Roller Screw Hypocycloid Tooth Profile Wear Reduction

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Solution Overview

Problem

Current roller screw mechanisms experience increased wear of the nut due to pressure on the teeth during meshing, leading to premature replacement and difficulty in hardening the nut, which affects load capacity and manufacturing processes.

Innovation Solution

The introduction of a roller screw mechanism with gear wheels and rollers featuring a convex-concave hypocycloid tooth profile, where the outer diameter of the gear teeth is less than or equal to the thread root diameter, reducing contact pressure and distributing it over a larger surface, and the use of aluminium for the gear wheels to match roller hardness, thereby reducing wear and increasing load capacity.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Strength

If the teeth of the rollers are threaded during the threading step, then the load capacity is increased, but the contact pressure at the top of the teeth increases causing increased wear of the nut

Engineering Contradiction:

Improveload capacityVSAvoidwear of the nut

Core Design Contradiction:

StrengthVSReliability

Solution Approach 1:

The tooth profile is designed with different geometries at different locations: the top portion has a reduced outer diameter (less than or equal to the thread root diameter) to reduce contact pressure and wear, while the root portion maintains full thread engagement to preserve load capacity. This local differentiation resolves the contradiction between strength and reliability.

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The invention transitions from a conventional involute tooth profile to a hypocycloid profile, fundamentally changing the geometric dimensioning approach. The hypocycloid profile is generated by a point on a circle rolling inside another circle, creating a unique curvature distribution that naturally reduces contact pressure at the tooth top while maintaining engagement strength at the root.

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

2Reliability

If the hardness of the nut is increased to reduce wear, then the wear resistance is improved, but boring holes in the nut becomes very difficult

Engineering Contradiction:

Improvewear resistanceVSAvoiddifficulty of boring holes

Core Design Contradiction:

ReliabilityVSEase of manufacture

Solution Approach 1:

Instead of changing the material hardness parameter of the nut, the invention changes the geometric parameters of the tooth profile. By reducing the outer diameter of the tooth top and using a hypocycloid profile, the contact pressure is reduced without requiring increased material hardness, thus maintaining ease of manufacturing while improving wear resistance.

Inventive Principle:

Principle #35Parameter changes

3Strength

If the outer diameter of the gear teeth is increased to increase load capacity, then the strength is improved, but the contact pressure increases causing increased wear

Engineering Contradiction:

Improveload capacityVSAvoidcontact pressure

Core Design Contradiction:

StrengthVSStress or pressure

Solution Approach 1:

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The hypocycloid profile introduces specific curvature characteristics that distribute contact pressure more favorably compared to a straight or involute profile. The curved geometry of the hypocycloid allows for better stress distribution across the contact surface, reducing peak pressures while maintaining overall load capacity.

Inventive Principle:

Principle #14Spheroidality (Curvature)

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 convex-concave tooth profile significantly reduces wear on the rollers and gear wheels, increases load capacity, and simplifies manufacturing by allowing thread rolling on heat-treated materials, while maintaining similar hardness between the wheels and rollers, resulting in a longer-lasting and more efficient mechanism.

Implementation Method 1

each tooth of each roller has, in cross section, a concave profile curve and each tooth of each gear wheel has, in cross section, a convex profile curve, defined by a circular arc, which approximate a hypocycloid curve

Methodology Applied

Scientific EffectHypocycloid curve:

Implementation Method 2

a screw having an external thread, with a nut arranged around the screw and having an internal thread, and with a plurality of longitudinal rollers engaging the external and internal threads of the screw and of the nut

Methodology Applied

Scientific EffectScrew mechanism: Screw

Data Source

PatentUS9267587B2Roller screw

Publication Date: 2016.02.23 EWELLIX AB

AI summary

A roller screw mechanism provided with a screw having an external thread, a nut surrounding and coaxial with the screw, the nut including an internal thread, two gear wheels mounted in a non-threaded part of the nut, each gear wheel providing a first set of teeth having a plurality of internal teeth, and a plurality of rollers radially located between the screw and the nut, each roller having an external thread designed to engage the external and internal threads of the screw and the nut, and axially at each end a first set of teeth having a plurality of gear teeth designed to engage with the internal gear teeth of the gear wheels. The outer diameter of the gear teeth of each roller is less or equal to the diameter of the thread root of the external thread of the roller.