A gear trains is two or more gear working together by meshing their teeth and turning each other in a system to generate power and speed. It reduces speed and increases torque. To create large gear ratio, gears are connected together to form gear trains. They often consist of multiple gears in the train.
Gear trains consist of :
- Driving gear which is attached to the input shaft;
- Driven gear / motor gear which is attached to the output shaft; and
- Idler gears which are interposed between the driving and driven gear in order to maintain the direction of the output shaft the same as the input shaft or to increase the distance between the drive and driven gears. A compound gear train refers to two or more gears used to transmit motion.
The gears used in gear trains may be of spur, helical, bevel or spiral type depending on the type of motion required and applications.
Gear trains are used in representing the phases of moon on a watch or clock dial, lathe machines, automobiles etc. It is also used for driving a conventional two-disc lunar phase display off the day-of-the-week shaft of the calendar.
Gears are toothed wheels used for transmitting motion and power from one shaft to another when they are not too far apart and when a constant velocity ratio is desired. In comparison with belt, chain and friction drives, gear drives are more compact, can operate at high speeds and can be used where precise timing is required. Also gear drives are used when large power is to be transmitted. The force required to hold the gears in position is much less than in an equivalent friction drive.
Advantages and limitations of gear drive over chain and belt drives Advantages
- Since there is no slip, so exact velocity ratio is obtained.
- It is capable of transmitting larger power than that of the belt and chain drives.
- It is more efficient (up to 99%) and effective means of power transmission.
- It requires less space as compared to belt and rope drives.
- It can transmit motion at very low velocity, which is not possible with the belt drives.
- The manufacture of gears requires special tools and equipments.
- The manufacturing and maintenance costs are comparatively high.
- The error in cutting teeth may cause vibrations and noise during operation.
Types of Gears
Depending on their construction and arrangement, geared devices can transmit forces at different speeds, torques, or in a different direction, from the power source. Gears are broadly classified as follows,
Classification based on the relative position of their shaft axes
Examples: Spur gears, helical gears, rack and pinion, herringbone gears and internal gears.
Examples: Bevel gears and spiral gears.
Non-parallel, non-intersecting shafts
Examples: Worm, hypoid and spiral gears.
Classification based on the position of teeth on the wheel
- Straight gears
- Helical gears
- Herrigbone gears
- Curved teeth gears
Classification based on the type of contact between surfaces of the gear
- External gearing
- Internal gearing
- Rack and pinion
But from our objective point of view, gears are broadly classified in to four groups, viz., spur, helical, bevel, worm gears and rack-and-pinion.
- Spur gears: In spur gears, the teeth are straight and parallel to the axis of the wheel as shown in Figure (a). The gearing so formed is called spur gearing. They are used to transmit rotary motion between parallel shafts. Spur gears are the simplest and most common type of gear.
- Helical gears: Helical gears are simple modification of spur gears. A helical gear has teeth in the form of helix around the gear as shown in Figure (b). The angled teeth engage more gradually than do spur gear teeth. This causes helical gears to run more smoothly and quietly than spur gears. The use of helical gears is most common in automobiles, turbines and high-speed applications.
- Double helical gears, also known as herringbone gears, overcome the problem of axial thrust presented by ‘single’ helical gears by having teeth that set in ‘V’ shape. Each gear in a double helical gear can be thought of as two standard but mirror image helical gears stacked. This cancels out the thrust since each half of the gear thrusts in the opposite direction. Figure (b) shows this type of gear.
- Bevel gears: Bevel gears are formed by cutting teeth along the elements of frustum of a cone. That is, the pitch surfaces in the bevel gears are truncated cone, one of which rolls over the other, as shown in Figure (d). Bevel gears are used to transmit power between two intersecting shafts. Bevel gears are commonly used in automotive differentials. When teeth formed on the cones are straight, the gears are known as straight bevel and when inclined, they are known as spiral or helical bevel. Zero bevel gears have teeth which are curved along their length, but not angled.
- Worm gears: A worm is a gear that resembles a screw. It is a kind of helical gear, but its helix angle is usually somewhat large (somewhat close to 90°) and its body is usually long in the axial direction and therefore, it is looking like screw. A worm is usually meshed with an ordinary looking, disk-shaped gear, which is called the “worm gear”, or the “worm wheel”. The prime feature of a worm-and-gear set is that it allows the attainment of a high gear ratio with few parts, in a small space.
- Rack and pinion: A rack is a toothed bar or rod that can be thought of as a sector gear with an infinitely large radius of curvature as shown in Figure (a). Torque can be converted to linear force by meshing a rack with a pinion: the pinion turns; the rack moves in a straight line. Such a mechanism is used in automobiles to convert the rotation of the steering wheel into the left-to-right motion of the tie rod(s).
- External gearing: If two gears are meshed externally with each other as shown in Figure (a), they are called external gears. The larger one of these two gears is called a gear and the smaller one is called pinion. Due to external contact between two gears, the rotation of the two gears will be opposite to each other.
- Internal gearing: If two gears are meshed internally with each other as shown in Figure (b), they are called internal gears. The outer one is called annular wheel and inner one is called a pinion. In this case, the rotation of two gears is always in the same direction.
Types of Gear Trains
- Simple gear train
- Compound gear train
- Epicyclic gear train
- Reverted gear train
Simple gear trains
The most common of the gear train is the gear pair connecting parallel shafts. The teeth of this type can be spur, helical or herringbone. The angular velocity is simply the reverse of the tooth ratio. The main limitation of a simple gear train is that the maximum speed change ratio is 10:1. For larger ratio, large size of gear trains is required.
Compound gear trains
Compound gear train is preferred when large velocity ratio required. If more than one gear is connected to the same shaft called compound gears. In Figure, the gears B and C are arranged in the same shaft and hence it is a compound gear. As compared to simple gear train where the intermediate gears have less space between the input and output shaft, in a compound gear train the intermediate gears are also useful for increasing or decreasing the gear ratio of the arrangement. The gears arranged in the same shaft i.e. gear B and C will have same angular velocity.