GEARBOXES

THE GEARBOX

In normal gear conditions, the engine of a car turns at 3,500rpm; if we make wheels spin at this speed, the car will run at a speed faster than 350km/h. Therefore, it is clear that there must be an intermediate element to modify the transmission relationship between the engine and the wheels, so that we can always get maximum performance from the engine. This element will also have to allow the direction of the vehicle's gearing to be reversed without reversing the engine's spinning, as this is not possible.

The part in charge of these functions is the gearbox, located inside the transmission chain, just after the clutch.
The gearbox looks like a box with two shafts - one of them linked to the clutch and the other continuing through the transmission chain to the wheels - and a lever driven from inside of the car allowing the most appropriate relationship to be selected at all times.

The working principle of the gearbox is the same applying to the law of leverage, which allows a large weight to be moved by applying a small force, using just a long arm. For this purpose, the main elements within the gearbox are the gear wheels, which work like a series of consecutive levers. If the motion enters via a small pinion and goes out through a larger one, the torque will be increased but the spinning speed will be reduced, whereas if the torque enters via the large pinion, the opposite process will take place.

The following diagram shows a four-gear gearbox. In this case, the cogs are not straight but helicoidal, which makes them much quieter.

On the other hand, in reverse gear, the cogs are placed straight, as in this case the noise is not a problem because the time the vehicle will be in reverse gear will not be long and the speed will always be low. A car makes a characteristic noise in reverse gear when it reaches a certain speed.

The lower (secondary) shaft turns round connected to the shaft coming from the clutch (left). This secondary shaft has fixed gear wheels which are continuously engaged with another set of twin gear wheels located on the output shaft (top). These can turn freely or be attached to the output shaft. If the wheels turn freely, there will be no torque transmission and we will get what is called neutral. By contrast, when they are attached to the shaft, motion is transmitted and the transmission relationship depends on the attached wheel.

NEUTRAL
The motion coming from the clutch is transmitted to the primary (lower) shaft and, as there is no pinion fixed to the top shaft, there is no communication with the transmission chain.

FIRST GEAR
When first gear is selected, we see how the larger wheel (in red) is attached. Motion is transmitted from a small (lower) wheel to a large (higher) one, so we get a reduction in the turning speed but an increase in torque transmitted.

The other gears are selected the same way, except for two particular cases: top gear and reverse gear.

Gear selection

There must be a system that allows the pinions to be attached to the shaft and released so that one gear or another can be selected.

The system used is the sliding gear.

The sliding gear engagement consists of one part always spinning round connected to the output shaft, but which can slide lengthwise. It also has teeth on both sides which can engage with other teeth on the output shaft pinions. Once these teeth are engaged, the corresponding pinion will spin, pulling the sliding gear engagement and, therefore, the output shaft.

All this can be seen on the following diagram. On the left, the sliding gear fixes the third gear pinion and, on the right, it is coupling top gear.

TOP GEAR
We get the top gear by directly coupling the input and output shafts without going through the secondary shaft, as shown on the diagram. There is therefore no reduction.

REVERSE GEAR
If we want the vehicle to move backwards, we must reverse the turning direction of the output shaft. We can achieve this by inserting a third wheel between the secondary and output shafts. We can see that these teeth are straight, compared to the helicoidal ones used in all other gears.

AUTOMATIC GEARBOX

ACTIVATION OF GEARS
Next we will see how gears are selected in an automatic gearbox with two connected planetary gears. This gearbox achieves two forward gears and one reverse gear.

FIRST GEAR
When activating the forward clutch, the engine (red arrow) pulls the first crown wheel. This makes the planetary gear turn in the opposite direction. The second planet carrier (light grey) is fixed, so the planets pull the second crown wheel and the output shaft.

SECOND GEAR
When activating the forward clutch, the engine pulls the first crown wheel. The common planetary gear is braked and the crown wheel pulls the planets and planet carrier, whose shaft is also the output, so there is only one reduction. The second planetary gear (dark grey) turns without load around the planetary gear.

THIRD GEAR
When activating the forward clutch, the engine pulls the first crown wheel. The reverse clutch is also activated and joins the input shaft to the planetary gear, so both turn at the same speed.

REVERSE GEAR
The forward clutch does not operate and the first crown wheel spins without load. The reverse clutch starts and the engine pulls the planetary gear. The second planet carrier is braked and the planetary gear pulls the planets so they turn round; the second crown gear is therefore forced to turn in the opposite direction.

MULTI-PLATE DISC CLUTCH
A clutch activated by pressure oil is in charge of fixing and releasing the corresponding parts to achieve different transmission relationships.

