chapter 17 torque converters. objectives (1 of 2) explain the function of the torque converter in a...
TRANSCRIPT
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Chapter 17
Torque Converters
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Objectives (1 of 2)
• Explain the function of the torque converter in a vehicle equipped with an automatic transmission.
• Explain how the torque converter is coupled between the crankshaft and the transmission.
• Identify the three main elements of a torque converter and describe their roles.
• Define torque multiplication and explain how it is generated in the torque converter.
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Objectives (2 of 2)
• Define both rotary and vortex fluid flow and explain how each affects torque converter operation.
• Describe the overrunning clutch, lockup clutch, and variable pitch stators.
• Outline torque converter service and maintenance checks.
• Remove, disassemble, inspect, and reassemble torque converter components.
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Shop Talk
• Torque converters can be confused with fluid couplings because both use similar operating principles.
• The most fundamental difference is that torque converters use curved blades, while fluid couplings and fluid flywheels use straight pitch blades.
• Torque converters also use stators and have the ability to multiply torque, neither of which is characteristic of fluid couplings.
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Torque Converters
• Automatic transmissions use a torque converter to couple the engine to the transmission.
• The torque converter:– Transmits the twisting force or torque delivered to it
by the engine crankshaft
– Multiplies engine torque when additional power is needed
• The amount of torque transferred from the engine to the transmission by the torque converter is directly related to engine rpm.
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Torque Converter Construction
• Truck torque converters can be:– Serviceable
• Many torque converters are welded together. A normal service shop cannot disassemble them for servicing and repair.
– Non serviceable• The only types of torque converters (T/Cs) that
are readily serviceable are the types that are bolted together.
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Flex Plate
• The flex plate carries the starter motor ring gear.
• The combined mass of the torque converter and flex disc acts like a flywheel to smooth out the power pulses produced by the engine.
• The flex plate also allows for a slight alignment tolerance between the engine and torque converter assembly.
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Components
• The torque converter has three main components.– Impeller
– Turbine
– Stator
– Optional lockup clutch
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Exterior
• The exterior of the torque converter shell is shaped like two bowls facing each other. – They are either welded or
bolted together.• A pilot shaft supports the
weight of the torque converter at the front.
• At the rear of the torque converter shell is the pump drive hub with notches or flats which are used to drive the transmission pump.
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• Curved blades rotate as a unit with the shell at engine speed. It starts the transmission oil circulating within the torque converter shell.
• The impeller is positioned with its back to the transmission housing; the turbine is positioned with its back to the engine.
• The hub of the turbine is splined so that it can drive the turbine (transmission input) shaft. The turbine shaft transfers engine torque to the transmission gearing.
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• The turbine blades are designed to have a greater curve than the impeller blades.
• This helps reduce oil turbulence between the turbine and impeller blades–turbulence that would slow impeller speed and reduce the converter’s efficiency.
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• A fundamental law of hydraulics states, “The more the moving stream of fluid is diverted (changed), the greater the force it places on the curved reaction surface.”
• As oil in the torque converter moves around the turbine blades, it pushes against the blades and transmits additional force.
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Principles of Operation
• As the pump impeller rotates, centrifugal force throws the oil outward and upward.
• The faster the impeller rotates, the higher the centrifugal force.
• Fluid under pressure is continuously delivered through the converter hub.
• It is important to note that the oil pump delivering the fluid is driven by the engine.
• A seal or combination of seals prevents fluid from being lost from the system.
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• Oil leaving the turbine is directed to an external oil cooler and then to the transmission oil sump.
• At idle there is insufficient centrifugal force within the torque converter to move the truck.
• As impeller speed increases the centrifugal force of the oil directed against the turbine blades becomes great enough to move the vehicle.
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Shop Talk
• There can be a mechanical connection between the impeller and turbine by the use of a lockup clutch. – A lockup clutch eliminates slippage between
the impeller and turbine at certain speeds.– This helps to reduce the heat generated in the
fluid and improves fuel mileage.
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Types of Oil Flow (1 of 2)
• The two types of oil flow that occur within the T/C are:– Rotary
• Rotary flow describes the centrifugal force applied to the fluid as the converter rotates on its axis.
– Vortex• Vortex flow is the
circular flow that occurs as the oil is forced from the impeller to the turbine and then back to the impeller.
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Types of Oil Flow (2 of 2)
• If a toy pinwheel were held at arm’s length and swung in a large circle, air movement at the outer circle would produce rotary flow, while the small circles cut by the pinwheel’s propeller vanes would produce vortex flow.
• The point when the speed of the turbine approaches the speed of the impeller is referred to as the coupling point.
