Torque Converter

TORQUE CONVERTER

STUDY ASSIGNMENT:

1. CLUTCHES – GENERAL
Automotive clutches depend on friction for their operation. This may be solid friction as in the conventional clutch, or fluid friction and inertia as utilized in the fluid coupling and torque converter. This lesson explains the principles and operation of the fluid coupling and torque converter.

2. FLUID COUPLING
a. General
(1) The hydraulic coupling is the simplest means of transmitting torque hydraulically. It can be called a fluid coupling, a fluid flywheel or a fluid clutch. The ratio of input to output is always one to one.
(2) The principle of this type of drive is illustrated by the action of two electric fans facing each other (Figure 1), one with power connected and the other with the power disconnected. As the speed of the power driven fan is increased the flow of air transmits power to the motionless fan and it begins to rotate. In this case the air is the fluid, but since the two fans are not in closed or closely coupled, this sort of coupling is not very efficient. To make a more efficient fluid coupling, oil is used as the fluid and the two halves or members of the coupling (Figure 2), are mounted very closely together and are in closed in a housing.
b. Operation
(1) Figure 3, shows two members of a fluid coupling. The driving member or impeller, is attached to the engine flywheel, while the driven member or turbine is attached to the transmission input shaft. This shaft is in turn connected through gearing and the propeller shaft to the differential and the wheels.
(2) The hollow space in the two members is filled with oil. When the driving member or impeller, begins to rotate (as the engine is started and runs), the oil is set into motion. The vanes in the driving member (Figure 2) start to carry the oil round with them. As the oil is spun round, it is thrown outward or away from the shaft, by centrifugal force. This means, that the oil moves outward in the driving member in a circular path, as shown by the dotted arrows in Figure 3. However, since the oil is being carried round with the rotating driving member, it is thrown into the driven member at an angle shown by the arrows in Figure 4. The oil thus strikes the vanes of the driven member at an angle, thereby imparting torque, or turning effort, to the driven member. The faster the driving member turns, the oil strikes the vanes of the driven member with more force. The more force the oil imparts to the vanes, the greater the turning effort imparted to the driven member, thus causing it to turn.

(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.

3. TORQUE CONVERTER
a. General
The section will explain the principles of the torque converter and the fundamentals of torque multiplication. Torque multiplication will be discussed first, followed by and explanation of how a torque converter works.
b. Torque Multiplication
(1) A good example of multiplying force may be found in the fulcrum and lever principle. To make the explanation of torque multiplication simpler, consider the torque converter a “floating” fulcrum. To do this it will be necessary to review the function of the fulcrum and lever to see how it applies to the torque converter.
(2) Fulcrum lever principle
(a) The fulcrum and lever principle illustrates how a force can be multiplied. In Figure 7, Diagram “A”, “x” and “y” are in balance because the fulcrum “f” is equally distant from “x” and “y”, and “x” is equal to “y”. In Diagram “B”, Figure 7, the increased load at “x” is balanced by positioning the fulcrum closer to “x” and farther from “y”.
(b) In Diagrams “C” and “D”, Figure 8, two alternate ways to lift the load at “x”, are illustrated. The force at “y” may be multiplied (Diagram C), or the distance from the fulcrum the force is applied can be increased, while reducing the force (Diagram D).
(3) Torque and speed
(a) Figure 9 illustrates one to one speed ratio at a one to one ratio at “B”. Note, that the output speed RPM at “B” will be the same as the input speed at “A”. Whatever torque or twisting effort is put into “A” will be transmitted through the output “B”. If 20 ft lbs of torque go into “A”, 20 ft lbs of torque will come out of “B”, or if there is 30 ft lbs input, there will be 30 ft lbs output, or in other words, the torque ratio is one to one ….. Suppose it becomes necessary to double the lifting force, this can be done in two ways. Increase the power source or relocate the fulcrum. The fulcrum is relocated by moving a large gear into position (Figure 10). For every two revolutions of “A”, gear “B” will make one revolution. If the input twisting effort is 20 ft lbs, the output torque will be 40 ft lbs.
(b) To make the fluid coupling perform as a torque multiplier, it is necessary only to add a reaction member, or fulcrum. The stator becomes the reaction member, by placing a stator (reaction member – Figure 15) between the load and power source, a fulcrum is provided. A torque converter has at least three elements – impeller (driving member), turbine (driven member) and stator (reaction member).

