In the chain of converting engine energy into wheel movement, there is a critically important unit, without which the car either will not move or will move at an inadequately high speed at minimum engine speed. This unit is the main gear, located directly in the rear or front axle housing, and in front-wheel drive vehicles - in the gearbox housing.
The main task of this mechanism is to finally increase the torque transmitted from the engine and change the direction of rotation by 90 degrees to drive the drive wheels. Understanding the operating principles of this unit is necessary for every driver who wants to extend the life of the transmission and correctly diagnose faults by ear.
Structurally, the device is a complex gearbox operating under conditions of constant high loads and friction. Not only the level of driving comfort, but also the safety of driving in emergency situations depends on the quality of its lubrication and the condition of the gear pairs.
The main purpose of the main gear in the transmission
The key function of the node is final torque increase, which comes from the gearbox. The internal combustion engine is only capable of producing maximum power and thrust within a certain rpm range, and the transmission's job is to adapt these figures to the current road conditions.
The second important task is to change the direction of rotation of the power flow. Since the engine crankshaft and gearbox shafts in the classic layout are located longitudinally, and the wheels rotate in a transverse plane, an angular transmission mechanism is required. In most cases, this is implemented through bevel gears.
⚠️ Warning: Installing a final drive with a gear ratio that does not meet factory specifications may result in engine overload or, conversely, an inability to develop sufficient traction on inclines.
The device also ensures a constant gear ratio when the vehicle is moving. Unlike a gearbox, where the driver or automation changes gears, the main gear operates with a constant coefficient, determining the dynamic characteristics of the car as a whole.
Design features and types of gears
In the modern automotive industry, various types of gearing are used, the choice of which depends on the type of drive and the required noise characteristics. The most common is hypoid transmission, which shifts the axis of the drive gear relative to the axis of the driven gear.
This offset allows the driveshaft to be lowered, which lowers the vehicle's center of gravity and increases interior space. However, hypoid gearing requires the use of special gear oils with extreme pressure additives, since the tooth slip is higher here than in other types.
For trucks and heavy equipment, spur or chevron gears are often used. It is easier to manufacture and has high efficiency, but it creates more noise and does not allow assembling components as efficiently as the hypoid analogue.
In front-wheel drive cars where the engine is located transversely, it is often used cylindrical chevron or helical gear. This is due to the fact that no or minimal angular changes are required to change the direction of rotation of the power flow, and the main emphasis is on compactness.
Why does a hypoid gear hum?
Hypoid gears are inherently prone to increased noise due to the complex nature of the tooth contact. Unlike a spiral-bevel gear, where pure rolling occurs, in a hypoid gear there is significant longitudinal sliding. It is this sliding that requires special oils and is the source of the characteristic low-frequency hum, which increases with wear or improper adjustment of the contact patch.
The role of the differential in the operation of the final drive
An integral part of the main gear is differential - a mechanism that allows the drive wheels to rotate at different angular speeds. This is critical when cornering, when the outside wheel travels longer than the inside.
If there was no differential, one of the wheels would be forced to slip when turning, which would lead to rapid tire wear, increased fuel consumption and poor handling. In terms of design, the differential is usually made in the form of a planetary mechanism installed inside the driven gear of the main gear.
- 🚗 Satellites: small gears that transmit torque to the side gears.
- ⚙️ Side gears: transmit rotation directly to the wheel drives.
- 🔩 Carrier: a housing in which the satellite axes are fixed and which is rigidly connected to the driven gear.
There are different types of differentials: from open symmetrical to limited-slip differentials (LSD) and electronically controlled. An open differential always transfers torque to the wheel with less resistance, which can become a problem in slippery areas when one wheel slips.
To improve off-road performance on vehicles without differential locks, you can use tires with a more aggressive tread pattern, which will increase traction on loose surfaces.
Gear ratio: influence on dynamics and consumption
One of the most important parameters that determine the character of a car is gear ratio main couple. It is calculated as the ratio of the number of teeth of the driven gear to the number of teeth of the drive gear.
The higher this number, the more torque is transmitted to the wheels, which improves acceleration and traction capabilities. However, the “payment” for such high-torque power is higher engine speeds when driving at a constant speed, which leads to increased fuel consumption and noise levels.