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Split Guide Rings
• Fast moving oil exits the impeller blades, striking the turbine blades with considerable force.
• It then has a tendency to be thrust back toward the center of both impeller and turbine.
• To control this fluid thrust and the turbulence that results, a split guide ring is located in both the impeller and turbine sections of the T/C.
• The guide ring suppresses turbulence, allowing more efficient operation.
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Stator (1 of 3)
• The stator is the key to torque multiplication. It redirects the oil leaving the turbine back to the impeller.
• The stator then redirects the fluid flow so that the oil reenters the impeller, moving in the same direction as the impeller.
• The kinetic energy remaining in the oil now helps rotate the impeller with more force, multiplying torque.
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Stator (2 of 3)
• The roller clutch is designed with an inner race, rollers, accordion (apply) springs, and outer race.
• Around the inside diameter of the outer race are several cam-shaped pockets. – The rollers and accordion
springs are located in these pockets.
• As the vehicle begins to move, the stator stays in its stationary or locked position.
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Stator (3 of 3)
• Locking mode takes place when the outer race attempts to rotate counterclockwise.
• The accordion springs force the rollers down the ramps of the cam pockets.
• As vehicle road speed increases, turbine speed approaches impeller speed.
• Oil exiting the turbine vanes now strikes the back face of the stator, causing the stator to rotate in the same direction as the turbine and impeller, unlocking the clutch.
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Variable Pitch Stator
• Each of a series of movable stator vanes has a crank rod fitted into a circular groove in the hydraulic piston.
• The movement of a hydraulic piston varies the angle of the stator vanes.
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Lockup Clutches
• A lockup torque converter eliminates the 10 percent slip that takes place at the coupling phase of operation.
• The engagement of a clutch has the advantage of improving vehicle fuel economy, lowering overall engine emissions, and reducing torque converter operating heat and engine speed.
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Caution
• The torque converter is free to move forward when the transmission is disconnected from the engine.
• To ensure that the torque converter does not separate from the transmission while the transmission is being removed from the vehicle, install a retaining strap to hold the T/C in position.
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Lockup Clutch Back Plate
• The back plate should be flat to within 0.006 inch.
• Inspect the football key slot for evidence of wear from movement of the lockup back plate and football key.
• Excessive wear or elongation of the football key slot will require component replacement.
• Inspect the back plate for cracks, from the key slot to the ID of plate.
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Turbine Assembly
• Inspect the turbine assembly for cracked or broken vanes and signs of overheating.
• If the turbine assembly is taken apart, inspect the rivet holes for signs of wear or elongation.
• The turbine assembly hub should be checked for stripped, twisted, or broken splines.
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Lockup Clutch Piston
• Inspect the seal ring groove.
• Check the lockup pinholes.
• Inspect the friction surface for wear, scoring, scratches, signs of overheating, and flatness. – It should be flat to within
0.003 inch TIR.
• Inspect the seal surface.
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Inspecting the Stator Assembly
• Inspect the stator assembly. • Inspect the rivets for cracks
or loose fit. • Check the stator cam roller
pockets.• Inspect the stator thrust
bearing race surface.• Check the stator freewheel
roller surface, thrust bearing surface, and roller bearing surface.
• Inspect the stator freewheel rollers.
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Pump/Impeller
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Pump (Impeller) Hub (1 of 2)
• Check the seal ring grooves on the pump hub for cracked edges, nicks, burrs, and sharp edges.
• Inspect the front seal surface for scoring, scratches, nicks, and grooves. – No rework of any
irregularities noted is allowed on the seal surface.
– The use of a crocus cloth or light honing on this surface could promote leakage past the front seal.
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Pump (Impeller) Hub (2 of 2)
• There should be no signs of cracks, scoring, metal transfer, or heat damage on the pump drive flange.
• Inspect the snap ring groove for burrs, cracks, and nicks.
• Check the bearing race surface.
• Inspect the roller bearing bore.
• Inspect the gasket surface. • Inspect the pump hub for
pulled, stripped, or crossed threads.
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Summary (1 of 2)
• Automatic truck (and passenger car) transmissions use a type of fluid coupling known as a torque converter to transfer engine torque from the engine to the transmission.
• A flex plate, sometimes called a flex disc, is used to connect the torque converter to the crankshaft.
• Transmission oil is used as the medium to transfer energy from the engine-driven impeller to the turbine, which in turn drives the transmission.
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Summary (2 of 2)
• Two types of oil flow take place inside the torque converter.– Rotary flow and vortex flow
• A converter lockup clutch enables a mechanical coupling of the engine and transmission.