How Does a Gearbox Work?
Ellie Dyer-Brown, 3 months ago
5 min read
- Gearbox
- How it works
FixMyCar explains how a car gearbox works.
The majority of cars - except electric vehicles - are fitted with a gearbox to help them change speeds. It’s a process that doesn’t require much thought for most drivers, even in manual cars; you press the clutch and move the gear stick, and the vehicle reacts accordingly. But how does the gearbox actually work? Find out in this guide.
Contents
How does a manual gearbox work?
Pros and cons of a manual gearbox
How does an automatic gearbox work?
Pros and cons of an automatic gearbox
What is a gearbox?
A gearbox is a mechanical component that uses different gear ratios to transfer power from the engine to the wheels. It consists of several gears, bearings and shafts. In manual cars, a clutch connects the gearbox to the crankshaft, allowing the power that usually goes to the wheels to be disengaged while changing gears.
The gearbox plays a vital role because, in simple terms, it changes the car’s speed and the engine’s power (known as torque).
The functions of a gearbox
A gearbox performs the following functions:
Transfers power to the wheels
Controls the car’s speed
Changes the gear ratio
Increases the engine’s torque
How does a manual gearbox work?
The simple explanation
The gearbox transfers power from the engine using a set of gears that can be changed to alter the vehicle's speed and torque. The gears are arranged in different patterns to offer different speeds, and they are chosen by the driver using a gear stick or electronic selector. The lower the gear, the slower the wheels turn in relation to the engine's speed.
The process goes like this:
Power is sent through a shaft that connects the gearbox and the engine.
The shaft must be disconnected from the engine when changing gears, which is achieved by the clutch.
After changing gears, releasing the clutch pedal allows the clutch assembly to reconnect the engine's power with the gearbox.
The gearbox then turns a series of shafts with gears fitted to them.
When you change gear, a selector fork in the transmission uses a synchroniser to engage your chosen gear.
The gearbox directs power from the engine to the wheels again when the gear has been changed.
The more complex explanation
The gearbox is the second stage in the transmission system. Most manual vehicles have a constant-mesh gearbox; this is the most popular type and the one we’re focusing on here, though there are others.
The gear lever (operated by the driver) is connected to a series of connector rods at the side or top of the gearbox. The selector rods are parallel with shafts that carry the gears.
Three shafts - the layshaft, the input shaft and the main shaft - run in bearings in the gearbox casing. The reverse-gear idler pinion rotates on a different shaft.
The engine drives the input shaft, which then drives the layshaft. The layshaft rotates the gears on the main shaft, but the gears freely rotate until they are locked by a synchromesh device splined to the shaft.
When the driver changes gear, a selector rod with a fork on it moves the synchromesh to engage the gear. The final refinement in modern gearboxes is the baulk ring, which stops the gear from engaging until the shaft speeds are synchronised.
How the gears are synchronised
The synchromesh device is responsible for synchronising the gears. It is a ring with teeth on the inside mounted on a toothed hub. This hub is secured with a spline to the shaft.
When you choose a gear, cone-shaped friction surfaces on the hub and the gear transmit drive from the turning gear to the hub shaft, ensuring the speeds of the two shafts are the same.
As you move the gear stick into position, the ring moves along the hub until its teeth mesh with bevelled dog teeth on the side of the gear, locking the splined hub and gear together.
How gear ratios work
Neutral
All the gears, except those needed for reverse, are constantly meshed. The gears on the output shaft revolve freely around it, and the ones on the layshaft are fixed. No drive is transmitted with this gear ratio.
First gear
The smallest gear on the layshaft is locked to it, driving the largest gear on the main shaft. This configuration provides high torque and low speed to get the car moving.
Second gear
The difference in diameter of the gears on the main shaft is reduced in second gear, creating more speed and a lower torque increase. This ratio is excellent for driving up steep hills.
Fourth gear
The input shaft and the main shaft are locked together in fourth gear, providing ‘direct drive’, which means there is one revolution of the propeller shaft for each revolution of the crankshaft. There is no increase in torque.
Reverse
An idler gear is placed between gears on the two shafts, forcing the main shaft to reverse direction. The reverse gear isn’t usually synchronised.
Pros and cons of a manual gearbox
Below are some things to consider when deciding whether to buy a manual or automatic vehicle.
| Advantages | Disadvantages |
|---|---|
| You are in control of all gear changes | You have to do lots of changes in heavy traffic |
| Manual cars are less expensive to buy than automatics | New drivers can find it difficult to learn |
| Low maintenance | Clutch control can be difficult on steep inclines |
How does an automatic gearbox work?
Automatic gearboxes shift gear without any intervention from the driver, usually using hydraulic power. They typically combine a torque converter (instead of a clutch) with different gear sets. The torque converter connects the engine to the transmission, using pressurised fluid to transfer power to the gears.
Here’s a basic overview of how it works:
The engine transmits power to the torque converter pump.
The pump transmits this power to the transmission fluid, which powers the torque converter’s turbine.
The turbine increases the fluid’s power and transmits even more power back to the turbine, creating a vortex power rotation that spins the turbine and the attached central shaft.
The power from the rotation is then transferred from the shaft to the transmission’s first planetary gear set.
The above process relies on hydraulic control. An oil pump pressurises the transmission fluid, which allows the speed to change depending on various factors, such as the number of tyre revolutions per minute.
Pros and cons of an automatic gearbox
| Advantages | Disadvantages |
|---|---|
| Easy to drive in traffic | May not be suitable for off-road driving |
| Comfortable for drivers | More expensive to buy and repair than manual cars |
| Smoother drive | Boring to drive |
What is a semi-automatic gearbox?
A semi-automatic gearbox offers the best of both worlds, allowing you to decide whether to drive the car in manual or automatic mode. There is no clutch in a semi-automatic vehicle. When you switch to manual mode, you usually move the gear selector to a dedicated position. You change gear by flicking the selector forward or backwards or by using flappy paddles behind the wheel.
Frequently asked questions
You have an automatic gearbox if you don’t have to change gears manually. Paddles behind the steering wheel typically indicate that you have a semi-automatic gearbox. You can learn more about your vehicle's gearbox type by looking in the owner’s manual.
Both terms are often used interchangeably, although they are technically slightly different. Transmission refers to the entire drivetrain (all the parts that allow the power to be transmitted from the engine to the driveshaft and then the wheels), whereas the gearbox is a part of the transmission.
DSG gearboxes are used by manufacturers in the Volkswagen Group. Find out how they work in this guide: https://www.whocanfixmycar.com/advice/what-is-a-dsg-gearbox
A tiptronic gearbox is a semi-automatic gearbox; they are different names for the same thing.
Looking for affordable car repairs or maintenance? FixMyCar can help you find the right garage at the right price.
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Written by Ellie
Ellie is FixMyCar's Content Writer. She has over three years of experience writing about cars and regularly collaborates with automotive experts to provide trustworthy advice for drivers in a language they understand. Her work has been featured in Yahoo! Finance, iNews, The Daily Express and The Sun. She has a BA in English literature and an MA in creative writing from Durham University. Outside of work, Ellie follows F1 and eagerly awaits Ferrari's next era of dominance in the sport. She drives a Suzuki Swift.