On the contrary, a “long” main gear (with a smaller gear ratio) allows you to reduce engine speed on the highway, saving fuel and engine life. But at the same time, the car becomes more sluggish when accelerating and requires more frequent downshifts when overtaking or going uphill.
| Transmission type | Gear ratio | Impact on dynamics | Impact on consumption |
|---|---|---|---|
| Short (traction) | 4.1 - 4.5 and higher | Excellent acceleration, high traction | Elevated on the track |
| Medium (standard) | 3.7 - 3.9 | Balance between traction and speed | Moderate |
| Long (economy) | 3.0 - 3.5 | Poor acceleration, needs a gearbox | Low on the track |
The choice of the optimal gear ratio depends on the purpose of the vehicle. For racetracks or off-road use, short gears are chosen, while for long-haul tractors or economy sedans, priority is given to long gears.
Lubrication system and temperature conditions
The operation of the main gear is accompanied by intense friction of gear teeth and bearings, which leads to the release of a large amount of heat. Used to dissipate heat and reduce friction gear oil, which also serves as a lubricant.
Most gearboxes use oil splashing: the rotating driven gear is dipped into an oil bath and sprays fluid along the inner walls of the crankcase. Some heavy-duty trucks may use forced-pressure lubrication.
⚠️ Attention: Using oil with a viscosity lower than that recommended by the manufacturer can lead to squeezing out the seals due to excessive foaming and increased pressure inside the crankcase.
The most important element of the system is the breather (breathing valve). When heated, the air inside the crankcase expands, and if there is nowhere for it to escape, the pressure can squeeze out the axle seals. The breather equalizes pressure, but it often becomes clogged with dirt, requiring periodic cleaning.
☑️ Checking the condition of the gearbox
Diagnosis of faults and characteristic symptoms
Determining a main gear malfunction is often possible by a characteristic sound. Gearbox howl - the most common complaint that can occur during acceleration, releasing the gas, or constantly.
If it hums when accelerating and dies down when you let off the gas, this usually indicates wear on the running side of the gear teeth or an incorrect contact patch. If the noise appears precisely when the gas is being released, it means that the back side of the tooth has worn out.
A knocking or crunching sound when turning with the wheels turned out often indicates problems in the differential, and not in the main pair itself. Vibration transmitted to the body can be caused by play in the shank bearings or an imbalance in the driveshaft.
For an accurate diagnosis, you need to lift the car, fix one wheel and rock the other, checking for play. A visual inspection of the contact patch of the teeth is also carried out, which should be located strictly in the center of the tooth.
A constant hum, varying in proportion to the speed of the car (and not the engine speed), is a sure sign of wear on the main pair or differential bearings.
Unit adjustment and maintenance
The final drive is a precision mechanism that requires precise adjustment during assembly. The key parameters are bearing tension and the position of the contact patch of the gears. Adjustment is carried out by selecting adjusting washers and turning the adjusting nuts (in modern bridges).
The contact patch is checked by applying special paint to the teeth of the drive gear and briefly turning the mechanism under load. Shifting of the stain towards the tooth stem or end indicates the need to change the thickness of the adjusting washer.
Regular maintenance consists of changing the oil according to the regulations. Despite the fact that many manufacturers claim that the oil is “filled for the entire service life,” in real-life operating conditions it is better to change it every 60-80 thousand kilometers.
When changing the oil, it is important to pay attention to its condition: the presence of metal shavings (especially large ones) indicates the destruction of gears or bearings, and the presence of an emulsion (light coffee color) indicates water entering through the breather when overcoming fords.
Why does the main gear howl after replacement?
Howling after replacement can be caused by incorrect adjustment of the bearing preload or incorrect alignment of the contact patch. It is also possible that the new pair has not yet gotten used to it, and this requires a run of 1000-2000 km in gentle mode.
Is it possible to drive if the bridge is buzzing?
You can drive, but not for long. The hum indicates the destruction of the surface layer of the metal (pitting). Over time, the gaps will increase and play will appear, which can lead to wheel jamming while driving or destruction of the differential.
What is the best oil to use in a hypoid gear?
Oils marked API GL-5 must be used. GL-4 oils do not contain sufficient extreme pressure additives and can lead to rapid destruction of the hypoid pair.
What is a "pair" in the context of final drive?
A pair is a set of drive and driven gears that are matched and ground to each other at the factory. They can only be changed as a set; replacing one gear without the second is unacceptable.
Why does the gearbox get hot?
Heating can be caused by oil that is too thick, crankcase too full of oil, bearings not adjusted correctly (excessive preload), or using the wrong viscosity oil.