What is the use of a gearbox and why use it over an electric motor with a speed drive?


I don’t quite understand the use of a gearbox. What is the purpose of it?

Say for example some machine use a gearbox for some reason is it possible to replace it with a motor and a speed contorller?

Thank You.


In: 0

An internal combustion engine for a car mostly operates between 1500 and 2000 rpm. But we don’t want the wheels of the car to turn at such a rate as that is in excess of 100 mph! What the gearbox does is either increase or in this case decrease the speed of the output from the engine to a desired point. That way you can run an engine at 1700 rpm at 25 mph and at 65 mph.

A gearbox allows this conversion to be done mechanically with minimal loss of efficiency. Running a generator to produce electricity to drive an electric motor would involve a loss of efficiency and additional complexity.

The purpose of a gearbox is to provide variable transmission of a constant RPM for variable needs. Think of a petrol car. The engine is designed to run best at a certain number of RPM, that is, revolutions per minute. It can go a bit above and a bit below this number, but so long as the engine is running, it “wants” to turn the drive shaft this particular number of times per minute. But, we can’t connect this to the wheels directly – when the car is moving very slowly, the wheels won’t be turning nearly that fast, and when the car is speeding down the highway they will be turning quite quickly. So the solution is a gearbox, a set of different gear ratios. If we match a larger gear to a smaller, the smaller will turn more times for each one rotation of the larger gear, and vice versa. Because the gearbox can shift between several different gear ratios, and the engine can tolerate some variance in RPM, we can provide every precise RPM that the wheels will move at.

This has another added benefit: gear ratios provide a lever action. It takes a lot of power to get a car moving from fully stopped, because it is heavy. More power than a single rotation of the engine could possibly provide. Luckily, when you use a lower gear ratio, the power or torque of the rotation is multiplied.

An engine makes tiny explosions – so it has to go fast…but not too fast or it explodes itself. It’s easier for us to control the output (via gears) to get our desired power or speed.

An electric motor runs how we want it to, so it’s easier for us to control the input (via voltage/frequency control).

An electrical motor is strong in relation to its physical size. It’s not entirely that simple, but almost. A large ass motor is strong. A small motor ain’t.

But you can use a gear box to make it strong, as long as you also accept a lower rotational speed secondary to the gearbox.

In other words, this comes in handy every time you have an application where the physical size of the motor is limited by the location where you intend to put it.

A very typical example of this is electrical trains or diesel-electrical trains. The motor is located, literally, between the wheels. And has a gearbox.

A motor on a train ALSO has electronics controlling the rotational speed by frequency, so this is a hybrid solution where you typically use both gears AND electronics.

But it proves my point; sometimes you can’t fit the motor you want, so you are going to have to accept gear losses to reach your goal.

It also works in the opposite direction, of course. When you can’t find a motor that is fast enough, you can gear up one that is strong enough.

To be fair, frequency controllers have been undergoing a lot of development in the past decade or two and was a bit of a new era when they showed up on the market in affordable, tiny encapsulations. There are a lot of applications that you today wouldn’t build with a gearbox at all, that you more or less HAD to have a gearbox on in the 70’s because you didn’t really have a choice on the matter.

Some industry applications consider the electronics a source of unnecessary potential faults. Pump installations in waterworks COULD be such an application, since the environment very obviously is going to contain water (and we all know that water and electronics is not a splendid combination) and likely a lot more more so when the pump is failing. If you intend to run the motor at its full speed for the entire duration of its life span…what do you need the electronics for? They are just an unnecessary potential breakdown that adds nothing to the application. The gearbox, you grease it at the same time as the motor, a few times a year on a schedule.

Another part of it is that it’s easier to cater to a market with gear boxes: you can bundle the same motor with several different gear boxes, effectively selling products for several different user cases with just one product.

Or, the gear box belongs to the machine. But when the motor fails, nearly any supplier you can think of has one available off the shelf. Because the gear boxes makes it easier for the motor manufacturers to concentrate on a few standard sizes. Just send someone to fetch the right size, install it and you are good to go again. (in reality, the suppliers still have hundreds of sizes, but they would have had at least tenfold need for stock if we didn’t also utilise gearboxes…)

A gearbox with multiple options to select allows you to change the relationship between engine speed and ground speed.

For combustion engines, this is really important as they can’t operate below a certain speed (stall) or above a max speed (red line aka the point where they might blow up). This range isn’t particularly large and to increase speed, you need to get more power which in these engines means increasing the engine speed and the engine doesn’t do that very well if the engine speed is too low compared to the power required which is why we use lower gears (higher ratios) to accelerate from a start or to pick up speed quickly.

Electric motors generally have a much wider speed range and most importantly, they can apply full power at zero speed. As a result, most electric vehicles have much fewer gears than ICE vehicles do.

For light vehicles and small to medium trucks, an ICE model generally has 5 or 6 gears, sometimes more in automatics. Heavy trucks often have 18 or more gears and a very narrow engine speed range (often just under 1000 to a red line at around 2000 rpm)

Electric light vehicles and even some smaller electric trucks like the Fuso E Canter only have a single forward gear and rely on controlling motor speed to accelerate. Very heavy electric trucks (e.g. Volvo FE electric and Scania electric) use a two speed gear box to allow the motor to run at lower rpm when the truck is going faster. I suspect that this is because the larger motors used for trucks of this size have a bit more drag when spinning at high speed so the extra power loss in the transmission is offset so it still works out more efficient