Two-Piece Torque-Limiting Screw for Overtightening Prevention
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Solution Overview
Problem
Existing torque limiting screws are either destructive, complex, or ineffective in preventing overtightening, especially in small assemblies, and lack design versatility and economical manufacturing.
Innovation Solution
A two-piece screw assembly with a rotating cap that engages cam surfaces to limit torque during tightening and facilitate easy loosening without the need for a torque wrench, featuring a cap with axially extending peripheral arms that frictionally interact with the screw head to prevent overtightening.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a two-section head screw with thinner connecting metal is used to limit torque, then the torque limiting function is achieved, but the screw requires external driving and cannot be installed into a counter bore
Solution Approach 1:
The screw head is divided into two separate components: a screw body with head and a separate cap that surrounds the head. The cap can rotate independently on the screw head, allowing the screw to be driven from above while maintaining torque limiting functionality. This segmentation resolves the contradiction by enabling both torque limiting and counterbore installation capability.
2Reliability
If friction disk clutches and springs are added to create torque limiting screws, then the torque limiting function is improved, but the device complexity increases significantly
Solution Approach 1:
The complex internal friction disk clutch and spring mechanisms are completely removed from the design. Instead, the torque limiting function is achieved through the simple geometric interaction between the cam surfaces on the screw head and the cap arms, which naturally limit torque through their shape and engagement geometry without requiring additional mechanical components.
Solution Approach 2:
The torque limiting function is achieved by changing the geometric parameters of the cam surfaces (radius, profile, engagement angle) rather than adding complex mechanisms. By adjusting the cam surface geometry, different torque limits can be achieved with the same simple cap-and-screw structure, maintaining low complexity while providing reliable torque limiting.
3Measurement precision
If standard screws are used with a torque wrench, then the desired torque can be achieved, but overtightening damage can still occur
Solution Approach 1:
The cam surface geometry is designed to inherently limit the maximum torque that can be applied to the screw. As the cap rotates and engages the cam surfaces, the geometric constraints prevent any torque beyond the designed limit from being transmitted to the screw, providing beforehand protection against overtightening damage without requiring external torque measurement devices.
4Ease of operation
If a rotating cap with peripheral arms engaging cam surfaces is used, then the torque limiting function is achieved and easy loosening is enabled, but the manufacturing complexity increases
Solution Approach 1:
The cap is designed to rotate dynamically on the screw head during both tightening and loosening operations. During loosening, the cap rotates in the opposite direction, allowing easy removal without torque limitations. The dynamic rotational capability provides ease of loosening while the cap and screw remain simple components that can be manufactured using standard machining processes.
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
The solution provides a non-destructive, cost-effective torque limiting mechanism that can be adapted for various applications, preventing damage from overtightening and ensuring easy assembly in both large and miniature assemblies without requiring a torque wrench.
Implementation Method 1
The arms of the cap are disposed to frictionally engage the cam's outer surfaces. The cams surfaces may be axially tapered to supply the desired amount of design torque transferred from the cap to the screw.
Data Source
AI summary
A compound screw is a two-piece assembly where the head of the screw can rotate in the tightening direction to a designed tightening torque to drive the screw while being able to positively counter-rotate in the loosening direction. The screw assembly comprises a screw and a cap surrounding the head of the screw. The screw has a head at the top and a threaded shank downwardly extending from a base of the screw head. The screw is turned by turning the cap about a central axis of the screw. The cap is rotatably affixed to the screw head by a loose riveting of the cap to the top most end of the screw head by flaring a thin-walled upwardly extending portion thereof. The cap includes a plurality of axially extending peripheral resilient arms which engage peripheral cam surfaces around the outside of the screw head.