Chapter 18

Automatic Transmissions

Objectives (1 of 2)

• Identify the components of a simple planetary gearset.

• Explain the operating principles of a planetary gear train.

• Define a compound planetary gearset and explain how it operates.

• Describe a multiple-disc clutch and explain its role in the operation of an automatic transmission.

Objectives (2 of 2)

• Outline torque path powerflow through a typical four- and five-speed automatic transmission.

• Describe the hydraulic circuits and flows used to control automatic transmission operation.

• List the two types of hydraulic retarders used on Allison automatic transmissions and explain their differences.

Planetary Gear Set (1 of 3)

• A simple planetary gear set can provide:– Overdrive – Reverse – Forward reduction – Neutral– Direct drive ratios

• There are three main components in a simple planetary gearset.– A sun gear– A carrier with multiple

planetary pinion gears mounted to it

– An internally toothed ring gear or annulus

Planetary Gear Set (2 of 3)

• The sun gear is located in the center of the assembly.

• Planetary pinion gears are small gears fitted into a planetary carrier.

• The planetary carrier can be made of cast iron, aluminum, or steel plate.

• Planetary pinions rotate on needle bearings.

Planetary Gear Set (3 of 3)

• The carrier and its pinions are considered one unit—the mid-size gear member.

• The ring gear is the largest component of a simple planetary gearset.

• The ring gear is located the greatest distance from the axis of the sun gear and is therefore capable of exerting the most leverage.

How Planetary Gears Work (1 of 4)

• Power transmission is possible only when one of the three members is held stationary, or if two of the members are locked together.

• Any one of the three members—sun gear, pinion gear carrier, or ring gear—can be used as the driving or input member.

• At the same time, another member might be kept from rotating and thus becomes the held or stationary member.

• The third member then becomes the driven or output member.

How Planetary Gears Work (2 of 4)

• Depending on which member is the driver, which is held, and which is driven, either a torque increase or a speed increase is produced by the planetary gearset.

• Output direction can also be reversed through various combinations.

How Planetary Gears Work (3 of 4)

• Laws of simple planetary gear operation– See Table 18-1 on page 531 of the textbook.

How Planetary Gears Work (4 of 4)

• When an external-to-external gear tooth set is in mesh, there will be a change in the direction of rotation at the output.

• When an external gear tooth is in mesh with an internal gear, the output rotation for both gears will be the same.

Maximum Forward Reduction

• With the ring gear stationary and the sun gear turning clockwise, the external sun gear teeth rotate the planetary pinions counterclockwise.

• Each planetary pinion pushes against its shaft, moving the planetary carrier clockwise.

• The small sun gear must rotate several times to turn the larger planetary carrier through one complete revolution.

• This combination represents the most gear reduction that can be achieved in one planetary gearset.

Minimum Forward Reduction

• The ring gear drives the planetary pinions clockwise and walks around the stationary sun gear.

• The planetary pinions drive the planetary carrier in the same direction as the ring gear—forward.

• More than one turn of the input is needed for one complete revolution of the output. – But because a large gear

is driving a small gear, the amount of reduction is not as great.

Maximum Overdrive

• The ring gear is stationary and the planetary carrier rotates clockwise.

• The planetary pinion shafts push against planetary pinions. – The pinions are forced to

walk around the inside of the ring gear, driving the sun gear clockwise.

• The carrier is rotating less than one turn input compared to one turn output.

Slow Overdrive

• In this combination, the sun gear is stationary and the carrier rotates clockwise.

• As the carrier rotates, the pinion shafts push against the pinions and they are forced to walk around the held sun gear.

• This drives the ring gear faster.

• The carrier turning less than one turn causes the pinions to drive the ring gear one complete revolution in the same direction as the planetary carrier.

Slow Reverse

• Here, the sun gear is driving the ring gear while the planetary carrier is held.

• The planetary pinions, rotate counterclockwise on their shafts.

• While the sun gear is driving, the planetary pinions are used as idler gears to drive the ring gear counterclockwise.

• Because the driving sun gear is small and the driven ring gear is large, the result is slow reverse.

Fast Reverse

• For fast reverse, the carrier is still held as in slow reverse, but the sun gear and ring gear reverse roles, with the ring gear now being the driving member and the sun gear driven.

• In this combination, the input ring gear uses the planetary pinions as idlers to drive the output sun gear in reverse.

• The sun gear rotates in reverse to the input ring gear.

Direct Drive

• Both the ring gear and the sun gear are input members.

