Planetary Roller Screw Gear Profiles for Higher Contact Ratio
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Solution Overview
Problem
The existing planetary roller screw mechanisms have an insufficient contact ratio at the interface between the gear teeth, leading to issues with load distribution, fatigue, vibrations, and noise, and require specific gear profiles to avoid interference between threads and gear teeth.
Innovation Solution
The mechanism employs rollers with outer gear teeth having a convex hypotrochoidal profile and synchronizing gear teeth with a hypotrochoidal or epitrochoidal profile, increasing the contact ratio and preventing thread interference, allowing for improved load distribution and assembly efficiency across various advance ranges.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If the outer gear teeth of the rollers have a reduced diameter to avoid interference with the outer thread, then interference between threads and gear teeth is avoided, but the contact ratio at the gear teeth interface becomes insufficient
Solution Approach 1:
The patent changes the geometric parameters of the gear teeth profiles by introducing hypotrochoidal and epitrochoidal curves instead of conventional profiles. This allows the gear teeth to engage with a higher contact ratio while maintaining the reduced roller diameter necessary to avoid thread interference. The specific parameters include the hypotrochoidal profile parameters (α, β, γ) and epitrochoidal profile parameters that optimize the contact ratio.
Solution Approach 2:
The patent employs curved hypotrochoidal and epitrochoidal profiles for the gear teeth flanks instead of straight or simple circular arcs. These complex curved profiles increase the length of the contact path between engaging teeth, thereby improving the contact ratio. The curvature of these profiles is specifically designed to maintain continuous contact between multiple teeth pairs during the engagement cycle.
2Reliability
If the gear teeth profile is modified to improve contact ratio, then load distribution and fatigue resistance improve, but the manufacturing complexity increases
Solution Approach 1:
The patent replaces complex mechanical gear tooth shaping operations with a more manufacturable approach by defining the hypotrochoidal and epitrochoidal profiles through mathematical equations and geometric construction methods. The profiles can be generated using standard machining operations combined with precise mathematical guidance, reducing manufacturing complexity compared to creating entirely new gear shaping mechanisms.
3Object-affected harmful factors
If the contact ratio is increased to reduce vibrations and noise, then the mechanism operates more smoothly, but the gear tooth geometry becomes more complex
Solution Approach 1:
The patent merges two different geometric concepts (hypotrochoidal and epitrochoidal profiles) into a unified gear system where the roller gear teeth use one profile type and the synchronizing gear teeth use the complementary profile type. This combination allows the complex geometry to serve dual purposes: achieving high contact ratio for smooth operation and maintaining compatibility with the reduced roller diameter constraint.
Data Source
AI summary
A roller screw mechanism is provided comprising a screw with an outer thread, and a nut disposed around and coaxially with the screw and comprising an inner thread; and with rollers that each have an outer thread and inserted between the screw and the nut, each roller having two ends each provided with an outer gear teeth, each outer gear teeth having, in cross-section, an outer diameter less than or equal to a root diameter of the outer thread of the rollers; and two synchronizing gear teeth disposed coaxially to the screw and in which the outer gear teeth of the rollers engage, wherein the outer gear teeth of the rollers have teeth with a flank having, in cross-section, a convex hypotrochoidal profile and the synchronizing gear teeth have teeth with a flank having, in cross-section, a hypotrochoidal or epitrochoidal profile.