Gearbox

Need of the Gearbox

• To be able to regulate both power output and speed range of the engine relative to the range of speeds over which the vehicle is at any given time likely to be required to operate.

• Driver can regulate torque by selecting the appropriate speed range or the vehicle speed at which max. torque is obtainable.

Forces opposing the motion of Vehicle

• Aerodynamic Forces• Gradient forces• Rolling forces

Aerodynamic Forces

• Air offers a resistance to the passage of bodies through it, as does water or any other fluid.

• The magnitude of this resistance is dependent directly upon the shape and frontal area of the body exposed to the fluid it is passing through, and to the square of its velocity.

• The graph shows that the air resistance is directly proportional to the square of the velocity of the vehicle.

• The air drag becomes significant only after a speed of 70 kmph and below this speed we can simply neglect it.

Gradient Resistance

• The resistance offered by the road is the gradient resistance.

• The gradient resistance is dependent solely on the steepness of the slope and is unaffected by the speed of the car.

• A force more than H is required to make the vehicle move uphill if we neglect the effect of aerodynamic forces and rolling resistance.

• It is independent of all other factors and depends only on angle ‘Alpha’.

Rolling Resistance

• All the remaining forces resisting motion at a constant speed come under the heading of rolling resistance.– Energy dissipated at the tyres– Energy loss in the transmission

• Energy loss in transmission is generally neglected.

• Depends greatly on the nature of road and the tyres.

• Soft roads/tyres cause larger dissipation of energy as compared to hard surfaces.

• On soft surfaces, the wheels are, in effect, being continuously driven up a ramp to lift the vehicle out of the rut into which its wheels have sunk.

Graph of total resistance vs the vehicle speed in kmph

Graph of total resistance vs speed for different surfaces

Variation of tractive effort with speed

• A plot of tractive effort, against vehicle speed for a given gearbox ratio, a given final drive ratio and a given road wheel radius is shown at RS in Fig.

• If the gearbox ratio is altered we shall get another curve of tractive effort.

• If the gearbox ratio is altered so that the total gear ratio between engine and the back wheels is doubled we get curve TU.

• The vehicle speed will be halved but the tractive effort will be doubled.

• The graph shows the relation between the tractive effort, the total resistance and the speed of the vehicle.

• A-F are the different total resistances for a road with uniform surface but varying gradients.

• Curve A is the level whereas the curve B is the steepest gradient.

Constant Power Curve

Graph Details

• Let engine gives its maximum power at all speeds then,Power = TE × Speed.

• It follows that the TE will be inversely proportional to the road speed. The graph of the TE plotted against the road speed would then be like the full line curve.

• The TE curves for an actual engine and gearbox combination will touch this constant power curve at one point (corresponding to the engine speed at which the engine gives its maximum power),

• It could be obtained with any engine if an infinitely variable gearbox of 100% efficiency were available.

• The curves A and E represent the horsepower required to overcome the resistances represented by the curves A and E of Slide 14.

• The curves RS and TU represent the horsepower available at the road wheels on the two gears corresponding to the curves RS and TU.

• The greatest speeds possible on the roads to which A and E refer are given by the intersections of the horizontal line representing the maximum horsepower of the engine.

• Best gear ratio= max speed achieved= max. power delivered by engine at that speed.

• The maximum speeds possible with the gear ratios for which the curves RS and TU are drawn are given by the intersections H and L respectively.

Gear ratio

• Lets say the driving gear has 25 teeth and the driven gear has 50 teeth.

• Then the gearing ratio is defined by:

Gr= 25/50= 0.5

• It means for every one complete rotation of the input gear the output gear turns by half a revolution.

• Lets say we have a idler in between. Teeth on driver are 25, teeth on driven gear are 50 and teeth on the idler are 37.

So what is the final gearing ratio?

Sample Problem

Gear ratios and the rpm of a standard 5-speed gearbox

Final Reduction Gear

• Present in the differential itself.• It has generally got a high value.• The final reduction gear has a value

somewhere around 4:1 to 6:1.• From here the drive goes directly into the

wheels and this determines the actual velocity of the wheels.

Sliding mesh type gearbox

• The pinion B meshes continuously with the spur wheel D secured to the lay shaft E, which is arranged parallel to the shaft A and carried at its ends in the ball bearings shown. Secured to the lay shaft are four other spur gears, F, G, H and J.

• M is a single spur gear while N is a double gear, being in effect two gears N1 and N2 made in one piece.

• The gears M and N can be slid along the main shaft when required by selector forks T and U respectively.

• The main shaft K is connected to the road wheels.

• The gearbox shown provides four forward speeds and one reverse gear.

First Gear

• The gear M occupies the position shown in Fig.

• The gear N is slid along the main shaft K until it occupies the position shown in Fig.

• It then meshes with the gear H of the lay shaft. The drive comes from the engine shaft through the constant-mesh gears B and D to the lay shaft, is then transmitted through that shaft to the gear H and thence through the gear N1 to the main shaft.

