(3) The turning effort carried to the wheels sets the vehicle in motion, As the driven member approaches the speed of the driving member, the effective force of the oil on the driven member vanes is reduced. If the two members turn at the same speed, then the oil would not pass from one to the other causing power to be transmitted through the coupling. Thus, this “same speed” condition would not exist when the engine is driving the vehicle. The driving member always has to be turning a little faster than the driven member for engine power to flow through the fluid coupling to the wheels, However, if the engine speed is reduced and the vehicle begins to drive the engine, there would be a point at which both members turn at the same speed. Then, as the engine slowed further, the driven member would temporarily become the driving member (since the vehicle would be driving it). As this happens, the normally driven member would begin to pass oil into the normally driving member causing the engine to exert a braking effect on the vehicle.
(4) The fluid coupling (Figure 5), is not very efficient under many conditions because of the turbulence that occurs in the oil. Turbulence is a state of violent random motion or agitation. Under certain conditions (particularly when there is considerable difference in speed between the driving and driven members), the oil will be striking the vanes of the driven member with great force. This would cause the oil to swirl about in all directions, particularly in the center sections of the members as shown in Figure 5. To reduce this turbulence, and to increase the efficiency of the fluid coupling, a split guide ring is centered in the members (Figure 6). Each half of the guide ring is attached to one half of the fluid coupling member. This arrangement does not allow the oil to change and avoids the turbulence as shown in Figure 5.
c. Disadvantages
(1) Essentially, the fluid coupling is a special form of clutch which provides a smooth, vibration less coupling between the engine and transmission, If operates at maximum efficiency when the driven member approaches the speed of the driving member. If there is a big difference in the speeds of the two members, power is lost and efficiency is low. Here’s the reason :
(2) When the driving member is turning considerably faster than the driven member, the oil is thrown onto the vanes of the driven members with considerable force. It strikes the vanes and splashes, or “bounces back”, into the driving member. In other words, the effect actually causes the oil to work against the driving member, therefore, when there is a big difference in the driving and driven speeds, a good part of the driving torque is used in overcoming this “bounce back” effect. Torque is lost and there is a torque reduction through the fluid coupling.
2.