Q As a student of engineering have read
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with interest the forecasts of leading engineers that hydrostatic transmission will eventually be applied to heavy commercial vehicles.
I have seen references to split-torque systems; how do these systems differ from the integral pump/motor unit and the pumpand-multiple-wheel-motor type? And what are their advantages?
AIn a split-torque system the torque is
divided between a pump/motor unit and a planetary gear train, the drive ratio being varied in the usual way by employing a variable-output pump. The future of hydrostatic drive for road vehicles—as distinct from
the earth-moving type undoubtedly depends upon increasing the efficiency of the
system. This can be done by using small highpressure units that operate with low pumping losses; the split-torque system promises to provide even higher efficiencies.
Power loss in a mechanical train of gears is less than the pumping losses in a hydrostatic system. The overall power loss is therefore reduced if part of the torque is transmitted through a planetary, gear train. In practice the torque throughput of the hydrostatic units may be as little as 30/40 per cent of the total.
In the case of a split-torque system the pump/motor unit and planetary gear train have to be built as an integral unit, and separate wheel motors cannot therefore be employed.