A new transmission control system which realizes almost the same high level of operability as of a TORQFLOW (T/F) transmission (see Note 1) has been developed and introduced on an efficient HYDROSHIFT (H/S) transmission (see Note 2) for bulldozers. To develop the system, comparison and evaluation were made between H/S transmission machines without torque converter and T/F transmission machines with torque converter to ascertain the advantages and disadvantages of H/S transmission machines. As a result, the disadvantages inherent to H/S transmission machines, such as poor shifting characteristics, fine travel characteristics at low travel speed, and anti engine-stall characteristics were solved by electronically and synthetically controlling all clutches including those incorporated into the steering brake system, without sacrificing any advantages.
Concerning the excess thermal load applied on the transmission clutch at Forward/Reverse (F/R) directional change, the new system greatly lowers the thermal load on the clutch by interlocking the transmission and steering brakes to share the load, making it possible to reduce the transmission size. In addition, the system effectively lowers peak torque and shock at shifting, realizing excellent shifting characteristics compared with conventional models. An additional benefit of electronic control is that this system has an abnormality diagnostic function leading to reduced machine downtime.
Note 1: Powershift transmission with torque converter
Note 2: Powershift transmission without torque converter