• The gear ratio, or the ratio between the speeds of the engine and main shafts, is—

speed of engine shaft/ speed of main shaft=

=n1

Second Gear

• The gear M continues to occupy the same position, but the gear N is slid over to the left as shown in Fig.

• It then meshes with the gear G of the lay shaft. The drive is from the engine shaft through the constant-mesh gears to the lay shaft, through that shaft to the gear G thence to the wheel N2 and the main shaft.

• The gear M continues to occupy the same position, but the gear N is slid over to the left as shown in Fig. It then meshes with the gear G of the lay shaft. The drive is from the engine shaft through the constant-mesh gears to the lay shaft, through that shaft to the gear G thence to the wheel N2 and the main shaft.

• The gear ratio is—

speed of engine shaft/ speed of main shaft=

=n2

Third Gear

• The gear N occupies the position shown in Fig. The gear M is slid to the right into the position shown in Fig, where it meshes with the gear F of the lay shaft. The drive is from the engine shaft through the constant mesh gears to the lay shaft, thence to the gear F, through that gear to the gear M and the main shaft.

• The gear ratio is—

Speed of engine shaft/ speed of main shaft=

=n3

Fourth Gear

• The gear N occupies the position shown in Fig, while the gear M is slid over to the left as shown in Fig.

• The dog teeth on M then engage in the spaces between the dog teeth on B, thus connecting B and M and giving a direct drive between the engine and the main shaft.

• The gear ratio is then 1 : 1. The lay shaft now revolves idly.

Direction of rotation of the Shaft

• On all the above gears the direction of rotation of the lay shaft is the opposite to that of the engine shaft, while the direction of rotation of the mainshaft is opposite to that of the layshaft.

• The mainshaft rotates in the same direction as the engine shaft

Reverse Gear

• The gear N is slid over to the right, but farther than when first gear was being obtained. It reaches the position shown in Fig. ; but in this position it does not mesh with the pinion J of the layshaft, since that pinion is made small enough to clear the gear N1.

• The gear N1 does, however, mesh with the reverse idler Q which is carried on a shaft on which it is free to revolve. The shaft is fixed in the gearbox casing and the idler Q is constantly in mesh with the pinion J of the layshaft.

• The gear ratio is given by:-=n4

• It will be noticed that the number of teeth in the idler does not affect the gear ratio, but only the direction of rotation.

Constant mesh gearbox

• The engine shaft A is integral with a pinion B, which meshes with the wheel C on the layshaft.

• Wheels E, F and G are fixed to the layshaft just as in a sliding-mesh gearbox.

• The gears E, F and G are, however, in constant mesh with the wheels H, I and J, which are perfectly free to turn on the mainshaft.

• The gears H, I and J, therefore, are constantly driven by the engine shaft, but at different speeds, since the wheels E, F and G are of different sizes.

• Any one of the gears H, I or J when coupled up to the mainshaft will give driving connection between that shaft and the engine shaft.

Dog Clutch

• The coupling is done by means of the dog clutch members L and M, which are carried on squared portions of the mainshaft.

• They are free to slide on those squared portions, but have to revolve with the shaft.

• When member M is slid to the left it will couple the wheel I to the mainshaft giving the first gear. The drive is then through the wheels B, C, F and I and the dog clutch M.

• When the member L is slid to the right, it will couple the wheel H to the mainshaft and give second gear.

• The reverse gear is engaged by sliding the member M to the right when it will couple the wheel J to the mainshaft.

Advantages of Constant over Sliding Mechanism

• Facilitates the use of helical or double helical gear teeth which are quieter than straight teeth.

• Incorporation of synchronising devices is more readily done than the sliding-mesh box.

• The dog clutch teeth can be made so that they are easier to engage than the teeth of gear wheels.

Advantages of Constant over Sliding Mechanism

• Damage that results from faulty manipulation occurs to the dog clutch teeth and not to the teeth of the gear wheels.

• Once dog clutches are engaged, there is no motion between their teeth.

• When gear teeth are engaged the power is transmitted through the sliding action of the teeth of one wheel on those of the other.

Advantages of Constant over Sliding Mechanism

• The teeth have to be suitably shaped to be able to transmit the motion properly, and if they are damaged the motion will be imperfect and noise will result.

• Damage is less likely to occur to the teeth of the dog clutches, since all engage at once, whereas in sliding a pair of gears into mesh the engagement is between two or three teeth.

5 speed constant mesh gearbox

Dog Gear in neutral position

Dog gear in engaged position.

Reverse gear

Grinding Gears???• The common misconception is that this grinding

noise is the teeth of the gears grinding together.• Rather it's the sound of the teeth on the dog

gears skipping across the dog teeth of the helical output gears and not managing to engage properly.

• This typically happens when the clutch is let out too soon and the gearbox is attempting to engage at the same time as it's trying to drive.

