clearance
- higher axial rigidity
- better behavior during direction reversal
- better resistance to small changes in external load
That sounds like an easy win.
It isn't.
Preload also increases the force between the balls and raceways. That means more friction, more running torque and potentially more heat.
So a ball nut shouldn't be specified with the highest possible preload just because the machine needs low backlash.
The right question is how much preload the application can actually use.
Backlash and Lead Accuracy Are Different Problems
This distinction is important when selecting a ball screw.
Lead accuracy tells you how closely the screw's actual travel matches its nominal travel over a given distance.
Backlash is unwanted movement when the direction of the load changes.
They're related to machine positioning, but they are not the same error.
For example, you can have a C5 ball screw with a backlash problem.
You can also have a C7 ball screw with a properly preloaded nut and very little internal clearance.
So if someone says:
“My machine has backlash, so I need C5.”
I'd stop there and ask how the backlash was measured.
If the problem is loose bearing support or a coupling that is slipping on the shaft, a higher ball screw accuracy grade isn't going to solve it.
Where the Backlash May Actually Be Coming From
This is where troubleshooting gets more useful than looking at the catalog.
Suppose the machine reverses direction and the table moves 0.05 mm before the indicator starts moving.
There are several places that 0.05 mm could be coming from.
Ball nut
The nut may have too much internal clearance, incorrect preload, or wear.
This is the part everyone checks first, but it isn't necessarily the problem.
Bearing support
If the screw can move axially inside the bearing arrangement, that movement will show up at the axis.
A good ball screw with a poor support arrangement is still going to give you a positioning problem.
Coupling
Check the coupling before ordering another screw.
A loose clamp, incorrect bore, damaged flexible element or poor shaft engagement can introduce measurable movement.
Nut housing
The nut itself may be fine while the housing moves relative to the machine.
That is especially easy to miss on machines using fabricated or bolted structures.
Linear guides and machine structure
If the gantry or table moves under changing cutting forces, the dial indicator will show movement even though the ball screw has very little internal backlash.
The machine doesn't know where the error came from.
It only knows that the axis moved.
How I Would 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 fairly simple.
Put a dial indicator against the moving axis. Move the axis in one direction and establish a reference point. Then reverse the direction by a small amount.
If the motor turns but the machine doesn't immediately respond, 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.
The important part is not to assume the first component you suspect is automatically guilty.
If the backlash is different at different positions along the travel, that's another useful clue. If it is fairly constant, the source may be a different part of the transmission.
Preload Is a Trade-Off
This is the part that gets lost in simple ball screw explanations.
More preload can give you more rigidity and less internal clearance.
But more preload also means more friction.
That can lead to:
- Higher motor torque
- Higher operating temperature
- More heat in the nut
- Increased 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.
Imagine two machines.
The first is a small CNC router cutting wood and plastic. The moving mass is relatively low, cutting forces are moderate, and the machine doesn't need micron-level positioning.
The second is a precision machining center where reversal accuracy and structural rigidity are critical.
It would make little sense to specify the same preload simply because both machines use a 20 mm ball screw.
The screw may have the same diameter.
The application does not.
Don't Use Preload to Fix an Alignment Problem
This is one of the mistakes worth avoiding.
Suppose the screw and guide rail aren't properly aligned.
The ball nut may feel tight at one end of the travel and noticeably easier at the other.
Increasing preload isn't going to correct that.
It may actually make the problem worse.
Misalignment can increase:
- Running torque
- Friction
- Heat
- Nut wear
- Motor load
If the axis feels different depending on where it is in the travel, I'd investigate alignment before changing the 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 needs to be supported and aligned properly before you start worrying about fine preload adjustments.
Preload Also Changes the Motor Load
This matters more than people sometimes expect.
A preloaded nut doesn't run with the same resistance as a lightly loaded or non-preloaded arrangement.
The actual running torque depends on several factors, including:
- 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 matter on a small stepper-driven CNC.
It can also matter on a vertical Z-axis, where the motor is already working against gravity.
So when changing preload, I would check the motor torque margin rather than looking only at backlash.
Single Nut or Double Nut?
A double-nut ball screw is often used when higher rigidity and very low backlash are required.
The two nuts can be adjusted relative to each other to create the required preload.
That can provide a very rigid assembly, but it also adds cost, length and adjustment requirements.
It doesn't automatically make a double nut the right answer for every CNC machine.
For many general-purpose applications, a properly selected preloaded single nut is sufficient.
The choice depends on the required rigidity, external load, positioning performance and operating conditions.
Don't Confuse Backlash With Repeatability
Here's a simple example.
Suppose you command the axis to 100.000 mm several times and it repeatedly stops around 100.020 mm.
The axis isn't 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 the reversal behavior.
But even here, don't 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, or load-dependent.
That information is much 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're 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 doesn't 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 is going to fix it.
This distinction can save quite a bit of money when specifying a replacement screw.
Long-Term Wear Is Another Story
A newly installed ball screw with excessive backlash and a ball screw that develops backlash after years of operation should not be diagnosed 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 are working 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
- Changes in backlash over time
If the machine has been running for years, don't 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's another problem that often gets mixed into the same discussion.
A machine may be accurate when it is cold and gradually shift after several hours.
That's 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, I'd investigate thermal behavior before changing the preload.
This is particularly 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 isn't one answer.
For a general-purpose CNC router, excessive preload may simply add friction and heat without giving the machine 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 rather than 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'd follow:
Measure it first.
Don't diagnose backlash by feel.
Check the coupling.
Make sure the motor and screw are actually locked together.
Check the bearing support.
Look for axial movement.
Check the nut mounting.
The nut housing should not be moving relative to the machine.
Check alignment.
Look for tight spots or changing resistance along the travel.
Check the preload specification.
Make sure the installed nut is actually the configuration the machine requires.
Then check the ball screw for wear.
Especially if the machine has already 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're ordering a new screw, don't just send:
“SFU2005, 800 mm, please quote.”
That gives the supplier the product dimensions, but not much about the application.
A more useful request would be:
- 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/reversal performance
If this is a replacement, add the 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 actually help diagnose the problem instead of simply quoting another screw.
The Point Is Not to Make the Ball Screw as Tight as Possible
This is where I would leave the discussion.
Preload is useful because it reduces internal clearance and increases rigidity.
Too little preload can leave the machine with unwanted movement.
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 won't solve it anyway.
So when a CNC axis develops backlash, don't 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's a ball nut.
Sometimes it's a bearing.
Sometimes it's a coupling that needs five minutes with a hex key.
And sometimes the ball screw was never the problem in the first place.
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