Automatic Transmissions

An automatic transmission does away with all the clutch and gearshift operations that are normally carried out by the driver of a manual transmission vehicle. As well as automatic shifts, the transmission has a torque converter which acts as a fluid coupling. it also acts as a form of variable gear ratio that automatically adjusts to the operating conditions of a vehicle. These transmissions are now fitted to  most modern vehicles. They consist of three basic sections: A torque converter, Gearing, Hydraulic system.
Torque converter:
The torque  converter can transmit different torque ratios. Three main parts in the converter are: impeller, turbine, stator. With gears, speed reduction means torque multiplication. This principle also applies to a torque converter. Speed reduction with in the converter results in an increase in torque. Torque multiplication is achieved by the action of the stator.
When the gear selector lever is in D, and the engine idling, the fluid in the converter does not have enough force to turn the turbine, there for the vehicle stays stationary. when the engine speed picks up the oil will be thrown into the turbine with great force. at this point there will be maximum torque multiplication. as the speed increases the speed between the impeller and the turbine is quickly reduced. a point is reached where there speeds are about the same this is called coupling point. at this point there is no torque multiplication and the converter acts as a fluid coupling. coupling point occurs when the turbine speed reaches approximately 90 percent of the impeller speed. Below coupling point, the force of fluid against the stator blades cause the one way clutch to lock and hold the stator to  prevent it from rotating backwards. The function of the one way clutch is to prevent torque loss.

Fluid flow in a torque converter

Gears:

Most automatic transmissions use a planetary gear set up. they have three main parts: the ring gear, the planet carrier and pinions and the sun gear. all three parts are concentric. The pinions connect the ring gear to the sun gear. The pinions not only rotate on their shafts, but they can also walk around the teeth of both the sun gear and ring gear

 

All the gears are in constant mesh and do not slide. the gear ratios are obtained by holding one of the parts stationary with a brake. one part then acts as a driver and the other part is the driven. The two ways  the gear set operates is called non-planetary and planetary depending on which part is being held and which part is the driver.

Final drives and Differentials

Final drive:
The crown wheel and pinion are the final drive gears in a rear wheel drive car. The gears are called Hypoid gears.
Hypoid gears are a special design of spiral bevel gears which have the centreline of the pinion below the centerline of the crown wheel. One advantage for passenger cars is that the proppeller shaft or input shaft is lower and the rear floor can be made flatter. Hypoid gears are used in commercial vehicles as well. The tooth shape of these gears provide a greater area of tooth contact and so greater strength, although correct gear adjustment is needed to prevent gear noise.
Hypoid gears have high tooth loading. The teeth do not merely roll against each other, but have a wiping action as well. this requires a special lubricant which must have high pressure additives. Gear oil in the housing is picked up by the crown wheel and circulated to the pinion and bearings.

Differential:
The differential divides torque between the rear wheels and also enables the driving wheels to rotate at different speeds when turning. When turning the outer wheel turns further than the inside wheel. With out a differential, the driving wheels would have to skid every time the car is steered. As well as causing tyre wear, the vehicle would be almost impossible to steer.

Differential gears.

Clutches, universal joints, and c.v joints

Clutches.

A clutch is a device that is used to engage or disengage one mechanical part from another. In a manual car the clutch is controlled by the driver. Engines need clutches because the engine has to be turned on and running at speed before the load can applied. The clutch allows the load to be applied gradually so the vehicle can start to move steadily. The clutch is disengaged and engaged so that gear shifts can take place. The clutch has to be held firmly onto the flywheel because all the torque from the engine passes through it into the gearbox.

The clutch is made up of the fly wheel face, clutch disc, pressure plate assembly, and release mechanism. The rear of the flywheel has a machined surface, the pressure plate assembly is bolted to the flywheel, it provides a flat face for the clutch disc, which is held against by the pressure plate. The clutch disc is splined to the input shaft. When the clutch is engaged the disc is held firmly between the pressure plate and flywheel. The pressure plate has a spring loaded plate which holds the disc firmly against the flywheel. There are two types of pressure plates: diaphragm spring and coil spring pressure plates.

