allows for the control of the hydraulic motor's ability to use the energy provided by the pump. By adjusting the swash plate in the motor, the stroke of its pistons are changed. If the swash plate is set to neutral, the pistons do not move in their bores and no rotation is allowed, thus the machine does not move. By moving the lever, the swash plate moves its attached pistons forward, thus allowing them to move within the bore and causing the motor to turn. This provides an infinitely variable speed control from 0 ground speed to what ever the maximum speed is allowed by the gear selection of the transmission. The standard clutch was removed from this drive system as it was no longer needed. Most if not all modern combines are equipped with hydrostatic drives. These are larger versions of the same system used in consumer and commercial lawn mowers that most are familiar with today. In fact, it was the downsizing of the combine drive system that placed these drive systems into mowers and other machines. The threshing process Despite great advances in mechanics and computer control, the basic operation of the combine harvester has remained unchanged almost since it was invented. Power requirements over the years have increased due to larger capacities and some processes such as rotary threshing and straw chopping take considerable power. This is sometimes supplied by a large tractor in a pull-type combine, or a large gasoline or diesel engine in a self-propelled type. A frequent problem is the presence of airborne chaff and straw, which can accumulate causing a fire hazard and to radiators which can become plugged. Most machines have addressed these problems with enclosed engine compartments and rotary centrifugal inlet screens which prevent chaff buildup. First, the header, described above, cuts the crop and feeds it into the threshing cylinder. This consists of a series of horizontal rasp bars fixed across the path of the crop and in the shape of a quarter cylinder. Moving rasp bars or rub bars pull the crop through concaved grates that separate the grain and chaff from the straw. The grain heads fall through the fixed concaves. What happens next is dependent on the type of combine in question. In most modern combines, the grain is transported to the shoe by a set of 2, 3, or 4 (possibly more on the largest