(c) Oil Flow Patterns
(1) To assist us in understanding hydraulic torque multiplication, let us consider three terms. First, Kinetic Energy : Energy opposed by oil in motion. Second, Rotary Flow: The Flow of oil around the outer circumference in the converter. Third, Vortex Flow: The Flow of oil across the converter. Third, Vortex Flow : The flow of oil across the converter.
(2) Kinetic energy
The multiplication of input torque results from the kinetic energy imparted to the oil by the pump (impeller) plus the kinetic energy entering the pump from the stator. The more the turbine resists turning, the greater the velocity of the vortex flow of oil circulating in the converter, and the greater the torque multiplication. The less the turbine resists turning, the less the velocity of the vortex flow of oil in the converter and the less the torque multiplication.
(3) Rotary and vortex flow
The instant the torque converter impeller, which is driven by the engine flywheel, starts rotating, the oil spins around with it. This movement of oil is rotary flow (Figure 11). The converter turbine, connected to the load, resists turning as the oil strikes its blades. Because of this resistance, and because of the shape of the blades within the converter elements, the oil takes a second path of travel cross-wise. This second path is called vortex flow (Figure 12). The greater the load resistance transmitted by the turbine, the greater will be the vortex flow.
(d) Principle of Operation
The torque converter provides varying drive ratios between the driving member (impeller) and the driven member (turbine). This is accomplished by using curved vanes in both the driving and driven member, and by using one or more extra members. The extra members act to reduce the splashing effect mentioned before in paragraph 2b (4). Figure 13 shows the curvature of the vane in the torque converter members.
(e) Power Flow
The impeller (Figure 13 (1) is driven by the engine. The turbine (Figure 13 (3) is attached to the converter output shaft. The stator (Figure 13 (2) is supported between the impeller and turbine and can either be held from rotating by being mounted rigidly on a fixed support, or mounted in a one way clutch which permits it to turn free when the driving and driven members are both turning at about the same speed. The impeller is driven by the engine; the turbine, connected to the output shaft rotating free (no mechanical connection to the impeller or stator); the stator is positioned between the turbine and impeller and directs the flow of oil between the two units.

(f) Action of the Torque converter
(1) The vanes in the impeller are curved in one direction, and the vanes in the turbine are more curved in the opposite direction. The curvature of these vanes is critical for they are designed to develop high torque efficiency. To visualize the operation of the torque converter, it is important to remember that the oil flows essentially in two directions (paragraph 2c (2) ; around the converter in the direction of rotation (Figure 11), and around the torus shaped ring formed by the impeller, turbine, and stator (Figure 12).
(2) When the impeller is turned by the engine, centrifugal force tries to throw the oil in the impeller outward. However, the oil cannot be thrown outward in a straight line because of the curvature of the impeller and vanes. Therefore, the oil is thrown against the vanes of the turbine (Figure 14). The force with which oil is thrown against the vanes of the turbine causes the turbine to turn in the same direction as the impeller.
(3) After the oil passes over the turbine vanes, it leaves the inner edge of these vanes, and travels in a direction almost opposite to that in which the turbine is traveling (Figure 15).
(4) If nothing were done to change the direction of the oil flowing from the turbine, it would strike the blades of the impeller on their leading surfaces and tend to stop the impeller. This would cause the engine to work harder to keep the impeller turning. At this point the reaction member becomes vital.
(5) The stator (reaction member – Figure 15 (2), has no mechanical connection to either the impeller or turbine. It fits in between the outlet of the turbine and the inlet of the impeller so all the oil has to pass through it when returning from the turbine to the impeller. The stator changes the direction of the oil flowing from the turbine so that it will go in the same direction as the impeller. Also, the openings between the stator blades speed up the flow of oil so that it re-enters the impeller in such a manner that less engine torque is required to drive the impeller (Figure 15).
(6) Actually, the change-over from one phase to the next phase of operation, as outlined above, is not sudden but gradual and is in accordance with the changing demands of the operation. When starting and accelerating, as the turbine speed nears the impeller speed, the changing pattern of oil flow eases the back pressure on the stator vanes. As a steady speed is reached on a level road the impeller and turbine speeds become nearly equal. This means that further changes in the pattern of oil flow will have taken place and will not contribute anything to the operation of the converter. So, there is a continuous circulation of oil through the three units of the torque converter. From the impeller, through the turbine, through the stator, and back to the impeller.

