US Patent 7757487 Drive apparatus

A drive apparatus for a vehicle, the apparatus comprising a hydraulic pump, two control valve assemblies, and two pairs of hydraulic drive assemblies. The control valve assemblies are hydraulically connected in parallel, and in an operational condition hydraulic fluid is pumped through the control valve assemblies by the pump to drive the hydraulic drive assemblies, and wherein each control valve assembly proportionately distributes hydraulic fluid to a respective pair of the hydraulic drive assemblies in response to the hydraulic loads of the respective hydraulic drive assemblies of the respective pairs of hydraulic drive assemblies. In response to a difference in the respective hydraulic loads of the hydraulic drive assemblies of the respective pairs of hydraulic drive assemblies, flow of hydraulic fluid to the hydraulic drive assembly of each of the respective pairs of hydraulic drive assemblies having the lower hydraulic load is limited, and wherein each of the hydraulic drive assemblies includes a ground engaging drive member and at least one hydraulic motor for driving the drive member.