Fatigue Life Extension by Surface Enhancement

To increase predictable life of precision gears, requires a process which simultaneously produces the required geometry accuracy and surface finish, as well as the desired state of compressive residual stress at the part surface. The goal is to increase power throughput by optimizing the surface properties of the gears, without compromising tooth scoring resistance. In this experiment, four different surface enhancement processes were selected, implemented, and tested. Rolling Sliding Contact Fatigue tests were used to qualify the percentage of improvement. A set of test specimens were also made without any of the surface enhancements above. These specimens were used as the baseline for comparison. 9310 steel was used as the material to prepare the test specimens. All specimens were carburized, hardened and ground to the aerospace specification. Gear Research Institute conducted testing on all samples. The baseline specimens were all run in high-load, short-cycle tests and they failed in the predicted manner with expected scatter. The performance of some surface-enhanced specimens exceeded the performance of the baseline specimens. Improved life-to-failure rates were as high as 10%. This paper will discuss the test specimen preparation processes, testing process and results of this project. It also includes recommendations for future work in surface enhancement.

Tooth Undercutting of Beveloid Gears

A beveloid gear can be viewed as an involute gear of which the profile-shifted coefficient linearly decreases from the heel to the toe. Therefore, tooth undercutting occurs and singular points appear on the tooth surfaces near the toe. When undercutting occurs, the gear tooth is comparatively weak. In this study, the conditions of tooth undercutting of beveloid gears were derived and specific phenomena were also investigated by numerical illustrated examples. In addition, according to the characteristics of tooth undercutting on the beveloid gear tooth surface, a novel type hob cutter with varying cutting depths was designed to avoid tooth undercutting of the beveloid gear.

A Wave Generator of New Concept for Flex Gear of Harmonic Drive With Pure Involute Tooth Gear Pairs

A new wave generator (or cam) is proposed (Maiti, Patent 1995) to drive the flex gear of strain wave gearing i.e., harmonic drives with gear pairs of pure involute profiles. The cam profile is made of circular arcs at the two contact zones and shifted elliptical curves for the other zones. The geometric constniction is made in such a way that tip interference is properly avoided for both engagement and disengagement with nominally stubbed or full depth involute gears. The theories are established to verify other geometric and gearing conditions. Evidently, in the existing products (all are patented design) the profiles are non-involute and none of them offers ideal gear kinematics. It is expected that this new harmonic drive will offer the best gearing kinematics and will have desired accurate performance.

Thermal Behavior Evaluation During Mechanical Design: Validation of Thermal Numerical Gearbox Models

In todays mechanical design, static and dynamic numerical models are widely used, and thermal models are needed to make robust design. Thermal models, based on the thermal network method, are now available. Several hypotheses are made as physical phenomena are complex and experimental validation is necessary. A thermal model of gearbox has been already presented and compared to few experimental results that had allowed global validation of the model. Now, the experimental validation is concerned with thermal transient and steady state behavior of gearbox versus transmitted power and lubrication conditions in order to finely validate the model. The test gearbox is compound of 3 spur gears supported by 6 spherical roller bearings, a housing and a lubrication circuit cooled by an oil-air exchanger. The maximum transmitted power is 500 kW. Gears, bearings, housing, shafts, and the lubrication circuit have been equipped with thermocouples, flux-meters and flow-meters. Heat flux were measured on the internal and external side walls of the housing. Oil flowing on a side wall has been measured. Experiments were run under several transmitted powers and oil flows at meshing. Thermal map at steady state and transient temperature rises of technological elements are obtained for each test. Finally, transient temperature rises and steady state from numerical and experimental results are compared. The comparison shows a good agreement, and the importance of taking into account oil flowing on the inside walls of the housing is brought to the fore. The difficulty of evaluating the oil flowing on the internal walls of a housing is discussed and illustrated with numerical results.

Simulation of Traction Drive Ratio Changes

In the simulation model presented in this paper, the kinematic characteristics of traction drives are formulated using classical Hertzian contact theory and elasto-hydrodynamic theory. The roller swing motion is governed by an equation derived based on Newton’s Second Law and is coupled to the side slip, torque input and output, as well as ratio variations. A control strategy with feedbacks for both the roller swing and the piston displacement is applied for ratio control based on stability and responsiveness considerations. The model has been implemented systematically in Matlab/Simulink environment. The effectiveness of the ratio control system in terms of stability and accuracy is illustrated by the simulation results included in this paper.

