By changing the connections between the engine and the driving wheels, the engine can be made to run twice as many revolutions to one turn of the driving wheels, which will give double the number of power strokes; the wheels will thus be turned with twice the force. As no change is made in the speed of the engine, the wheels will now turn at half their former speed, and the tractor will run at 1½ miles per hour. It will, however, have twice the ability to overcome obstacles.
This change in the connections between the engine and the drive is performed by the change speed gear, which is driven by the engine and which in turn drives the wheels.
There are many varieties of change speed gears, but the main principle in them all is the same, for they depend on the action of cog-wheels, or gears.
When two gears running together, or in mesh, have the same number of teeth, they will revolve at the same speed. If one has half as many teeth as the other—10 teeth and 20, let us say—the 10-tooth gear will make two revolutions while the 20-tooth gear is making one.
There are two shafts in a change speed gear, one driven by the engine and the other driving the wheels; each carries gears that mesh with gears on the other shaft. These pairs of gears are of different sizes, and any pair may be used; the shaft driven by the engine runs as the engine runs, while the speed of the other shaft depends on the pair of gears that is being used.
By changing from one pair of gears to another, the driven shaft, and, consequently, the wheels, may be run at a greater or less number of revolutions, while the speed of the engine and the driving shaft do not change. The number of power strokes that occur during one revolution of the wheels is thus changed, and they turn with more force or with less.
High speed, or high gear, means the combination of gears that gives the greatest speed to the wheels, but the fewest power strokes to each revolution. The combination that gives the slowest speed to the wheels, but the greatest number of power strokes, is called low speed, or low gear.
Many tractors have but two speeds, a low and a high; but others have an intermediate combination for conditions too severe for running on high gear but too easy for low.
The change speed gear mechanism also provides for reversing or backing the tractor. Two gears running together turn in opposite directions, while in a train of three gears the outside gears turn in the same direction. The usual combination in a change speed gear uses two gears for going ahead; to run the driven shaft the other way, which will make the tractor back, a third gear is meshed between the two.
The differences between various makes of change speed gears are in the methods used to put into action the desired pair of gears.