A complete ball screw drive system converts motor rotation into controlled linear movement. The ball screw is the main transmission component, but it cannot operate as an accurate machine axis by itself. It must work together with a motor, coupling, support bearings, linear guideway, moving table and control system.
When the motor rotates the screw shaft, recirculating balls transmit force between the screw and nut raceways. The nut moves along the screw, while the linear guideway supports the table and prevents unwanted rotation. The controller determines the travel distance, speed, acceleration and direction.
Main Components of a Ball Screw Drive System
The exact structure varies between machines, but a conventional motor-driven axis normally contains the following components.
| Component | Main Function | Important Selection Point |
|---|---|---|
| Servo or stepper motor | Provides rotational speed and torque | Torque, speed, inertia and control requirements |
| Coupling | Connects the motor shaft to the ball screw | Torque capacity, torsional rigidity and allowable misalignment |
| Fixed-side support | Locates the screw axially and supports radial and axial load | Bearing arrangement, preload and shaft-end accuracy |
| Support-side bearing | Supports the opposite screw end while allowing the selected thermal arrangement | Alignment, radial support and expansion requirements |
| Ball screw shaft | Provides the helical raceway and transmits rotational input | Diameter, lead, length, accuracy and critical speed |
| Ball nut | Converts screw rotation into linear travel | Nut type, circulation structure, preload and load rating |
| Nut housing or nut seat | Connects the ball nut to the moving table | Mounting rigidity and flange compatibility |
| Linear guideway | Supports the table and controls its travel path | Load, moment capacity, preload and alignment |
| Controller and feedback | Commands and monitors position, speed and acceleration | Open-loop or closed-loop control and feedback resolution |
The linear guideway carries radial load and moment load from the table. The ball screw should primarily transmit axial force. Using the ball screw as the only guide for the moving load can introduce radial force and moment into the nut, causing uneven wear and unstable operation.
How Does the Complete Ball Screw Drive System Work?
- The controller issues a movement command.
The command defines the required travel distance, direction, speed and acceleration. - The motor generates rotation.
A servo motor normally uses encoder feedback for closed-loop position and speed control. A stepper motor may operate open-loop or with feedback, depending on the system design. - The coupling transmits motor torque.
The coupling transfers rotation to the screw shaft. It can accommodate limited installation error, but it should not be used to compensate for severe motor-to-screw misalignment. - The support bearings locate the screw.
The fixed-side bearing arrangement controls axial position and supports thrust load. The support-side bearing stabilizes the opposite end according to the selected mounting arrangement. - The screw shaft rotates.
As the shaft rotates, the steel balls roll between the matching helical grooves of the screw and nut. - The ball nut moves linearly.
The rolling balls transmit axial force to the nut. Because the nut is connected to the table and prevented from rotating, it moves along the screw axis. - The balls recirculate inside the nut.
After leaving the loaded raceway, the balls pass through an internal deflector, end-cap passage or external return path and re-enter the load zone. - The guideway controls the table path.
The linear guide rails and blocks support the table, resist moments and maintain the intended direction of movement.
For a detailed explanation of the nut's internal return path, see How Does a Ball Screw Recirculation System Work?
How Are Ball Screw Speed, Lead, Force and Torque Related?
Linear Travel per Revolution
Ball screw lead is the theoretical distance travelled by the nut during one complete revolution of the screw.
For example, a ball screw with a 10 mm lead theoretically moves the nut 10 mm for each complete screw revolution.
Linear Speed
At 1,000 rpm, a 10 mm lead produces a theoretical linear speed of 10,000 mm/min. Actual permitted speed must also remain within the limits of the screw's critical speed, ball circulation, support arrangement, lubrication and machine structure.
Axial Force and Drive Torque
The ideal relationship between required screw torque and axial force can be expressed as:
Where:
- T = drive torque
- F = axial force
- L = ball screw lead
- η = mechanical efficiency
This equation is only a starting point. Motor selection must also include acceleration torque, bearing resistance, guideway resistance, seal drag, preload, external process force and gravity in a vertical axis.
Rotating-Screw vs. Rotating-Nut Drive Systems
Rotating Screw with Travelling Nut
This is the most common configuration. The motor rotates the screw while the ball nut is fixed to the moving table. It provides a practical structure for many CNC machines, automation units, packaging machines and positioning systems.
