In the 2012 racing season, V8 supertourer gearboxes were failing prematurely by gears being cracked or the gear teeth being stripped. The V8 supertourers are a similar concept to the V8 supercars but produced at a budget of less than half the cost. The failed gears were manufactured offshore.
METLAB along with AH Gears was requested to examine and analyse the gears to determine why they were failing. Several cracked, but not-yet fractured gears were supplied for examination. After crack-testing, the gears were sectioned for metallographic examination and chemical analysis.
Microhardness traversing through the carburised case was carried out in three locations. First, in the root adjacent to a crack. Second, through the pitch-line of a tooth and third, through the splines nearer the centre of the gear. An effective case depth to 550HV was determined along with a total case depth.
The gear material was found to be carburizing steel EN39B and was metallurgically clean. The tempered martensite microstructure was homogeneous and not banded and therefore the material itself was not implicated in the cause of failure. Metallography through the cross-section showed cracks in both sides of the root fillet radius in all of the teeth in the mounted section. Microhardness testing showed a hardness of up to 750HV0.5 (61-62HRC) in the pitch line and splines and up to 730HV0.5 in the root radius. The core hardness was measured at 450-460HV0.5 (43-44HRC) which is considered typical for this steel.
Microhardness profiles through the pitch line, splines and root radius showed a reduced case depth in the root radius. This is not normal, as the carburised case should be relatively uniform around the entire carburised surface.
It was determined that the finish grinding operation following the carburising heat treatment in the root radius has removed approximately 0.35mm of this carburised case. The root of the teeth therefore had less ability to absorb the applied stresses, and once the applied stress exceeded the tensile strength of the material in this region a crack initiated.
AH Gears Ltd looked at the results from METLAB and it became clear that the European manufacturer had used a standard hob cutter when they gearcut the blanks prior to heat treatment. This meant that after heat treatment, excessive amounts of material had to be removed from the gear roots to achieve the correct clearances, and in doing so they had ground into the case-hardened layer.
Inspection of the finish-ground tooth profiles also showed that there had been no modifications made to the involute or lead to allow for deflections of the gears when under load.
AH Gears Ltd used a pre-grind hob cutter for pre-heat treatment cutting of a new set of gears and designed specific gear-tooth modifications. These changes dramatically extended the life of these highly stressed gears.
Andrew Ouwejan, BE(Hons) ME, of METLAB LTD discussed this case study at the MENSZ Conference in November 2014.