The engine torque is connected to and disconnected from the planetary gear elements through multi-plate disc friction clutches soaked in oil. These are activated by pressure oil controlled by the command valves.

The diagram shows that the clutch on the left is disconnected, whereas that on the right is connected, thanks to the oil pressure.

PRODUCTION PROCESS
With a single phone call and by sending a breakdown test form to our customer, we can immediately determine the cost of the repair and delivery time for the gearbox.

We have all the technical information and human resources required to assess whether the vehicle's problem is the gearbox.

Once the breakdown has been determined, the customer sends us the gearbox, or, for a more direct and effective diagnosis, the vehicle.

The gearbox is taken out of the vehicle. Our operators are duly qualified to perform this removal as quickly as possible and to check for other irregularities in the vehicle that may affect the operation of the gearbox.

The gearbox is cleaned on the outside to get a better view of it, dismantling it completely and following the manufacturer's instructions step by step, using the specific required tools.

Once the gearbox is dismantled, our operators search for the cause of the breakdown and start a thorough cleaning of each and every component.

In our spares warehouse, we have all spares necessary for rebuilding the gearbox in the minimum time.

All parts subject to wear, such as seals, bearings, synchro rings, friction discs, etc. are replaced, as well as o-rings, gasket rings, filters, brake straps.
All the automatic gearbox's electrical parts are checked: sensors, solenoids.
The gearbox is then assembled again, always using the best spares and taking into account assembly measurements, tolerances and tightening torque recommended by each manufacturer.

Thanks to our special gearbox test bench, which helps us simulating vehicle operation, we eliminate any human error or material fault that might occur after quality processes have been passed.

Once the product has passed quality control, the gearbox is ready to be sent, with all the information on its assembly, as well as the results of the simulator tests and, if the customer so requires, the replaced parts.
If we have the vehicle, we carry out the road tests and delete the breakdowns from the control unit memory.

After finishing the assembly, we check the appearance of our product.

Concepts and history

What is the need for a gearbox?
The gearbox is necessary to transmit the rotary motion from the crankshaft to the driving wheels after changing the ratio according to the condition of the terrain and the driver's needs.

Types of gearbox assembly.

Manual
The driver selects the correct ratio using the gear lever. The driver also has to activate the clutch to suppress any kind of force coming from the engine that affects the moving parts of the gearbox.

Semi-automatic
The driver selects the right ratio using the gear lever. The clutch is activated automatically, while the driver concentrates on driving.

Automatic
This gearbox does not need action by the driver as the control system decides when it is necessary to change the ratio.
The clutch is replaced by the converter, which takes care of the clutch functions on its own.

Transfer box
Generally coupled after the manual or automatic gearbox. Its job is to increase the number of ratios. Used on 4x4 vehicles, heavy vehicles and, generally, vehicles subject to extreme conditions.

Types of clutch

• Embrague
• Mechanical disc clutch
• Electromechanical disc clutch
• Centrifugal clutch (shoes, rollers)
• Electromagnetic clutch
• Converter

Internal gearbox parts (The ones that suffer most wear)

Manuals

Bearings:
Their job is to maintain the rotating movement of the shafts with the minimum friction and noise. If any of these properties changes, they must be replaced.

Types:

• Ball bearings
• Roller bearings
• Conical roller bearings
• Needle bearings
• Axial bearings

Oil seals:
Their job is to prevent lubricant from the gearbox leaking out. They are always replaced when any operation is carried out.

Synchronising rings:
Their job is to equalise the speeds of the pinions with the speed of the shaft and to position the gears for coupling.
Because of the force experienced by these parts, they must be made of an alloy that can stand heavy friction and temperature loads (manganese brass).

Types:

• ZF synchronising ring
• Inverted cone synchronising ring
• Elastic synchronis
  ing ring

Automatic

Bearings:
Their job is to maintain the rotational movement of the shafts with the minimum friction and noise. They must be replaced if they are not performing this task.

Oil seals:
Their job is to prevent any lubricant leaking out and they are replaced when any operation is carried out.

Gasket rings:
Their job is to direct the hydraulic pressure to the drums, servo pistons, etc.

Types

• Toric
• Teflon rings
• Metal rings

Bushing:
Its job is to transmit the force of the engine to the gears corresponding to each speed.

Discs:
Their job is to transmit the force of the engine to the gears corresponding to each speed.

Types

• Friction
• Metal

Brake strap:
Its job is the same as the discs.

Filter:
Eliminates possible impurities that could affect the behaviour of the gearbox.

Transfer box

Bearings

Synchronising rings

Oil seals