g. Variations in Torque Converters
While the foregoing description covers torque converters generally, some torque converters have more members (other than the impeller, turbine, and stator).
(1) Three element torque converter
The three element torque converter found on many pieces of equipment will have either a fixed or a rotating stator (reaction member).
(a) The fixed stator is mounted rigidly in the converter and will not turn.
(b) The stator that is mounted on a one-way clutch is able to turn in the same direction as the engine only (Figure 16).
(c) The overrunning clutch used in a torque converter is generally a sprag-type. The sprags are somewhat like flattened rollers (Figure 17). The inner and outer races of the overrunning clutch are smooth. A series of sprags are positioned between the inner and outer races and are held in place by two springs put into the sprag notches (Figure 17). The outer race is stationary but the inner race is splined to the stator hub and therefore will turn with the stator. During steady running, the stator is not needed and as mentioned will rotate. The sprags shown in Figure 17, have no effect on the forward rotation of the inner race. During acceleration the oil must change direction and the oil is thrown against the front faces of the stator vanes. This produces a backward thrust, or pressure on the stator vanes, which halts the stator and attempts to turn it backwards. As this happens, the sprags jam between the inner and outer races, thereby locking the inner race so that it cannot turn backward. The stator then becomes stationary so that its vanes can effectively change the direction of the oil flow.

(2) Four element torque converter
(a) The four element torque converter contains a driving member, or impeller, a driven member or turbine, plus a primary stator and a secondary stator. Each of these stators is mounted on an overrunning type clutch shown in Figure 17. When the torque converter is first started, the impeller is rotation much faster than the turbine. The two stators are held stationary and they redirect the oil as it leaves the turbine. This is shown by the arrows in Figure 18. In other words, as the oil comes off the turbine vanes, it strikes the stationary stator vanes and has its direction changed. This enables I to enter the impeller in a direction that does not hinder the impeller rotation.
(b) The turbine speed increases until it nears the speed of the impeller. The oil leaving the turbine vanes gradually changes direction until it begins to strike the back faces of the primary stator vanes. This will cause the primary stator to begin to rotate. The secondary stator still is stationary and will continue to change the direction of oil.
(c) When the speed of the turbine is near equal to that of the impeller the oil will now strike the back of the secondary stator vanes. Now the secondary stator also begins to rotate. Neither stator enters into the action now, and the converter acts as a fluid coupling.
(3) Five element torque converter
(a) The five element torque converter is similar to the four element converter, the main difference is the addition of a secondary impeller. The secondary impeller is not used during heavy load or hard acceleration operation. It is mounted on an overrunning clutch (Figure 17) which allows it to spin faster than the primary impeller, however, the secondary impeller locks when it attempts to rotate slower than the primary impeller. This locking forces both impellers to turn at the same speed.
(b) The force that causes the secondary impeller to spin faster than the primary impeller comes from the oil striking the back faces of its vanes.
h. Torque Multiplication Factors
Since torque converters vary considerably in design, the amount of torque multiplication they can achieve also varies with the practical limit being 5:1. The torque multiplications would normally depend on the size of the converter, the number of elements and the piece of equipment the torque converter was designed for.