For long screws operating at high rotational speed, critical speed and shaft vibration become important. Screw diameter, unsupported length and bearing arrangement must be checked before determining the maximum operating speed.
Fixed Screw with Rotating Nut
In a rotating-nut system, the screw shaft remains stationary while the nut rotates. The rotating nut is supported in a bearing housing and driven by a belt, gear or other transmission arrangement.
This configuration can be useful for long-stroke, high-speed axes because the long screw does not rotate at high speed. However, the rotating-nut unit is more complex and its bearing, drive and lubrication arrangement must be designed as a complete assembly.
| Configuration | Rotating Component | Typical Consideration |
|---|---|---|
| Rotating screw | Screw shaft | Simple structure; critical speed must be checked for long shafts |
| Rotating nut | Ball nut unit | Suitable for some long-stroke, high-speed systems; more complex nut drive unit |
What Determines Ball Screw System Accuracy?
The accuracy of a complete drive system is not equal to the accuracy grade of the ball screw alone. Positioning error can originate from several components.
- Ball screw lead accuracy: Difference between theoretical and actual nut travel.
- Axial clearance: Relative movement between the screw and nut when the force direction reverses.
- Support-bearing play: Axial movement at the fixed-side bearing arrangement.
- Coupling deformation: Torsional deformation under changing torque.
- Screw and support stiffness: Elastic displacement under axial load.
- Thermal expansion: Change in screw length as operating temperature rises.
- Guideway accuracy: Deviation in the actual movement path of the table.
- Machine-frame deformation: Structural displacement under load.
- Feedback method: A motor encoder does not directly measure table position unless a separate linear feedback device is used.
DLY supplies rolled C7 and C10 ball screws for general motion requirements and ground C5, C3 and higher-precision options for applications requiring tighter control. The final system accuracy still depends on installation, supports, preload, temperature and control method.
How to Select a Ball Screw Drive System
Ball screw selection should begin with the complete operating conditions rather than only the screw diameter.
| Parameter | Why It Matters |
|---|---|
| Maximum and normal axial load | Affects diameter, nut type, load rating and expected fatigue life |
| Stroke and total screw length | Affect shaft length, buckling, critical speed and support arrangement |
| Required linear speed | Determines the required lead and rotational speed |
| Acceleration and duty cycle | Affect motor torque, heat and service life |
| Positioning requirement | Affects accuracy grade, preload, feedback and thermal control |
| Horizontal or vertical installation | Changes gravity load, braking and back-driving requirements |
| Mounting method | Affects critical speed, buckling strength and axial rigidity |
| Operating environment | Determines sealing, lubrication and corrosion-protection requirements |
DLY provides ball screws in different diameters, leads, nut structures and accuracy grades. Length and shaft-end machining can be matched to BK/BF, FK/FF or EK/EF support arrangements according to the customer's drawing.
The support components should be confirmed together with the shaft-end drawing. DLY ball screw support units include fixed-side and support-side arrangements for different screw sizes and mounting structures.
Common Ball Screw Drive System Problems
| Symptom | Possible Cause | First Check |
|---|---|---|
| Uneven movement or tight points | Misalignment, bent shaft, contaminated raceway or guideway error | Check alignment over the complete stroke |
| Position changes after direction reversal | Nut clearance, bearing play, coupling deformation or loose connection | Measure each source of lost motion separately |
| Abnormal noise | Poor lubrication, damaged balls, return-system problem or bearing damage | Identify whether noise comes from the nut, bearings or motor |
| Rising temperature | Excessive preload, high speed, unsuitable lubrication or misalignment | Check temperature, torque and lubrication condition |
| Screw vibration at high speed | Operating near critical speed, poor support or excessive runout | Verify screw speed, unsupported length and bearing arrangement |
| Motor overload | Incorrect torque calculation, binding, excessive load or aggressive acceleration | Compare actual current and torque with each operating stage |
Maintenance should include suitable lubrication, contamination control, inspection of supports and couplings, and monitoring of noise, temperature and positioning performance. If a fault appears, inspect the complete axis rather than replacing the ball nut without identifying the original cause.
Contact Us
Need help selecting a ball screw, nut type, accuracy grade or support arrangement? Send us your load, stroke, speed, installation method and shaft-end drawing.
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