Double Clutching• Used in old cars for shifting from one gear to the

other.• Clutch is pressed for the first time to disengage

the dog gear from the gear.• Then the engine had to be revved up to match

the speed of the shaft with the speed of the gears at which they are rotating.

• Then clutch is pressed again and dog gear is engaged with the desired gear.

Synchromesh

• Introduction of synchromesh has obviated the need of double clutching.

• A synchro is a device that allows the dog gear to come up to a speed matching the helical gear before the dog teeth attempt to engage.

• the synchro does the job of matching the speeds of the various gearbox components for you.

Crash Boxes

• They use straight-cut gears instead of helical gears.

• Straight-cut gears have less surface area where the gears contact each other, which means less friction, which means less loss of power.

• Straight-cut gears are mostly submerged in oil rather than relying on it sloshing around like it does in a normal gearbox

Sequential gearbox

• In a sequential box, the selector forks are connected to a single shaft that has corkscrew-type grooves in it.

• The collar that fits around this selection shaft has a ball bearing in it which sits in a recess in the collar as well as in one of the corkscrew grooves.

• When the gearstick is moved forwards or backwards, the selector shaft is mechanically turned by some number of degrees.

• That twisting motion rotates the corkscrew groove which in turn interacts with the ball bearings and the selector fork collars, forcing them to slide back and forth.

• Each click of the gearstick rotates the shaft another number of degrees and all the selector forks change position in one go.

• the gears are always selected in sequence. You can't jump from first to third; you have to go via second.

Automatic gearbox

Planetary Gearset

• Any planetary gear set has three main components. The sun gear, the planet gears and the ring gear.

• Any one of these three components can be locked in place, but more importantly, anyone can be the input or the output drive.

Compound Planetary Gearset

Mechanism

• There are two sun gears and two sets of intermeshing planet gears.

• The inner ones are shorter and only engage the small sun gear and the outer planet gears.

• The outer planet gears engage the larger sun gear at the bottom and the outermost ring gear.

How it works

First gear and Second Gear

• When first gear is engaged, the smaller sun gear (green) is driven.

• The planet carrier (red) tries to spin the opposite direction but because of a one-way clutch system, it locks in place which forces the ring gear (blue) to turn instead.

• The ring gear becomes the output from the gearbox in this case and there you have first gear.

• When second gear is engaged the input is again the small sun gear but this time the ring gear is held in place by a band and the output becomes the planet carrier.

Locking the ring gear

• A band is wrapped around the outside of the ring gear and when tightened, they lock the ring gear in place.

• Bands are actuated by a lever or pivot connected to a small hydraulic piston in the gearbox housing.

• When the piston is pushed down; it tightens the band and clamps the ring gear into place, locking it to the gearbox case.

Park it!!!

Mechanism

• There are notches on the outside of the clutch housing and a single or pair of spring-loaded catches.

• When you put the gearbox in 'P' for park, the catches are deployed and they fit into the notches on the outside of the clutch housing

Torque Converters

Basic construction

• A torque converter consists of three basic elements. The impeller, the turbine and the stator.

• The impeller is attached to the torque converter housing which itself is attached to the engine flywheel.

• The impeller is basically a centrifugal pump. As the flywheel spins, so does the impeller and the vanes take the fluid from the central part of the torque converter and fling it to the outside creating a pumping action.

• The fluid then circulates around the outer edge of the torque converter and back into the turbine.

• The turbine is basically the opposite of the impeller - it's like a ship's propeller in that the fluid passing through it causes it to spin.

• The turbine is connected to the input shaft of the gearbox via a splined shaft so as the turbine spins, so does the input shaft to the gearbox.

• The fluid passes through the turbine from the outside towards the inside. Finally, as the fluid reaches the central core, it passes through the stator which is designed to help redirect the flow into the inner vanes of the impeller.

• Without the stator, the whole system would be a lot less efficient. With this mechanism, the fluid is constantly being circulated

DSG

• A DSG gearbox has one layshaft like a normal gearbox, but two output shafts that mesh to a third shaft which goes to the differential.

• One output shaft has 1st, 3rd and 5th gears on it whilst the other has 2nd, 4th and 6th.

• The layshaft is actually two shafts one inside the other connected to two concentric 4-plate basket-type clutches at the end.

Cutaway of DSG

• In first gear, one clutch is engaged and the central layshaft is connected to the engine.

• Selector forks have the first dog-gear engaged with the first helical gear and the car is moving forwards.

• At the same time though, on the second output shaft, the second dog gear is already engaged with the second helical gear.

First Gear

• Because the outer clutch on the layshaft is disengaged though, there is nothing driving this second gear and the outer layshaft is simply spinning freely.

• At the point when the gearbox needs to shift up, it simply engages the second clutch at the same moment it disengages the first and the outer layshaft is now being driven from the engine.

Second Gear