FEATURE SPONSOR
GEARS
THE EFFECT OF RESIDUAL STRESS ON THE FATIGUE STRENGTH OF HIGH PERFORMANCE GEARS
Transmission components are subjected to alternating loads as a function of their purpose, and so fatigue failures (root bending and contact) are an on-going problem.
High performance gears are normally made from good quality materials and have optimised design profiles, so to achieve good fatigue life and strength when increases in load carrying and lifetime performance are required, the residual stresses in the gear root and contact surface must be considered.
The existence of tensile residual stresses of unknown magnitude, as a result of
APPLICATION OF CONTROLLED SHOT PEENING AND SUPERFINISHING Controlled Shot Peening is a proven process that improves the root bending fatigue performance of transmission components. In combination with CASE Superfinishing, the contact fatigue performance is also improved, significantly reducing the likelihood of micro pitting and achieving improved gearbox efficiency by reduced frictional losses, typically resulting in a 20o
C drop in lubricant temperature.
CONTROLLED SHOT PEENING The process of Controlled Shot Peening involves directing a stream of spherical particles, that possess specific material
carburised and nitrided) demonstrate consistently good high cycle fatigue strength performance improvements.
C.A.S.E. SUPERFINISHING Superfinishing is a technique of 'final machining' in a controlled non-abrasive manner to reduce surface finish Ra. The particular technique of Superfinishing uses oxidising chemicals and non-abrasive vibrofinishing stones to preferentially remove surface asperities.
The chemicals oxidise the surface which causes the asperities to be more susceptible to micro honing with the result that the most positive (peaks) surface areas are removed progressively. The vibrofinishing stones are selected to suit the geometry of the component and span the machine lay, therefore cutting of the negative (valleys) surface areas is avoided to retain lubricity.
Consequently the symmetry of the roughness profile can be altered producing a negative skew, which is ideal for contact (rolling contact) conditions, peaks are removed and valleys retained, thus maintaining the integrity of the lubricant film. The gear tooth profile remains consistent as there is no root-to-tip preferential material removal.
CONCLUSIONS
Surface treatments of gears using the Controlled Shot Peening and CASE Superfinishing processes, tailored to suit the application appropriately, will result in an optimised residual compressive stress and surface texture.
manufacturing processes, are detrimental to fatigue performance and will give rise to premature failures. Surface treatments such as Controlled Shot Peening and Isotropic Finishing (CASE Superfinishing) remove these harmful tensile residual stresses, impart beneficial compressive stresses and reduce contact stresses at and below the surface of the component thus offsetting a portion of the applied load.
characteristics, at critical areas on the component resulting in a compressive residual stress that can be modelled and gives a good estimated value of magnitude and depth.
The action of impingement at the surface yields the material locally and a surface residual compressive stress results. The magnitude of the compressive stress is directly related to the component material yield strength and is approximately equal to 70% of that value in compression. All engineering materials can be shot peened, and heat treated steels (including
Practical experience in processing transmission parts for a wide array of industries has shown significant improvements in root bending fatigue, contact fatigue and surface pitting negating the detrimental influence of variations in manufacturing techniques which can be reduced or eliminated enabling improved performance and life.
Jeremy Allen
Technical Services Manager Metal Improvement Company
www.windenergynetwork.co.uk
101
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