Diaphragm clutches are commonly used for passenger and light commercial vehicles.

The release mechanism is made up of a release fork and release bearing. The fork is a lever with e forked end that carries the release bearing. When disengaging the clutch the fork pivots on a ball stud which in turn forces the bearing against the diaphragm fingers. The bearing in the assembly is a thrust bearing designed to be operated under pressure. The ball race is mounted on a carrier, in some cases the carrier is part of the bearing assembly. Most clutch bearings cannot be lubricated, as they are packed with lubricant during manufacture.

Constant-Velocity joints.

these joints are designed to transmit rotary movement to shafts at a constant velocity. They also help reduce shaft vibration. They are used on rear-axle drive shafts for rear wheel drive vehicles with independent rear suspension, on the drive shafts of front wheel drive vehicles and sometimes used on rear wheel propeller shafts. There a three types of c.v joints, Birfield, Double-offset, and tripod. 

Birfield c.v joint

Birfield joints are used as the outer c.v joint on the drive shaft.

Double-offset c.v joint

Tripod c.v joint

Universal Joints.

A universal joint is made up of two yokes attached to a spider, or cross, with the parts free to move in relation with each other. when it is connected to two shafts it allows the shafts to operate at an angle transmitting the driving force from one to the other.

This joint will transmit constant velocity only if the shafts are in line with each other. When the shafts are  at an angle the driven shaft speed will increase slightly then decrease slightly as the joint rotates. this effect is called velocity fluctuation.This can cause noticeable shaft vibration. The greater the angle of the shafts the greater the greater the vibration.

What makes up the cross-and-yoke universal joint. It has two yokes with a cross in between, the needle rollers are carried in bearing cups. The cups fit onto the yokes, and over the ends of the cross so that the needle bearings operate on the cross trunnions. 

Manual Transmissions

In the first week we had to disassemble and measure the components of the gear box.

The purpose of a transmission is to produce the torque required to get the vehicles wheels moving along the road. the transmission provides a range of gearing which allows the engine to operate at different speeds  to suit the condition of the terrain the vehicle is on. for example on take off you want a lot of torque to get moving.

Before dismantling we had to do a visual check of the exterior, looking for cracked housings, bolts missing, threaded holes blocked etc. We also had to check that it selects each gear. The gearbox we where given only selected 2nd gear. The cause of the fault could be incorrect re-assembly, we found out that was problem. The end bearing cir clips were not installed properly so the gears were not meshing properly.

There are many types of bearings within the gearbox, they all have different properties to them. Cage roller, Ball race, Tapered roller, Needle bearings just to name a few. We checked all the bearings for pitting and signs of damage. Reverse gear doesn't all ways have a bearing, it has a bush. This is because reverse gear is not a main drive gear and not used as heavily as the forward moving gears.

The gearbox uses a synchromesh system to select gears. The syncromesh system is made up of a hub, sleeve, baulk rings, inserts,and springs. 

The hub is splined to the mainshaft and the sleeve is splined to the hub. The inserts fit between the hub and the sleeve. They lie in slots in the hub and are held outward by a spring at each end. They have a small bead which locates them in a groove in the sleeve. The baulk rings located at each end of the hub have slots for the inserts. they are made of bronze or aluminium and have a coned inner surface to match the steel cones on the gears. the teeth on the outer edge of the baulk rings, the small teeth on the gears, and the splines inside the sleeve are all the same size. How the synchro system works. When the sleeve is moved to select a gear the inserts move and hold the bead of the insert into the slot in the sleeve. the inserts push the cone of the baulk ring against the cone of the gear, friction between these two surfaces slow the shaft or speed the shaft up so that the parts turn at the same speed. once the speed is synchronised the sleeve can slide over the small teeth of the baulk ring and gear. Once the sleeve is engaged with the gear teeth, the gear will be connected to the mainshaft and gear selection complete.

Gear Ratios.
The gear ratio is determined by the number of teeth in each gear. To work out the gear ratio of each gear you can use a formula, ratio=driven gear/driving gear.  gear ratios are used to get the optimum output of the engine   under different circumstances.