• Opposing forces lock the planetary pinions against rotation so that the entire planetary gearset rotates as one complete unit.

• For direct drive, both input members must rotate at the same speed.

Neutral

• When no member is held stationary or locked, there will be input into the gearset, but no output.

• The result is a neutral condition.

Summary of Simple Planetary Gearset Operation

• When the planetary carrier is the input, the gearset produces an overdrive. Speed increases and torque decreases.

• When the planetary carrier is the driven (output) member, the gearset produces a forward reduction. Speed decreases and torque increases.

• When the planetary carrier is held, the gearset will produce a reverse. To determine if the speed produced is fast or slow, remember the rules regarding large and small gears.

• A large gear driving a small gear increases speed and reduces torque of the driven gear.

• A small gear driving a large gear decreases speed and increases torque of the driven gear.

Planetary Gear Controls

• Multiple-disc clutches serve as both braking and power transfer devices.

• A multiple disc clutch uses a series of circular friction discs to transmit torque or apply braking force.

• The discs have internal teeth that are sized and shaped to mesh with splines on the clutch assembly hub. – This hub is connected to a planetary geartrain

component so that gearset members receive the desired braking or transfer force when the clutch is applied or released.

Multiple Disc Clutch Design

• The drum holds all other clutch components: the cylinder, hub, piston, piston return springs, seals, pressure plate, clutch pack (including friction plates), and snap rings.

• The cylinder in a multiple-disc clutch is relatively shallow. The cylinder bore acts as a guide for piston travel.

• The piston is made of cast aluminum or steel with a seal ring groove around the outside diameter.

• A seal ring seats in the groove. This rubber seal retains fluid pressure required to stroke the piston and engage the clutch pack.

• The piston return springs overcome the residual fluid pressure in the cylinder and move the piston to the disengaged position when clutch action (holding or transfer) is no longer needed.

Multiple Disc Clutch Cooling

• Operating temperatures can reach 1,100° F, so clutch disc cooling is an important design consideration in transmission manufacturing.

• Friction disc surfaces are grooved to improve cooling.

• Grooving provides the following advantages: – It allows the disc to store fluid to lubricate, cool, and

quiet clutch engagement. – Grooves in the friction disc surface provide channels

through which the fluid can escape and drain quickly. – When a clutch is disengaging, any fluid on the disc

and plate surfaces can create a suction effect, making the two surfaces more difficult to separate.

Compound Planetary Gearsets (1 of 3)

• Multiple planetary gearsets are coupled to produce the required gear ratios and direction.

• The four-speed heavy-duty transmissions have three simple gearsets that we call front, center, and rear.

• Five-speed transmissions can add a low planetary gearset to the three-gear configuration for a total of four gearsets.

• Input torque is directed to one of the planetary sun gears through the use of clutches.

Compound Planetary Gearsets (2 of 3)

• A four-speed, three-gearset transmission uses only five clutches, excluding the torque converter lockup clutch.

• A five-speed, four-gearset transmission uses six clutches. They function as follows: – Forward clutch locks converter turbine to main input

drive shaft. – First clutch anchors rear planetary ring gear. – Second clutch anchors front planetary carrier. – Third clutch anchors sun gear shaft. – Fourth clutch locks main input drive shaft to center

sun gear shaft. – Low clutch (five speeds only) anchors low planetary

carrier.

Compound Planetary Gearsets (3 of 3)

• Because the simple planetary gearsets are interconnected by common shafts and drums, the output of one planetary gearset can be used as the input of another planetary gearset.

• Therefore, one gearset can take engine input torque, modify speed, torque, and direction, and pass this onto a second gearset.

• The second gearset then further modifies speed, torque, and direction before directing the power flow to the output shaft and vehicle drivetrain.

• With two simple planetaries working together, torque input is compounded.

Laws of Simple Planetary Gear Operation

• See Table 18-1 on page 531 of the textbook.

Planetary Gearset Combinations

• See Table 18-2 on page 537 of the textbook.

Oil Pump

• An oil pump is used to circulate oil for:– Lubrication– Cooling– Hydraulic clutch application

• The pump is located ahead of the gearing and clutches, and it will not be driven when the vehicle is towed or pushed.

• Whenever the vehicle must be towed or pushed more than a short distance (one-half mile maximum), the driveline must be disconnected or the driving wheels raised. – Failure to do this will result in premature wear and

damage to internal components.

Four-speed Clutch Application

• See Table 18-4 on page 543 of textbook.

