ball screw assembly - Product Range and Sourcing Guide

Ball Screw Assembly Components and Configurations

A ball screw assembly is a complete power transmission unit that converts rotary motion to linear motion using recirculating balls between a screw shaft and nut. Dongfeng Bearing provides complete ball screw assemblies that include the screw shaft, nut, balls, return system, seals, support bearings, and end machining. Understanding the components and their interactions helps buyers specify the correct assembly for their application and evaluate supplier proposals.

The assembly's performance depends on the interaction of all components: the screw shaft's lead accuracy determines positioning precision, the nut's ball circuit design affects load capacity and smoothness, the support bearings handle axial and radial loads, and the end machining ensures proper coupling to the drive motor. A weak link in any component compromises the entire assembly's performance.

Types of Ball Screw Assemblies

Ball screw assemblies are classified by their nut configuration, preload method, and accuracy grade. Dongfeng Bearing offers the following assembly types:

• SFU single-nut assembly (rolled C7): The most economical configuration, suitable for general CNC, automation, and 3D printing. Standard backlash of 0.01-0.05 mm. Available in shaft diameters 12-32 mm with leads of 4-10 mm.
• DFU double-nut assembly (rolled C7, zero backlash): Uses two single nuts with a precision spacer for zero-backlash operation. Recommended for applications requiring bidirectional positioning accuracy. Longer and more expensive than single-nut.
• SFE ground assembly (ground C5): Precision assembly with ground raceway for C5 accuracy. Used in CNC machine tools, semiconductor equipment, and metrology instruments. Available with double-nut preload.
• BSH miniature ground assembly (ground C5): Compact assembly for small-diameter screws (6-12 mm). Used in medical devices, optical instruments, and micro-positioning stages.

Selecting the Right Ball Screw Assembly

Choosing the correct ball screw assembly requires evaluating several application parameters:

1. Required positioning accuracy: Determine the maximum acceptable positioning error. If 0.05 mm per 300 mm is acceptable, C7 rolled (SFU/DFU) is sufficient. If 0.018 mm per 300 mm is required, C5 ground (SFE/BSH) is necessary.
2. Load calculation: Calculate the axial load including payload weight, guide rail friction, cutting forces (for machine tools), and acceleration/deceleration forces. Select a screw with a dynamic load rating that provides the desired L10 travel life (typically 10,000-50,000 km) at the calculated equivalent load with a safety factor of 1.5-2.0.
3. Speed and critical speed: The maximum rotational speed is limited by the screw's critical speed (resonance) and the DN value (diameter × RPM). For screws over 1500 mm long, critical speed may be the limiting factor. Use a larger diameter or intermediate support to increase critical speed.
4. Backlash requirements: If the application involves bidirectional positioning (CNC machining), select a double-nut preloaded assembly (DFU or SFE with double nut). If backlash is acceptable (manual positioning, unidirectional load), a single-nut SFU is more economical.
5. Drive torque calculation: Calculate the required motor torque based on the axial load, lead, and efficiency (90%+ for ball screws). Verify that the selected motor and coupling can deliver the required torque with margin.
6. Environmental considerations: For contaminated environments (machining, woodworking), specify seals and scrapers. For corrosive environments, specify stainless steel. For cleanroom applications, specify low-outgassing grease and sealed nut covers.

Support Bearing Configurations

The support bearing configuration is a critical part of the ball screw assembly that affects rigidity, accuracy, and thermal behavior:

The fixed-supported configuration is the most common, providing adequate rigidity for most applications while allowing thermal expansion. Fixed-fixed provides maximum rigidity but requires careful preload management to avoid excessive stress from thermal expansion. Fixed-free is limited to short screws (under 500 mm) and low-precision applications.

End Machining Specifications

The shaft ends must be machined to interface with the support bearings and motor coupling. Key end machining specifications include:

• Bearing journal: Precision ground to bearing inner race tolerance (typically h5 or h6). Surface finish Ra 0.4 μm or better. Shoulder for bearing positioning must be square to the journal within 0.005 mm.
• Threaded end: For locknut installation to secure the bearing inner race. Thread class 6g, with a washer surface for the locknut to seat against.
• Keyway or flat: For coupling engagement. Keyway width per ISO standard (3-8 mm depending on shaft size). Alternatively, a flat can be milled for clamp-type couplings.
• Coupling journal: Diameter matched to the coupling bore, typically the motor shaft diameter. This ensures coupling interchangeability with standard motor sizes.
• Overall length: The total shaft length equals the thread (effective travel) length plus the two end machining lengths. The end machining length is typically 50-100 mm per end, depending on the bearing and coupling configuration.

Specify the bearing type and dimensions, motor coupling type, required overall length, and any special features (such as a second flat for a rotary encoder) when ordering.

Calculating Travel Life and Load Capacity

The travel life of a ball screw assembly is determined by the dynamic load rating, the equivalent applied load, and the operating conditions. The L10 life (distance at which 90% of screws survive) is calculated as:

L10 = (C / P × fw)³ × 10⁶ (revolutions)

Where C is the basic dynamic load rating, P is the equivalent axial load, and fw is the load factor (1.0 for smooth operation, 1.2 for normal operation, 1.5 for impact/vibration). To convert revolutions to travel distance, multiply by the lead.

For preloaded assemblies, the equivalent load should account for the preload force. Our engineering team provides free load calculation and life estimation based on your application parameters.

Installation and Commissioning

Proper installation is critical for ball screw assembly performance and life:

• Shaft alignment: Align the screw parallel to the linear guide rails within 0.05 mm over the full length. Misalignment causes radial loads on the nut, increasing friction and wear. Use a dial indicator on the nut or shaft during alignment.
• Bearing preload: Set the angular contact bearing preload to the manufacturer's specification using precision ground spacers. Insufficient preload allows axial play; excessive preload increases friction and heat.
• Coupling alignment: Align the motor shaft and screw shaft within 0.05 mm radial and angular misalignment. Use a flexible coupling with zero backlash (bellows or disk type) for positioning applications. Avoid rigid couplings.
• Initial lubrication: Apply ball screw grease before first operation. Run the assembly through several full travel cycles at low speed to distribute grease. Check for smooth motion, unusual noise, or excessive heat.
• Run-in: For precision applications, run the assembly at 25% of maximum speed for 1-2 hours to allow components to seat and preload to stabilize. Recheck alignment and bearing preload after run-in.
• Encoder alignment: If using a rotary encoder on the screw end, align the encoder coupling carefully. Misalignment causes positioning error that varies with rotation angle, which is difficult to diagnose.

Maintenance and Troubleshooting

Regular maintenance extends ball screw assembly life and prevents premature failure:

Relubricate every 500-1000 km of travel or every 3-6 months. In contaminated environments, clean the shaft and relubricate more frequently. Use wiper seals to remove debris from the shaft before it enters the nut. For continuous-duty applications, consider forced oil lubrication with a filtration system.