On the selection of parameters of double-flow hydromechanical transmission for industrial and forestry tractors

Abstract

Double-flow hydromechanical transmissions are widely used in industrial and forestry tractors. The main advantage of such transmission is the higher value of the maximum efficiency coefficient in comparison with the single-flow one. The article presents a method of selection of parameters of such transmissions for industrial and forestry tractors. In this case, it considers the hydromechanical transmission with a differential unit at the input and at the output. Three-link differential devices with mixed type of gear engagement are used as the differential unit. All of the twelve possible schemes of double-flow hydrodynamical transmissions are considered. It is shown that double-flow hydrodynamical transmission has an operation mode when the torque converter turbine is rotating counter to the rotation of pump wheel. This factor is taken into account during the selection of parameters of double-flow hydrodynamical transmission. The article describes the method of construction of external characteristic of the transmission and matching of its load characteristic with the full-load curve of diesel engine. The matching is performed by three ways: the selection of required value of active diameter of torque converter; the selection of reduction ratio of matching gear; the selection of characteristic of planetary gear set. It is found that for the most common scheme of double-flow hydromechanical transmission when the characteristic of planetary gear set is reduced, its maximum efficiency coefficient is increased, the active diameter of torque converter is reduced and the relative rotation frequency of satellite gears of three-link differential device is increased. In this case, the rotation frequency of satellite gears limits the minimum value of characteristic of planetary gear set. Increasing of characteristic of planetary gear set leads to the reducing of transparency of the hydromechanical transmission and to increasing of its transformation ratio. It is shown that the double-flow hydromechanical transmission allows to utilize more fully the converting properties of engine in comparison with the single-flow one; it allows to change the torque value if external engine load is changing, that is very important for industrial and forestry tractors.