CVT Steel Ring Set Nitriding for Fatigue Without Brittle Fracture
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Solution Overview
Problem
Conventional drive belts for continuously variable transmissions (CVTs) in motor vehicles face challenges in achieving optimal fatigue strength due to excessive nitride layer thickness leading to brittle fractures, which compromises ductility and fatigue performance, despite high compressive residual stress levels.
Innovation Solution
Optimizing the nitriding process to achieve a nitride layer thickness between 10 and 18 microns and compressive residual stress between 350 and 650 MPa, with additional shot-peening for the radially innermost steel ring to enhance surface hardness below 750 HV0.1, reducing brittle fracture and improving fatigue strength.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the nitride layer thickness is increased to improve wear resistance and compressive residual stress, then the surface hardness and fatigue strength are improved, but brittle fractures occur and ductility deteriorates
Solution Approach 1:
The patent applies parameter changes by precisely controlling the nitride layer thickness (10-18 microns) and compressive residual stress (350-650 MPa) within optimal ranges. This resolves the contradiction by finding the sweet spot where the nitride layer provides sufficient wear resistance and fatigue strength without becoming excessively thick to cause brittle fractures, thus improving reliability while maintaining strength.
Solution Approach 2:
The patent applies partial action by using shot-peening treatment only on the radially innermost steel ring rather than all rings. This selective application provides enhanced surface hardness (below 750 HV0.1) and compressive residual stress where most critical for fatigue resistance, while avoiding over-treatment that could induce brittleness in other rings, thus balancing strength improvement with fracture resistance.
2Stress or pressure
If the nitriding process is extended to increase nitride layer thickness, then surface compressive residual stress increases, but the ductility of the steel ring decreases
Solution Approach 1:
The patent resolves this contradiction by changing the process parameters to achieve compressive residual stress within the optimal range of 350-650 MPa through controlled nitriding. This prevents excessive stress buildup that would compromise ductility, while still providing sufficient compressive stress to counteract tensile stresses during operation and improve fatigue life.
Solution Approach 2:
The patent applies partial action by using shot-peening treatment only on the radially innermost steel ring to enhance surface compressive residual stress locally where it is most needed for fatigue resistance, rather than applying excessive treatment to all rings which would reduce overall ductility.
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 approach enhances the fatigue strength of steel rings in drive belts by minimizing brittle fracture and maintaining satisfactory compressive residual stress, thereby extending the functional lifespan while reducing material and processing costs.
Implementation Method 1
the nitrogen atoms move away from the outer surface into the ring material by diffusion, thus providing the steel ring with a nitrided surface layer of increasing thickness
Implementation Method 2
In nitriding, at least in the typically applied gas-soft nitriding variant thereof, the steel rings are kept in an ammonia gas (NH3) containing process atmosphere at a temperature of, likewise, more than 400°C
Implementation Method 3
Precipitation hardening is also known as aging and is realised through heating the steel rings to a temperature exceeding 400 degrees Celsius, at which temperature microscopic inter-metallic precipitates incubate and grow at random locations throughout the ring material
Implementation Method 4
heating the steel rings to a temperature exceeding 400 degrees Celsius
Implementation Method 5
additional shot-peening for the radially innermost steel ring to enhance surface hardness below 750 HV0.1
Data Source
Figure 1~2
Figure 3I~3XI
Figure 4~8
AI summary
A ring set (31) for a drive belt (3) for a continuously variable transmission composed of a number of mutually concentrically nested steel rings (32), each having a nitride layer. According to the present disclosure, those steel rings (32) of the ring set (31) that are located between the innermost steel ring (32) and the outermost steel ring (32) thereof are provided with a compressive internal residual stress in the vicinity of their respective outer surfaces having a value in the range between 350 and 650 MPa, whereas the radial inner side of the said innermost steel ring (32) and the radial outer side of the said outermost steel ring (32) are provided with a compressive internal residual stress exceeding such compressive internal residual stress of those in-between rings (32).