| Criterion | Ball screw | Trapezoidal lead screw | Rack |
|---|---|---|---|
| Efficiency | 90–98% | 26–46% | 85–95% |
| Accuracy (typical) | ±10–20 µm | ±50–100 µm | ±20–50 µm |
| Repeatability | ±0.02 mm (BZT standard) | ±0.05–0.1 mm | ±0.03–0.05 mm |
| Maximum speed | >200 mm/s | 50–100 mm/s | >300 mm/s |
| Wear | minimal (with correct lubrication) | higher (sliding friction) | medium (tooth wear under high load) |
| Backlash | practically zero (with preload) | 0.1–0.5 mm | 0.05–0.2 mm |
| Service life | 8000–20,000+ h | 3000–8000 h | 5000–15,000 h |
| Optimum travel | <2000 mm | <1000 mm | >2000 mm |
| Purchase cost | medium to high | low | medium |
A ball screw (also called a ball screw assembly) is the feed drive that converts the rotary motion of the motor into a precise linear movement of the axis in professional CNC gantry milling machines. Instead of sliding friction, as with the trapezoidal lead screw, it transmits the force via circulating steel balls. The result: an efficiency of 90 to 98% (compared with 26 to 46% for trapezoidal screws), a repeatability of around ±0.02 mm and significantly less wear. On BZT CNC gantry milling machines, the ball screw is therefore used in almost all series; on very large-format machines it is supplemented by rack drives on the long axes.
In this article we explain how a ball screw works, why the ball nut and its preload decide accuracy, how ball screw assemblies, trapezoidal lead screws and racks differ, and which BZT series brings the right drive technology for which application.
How does a ball screw work?
A ball screw consists of three main components: the screw with its thread, the ball nut with a corresponding internal thread, and the steel balls that circulate between the two in precisely shaped raceways. When the screw turns, the balls roll along the thread and push the nut axially along the screw. Special return channels in the nut ensure that the balls are fed back in a closed circuit.
The decisive difference from the classic trapezoidal lead screw lies in the way the force is transmitted: the ball screw works with rolling friction instead of sliding friction. As a result, much less energy is lost as frictional heat, the mechanism runs more easily and the drive responds more directly to control commands. It is precisely this rolling motion that is the reason for the typical advantages of the ball screw assembly in CNC technology.
Efficiency: Why the Ball Screw Is More Efficient
Efficiency describes how much of the rotational energy introduced is actually converted into usable linear motion. This is where the ball screw assembly shows its greatest technical advantage:
- Ball screw: 90 to 98% efficiency; modern high-precision versions reach up to 98%.
- Trapezoidal lead screw: 26 to 46% efficiency due to the high sliding friction.
In practice this means: the ball screw converts motor power into feed more efficiently, generates less heat and reduces energy losses considerably. Less heat generation also has a positive effect on dimensional accuracy, because the screw expands less thermally. For the spindles and motors, an efficient drivetrain means that the available power arrives where it is needed, at the tool.
The high efficiency also has an economic side: lower friction means lower energy use per milled part and less thermal load on the entire mechanism. Over the service life of a machine, this advantage adds up to noticeably lower operating costs and more stable manufacturing quality, because the machine stays thermally more constant over the working day.
Accuracy and Repeatability
For machining, it is not only important that an axis moves to a position, but that it reaches this position reproducibly again. Here are the verifiable values:
- Positioning accuracy of the ball screw: typically in the two-digit micrometre range (10 to 50 µm), and with high-precision versions of accuracy grade C1 up to ±3 µm over 300 mm measuring length.
- Repeatability at BZT: machines with ball screws and Bosch Rexroth linear guides achieve around ±0.02 mm (20 µm); the programmable step size of the controller is 0.0025 mm.
- Trapezoidal lead screw: significantly lower positioning accuracy and higher tolerances due to sliding friction.
The high accuracy of the ball screw only unfolds its full potential in combination with precise linear guides and a suitable controller. In the industrial series such as the BZT PFH series, Bosch Rexroth linear guides are fitted as standard, which together with the high-quality ball screw assemblies make the stated repeatability possible in the first place.
Pitch, Speed and Critical Speed
The pitch indicates how many millimetres the nut travels per screw revolution, stated in mm per revolution. It is the central lever between speed and resolution:
- A larger pitch means higher travel speed at the same screw speed, but a coarser position resolution.