4. PREVENTIVE MAINTENANCE SERVICES
a. General
The fluid coupling and torque converter are assemblies which cannot be disassembled in the field. Therefore, there is little in the way of service that can be given.
b. Lubrication
The importance of proper lubrication cannot be over emphasized, since it is the most essential single factor that will determine the life of the units.
(1) Always refer to lubrication order pertaining to the piece of equipment begin serviced.
(2) When ever oil is changed in the fluid coupling or torque converter, the oil filter should be replaced and the screen cleaned before new oil is added.
NOTE : NEVER FLUSH THE FLUID COUPLING OR TORQUE CONVERTER.
(3) Before changing oil in the fluid coupling or torque converter, the oil temperature should be at the operating range. If the piece of equipment begin serviced is not equipped with an oil temperature gage, start the engine and let it run until the engine temperature gage has risen to the operating range.

5. TROUBLESHOOTING
a. General
The ability to diagnose troubles in the fluid coupling or torque converter requires a knowledge of the construction and operation of these units. Also, it requires a knowledge of what to check for, what troubles may occur, and what effects these troubles will cause.
b. Overheating
Should the fluid coupling or torque converter, become overheated, a high temperature reading on the temperature gage will result, and a hot oily smell may be emitted. Overheating may be caused by loss of oil, failure of the oil to circulate, failure of the oil cooler to function, or overload. Loss of oil could result from a leaky oil seal or even from a relief valve that is stuck open, this condition permits the oil to flow directly from the oil pump back to the reservoir without entering the coupling or torque converter. The lack of sufficient oil will cause the units to overheat. If the oil cooler is clogged with dirt, or if the coolant is low, the engine oil temperature will be high. Operating in the wrong speed range will overwork the coupling or torque converter causing excessive heat.
c. Slipping
It is not always easy to detect a slipping fluid coupling or torque converter without certain tests. They both naturally slip to a certain extent; this slippage varies with the terrain in which the equipment operates, the driving conditions, and the temperature. An accurate test for slippage can be made by using an engine RPM indicator (tachometer0. To do this, measure the engine speed and then with the service and parking brake applied accelerate the engine with transmission in high. If engine accelerates rapidly without lugging, a serious slippage is occurring, and the oil level should be checked.
Fluid couplings and torque converters can be mounted to a transmission in a housing or can be made integral by itself. If mounted together with a transmission, some of the troubles may be diagnosed as part of the transmission and therefore are much harder to detect.

GLOSSARY :
The glossary of terms used in this lesson is designed to provide a ready reference. They are not intended to be all-inclusive, but have the purpose of serving as reminders.

Centrifugal Force The force acting on a rotating body, which tends to move its parts outward and away from the center of rotation.

Clockwise Direction of movement, usually rotary, which is the same as movement of the hands on the face of a clock.

Counterclockwise Direction of movement, usually rotary, which is opposite in direction of the hands on the face of a clock.

Efficiency Ratio between the effect provided and the power expanded to produce the effect.

Fluid Coupling A device in the power train consisting of two rotating members. It transmits power through a fluid from the engine to the power train.

Friction The resistance to motion between two bodies in contact.

Fulcrum A support such as a wedge-shaped piece of hinge, about which a lever turns.

Impeller The pump, or driving member, in the torque converter.

Integral Whole ; entire ; lacking nothing of completeness.

Oil Cooler A special cooling radiator, through which hot oil passes. Air also passes through separate passages in the radiator to cool the oil.

GLOSSARY CONT’d :

Oil Filter The part of the lubricating system that removes dirt and dust from the oil circulated through it.

Oil Seal A seal placed around a rotating shaft to prevent the escape of oil.

Overrunning Clutch A type of clutch which will transmit rotary motion in one direction only.

Pump A device to produce motion of liquids. In the torque converter it is the driving member.

Reactor The stator in the torque converter which provides reactive blades against which the oil can change direction.

Sprag Unit A form of overrunning clutch ; power can be transmitted through it in one direction, but not in the other.

Stator In the torque converter, a third member which changes the direction of oil under certain operating conditions (when stator is stationary).

Torque Turning or twisting effort, usually measured in foot pounds.

Torque Converter A device in the power train consisting of three or more rotating members, which transmits power through a fluid. It provides varying drive ratios ; with sped reduction, it increases torque.

Torus Rotating member of the fluid coupling (driving or driven torus).

Turbine A mechanism containing curved blades, the turbine is driven by the impact of oil against the curved blades. The driven member of the torque converter.