Gears play an important role in mechanical power transmission systems. They enable the prime mover characteristic (a gas turbine for example) to be matched to the characteristic of the driven load (say, a slow speed propeller), thus reducing the cost of both manufacturing and operating the system. The customer requirements for higher power density and lower noise demands more accurate gears. This imposes more stringent requirements on the measuring equipment that controls the quality of the manufacturing machines. Many gears have flank form and tooth spacing tolerances that are less then 10μm, and according to the so called `Golden rule', measuring equipment on the shop floor should have a measurement uncertainty of between 1 to 2μm. These are stringent requirements that demand the highest standards of metrology. Thus the need to accurately quantify the measurement uncertainty of inspection machines is of paramount importance if costly mistakes are to be avoided. The work reported in this thesis was completed as part of the activities undertaken by the author in his role as head of the UK National Gear Metrology Laboratory (NGML). The laboratory is accredited by the United Kingdom Accreditation Service (UKAS) for gear measurement and on-site calibration of gear measuring machines. The work is mainly experimental in nature. In fact, much of what is reported is centred on work undertaken with two artefact sets: one set consisting of 100mm diameter lead and profile artefacts and a second set of 200mm diameter artefacts. These gear artefacts are probably the most valuable in the world because of the volume and quality of the calibration data associated with them.

Micropitting is a form of surface contact fatigue encounteredin bearingsa nd gears, under lubricating conditions, which lead to their premature failure. All gears are susceptible to micropitting, including spur, helical and bevel. Micropitting can occur with all heatt reatmentsa ppliedt o gearsa nd with both, synthetica nd mineral lubricants. It can occur after a relatively short period of operation and, after a certain number of cycles,g earsn eedt o be replacedd ue to the increasedn oisea nd vibrations causedb y the deviations of the tooth profile. Continuing operation of affected gears can lead to a catastrophic type of failure (i. e., tooth breakage). These considerations explain the increasing current interest in micropitting. It has been reported that micropitting in bearings is associated with a specific microstructural transformation in steel, i. e. martensite decay. However, to the authoes knowledge, this transformation has not been reported in gears. In the present work, extensive metallurgical investigations have been carried out and they revealed that the same transformation occurs in gears. The aim of this project was to describe the mechanism of micropitting by taking into account the influence of several controlling factors such as, material, surface finish, lubricant, load, temperature,s peeda nd, slide-to-roll ratio. Their influence is assessed with a fractional factorial experimentadl esign.S everaln on-destructivete chniquesh ave been used in order to monitor the specimen condition during and after running, such as X-ray diffraction, optical profilometry, light microscopy. The mechanical properties of the products of martensite decay, known as dark etching regions, white etching bands and butterflies are highly relevant to the fatigue behaviour of the steel. Nanoindentation and AFM techniquesh aveb eenu sedt o determinet hesep roperties. A micropitting mechanism correlated with the mechanism of martensite decay in gears is suggestedb asedo n thesea nalyses.