- A smaller pitch gives finer resolution and more feed force, but less speed.
- Typical industrial pitches are 5 to 10 mm per revolution.
The feed rate results from pitch times screw speed. Ball screws allow feed rates of well over 200 mm/s.
The screw speed is limited at the upper end by the so-called critical speed. At too high a screw speed, a long, slender screw begins to vibrate through its natural oscillation and can deflect. How high this limit lies depends on the screw diameter, the unsupported length and the bearing arrangement. A long screw with a small diameter has a significantly lower critical speed than a short, thick screw. In practice, the CNC controller ensures that the maximum screw speed does not exceed this limit.
The Ball Nut and Preload Against Backlash
The ball nut is the component that translates the rotating motion of the screw into a straight movement of the axis. Its quality and above all its preload decisively determine the precision of the entire drive.
Without preload, tiny gaps remain between the balls and the thread raceway. When the axis changes direction, the drive first has to bridge these gaps before the nut moves again. This so-called backlash leads to positioning deviations that show up in the milled pattern as offset or contour errors.
Preload practically eliminates this play. Two methods are common:
- Double nuts: two nut halves are braced against each other with opposing load, so that the balls permanently rest against the thread flanks.
- Spring-preload systems: a defined spring pressure holds the balls against the raceway without play.
The effect: reproducible positions even after a change of direction and a higher rigidity of the entire drive. The correct dosage is important, because too high a preload increases friction, generates more heat and accelerates wear. The correct preload is therefore always a compromise between freedom from play and service life.
Ball Screw, Trapezoidal Lead Screw or Rack Compared
Three types of drive are available in CNC machine building: the ball screw (ball screw assembly), the trapezoidal lead screw and the rack with pinion. Which one is right depends above all on the travel, the required accuracy and the budget. The ball screw and the trapezoidal lead screw both work on the screw principle, but differ fundamentally in the type of friction. The rack follows a completely different principle: a pinion meshes directly with a fixed toothing and is therefore not limited by a critical speed. The following table compares the verifiable key data:
| Criterion | Ball screw | Trapezoidal lead screw | Rack |
|---|---|---|---|
| Efficiency | 90–98% | 26–46% | 85–95% |
| Accuracy (typical) | ±10–20 µm | ±50–100 µm | ±20–50 µm |
| Repeatability | ±0.02 mm (BZT standard) | ±0.05–0.1 mm | ±0.03–0.05 mm |
| Maximum speed | >200 mm/s | 50–100 mm/s | >300 mm/s |
| Wear | minimal (with correct lubrication) | higher (sliding friction) | medium (tooth wear under high load) |
| Backlash | practically zero (with preload) | 0.1–0.5 mm | 0.05–0.2 mm |
| Service life | 8000–20,000+ h | 3000–8000 h | 5000–15,000 h |
| Optimum travel | <2000 mm | <1000 mm | >2000 mm |
| Purchase cost | medium to high | low | medium |
This results in clear recommendations for use in CNC gantry milling machines:
- Ball screw: the standard for precision machines in wood, plastic and aluminium, ideal for travel up to about 2000 mm.
- Rack: sensible for very large travel over 2000 mm, because a continuous screw tends to deflect and vibrate here.
- Trapezoidal lead screw: economical only for simple applications without high precision requirements.
Which BZT series uses which drive technology?
At BZT, the ball screw is the central feed drive in most series, from the entry-level machine to the industrial machining centre. An exception is the large, wide-frame series, whose long axes are driven by racks for reasons of stability. There are differences in screw dimension, linear guide and nut design, which scale with the requirements of the respective machine class.
| Series | Drive technology | Application |
|---|---|---|
| PFI / PFK | Ball screw, compact design | Beginners, education |
| PF / PFJ | Ball screw, preloaded nut | Trades, workshop |
| PFA | Rack in X/Y, ball screw in Z | Sign making, large areas |
| PFE | Ball screw, optimised linear guides | Professional entry level, aluminium |
| PFU / PFU-S | Ball screw, reinforced; Bosch Rexroth guides (PFU-S) | Universal to industrial |
| PFH | Ball screw, Bosch Rexroth guides as standard | Industry, precision manufacturing |
A special feature of the higher-grade series is the driven nut in the X and Y axes: instead of turning the long screw, a motor turns the nut while the screw stands still. This reduces play further and avoids the limitation by the critical speed, a principle that only makes economic sense in combination with ball screws. On very large machines such as the industrial series or large-format machines, the rack is also used for the long main axes, while the shorter Z axis continues to work with a ball screw.
