Use of the hydrostatic transmission (HST) in compact riding mowers has recently become more widespread due to the fact that an HST provides considerably better mowing performance than a conventional manual transmission. In addition, there are now several manufacturers that are capable of building compact and efficient HSTs.
The standard power train of an HST-equipped compact riding mower consists of the engine, HST, and transaxle, laid out separately and interconnected by a V-belt. As a result, the power transmission system is highly complicated, making maintainability difficult.
The recently developed model PA540A and PA420A unitized power units are compact power packages combining a vertical gasoline engine, HST, and transaxle in a single structure that can be mounted with as few as 3 bolts, allowing much greater freedom in mower design. Moreover, each component is mechanically linked to the others and shares lubricating and actuating oil with the other components inside the power unit, simplifying maintenance considerably. In designing such a unitized power system, certain technical problems associated with integration must be overcome in addition to the usual design problems presented by the construction of the individual components. In this paper we will explain the technical considerations associated with unitization, provide concrete examples of such, and describe the experiments that led to success.