TRANSAXLES
Learning Objective: Identify components of the manual and automatic
transaxles. State the differences between transmissions and transaxles.
A transaxle is a transmission and differential combination in a single
assembly. Transaxles are used in front-wheel drive vehicles. A transaxle allows
the wheels next to the engine to propel the vehicle. Short drive axles are used
to connect the transaxle output to the hubs and drive wheels.
Vehicle manufacturers claim that a transaxle and front-wheel drive has
several advantages over a vehicle with rear-wheel drive. A few of these advantages
are the following:
- Improved efficiency and reduced drive train weight
- Improved traction on slippery surfaces because of increased weight on the
drive wheels
- Increased passenger compartment space (no hump in floorboard for rear drive
shaft)
- Less unsprung weight (weight that must move with suspension action), thereby
providing a smoother ride
- Quieter operation since engine and drive train noise is centrally located in
the engine compartment
- Improved safety because of the increased mass in front of the passengers
Most transaxles are designed so that the engine can be transverse
(sideways) mounted in the engine compartment. The transaxle bolts to the rear
of the engine. This produces a very compact unit. Engine torque enters the transaxle
transmission. The transmission transfers power to the differential. Then the differential
turns the drive axles that rotate the front wheels.
Both manual and automatic transaxles are available. Manual transaxle
uses a friction clutch and a standard transmission-type gearbox. An automatic
transaxle uses a torque converter and a hydraulic system to control gear
engagement.
MANUAL TRANSAXLE
A manual transaxle uses a standard clutch and transmission. A foot-operated
clutch engages and disengages the engine and transaxle. A hand-operated shift
lever allows the operator to charge gear ratios. The basic parts relating to a
manual transaxle are as follows:
- Transaxle Input Shaft—main shaft splined to the clutch disc turns the gear in
the transaxle.
- Transaxle Input Gears—either freewheeling or fixed gears on the input shaft
and meshes with the output gears.
- Transaxle Output Gears—either fixed or free- wheeling gears driven by the
input gears.
- Transaxle Output Shaft—transfers torque to the ring gear, pinion gears, and
differential.
- Transaxle Synchronizers—splined hub assemblies that can lock freewheeling gears
to their shafts for engagement.
- Transaxle Differential—transfers gearbox torque to the driving axle and
allows the axles to turn at different speeds.
- Transaxle Case—aluminum housing that encloses and supports parts of the transaxle.
The manual transaxle can be broken up into two separate units—a manual
transaxle transmission and a transaxle differential. A manual transaxle transmission
provides several (usually four or five) forward gears and reverse. You will
find that the names of shafts, gears, and other parts in the transaxle vary,
depending on the location and function of the components. For example, the
input shaft may also be called the main shaft, and the output shaft is called
the pinion shaft because it drives the ring and pinion gear in the
differential. The output, or pinion, shaft has a gear or sprocket for driving
the differential ring gear.
The clutch used on the manual transaxle transmission is almost
identical to the manual transmission clutch for rear-wheel drive vehicles. It
uses a friction disc and spring-loaded pressure plate bolted to the flywheel.
Some transaxles used a conventional clutch release mechanism (release bearing
and fork); others use a long pushrod passing through the input shaft.
The transaxle differential, like a rear axle differential, transfers
power to the axles and wheels while allowing one wheel to turn at a different
speed than the other. A small pinion gear on the gearbox output shaft or
countershaft turns the differential ring gear. The ring gear is fastened to the
differential case. The case holds the spider gears (pinion gears and axle side
gears) and a pinion shaft. The axle shafts are splined to the differential side
gears.
AUTOMATIC
TRANSAXLE
An automatic transaxle is a combination automatic transmission and differential
combined into a single assembly. The basic parts of an automatic transaxle are
as follows:
- Transaxle Torque Converter—(fluid-type clutch that slips at low speed but
locks up and transfers engine power at a predetermined speed; couples and
uncouples engine crankshaft to transmission input shaft and gear train).
-
Transaxle Oil Pump—(produces hydraulic pressure to operate, lubricate, and cool
the automatic transaxle; its pressure activates the pistons and servos).
- Transaxle Valve Body—(controls the flow of the fluid to the pistons and
servos in the transaxle; it contains hydraulic valves operated by the operators
shift linkage and by engine speed and load-sensing components).
- Transaxle Pistons and Servos—(operates the clutches and bands when activated
by fluid pressure from the valve body).
- Transaxle Clutches and Bands—(applies planetary gears in the transaxle;
different bands and clutches are activated to operate different units
in the gear-sets).
- Transaxle Planetary Gear sets—(provides different gear ratios and reverse in the
automatic transaxle).
- Transaxle Differential—(transfers powers from the transmission components to
the axle shafts).
Many of the components used in the automatic transaxle
are also found in the automatic transmission. Operating principles of these
components are the same as the automatic transmission. The differential of the
automatic transaxle is similar to that used on the manual transaxle.
REVIEW QUESTIONS
Q1. What is the purpose of the output shaft of a manual transaxle?
Q2. In an automatic transaxle, what component(s) operate(s) the clutches
and bands?