Five-speed Clutch Application

• See Table 18-5 on page 546 of textbook.

Converter Cooler Circuit

• Converter-in oil flows to the torque converter. Oil must flow through the converter continuously to keep it filled and to remove heat generated by the converter. The converter pressure regulator valve regulates converter-in pressure by exhausting excessive oil to sump.

• Converter-out oil, exiting the torque converter, flows to an external cooler (a vehicle OEM-supplied component). Ram air or engine coolant is used to remove heat from the transmission oil at the cooler heat exchanger.

• Lubrication oil is directed from there through the transmission to those components requiring continuous lubrication and cooling.

Modulator Pressure Circuit

• Modulator pressure is used to modify shift points in response to throttle position.

• Modulator pressure is highest when the engine is running at idle and decreases in proportion to accelerator pedal travel.

• The modulator valve assists the governor pressure in moving any of the shift signal valves during an upshift.

• It can also delay a downshift. A decrease in modulator pressure will cause a downshift if governor pressure is not sufficient to oppose the shift valve spring force.

• The trimmer boost and trimmer regulator valve are controlled by modulator pressure.

Trimmer Regulator Valve

• The trimmer regulator valve reduces main pressure to a regulated pressure. The regulated pressure is raised or lowered by changes in modulator pressure.

• This varies the clutch apply pressure pattern of the trimmer valves.

• A higher modulator pressure at zero accelerator pedal travel will reduce trimmer regulator pressure.

• A lower modulator pressure (open throttle) results in higher regulator pressure and a higher initial clutch apply pressure.

Lockup Circuit

• Lockup clutch engagement and release are controlled by the modulated lockup valve and the lockup relay valve.

• The purpose of the modulated lockup valve is to prolong lockup clutch engagement while vehicle speed decreases (closed throttle).

• This feature provides engine-braking action at speeds lower than the normal lockup disengagement point.

• In its downward position, the modulated lockup valve directs main pressure to the top of the lockup valve. Main pressure pushes the lockup valve downward. In its downward position, the valve directs main pressure to the lockup clutch. This engages the clutch.

Priority Valve

• Gives the control system priority– The priority valve ensures that the control system

upstream from the valve will have sufficient pressure during shifts to perform its automatic functions.

• Without the priority valve, the filling of the clutch might require a greater volume of oil (momentarily) than the pump could supply and still maintain pressure for the necessary control functions.

• In many models, first clutch pressure can bypass the priority valve when the transmission is in reverse.

Inhibitor Valve

• Inhibits downshifts– In five-speed transmissions, a 2–1 inhibitor

valve prevents a downshift from second gear to first gear when road speed is too high.

• Prevents overspeeding– It will also protect the engine from

overspeeding during downgrade operation in first gear by making a 1–2 upshift if road speed exceeds that which is safe for first gear operation.

Clutch Circuits

• There are five separate clutches in four-speed transmissions and six clutches in five-speed models.

• The first, second, third, and fourth clutches are each connected to a relay valve and to a trimmer valve.

• The forward clutch is connected directly to the selector valve and does not connect to a trimmer valve.

• Four-speed transmissions will automatically shift from first to second, second to third, and third to fourth.

• Five-speed models will automatically shift from second to third, third to fourth, and fourth to fifth. These shifts occur as a result of governor pressure, and (if the throttle is not fully open) modulator pressure.

Automatic Upshifts

• A combination of governor pressure and modulator pressure, or governor pressure alone, upshifts the transmission to second or third gear, respectively.

• At closed or part throttle, modulator pressure exists and will assist governor pressure. When the throttle is closed, shifts occur at lower wheel speeds.

• At full throttle, there is no modulator pressure, meaning that upshifts occur at higher road speeds.

• The greater the vehicle speed, the greater the governor pressure. – When governor pressure is sufficient, the first upshift will occur.

With a further increase in governor pressure, the second upshift will occur, etc.

• In any automatic upshift, the shift signal valve acts first. This directs a shift pressure to the relay valve. The relay valve shifts, exhausting the applied clutch and applying a clutch for a higher gear.

Automatic Downshifts

• Automatic downshifts, like upshifts, are controlled by governor and modulator pressures.

• Downshifts occur in sequence as governor pressure and/or modulator pressure decrease.

• Low modulator pressure (full accelerator pedal travel) will hasten the downshift; high modulator pressure (zero accelerator pedal travel) will delay downshifts.

