Segmented Ball Screw Spring for Even Compression Under Load
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Solution Overview
Problem
Existing ball screw assemblies experience uneven spring compression and increased stress due to friction, leading to fatigue failure when subjected to high axial loads, as the coil spring is not compressed evenly along its length.
Innovation Solution
A ball screw assembly design featuring a main spring assembly with varying spring constants, where the spring constant of the first spring portion is greater than the second and third spring portions, distributed across the length to promote consistent compression and reduce friction, allowing the balls to roll effectively with both the ball nut and screw shaft under high loads.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single coil spring is used in the ball screw assembly, then the structure is simple, but the spring compresses unevenly along its length causing higher stresses and fatigue failure
Solution Approach 1:
The single coil spring is divided into multiple segmented springs (first coil spring, second coil spring, third coil spring) arranged in series along the ball train. Each segmented spring has a different spring constant, with the first spring having the highest constant and the third having the lowest. This segmentation allows each spring to compress more uniformly, reducing stress concentration and preventing fatigue failure while maintaining structural complexity at an acceptable level.
Solution Approach 2:
Different portions of the spring assembly are given different local properties through varying spring constants. The first coil spring (adjacent to ball train) has a higher spring constant to handle higher loads near the ball train, while the third coil spring (adjacent to stopper) has a lower spring constant. This local quality variation ensures even compression distribution along the spring length, preventing the uneven loading that causes fatigue failure.
2Ease of manufacture
If a coil spring with uniform spring constant is used, then the manufacturing is simple, but the spring does not compress evenly under high axial load
Solution Approach 1:
The uniform spring is segmented into multiple springs with different constants. The first coil spring has a higher spring constant than the second and third springs. This segmentation transforms the manufacturing challenge of creating a single non-uniform spring into the simpler task of manufacturing multiple springs with different but standardized constants, while achieving even compression under high axial loads.
Solution Approach 2:
The spring constant parameter is varied along the length of the spring assembly by using different spring constants for different segmented springs. The first spring has a higher spring constant to resist compression near the ball train, while subsequent springs have progressively lower constants. This parameter change enables even compression distribution without requiring complex manufacturing of a single non-uniform spring.
3Device complexity
If friction between spring coils and support structure is present, then the mounting is simple, but the spring loading becomes uneven
Solution Approach 1:
The spring assembly is segmented into multiple independent springs that can be mounted separately on the ball train and stopper. This segmentation reduces the cumulative friction effect that would occur in a single long spring, as each shorter spring has less contact length with the support structure. The varied spring constants further compensate for any remaining friction effects, ensuring even load distribution.
Solution Approach 2:
The mounting arrangement is optimized locally at each spring position. The first spring mounted near the ball train has a higher spring constant to compensate for friction effects in that region, while springs further along the train have progressively lower constants. This local quality adjustment ensures even load distribution despite the presence of friction between spring coils and the support structure.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This design ensures even spring compression across the length, reducing stress and fatigue, and enables efficient screw power transmission by allowing the balls to roll with respect to both the ball nut and screw shaft, thereby enhancing the assembly's operational reliability and longevity.
Implementation Method 1
a main spring assembly including a first spring portion and a second spring portion, the main spring assembly being disposed in the raceway between a first end of the ball train and the first stopper
Implementation Method 2
wherein a spring constant of the first spring portion is greater than a spring constant of the second spring portion
Implementation Method 3
allowing the balls to roll effectively with both the ball nut and screw shaft under high loads
Data Source
AI summary
A ball screw assembly having a ball nut including a first end, a second end, a central bore, and a ball track defined by the inner surface, a ball screw shaft including an outer surface defining a ball track, the ball screw shaft being disposed in the central bore so that the ball tracks form a ball raceway, a first stopper disposed within the ball raceway, a plurality of main balls forming a ball train, the ball train being disposed in the ball raceway, and a main spring assembly disposed in the ball raceway between a first end of the ball train and the first stopper, wherein a spring constant of the first spring portion is greater than a spring constant of the second spring portion.