British Gear Association; ambitious investigation into fatigue strength of gear materials

British Gear Association ­ ambitious investigation into fatigue strength of gear materials

Keyword British Gear Association

An ambitious investigation has been completed into the fatigue strength of gear materials. The research programme has determined the surface and root bending fatigue strength characteristics for gears manufactured from carburising and nitriding steels.

The work, by the British Gear Association's Gear Research Council, which has a thriving membership of 37 companies, cost £0.8 million and was funded by the DTI, MOD(N) and the GRC. Said BGA Technical Director Tom Marsh:

The GRC work has generated reliable design data for the four steels being considered in addition to providing considerable insight into the effect of process variables on the performance of gears.

In providing base data for current gear steels and processing ­ as well as improving our manufacture and test procedures ­ this work has laid a firm foundation for improving gear performance by enhancing the fatigue properties of steel.

The steels under investigation were:

• 655 H13 (En 36) ­ carburising steel;

• 20 MnCr 5 ­ carburising steel;

• SAE 8620 H ­ carburising steel;

• 722 M24 (En 40B) ­ nitriding steel.

Several projects, which have been carried out by the BGA's Gear Research Foundation, a descendant of the Gear Research Council, have achieved maturity while research on others has just begun.

A three-and-a-half year project to improve worm gear performance to give greater load capacity has investigated the potential for significant improvements in load carrying capacity in worm gears.

A comprehensive contact analysis for cylindrical worm gears has been developed and made available to the collaborators.

A back-to-back test rig for worm gears of 160 to 200mm centres has been designed, manufactured and commissioned. The rig can test single and multi-start worm gears at torques up to 18,000Nm and a number of experimental worm gear geometries have been developed.

During the research it was found that the available facilities for grinding hardened worms and wheels have limited the gear geometries which can be manufactured and it has not been possible to explore the full potential of hard to hard worm gear pairings with the experimental gears.

A project to analyse and measure the transmission errors in precision worm gears for low noise applications has been successfully completed with the full commission of the test facility for worm gear transmission error and experimental validation of the contact and TE analysis software.

A follow-on project is based on the completed research and the proven test facility and is directed primarily at investigating the effect of detailed gear geometry and gear loading on the bedding-in process in worm gears with the resultant change in transmission error.

A validated full three-dimensional spline coupling stressing and contact model has followed a further project. The outcome of this work, which is nearing completion, will be a user-friendly computer software programme which will assist a specialist designer in producing efficient and effective spline joint couplings for a wide variety of applications.

Research into reducing gear grinding costs for generative gear grinding has identified improvements in cutting oil delivery and developed a calculation procedure for optimising grinding feed and speed to achieve high metal removal rates without grinding burns.

A follow-on project will investigate the improvement in form grinding and the control of grinding burn and wheel wear in this process. The work has wide implications for the majority of high performance gears being manufactured today.

A very substantial research programme being carried out by the Gear Research Foundation is investigating ways of improving the fatigue strength of gear steels, heat treated by case-carburising, nitriding and induction hardening.

New test facilities will for the first time allow the fatigue testing of helical gears at powers up to 1,800kW.

A number of modern processes, which can enhance gear fatigue strength and reduce distortion, as well as a programme to quantify potential advantages, have been planned.

An exciting and large programme of research has just started to investigate the many aspects of micro-pitting failure in gears. The first stage of work is a preliminary investigation into the methods of testing which will identify the way forward for subsequent phases.

Two programmes of work are underway to develop better stress analysis for gear design and a three year project aims to develop forging techniques/die designs to produce gears of single helical and spur form of at least ISO 5 accuracy and up to 200mm diameter.

The post-forging operation aims to provide a surface finish so that no other final machining operation is necessary. The work will take account of surface hardening and will look at producing forged gears with profile modification and crowning.

Other subjects being considered for collaborative research include shaft/joint interfaces, plastic gears, application and safety factors, gear efficiency improvements, noise in spur gears, coupling design and the development of functional metrology gear checkers.