Peter Oster

Influence of shot peening on bending tooth fatigue limit of case hardened gears

Abstract The effect of different surface treatments on the fatigue behaviour at the tooth root of spur gears is investigated. Case hardening and case hardening followed by shot peening were considered for 16MnCr5 steel gears. Pulsating tests ( R =0.1) were carried out on gear teeth to determine the fatigue endurance at 6×10 6 cycles. Residual stress profiles were measured at the tooth root by means of the XRD technique and carefully analysed in order to establish the effect of different treatment parameters on the residual stress field and to find a correlation with the measured fatigue properties. The XRD technique was also adopted for measuring the retained austenite content. The effective stress field at the gear tooth during the fatigue test was reproduced by finite element modelling to check—using a multiaxial fatigue criterion—whether the fatigue crack initiation can be considered as the precondition for failure. A satisfactory agreement between experimental and predicted fatigue limits was found for unpeened gears, whereas for peened specimens a significant underestimation of fatigue strength was found. This is discussed by considering the micro-structural improvement and the importance of compressive residual stress peaks in the early stages of fatigue crack propagation.

Systematic Investigations on the Influence of Case Depth on the Pitting and Bending Strength of Case Carburized Gears

The influence of case depth on the pitting and bending strength of case carburized gears was investigated. For this purpose an extensive test program on gears of different sizes, different gear geometry and with systematically varied case depth has been carried out. Residual stresses and further characteristics of the case hardened layer — that are influenced by the value of case depth as well — are examined. Based on the test results, rating formulas are derived to calculate the optimum case depth for maximum load capacity. Influence factors are defined, by which the influence of case depth on the load capacity regarding pitting and bending strength can be taken into consideration if rating a gear according to the standardized calculation method of DIN 3990/ISO 6336.Copyright

Flank-Load-Carrying Capacity of Case Hardened Gears With Grinding Burn

A high geometric accuracy of case-hardened gears requires a grinding process after heat treatment. Inappropriate grinding conditions can induce surface tempering, alter hardness and lead to an unfavorable residual stress state. This effect is commonly known as grinding burn. The influence of grinding burn on the flank-load-carrying capacity was systematically investigated within a research project. The results of experimental tests and the analysis of surface and near subsurface parameters allowed a correlation between grinding burn grade, material characteristics and flank-load-carrying capacity. A main result of this project is a proposal for the calculation of surface durability of gears which implicates the influence of grinding burn. This paper summarizes results of the experimental testing and the accompanying analyses. The main focus herein is the consideration of an altered hardness and residual stress state in a material-physically based model for calculating the load capacity of gears.Copyright

The ???Practice Relevant Pitting Test??????A New Improved Test Method to Evaluate the Influence of Lubricants on the Pitting Load Capacity of Case Carburized Gears

Pitting is a form of fatigue failure which occurs on the flanks of highly loaded case carburized gears. The lubricants, consisting of base oil and additives, influence the pitting lifetime of the gear flanks. Common calculation methods of the pitting load capacity consider, along with the flank roughness and the tangential velocity, only the nominal viscosity of a lubricant as a lubrication relevant influence parameter. The actual lubricant performance and particularly the influence of the additives can be determined only experimentally. Within a FVA-research project a new “Practice Relevant Pitting Test” was developed to evaluate the influence of lubricants on the pitting load capacity of case carburized gears. The new test method is a mechanical test procedure which is based on the existing FVA-FZG-Pitting Test. It is defined as a single stage test which can be extended to an application test by testing the lubricant with a second load stage. In order to improve the practice relevance and to reduce the scatter of the test results the test uses superfinished test gears with adequate profile modifications. The extremely low flank roughness of the new test gears prevents particularly the appearance of undesired micro-pitting and therefore improves the reproducibility of the reached lifetime within the performed test runs. The new pitting test procedure was verified with different gear transmission lubricants and is seen as an improvement of the existing FVA-FZG-Pitting Test. It provides also a possibility to consider the test results for calculation of the pitting load capacity of industrial and automotive transmission gears with an additional factor ZLp which is to be applied in the calculation method according to DIN 3990. The paper describes the new developed test procedure “Practice Relevant Pitting Test” and discusses the correlation and classification to the existing FVA-FZG-Pitting Test.

Grinding Burn on Gears: Correlation Between Flank-Load-Carrying Capacity and Material Characteristics

A high geometric accuracy of case-hardened gears requires a grinding process after heat treatment. In appropriate grinding conditions can induce surface tempering, alter hardness and lead to an unfavorable residual stress state. This effect is commonly known as grinding burn. The influence of grinding burn on the flank-load-carrying capacity was systematically investigated within a research project. This paper summarizes the main results. Different methods for analyzing the surface and the near subsurface characteristics are presented. Based on the results of experimental testing and the analysis of surface and near subsurface, the paper summarizes the correlation between grinding burn and flank-load-carrying capacity.

Pitting Load Capacity of Helical Gears

In experimental analyzes the pitting load capacity of case carburized spur and helical gears is determined in back-to-back test rigs. The research program with one type of spur and 8 types of helical gears includes tests for the determination of influences of varying load distribution, overlap ratio and transmission ratio. The test results are presented and evaluated on the basis of the pitting load capacity calculation methods of ISO 6336-2/DIN 3990, part 2. A new DIN/ISO compatible calculation method for pitting load capacity is presented. This new calculation method comprehends helical gears more adequate than ISO 6336-2 / DIN 3990, part 2 and has the possibility to consider tooth flank modifications. The new calculation method is applied on test results and gears of a calculation study. It shows better accordance with the experimental test results than the present ISO 6336-2 / DIN 3990, part 2.Copyright

Erweiterte Berechnungsmethode zur Grübchentragfähigkeit einsatzgehärteter Gerad- and Schrägverzahnungen

Schragverzahnte Stirnrader haben gegenuber geradverzahnten ein gunstigeres Schwingungsanregungsverhalten and eine nach dem Rechenverfahren der DIN 399o T2 (DIN3990 1987a) hohere Grubchentragfahigkeit. Stirnradverzahnungen werden meist schragverzahnt ausgefahrt. In der Praxis and in Prufstands-Untersuchungen an einsatzgeharteten Zahnradern wird die hohere Tragfahigkeit der Schragverzahnung jedoch nicht ohne weiteres belegt. Insbesondere wird die Grubchentragfahigkeit nach DIN 3990 T2 in neueren experimentellen Untersuchungen (Dobereiner 1998; Haslinger 1991; Simon 1984) an den schragverzahnten Prufradvarianten nicht erreicht. Die Dimensionierungvon Schragverzahnungen nach DIN 3990 T2 wird damit in Frage gestellt.