Development of High-Loaded, Low-Speed Spiroid Gearboxes

Actuator rotation sometimes is required to transmit considerable torques at low speeds in a limited angular range. Such operating conditions are typical, for example, for the rotational drives of gas pipeline stop valves. These conditions are made worse by increased torques requried at the initial instant of motion when the torque is 1.3 to 1.5 times greater than the nominal torque, and by the range of operating temperatures of −60°C to +50°C. A number of gearboxes with a spiroid gear mesh were developed to satisfy these conditions for different torques (i.e. for different standard stop valves), with the steel spiroid pair case-hardened to 60–62 hardness Rc. A set of numerical studies had been conducted in order to choose gear design parameters and other elements of the gearbox. Experimental research performed using special testing rigs for definite operating modes showed high reliability and wear resistance of the drives developed and their high durability compared to known ones which is of great importance for given application domain.

Predesign Investigations of Non-Orthogonal Spiroid Gears

Gear non-orthogonality is well known to be an efficient aid in obtaining new gear properties and achieving better design and operating specifications. However, this holds for certain combinations of geometric gear parameter values, one of the variables being axis angle Σ. Predesign research aims at finding such combinations of gear parameter values that provide better gear properties. The paper discusses some predesign research results of non-orthogonal (Σ ≠ 90°) spiroid gears.

Viscoelastic Tooth Root Stress Analysis on Fiber Reinforced Thermoplastic Poly-Imide (TPI) Gears

Although thermoplastic poly-imide (TPI) gears do not have sufficient strength for power transmission, carbon fiber reinforcement greatly improves the strength of TPI gears. Previous experimental research showed that although standard specimens made from carbon fiber reinforced (CFR) TPI has 2.4 times strength in static bending than specimens made from natural TPI, gears made from CFR-TPI yields bending fatigue strength about 10 times greater than gears made from natural TPI. The present paper explains this phenomenon using viscoelastic tooth root stress analysis. The experiments indicated that the natural TPI gears showed much larger viscoelasticity than the CFR-TPI gears. Thus, tooth root stresses were calculated for cases of large and small viscosity moduli. These calculations showed tooth root stress increased with the increase in the viscosity modulus. Also, viscoelasticity may induce heat due to hysteresis loss, and this heat should reduce gear durability. The increase in tooth root stress and the heat due to hysteresis loss must make the durability of the natural TPI gears very small. Therefore, the CFR-TPI can yield much more durable gears than the natural TPI.

Effect of Gear-Side Case-Hardening on Residual Stresses of Case-Hardened Gears

This paper presents a study on effects of the case depth, the case-hardened part, the face width, the rim thickness and the standard pressure angle on residual stresses of case-hardened gears. A heat conduction analysis and an elastic-plastic stress analysis for the case-hardening process of spur gears were carried out by the three-dimensional finite-element method (3D-FEM), and then residual stresses were obtained. It was found that the compressive residual stress σ*θ = 30° at Hofer’s critical section of the end of the face width is smaller in magnitude than that of the middle of the face width, and that the absolute value of σ*θ = 30° of the middle of the face width decreases owing to case-hardening the gear-side and the decreasing rate increases with an increasing case depth and a decreasing face width.

Impact Bending Fatigue Strength of Gear Teeth Case-Hardened by Nitriding and Carburizing

Almost all gears used for power transmission of automobiles have been case-hardened by carburizing. Recently, strict demand for reducing running noise and vibration from the power transmission gears requires, in most cases, an additional finishing operation such as grinding and/or honing after carburizing. Nitriding is conducted at a temperature of about 820 K which is lower than the transformation temperature, and thus quenching is not required, resulting in smaller heat treatment deterioration. However, nitrided gears hardly used in practice as for power transmission gears. In the present investigation, experiments were conducted, using test gears case-hardened by two different methods, carburizing and plasma-nitriding. Test results showed that the fatigue strength of carburized gears was higher than that of nitrided gears in most cases when the test gears were made from the same steel. However, the impact fatigue strengths of nitrided gears made from a high tension steel with additional alloy elements Mo and V were higher than those of carburized gears made from the carbon and alloy steels which have been, used as for gear material. The other high tension steel containing neither Mo nor V could not bring about a sufficiently high fatigue strength in comparison with the conventional carburized gears. It should be noted that the impact fatigue strength of carburized gears made from the high tension steel was higher than the ones made of conventional carburizing steel.