Sequential Gearboxes & Gearbox Design

A gearbox is usually attached directly to the engine to form what they call a gearmotor. The only cases where these aren't is when the gearbox and the motor are expensive enough in themselves that you don't want them fixed (an example of this is the gearbox and engine of a car are usually kept separate). Nevertheless, both secured and separate sequential gearboxes work in the same way.

A gear is connected to an engine via a drive shaft. That gear is then connected to a gear on the end of what is known as a layshaft. On the layshaft are a number of gears, one is permanently connected to the gear on the driveshaft, the other gears are on the other end of the layshaft, and are all different sizes. These gears are connected to inversely sized gears on a differential (if the layshaft gear is big, then the gear on the differential is small). The only one that isn't inverted is the reverse gear, who has two equal sized gears, and an inverter gear inbetween them to turn the layshaft and the driveshaft in opposite directions.

Also on the differential is a number of engager gears. These are controlled by the driver of the gearbox (computer or human), and decide on which gear is used. Often, the lower the gear, the smaller the gear on the layshaft is. This is because the momentum is used in higher gears to drive the engine, rather than the accelerator.

Most people are familiar with gearboxes on cars, because they have contact with them on a day to day basis. These work on a "H" principle. In theory you can go from 1st gear to 5th gear instantaneously. However, this isn't practical, as the engine would chug, and you'll eventually stall. Nevertheless, whilst common on human drive systems, it is open to errors. Everybody's probably put their car in the wrong gear! It's far better having a sequential gearbox.

Sequential gearboxes work by having only two gears, an "upshift" and a "downshift". The upshift changes up a gear (1st to 2nd) whilst the downshift changes down a gear (2nd to 1st). Usually you can go to the highest gear, and the lowest gear is reverse (so on a 5 speed gearbox, the gear order from highest to lowest is 5th, 4th, 3rd, 2nd, 1st, Neutral & Reverse). Errors really can only be caused from dropping from 1st to neutral whilst still moving (allowing coasting, and less control of the engine), but other than that, a lot fewer driver errors occurr, and this is the way that automatic sequential gearboxes work.

Automatic sequential gearboxes, like those found on automatic cars, don't require any input from a driver, instead they are programmed to change up/down gears when a certain speed or revolution of the engines is reached. Then immediately the programmed gear switches up or down (depending on the general speed of the motor). This makes life a lot easier as it's a lot quicker and smoother than a manual change, however, there's less room for manoeuvre. So for things that performance can change drastically and occasionally there's a lack of control where control is needed (such when as a car is going uphill, it's far better being in a lower gear than in a higher gear going the same speed), it's far better to have a manual gearbox.

There are 3 main methods to design a gearbox: sketches, CAD or analysis.

But before this, the most important thing is to know the application for the gearbox. What will it actually do? This is known as the "Concept Stage".

The concept stage asks a number of questions:-

What will the gearbox do? A fairly big requirement, as the gearbox for a manual car will be different from a pulley gearbox. So knowing what you are designing for is a bonus!

Type of engine/motor available The motor that the gearbox will link to will affect, and in some ways limit the number of type of gears you will use for the engine.

Packaging Requirements The reason they are called gearboxes is that they are encased within a packaging. This all takes up space, so you can't put huge gears within an electric wheelchair gearbox. So you need to know the dimensions of the gearbox, in order to make the gear configuration fit.

There are 3 main approaches to gearbox design, either by using sketches, CAD or analysis.

Sketches / Layout Design - Best carried out by communicative engineeres, the sketches are an exploded design drawing, often with a large amount of annotations regarding configuration, gear types and ratios. There'll be some work done with engineers

CAD - Computer Aided Design allows anybody with not much design experience in technical drawing to create 3D models of the gearboxes before beginning. These can be printed out into 2D design drawings with dimension lines & annotations. However, one of the beauties of CAD is that algorithmic simulation can take place using information on the gears used.

Analysis - This avoids the gearbox design element entirely and instead specifications of bearings, shafts, gears and drive trains. Basic prototypes are put together and the performance of prototypes, along with the estimates and knowledge of factors such average lifespan of the components, allows modification and improvement to the original design.

For further reading on the subject, we can recommend the following websites:-

Gearbox Design article on Weebly.