• In any automatic downshift, the shift signal valve acts first. This exhausts the shift signal holding the relay valve downward. The relay valve then moves upward, exhausting the applied clutch and applying the clutch for the new lower gear.

Downshift and Reverse Inhibiting

• The system is designed, to prevent downshifts at too rapid a rate or to prevent a shift to reverse while moving forward.

• The progressive downshift occurs because the pressure and the valve sectional areas are calibrated to only shift the signal valves downward against governor pressure when governor pressure decreases to a safe downshift speed. – So if speed is too great, governor pressure is

sufficient to hold the shift signal valve upward.• As governor pressure decreases, all shift signal

valves move downward in sequence.

Hydraulic Retarders

• Hydraulic retarders are one of several types of auxiliary braking systems used on heavy-duty trucks.

• Some retarder systems are designed as an integral component of the automatic transmission.

• They provide some vehicle braking on severe downgrades and help maximize service brake life by providing braking action at the driveline.

• Integral retarders are usually available in two automatic transmission configurations: – The input retarder – The output retarder

Input Retarders

• The input retarder is located between the torque converter housing and the main housing. It is designed primarily for highway applications.

• The unit uses a paddle wheel design with a vaned rotor mounted between stator vanes in the retarder housing.

• Retardation or brake mode occurs when transmission oil is directed into the retarder housing.

• The oil causes resistance to the rotation of a vaned rotor. • Retarding capacity can be enhanced by downshifting. • Variable control is achieved by moving a hand lever or activating

a separate foot pedal.• Heat is dissipated by circulating the oil through a high-capacity

transmission oil cooler.

Output Retarders

• Most output retarders are mounted on the rear of the transmission without adding additional length to it.

• The output retarder uses a two-stage principle. – The first stage consists of a rotor/stator hydraulic design. The

second stage can be a powerful, oil-cooled, friction clutch pack. • On activation of the output retarder, quick initial response is

provided by a momentary application of the friction clutch pack while the hydraulic section is being charged with oil.

• The hydraulic section provides better retardation at higher speeds. As the vehicle slows, the second stage friction clutch pack phases in, providing the low-speed retardation power capable of slowing the vehicle to a virtual stop.

Summary (1 of 6)

• Automatic transmissions may be hydromechanical or electronic. All automatic transmissions upshift and downshift with no direct assistance from the driver.

• Factors such as road speed, throttle position, and governed engine speed control shifting between gears.

• Automatic transmissions use compounded planetary gearsets.

• The simple planetary gearset consists of three main components: a sun gear, a carrier with planetary pinions mounted to it, and an internally toothed ring gear or annulus.

Summary (2 of 6)

• Advantages of the planetary gearset are as follows: – Constantly meshed gears – Gear torque loads divided equally– Compact and versatile – Additional ratio and direction variations can be added

by compounding • In a planetary gearset, any one of the three main

members—sun gear, pinion gear carrier, or ring gear—can be used as the driving or input member.

• Depending on which member of the planetary gearset is the driver, which is held, and which is driven, torque or speed increases are produced.

Summary (3 of 6)

• To produce an output, one of the planetary gear members must be held stationary. – In heavy-duty truck transmissions this is achieved

with multiple-disc clutches that can serve as both braking and power transfer devices.

• Compound planetary combinations are several planetary gearsets coupled to produce the required gear ratios and direction.

• Four-speed, heavy-duty transmissions have three simple planetary gearsets—front, center, and rear.

• Five-speed transmissions add an additional low planetary gearset for a total of four gearsets.

Summary (4 of 6)

• In all automatic transmissions, power flows from the torque converter through the transmission planetary gearing and out to the output shaft.

• Transmission oil is drawn from the sump through a filter by the input-driven oil pump.

• Oil pressurized by the pump flows into the bore of the main pressure regulator valve.

Summary (5 of 6)

• The converter/cooler/lubrication circuit originates at the main pressure regulator valve.

• The selector valve/forward regulator circuit is manually shifted to select the operating range desired.

• Main pressure is directed to the governor valve. – The speed of the transmission output shaft controls

the position of the governor valve, which determines the amount of pressure in the governor circuit.

Summary (6 of 6)

• The modulator actuator varies modulator pressure as the accelerator pedal moves.

• Lockup clutch engagement and release are controlled by the modulated lockup valve and the lockup relay valve.

• There are five separate clutches in four-speed transmissions and six clutches in five-speed models, each clutch having its own circuit.

• Hydraulic retarders are one of several types of auxiliary braking systems used on heavy-duty trucks; they are applied by the driver as needed.