Influence of centrifugal forces on the tooth root load carrying capacity of high-speed gears

Abstract In drivetrains of modern battery electric vehicles (BEVs), motor speeds exceeding 15,000 rpm are already common. In scientific research projects, motor speeds up to 50,000 rpm are being investigated. These very high motor speeds must be harmonized with the rotational speed of the tires through a gearbox. The gearbox is considered a key enabling technology in this context, allowing future BEVs to be built with high market acceptance by reducing weight and providing high comfort and classic driving behavior. Due to the higher rotational speeds in the drivetrain of a BEV, compared to drivetrains with conventional internal combustion engines, additional loads from centrifugal forces occur in the gear. The notch effect of the tooth root fillet leads to significant stress concentrations, particularly in the area of the tooth root. At the same time, this area is subjected to highest loads due to the transmission of the torque required to set the vehicle into motion. From the literature, both simulative and experimental studies are known that demonstrate the influence of centrifugal force-induced stresses on the crack initiation site and crack propagation direction in the tooth root. In addition to influencing the crack behavior, a reduction in the transmittable torque due to superimposed centrifugal force load is to be expected but has not been quantified so far. The centrifugal force load, as a static preload, interacts in a complex manner with the dynamic (pulsating) load from tooth bending due to power transmission. In the present study, the influence of centrifugal force-equivalent stress states on the tooth root load carrying capacity is quantified. First, a device was developed to generate a centrifugal force equivalent stress state in the tooth root during the (rotation-free) testing of the gears at a pulsator test rig. Using the centrifugal force equivalence device, various rotational speeds were simulated in the pulsator, and partial S‑N-curves for the tooth root load carrying capacity were determined for each speed. These are evaluated and transferred to the Haigh diagram depending on their static preload. Based on the findings, a recommendation is made for considering centrifugal force-induced stresses in the design of future high-speed gearboxes.