A hydraulic mechanical transmission includes a first hydraulic unit having a first shaft and a second hydraulic unit having a second shaft. The second hydraulic unit is connected in hydraulic fluid communication with the first hydraulic unit by high and low pressure lines. At least one of the first and second hydraulic units has variable displacement. A mechanical torque transfer arrangement transfers torque between the first shaft and the rotatable component of the second hydraulic unit. One of the first and second hydraulic units operates as a hydraulic pump and the other of the first and second hydraulic units operates as a hydraulic motor.

A hydraulic mechanical transmission includes a first hydraulic unit having a first shaft and a second hydraulic unit having a second shaft. The second hydraulic unit is connected in hydraulic fluid communication with the first hydraulic unit by high and low pressure lines. A valve having a variable orifice is positioned along the high and low pressure lines, and at least one of the first and second hydraulic units has a variable displacement. A mechanical torque transfer arrangement transfers torque between the first shaft of the first hydraulic unit and a rotatable component of the second hydraulic unit. In use, one of the first and second hydraulic units operates as a hydraulic pump and the other of the first and second hydraulic units operates as a hydraulic motor.

A hydraulic mechanical transmission includes a first hydraulic unit having a first shaft and a second hydraulic unit having a second shaft. The second hydraulic unit is connected in hydraulic fluid communication with the first hydraulic unit by high and low pressure lines. At least one of the first and second hydraulic units has variable displacement. A mechanical torque transfer arrangement transfers torque between the first shaft and the rotatable component of the second hydraulic unit. One of the first and second hydraulic units operates as a hydraulic pump and the other of the first and second hydraulic units operates as a hydraulic motor.

A hydraulic mechanical transmission includes a first hydraulic unit having a first shaft and a second hydraulic unit having a second shaft. The second hydraulic unit is connected in hydraulic fluid communication with the first hydraulic unit by high and low pressure lines. At least one of the first and second hydraulic units has variable displacement. A mechanical torque transfer arrangement transfers torque between the first shaft and the rotatable component of the second hydraulic unit. One of the first and second hydraulic units operates as a hydraulic pump and the other of the first and second hydraulic units operates as a hydraulic motor.

A hydraulic mechanical transmission includes a first hydraulic unit having a first shaft and a second hydraulic unit having a second shaft. The second hydraulic unit is connected in hydraulic fluid communication with the first hydraulic unit by high and low pressure lines. A mechanical torque transfer arrangement transfers torque between the first shaft of the first hydraulic unit and a rotatable housing of the second hydraulic unit. One of the first and second hydraulic units operates as a hydraulic pump and the other operates as a hydraulic motor, and both of the first and second hydraulic units have variable displacement.

A hydrostatic assembly employing a 1.sup.st hydraulic piston drive unit is described in which the output is increased without using additional servo assemblies by incorporating a 2.sup.nd hydraulic piston drive unit, coupling the drive shafts of both drive units together, and employing a common means to simultaneously adjust displacement of both the 1.sup.st and 2.sup.nd hydraulic piston drive units. In such embodiments, coupling the drive shafts together such that the 1.sup.st hydraulic piston drive unit is rotationally offset with respect to the 2.sup.nd hydraulic piston drive allows for a reduction in the amplitude of pressure pulsations associated with the hydrostatic assembly output, thereby smoothing out operation and improving durability.

A hydraulic mechanical transmission includes a first hydraulic unit having a first shaft and a second hydraulic unit having a second shaft. The second hydraulic unit is connected in hydraulic fluid communication with the first hydraulic unit by high and low pressure lines. At least one of the first and second hydraulic units has variable displacement. A mechanical torque transfer arrangement transfers torque between the first shaft and the rotatable component of the second hydraulic unit. One of the first and second hydraulic units operates as a hydraulic pump and the other of the first and second hydraulic units operates as a hydraulic motor.

A hydraulic mechanical transmission includes a first hydraulic unit having a first shaft and a second hydraulic unit having a second shaft. The second hydraulic unit is connected in hydraulic fluid communication with the first hydraulic unit by high and low pressure lines. A valve having a variable orifice is positioned along the high and low pressure lines, and at least one of the first and second hydraulic units has a variable displacement. A mechanical torque transfer arrangement transfers torque between the first shaft of the first hydraulic unit and a rotatable component of the second hydraulic unit. In use, one of the first and second hydraulic units operates as a hydraulic pump and the other of the first and second hydraulic units operates as a hydraulic motor.

A hydraulic mechanical transmission includes a first hydraulic unit having a first shaft and a second hydraulic unit having a second shaft. The second hydraulic unit is connected in hydraulic fluid communication with the first hydraulic unit by high and low pressure lines. A mechanical torque transfer arrangement transfers torque between the first shaft of the first hydraulic unit and a rotatable housing of the second hydraulic unit. One of the first and second hydraulic units operates as a hydraulic pump and the other operates as a hydraulic motor, and both of the first and second hydraulic units have variable displacement.