How to choose ball screw supports?
How to choose ball screw supports? This is a question frequently asked by many customers. As a supplier specializing in the production of ball screw supports, we have found through communication with customers that many people experience reduced running accuracy and shortened service life of ball screws due to selecting the wrong support method. Ball screw supports act as the "stable foundation" for the screw, and their suitability directly impacts the transmission efficiency of the screw and the overall performance of the equipment. Today, we will discuss in detail how to select ball screw supports.
First, select based on load characteristics: Match the "load requirements" of the ball screw
1. Axial load-dominated scenarios: Prioritize angular contact ball bearing supports
When the ball screw primarilys axial loads (e.g., the feed axis of a precision machine tool) and the load value does not exceed 70% of the bearing's rated axial load capacity, angular contact ball bearings are the ideal choice. These bearings can counteract axial movement by adjusting the preload, and when installed in a face-to-face or back-to-back configuration, they can withstand bidirectional axial loads, ensuring axial rigidity during screw operation.
For example, in a CNC lathe where the ball screw's axial load is approximately 5000N, a pair of 7008AC angular contact ball bearings (rated axial load 8000N) are selected and installed in a back-to-back configuration. By adjusting the preload to control the axial clearance within 0.01mm, feed accuracy is significantly improved.
2. Scenarios with large radial loads: Combination of deep groove ball bearings and thrust bearings
If the ball screw is subjected to significant radial loads (e.g., in cantilevered installations), a single angular contact ball bearing may fail due to insufficient radial load capacity. In such cases, a combination of deep groove ball bearings (for radial loads) and thrust bearings (for axial loads) can be used. The radial rated dynamic load capacity of the deep groove ball bearings must be at least 30% higher than the actual radial load.
For a ball screw in an automated sorting device, where the cantilever length reaches 500 mm and the radial load is approximately 1500 N, a combination of 6206 deep groove ball bearings (with a radial rated dynamic load capacity of 19500 N) and 51106 thrust bearings is selected. This configuration meets the radial support requirements while ensuring axial positioning accuracy.
Second, select based on precision requirements: meet the "precision grade" of the ball screw.
1. High-precision scenarios (C5 grade and above): select P4-grade bearings with rigid support seats.
For applications such as semiconductor equipment and precision measuring instruments that require lead screw positioning accuracy ≤0.01mm/m, support bearings must be selected with P4-grade precision (rotational accuracy ≤0.005mm), paired with rigid support seats made of cast iron (such as flange seats with reinforcing ribs) to minimize accuracy loss caused by support deformation.
The ball screw support of a lithography machine uses P4-grade angular contact ball bearings and an integrally forged support base. Through precision grinding, the coaxiality of the bearing seat hole is ensured to be ≤0.002mm, enabling the ball screw's repeat positioning accuracy to reach ±0.0005mm.
2. General precision applications (C5-C7 grade): P5-grade bearings with standard support bases can be selected.
In general machine tools, conveying equipment, and other applications with low precision requirements (positioning accuracy ≤0.05mm/m), P5-grade bearings (rotational accuracy ≤0.01mm) with aluminum alloy or cast iron standard support bases can meet the requirements, eliminating the need for excessive pursuit of high-precision configurations and reducing costs.
A packaging machine's ball screw uses C7-grade precision, paired with P5-grade deep groove ball bearings and stamped steel plate support brackets. During operation, positioning errors remain stable within 0.03 mm/m, fully meeting the equipment requirements.
Third, select based on installation space: adapt to the "structural constraints" of the equipment.
1. Limited space scenarios: choose compact support units.
When the equipment installation space is limited (such as small modules or medical devices), integrated compact support units can be selected. These units integrate bearings, bearing seats, and seals into a single unit, reducing axial dimensions by 30%-50% compared to traditional combinations.
For example, the ball screw of a minimally invasive surgical robot has an installation space diameter of only 30 mm. A compact support unit with a diameter of 25 mm was selected, and through an integrated design, precise alignment between the bearing and the ball screw was achieved, meeting the equipment's miniaturization requirements.
2. Large span installation scenarios: Add intermediate supports
When the lead screw length exceeds 3 meters or the length-to-diameter ratio exceeds 30, intermediate auxiliary supports must be added to prevent the lead screw from bending due to its own weight. Intermediate supports typically use sliding bearings or self-aligning ball bearings, allowing for minor radial displacement while limiting excessive deflection (usually controlled within L/1000, where L is the support spacing).
Fourth, select based on the operating environment: resisting external "adverse effects"
1. Harsh environments (dust, humidity): Select sealed supports and corrosion-resistant materials.
In dusty workshops (e.g., woodworking machinery, mining equipment) or humid environments (e.g., food processing, cleaning equipment), support bearings must be equipped with double-lip seals (dust and waterproof rating IP65 or above), and bearing housings should be made of stainless steel (e.g., 304) or undergo electrophoretic corrosion protection treatment. (salt spray test ≥500 hours).
The ball screw support of a certain aquatic product processing equipment uses a stainless steel bearing housing with rubber seals. After two years of use in an environment with prolonged exposure to moisture, no rust or sticking occurred.
2. High-temperature environments (>80°C): Select high-temperature-resistant bearings and lubricants.
When the operating temperature exceeds 80°C, high-temperature-resistant bearings (e.g., high-temperature bearing steel SUJ2 + nitriding treatment, capable of withstanding temperatures up to 200°C) must be selected. Lubricants should use lithium-based high-temperature grease (dropping point ≥180°C) to prevent accelerated support wear due to lubrication failure.
A ball screw in a plastic molding machine operates at 120°C. The support bearings are treated with high-temperature nitriding and paired with polyurea-based grease, achieving an operational lifespan of over 8,000 hours-far exceeding the 3,000-hour lifespan of standard bearings.
Fifth, select based on preload method: control the "rigidity and clearance" of the support.
1. High-precision transmission: select constant pressure preload or constant position preload.
Constant pressure preloading applies a constant preload force via springs or disc washers, suitable for applications with significant speed variations (e.g., spindle drives), with preload fluctuations controllable within ±5%; position-based preloading achieves a fixed preload by grinding the inner or outer ring steps of the bearing, suitable for applications with extremely high precision requirements (e.g., coordinate boring machines), with preload deviations ≤3%.
A precision grinding machine uses position-based preloading for its ball screw, achieving precise control of the preload force at 200 N by grinding the outer ring of the bearing. The reverse clearance of the screw is ≤0.001 mm, meeting the precision requirements for mirror grinding.
2. Low-friction applications: Select bearings with clearance support
For equipment with low operational resistance requirements (e.g., automatic door drives, lightweight conveyors), bearings with a certain amount of clearance can be used to reduce bearing friction torque (typically controlled below 0.5 N・m). However, the clearance must not exceed 0.02 mm to avoid affecting transmission smoothness.
In a certain automatic sliding door, a clearance-type deep groove ball bearing was selected for the ball screw support, reducing the friction torque to 0.3 N·m, making the door operation more lightweight. Additionally, the radial movement of the screw was controlled through track-assisted support.
Summary
Selecting ball screw supports is fundamentally about "balancing and matching multiple factors"-load characteristics, precision requirements, installation space, operating environment, and preload methods must be considered to find the most suitable solution. Incorrect selection may result in loss of screw precision, reduced service life, or even equipment failure.
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