Gear Train
If two or more gearwheels are meshed in series, either in the same plane or in different planes, the gearwheel assembly is said to form a gear train.25.3.1.
Simple Gear Train
If the gearwheels are supported on separate shafts and are in the same plane, the gear train is known as a simple gear train (Fig. 25.7).
Fig. 25.5. Straight-tooth spur gear.
Fig. 25.6. Helical-tooth spur gear.
Engine timing gears use these arrangements. When only two gearwheels are involved then the gear train is a single-stage simple gear train Fig. 25.8. This arrangement is usually used with gearboxes associated with front wheel drive.
The gear ratio of a gear train is defined as the input speed divided by the output speed, or it may be obtained by using the following formula.
Fig. 25.7. Simple gear train.
Fig. 25.8. Single stage gear train.
25.3.2.
Compound or Multi-stage Gear Train
When two or more pairs of gearwheels are connected in series and the driven gearwheel of one gear train is connected by a common shaft to the driver gear wheel of the next gear train, the gear wheel assembly forms a compound gear train (Fig. 25.9). If two gear wheels are joined together by a single shaft, a double-stage compound gear train is formed (Fig. 25.9). This layout is usually found in gearboxes used in vehicles with front mounted engines that have a rear wheel drive.25.3.3.
Reverse Gear Train
If two gearwheels are connected by a third or middle gearwheel, the additional gearwheel does not affect the overall gear ratio, but changes the direction of rotation (Fig. 25.10). This additional gear wheel is known as an idler gear.
Fig. 25.9. Compound (double-stage) gear train.
