Axle torque distribution in four wheel drive tractors

摘要

Four wheel drive (4WD) tractors now dominate their two wheel drive counterparts in many regions of the world - in particular Europe and the USA. The benefits in terms of increased drawbar pull and work rate are well understood. 4WD tractors vary in the design of the power split between front and rear axles; from equal wheels F:R implying approximately a 50:50 power spilt to much smaller front wheels - often referred to as front-wheel-assist - which may have a F:R split of around 20:80. in optimising the tractive performance of 4WD tractors it is necessary to understand the details of the power flows, torque distributions and losses within the driveline system However, a review of previous works suggests that these driveline torques and power splits are surprisingly not well understood - in particular there are three areas of uncertainty. First, since the 4WD systems on agricultural tractors are normally arranged through a fixed gear ratio F:R, there is the problem of selecting this value so that the peripheral speeds of the front and rear wheels are the same - in practice, variations are caused by different tyre sizes, pressures and wear conditions for example. Second, the tractive conditions for the rear wheels may be significantly different from those at the front, because they travel in the compacted ruts established by the leading front tyres. And third during turning, the front steered wheels are kinematically constrained to move on different radii, and therefore travel faster than the rears. Taking all these factors into account, it is fairly common in choosing a compromise gear ratio to build in a small amount of lead, in which the front wheels move faster, say 2 to 4%, than the rears. In the literature, there is no universal agreement about the relationship between lead ratio and overall tractive efficiency. This situation is not helped by the fact that there have been relatively few attempts to measure F:R axle powers and torques in normal field operating conditions. The aim of the research presented here is to use a new method based on non-contact torque sensors in the front and rear axles of a 67kW front-wheel-assist tractor in order to measure the power and torque flows in the driveline during normal draught cultivation operations.