Now you can see what happens in the coupling when the engine is started and the coupling is filled with oil. The primary rotor turns with the engine crankshaft. (Refer to fig. 12-11.) As the primary rotor turns, the oil in the radial passages is forced to flow outward by centrifugal force. (See the arrows in fig. 12-11.) This forces oil across the gap at the outer edge of the rotor

Figure 12-12.-Scoop control hydraulic coupling.

and into the radial passages of the secondary rotor, where the oil flows inward. The oil in the primary rotor not only is flowing outward, but also is rotating. As the oil flows over and into the secondary rotor, it strikes the radial blades in the rotor.

The secondary rotor soon begins to rotate and pick up speed, but it will always rotate more slowly than the primary rotor because of drag on the secondary rotor. Therefore, the centrifugal force of the oil in the primary rotor will always be greater than that of the oil in the secondary rotor. This causes a constant flow from the primary rotor to the secondary rotor at the outer ends of the radial passages and from the secondary rotor to the primary rotor at the inner ends.