Which configuration suits your application cannot be answered in general terms, but depends on material, workpiece size and tolerance requirements. The series comparison gives a structured overview, and our technical advice is best placed to clarify the specific design.
Maintenance and Lubrication of the Ball Screw
The long service life of a ball screw assembly is not a matter of course, but the result of correct lubrication and cleanliness. Contamination is the most common cause of premature ball screw assembly failures.
- Lubrication interval: every 500 to 2000 operating hours, depending on intensity of use.
- Lubricant: synthetic greases are preferred because of their better temperature stability; mineral oil greases are suitable for air-conditioned workshops.
- Cleaning: clean thoroughly before lubricating, so as not to carry dirt into the raceways.
- Service life: 8000 to 15,000 operating hours in normal workshop use, over 20,000 hours with good maintenance.
Effective dust extraction extends the service life further, because less dust and fewer chips reach the screw. A suitable cyclone separator keeps the drive components clean. Wear announces itself through increased play, uneven running or noises; at the latest then, you should have the ball nut checked.
Pros and Cons: The Ball Screw at a Glance
Advantages:
- Very high efficiency of 90 to 98%
- High repeatability of around ±0.02 mm
- Low wear and long service life with correct lubrication
- Practically free of play thanks to preload of the ball nut
- Low heat generation, and therefore more stable dimensional accuracy
Limitations:
- Higher purchase cost than the trapezoidal lead screw
- Limited screw speed with long, slender screws (critical speed)
- Inferior to the rack for very large travel over 2000 mm
- Sensitive to contamination, regular lubrication necessary
Frequently Asked Questions
Why does BZT fit ball screws as standard instead of trapezoidal lead screws?
For reasons of precision and efficiency. Ball screws achieve the repeatability of around ±0.02 mm that professional CNC work requires, whereas trapezoidal screws manage only about ±0.05 to 0.1 mm. In addition, ball screw assemblies work with an efficiency of up to 98% instead of around 40%, generate less heat and wear more slowly. This saves tools, maintenance and operating costs.
What does preload against backlash mean and why is it important?
Without preload, small gaps remain between the screw thread and the ball nut. When the direction changes, the drive first has to bridge these gaps before the position is stable. With preload, usually via double nuts, the balls are permanently held under pressure against the thread flanks. In this way, positions remain reproducible at around ±0.02 mm even after a change of direction; without preload it would be more like ±0.1 to 0.2 mm.
How often does a ball screw need to be lubricated?
The lubrication interval is 500 to 2000 operating hours, depending on intensity of use. Synthetic greases are preferred because of their temperature stability; in air-conditioned workshops, mineral oil greases are also suitable. Thorough cleaning before lubricating is important, because dirt in the raceways is the main cause of premature failures.
How long does a ball screw last?
In normal workshop use, a ball screw reaches 8000 to 15,000 operating hours, and high-quality versions with good maintenance over 20,000 hours. For comparison, trapezoidal lead screws often last only 3000 to 8000 hours. Signs of wear are increased play of more than 0.02 mm, uneven running or noises.
Why can a ball screw not rotate arbitrarily fast?
At too high a screw speed, a long, slender screw begins to vibrate through its natural oscillation and can deflect; this is the so-called critical speed. It depends on diameter, unsupported length and bearing arrangement. A long screw of 2000 mm with a small diameter has a lower limit than a short, thick one. The CNC controller limits the screw speed so that this limit is not exceeded.
Can the ball screw be replaced by a rack if the machine gets larger?
Yes, with very large travel over 2000 mm this is usual. The long X and Y axes then receive a rack with pinion, while the shorter Z axis continues to work with a ball screw, because high precision over a short stroke is required here. This is the standard for large-format machines. A pure replacement without adaptation would reduce accuracy.
The Right Drive Technology for Your Project
The ball screw is the heart of every precise CNC gantry milling machine: high efficiency, reproducible accuracy and long service life make it the standard for demanding machining. Which screw dimension, nut design and linear guide is the right one for your material and workpiece size is best clarified in conversation. Compare the available CNC gantry milling machines, take a look at the series overview or contact our technical advice directly for a configuration tailored to your application. You can find out more about BZT as a German manufacturer on the page about the company.
0 comments