Lesson 2.1: The Variable Displacement Pump

This is the "sending" unit, connected to the engine by a belt and pulley. It's an axial piston pump.

Inside, a set of pistons move back and forth inside a rotating cylinder block.

The key component is the swashplate. The operator controls the angle of this plate with the speed control pedal or lever.

• Neutral: When the swashplate is perfectly vertical (90∘ to the pistons), the pistons just spin around without pumping any fluid. The wheels don't move.

• Forward: When you press the forward pedal, the swashplate tilts. This tilt forces the pistons to pump oil out as they rotate. The steeper the angle, the more oil they pump, and the faster the mower goes.

• Reverse: Pressing the reverse pedal tilts the swashplate in the opposite direction, reversing the flow of oil and making the mower move backward.

Analogy: Think of the swashplate like the pitch control on a helicopter's rotor. A flat pitch (neutral) creates no lift. Tilting the blades (the swashplate) creates lift (oil flow).