Study of fluid flow inside the gearbox
Licentiate thesis, 2014
Long haul trucks spend (almost) 80% of their time cruising from one destination to another. This means that they drive mostly at a constant speed and use their top gear most of the time. Depending on the gearbox, the top gear of long haul trucks is usually direct drive gear. With the direct drive gear losses mainly come from the interaction between lubricant and rotating gear (churning losses). It is obvious that using a small amount of low viscosity lubricant will reduce the churning losses dramatically. Unfortunately the solution is not as simple as that. This is because, besides lubrication, the lubricant is also there as a cooling agent. Hence, too little oil will affect the heat dissipation and the lifetime of the gear. Studies of oil flow inside the gearbox are needed to find ways to reduce churning losses and good heat dissipation.
A simplified version of a gearbox is built to study the flow, due to the complexity of the real gearbox. The new gearbox consists of a gear pair and has transparent rectangular walls to ensure good optical access. Velocity measurements with particle image velocimetry (PIV) are made to get the velocity field around the gear. Flash photography is done, as additional data, to study the flow distribution. Finally, torque measurements are made to measure the losses.
It was found in the PIV study that oil flow is dominated by a recirculation region. The recirculation is suspected to be the source of viscous losses. The oil flow inside the gearbox is highly three dimensional. Stereo-PIV has successfully revealed the third components of the flow. PIV measurements also reveal that the squeezing of the oil at the gear contact is considered minor compare to the losses due to recirculation. From flash photography, the splash pattern and oil distribution is studied. The oil distribution inside the gearbox is governed by a gear with a larger diameter. The larger diameter gear splashes more oil compared to the smaller one. This is most probably because the larger diameter gear is more immersed compared to the smaller diameter gear. Hence, the larger diameter gear drags more oil upward. Torque measurements show that the torque is dependent on the rotational speed, the immersion depth, and the oil properties. The oil properties are suspected to be changed when the oil is aerated, resulting in a higher torque reading.