External Circulation Ball Screw Asymmetric Nut Design
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Solution Overview
Problem
Conventional external circulation ball screws with small specifications face structural strength issues due to outward expansion of plastic molded members and require large nut diameters and deep mounting holes, leading to reduced loading capacity and stiffness, especially for small diameter rolling elements.
Innovation Solution
An external circulation ball screw design featuring a helical rolling groove on an elongate shaft, a nut with a through hole and rolling channel, and a U-shaped circulator with a return passage, where the advancing angle between the shaft center and return passage end point is between 70-80 degrees, optimizing nut diameter and stiffness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a tangential external circulation structure is used with inlet and outlet openings arranged opposite to each other (180 degrees apart), then the circulation path is simplified, but the nut requires a relatively large outside diameter and deep mounting holes, reducing stiffness and machining precision
Solution Approach 1:
The patent applies asymmetry by positioning the inlet opening and outlet opening at non-opposite locations on the nut, specifically with the outlet opening arranged at an angle of 120-150 degrees relative to the inlet opening rather than 180 degrees. This asymmetric arrangement reduces the required nut outside diameter and mounting hole depth, thereby improving machining precision and stiffness while maintaining circulation functionality
Solution Approach 2:
The patent redistributes the circulation openings in a three-dimensional space around the nut rather than arranging them in a simple linear or radial pattern. By positioning openings at specific angular intervals (inlet at 0 degrees, outlet at 120-150 degrees) and considering their radial and axial positions, the design optimizes the circulation path without requiring excessive nut dimensions, thus improving manufacturing precision
2Volume of moving object
If the nut outside diameter is reduced for small specification ball screws, then the overall size is minimized, but the structural strength and stiffness are compromised
Solution Approach 1:
The patent changes the geometric parameters of the circulation structure, specifically the angular position of the outlet opening (120-150 degrees) and the dimensions of the circulation channel, to optimize the balance between nut size and structural strength. This parameter optimization allows for reduced nut diameter while maintaining sufficient stiffness and strength for small specification ball screws
Solution Approach 2:
The patent employs a composite structure combining metal components (shaft, rolling elements) with plastic molded circulation components. This composite approach allows the plastic circulator to be designed with optimized geometry (asymmetric opening arrangement) that compensates for the reduced size of the nut, maintaining structural integrity through the combined properties of different materials
3Ease of manufacture
If plastic molding is used to replace metal guide tube, then manufacturing complexity is reduced, but the plastic molded members expand outward due to impact from rolling balls, compromising structural strength
Solution Approach 1:
The patent applies local quality by designing the plastic circulator with varying wall thicknesses and reinforcement features at critical locations. The circulation channel walls are thickened in areas subjected to impact from rolling balls, while other areas maintain thinner walls to reduce overall size. This localized reinforcement maintains structural strength while preserving the manufacturing advantages of plastic molding
Solution Approach 2:
The patent incorporates impact-absorbing features in the plastic circulator design, such as rounded corners, chamfered edges, and controlled wall thickness transitions, that cushion the impact of rolling balls before they reach critical structural areas. This beforehand cushioning prevents stress concentration and reduces the risk of structural failure while maintaining the simplicity of plastic molding manufacturing
Data Source
AI summary
An external circulation ball screw includes an elongate shaft having has an outer circumferential surface formed with a helical rolling groove; a nut having a through hole formed therein for receiving extension of the elongate shaft therethrough, the through hole having an inner circumferential surface formed with a rolling channel corresponding to the rolling groove, the rolling groove and the rolling channel collectively defining a loading passage, the nut having an outer surface, which includes, formed therein, two through apertures that are spaced from each other and in communication with the through hole and a cavity in communication with the two through apertures; rolling elements arranged in the loading passage and having a diameter between 0.3-1 mm; and a circulator having a U-shape and is arranged in the cavity and the through apertures and formed with a return passage connected to the loading passage to collectively define a circulation channel.