1. Gears

2. Gears Definition
Gears are parts of a machine which fit together via a shaft and rotate to provide or transfer drive and torque to another place They are manufactured by machining a blank of cylindrical part of steel to a desired piece with a set number of teeth. They are then heat treated required so the gear wheel will be hard wearing but not brittle. A Gear train is a set of gears which will work together to transfer drive. This can come in a simple arrangement, this can be where there are only a pair of gears involved, or a compound gear train, this is where a number of gears are involved or one of the gears involved may have 2 driving surfaces.

3. Gear Nomenclature

4. Gear Types Bevel Worm and Wheel Helical Rack and Pinion Herringbone
Spur
Hypoid

5. Bevel Gear

6. Bevel Gear http://m.youtube.com/watch?v=o-Kdj_f6WCQ
Bevel gearboxes are special speed reducers with their shafts lying perpendicular to each other and therefore used mainly in right-angle applications. The gearbox is a kind of right angle gear and is suitable for a right angle solution with a low ratio.
A very common application of use for this type of gearing is with a manually powered drill.
Bevel gearboxes are also used in diverse fields including : Printing presses, newspaper conveyors, locomotives, marine applications, automobiles, printing presses, steel plants, railway track inspection machines, material handling / conveyors, wrapping machines.

7. Bevel Gear
Advantages
This gear makes it possible to change the operating angle.
They can be made out of a variety of different materials
Save more energy as compared to worm gears and are available in varying gear ratios.
Differing of the number of teeth(effectively diameter) on each wheel allows mechanical advantage to be changed. By increasing or decreasing the ratio of teeth between the drive and driven wheels one may change the ratio of rotations between the two, meaning that the rotational drive and torque of the second wheel can be changed in relation to the first, with speed increasing and torque decreasing, or speed decreasing and torque increasing.

8. Bevel Gear Disadvantages
One wheel of such gear is designed to work with its complementary wheel and no other.
Must be precisely mounted.
The shafts' bearings must be capable of supporting significant forces.

9. Worm and Wheel

10. Worm and Wheel
Worm gears are used when large gear reductions are needed. It is common for worm gears to have reductions of 20:1, and even up to 300:1 or greater
Many worm gears have an interesting property that no other gear set has: the worm can easily turn the gear, but the gear cannot turn the worm
Worm gears are used widely in material handling and transportation machinery, machine tools, automobiles etc

11. Worm and Wheel Advantages
Worm gear drives operate silently and smoothly.
They are self-locking.
They occupy less space.
They have good meshing effectiveness.
They can be used for reducing speed and increasing torque.
High velocity ratio of the order of 100 can be obtained in a single step

12. Worm and Wheel Disadvantages Worm gear materials are expensive.
Worm drives have high power losses 
A disadvantage is the potential for considerable sliding action, leading to low efficiency
They produce a lot of heat.

13. Helical Gear

14. Helical Gear
The teeth on helical gears are cut at an angle to the face of the gear
This gradual engagement makes helical gears operate much more smoothly and quietly than spur gears
One interesting thing about helical gears is that if the angles of the gear teeth are correct, they can be mounted on perpendicular shafts, adjusting the rotation angle by 90 degrees

15. Helical Gear Advantages
The angled teeth engage more gradually than do spur gear teeth causing them to run more smoothly and quietly
Helical gears are highly durable and are ideal for high load applications.
At any given time their load is distributed over several teeth, resulting in less wear
Can  transmit motion and power between either parallel or right angle shafts

16. Helical Gear Disadvantages
An obvious disadvantage of the helical gears is a resultant thrust along the axis of the gear, which needs to be accommodated by appropriate thrust bearings, and a greater degree of sliding friction between the meshing teeth, often addressed with additives in the lubricant. Thus we can say that helical gears cause losses due to the unique geometry along the axis of the helical gear’s shaft.
Efficiency of helical gear is less because helical gear trains have sliding contacts between the teeth which in turns produce axial thrust of gear shafts and generate more heat. So, more power loss and less efficiency

17. Rack and Pinion

18. Rack and Pinion
Rack and pinion gears are used to convert rotation (From the pinion) into linear motion (of the rack) A perfect example of this is the steering system on many cars. This is also used for providing manual motion when driving a lathe saddle to and fro, on a rack railway to provide drive / motion, on the inside of an air driven valve actuator / solenoid.

19. Rack and Pinion Advantages Cheap Compact Robust
Easiest way to convert rotation motion into linear motion
Rack and pinion gives easier and more compact control over the vehicle

20. Rack and Pinion Disadvantages
Since being the most ancient, the wheel is also the most convenient and somewhat more extensive in terms of energy too. Due to the apparent friction, you would already have guessed just how much of the power being input gives in terms of output, a lot of the force applied to the mechanism is burned up in overcoming friction, to be more precise somewhat around 80% of the overall force is burned to overcome one.
The rack and pinion can only work with certain levels of friction. Too high a friction and the mechanism will be subject to wear more than usual and will require more force to operate.
The most adverse disadvantage of rack and pinion would also be due to the inherent friction, the same force that actually makes things work in the mechanism. Due to the friction, it is under a constant wear, possibly needing replacement after a certain time

21. Herringbone Gear

22. Herringbone Gear
To avoid axial thrust, two helical gears of opposite hand can be mounted side by side, to cancel resulting thrust forces
Herringbone gears are mostly used on heavy machinery: cranes, fluid pumps and power transmission to the propulsion screws in military ships for a quieter and less vibration operation.

23. Herringbone Gear Advantage
Double helical gears give an efficient transfer of torque and smooth motion at very high rotational velocities.
They operate with minimum of backlash
They can also transmit movement between axis that do not
intersect

24. Herringbone Gear Disadvantages
the high cost due to special gear shaping equipment and special cutting tools.
aligning is to be given special attention

25. Spur Gear

26. Spur Gears
Spur gears connect parallel shafts, have involutes teeth that are parallel to the shafts, and can have either internal or external teeth.
They give lower performance, but may be satisfactory in low speed or simple applications
Spur gears are found everywhere as they are the simplest of form and thereby extremely versatile, used in a wide range of everyday situations ranging between heavy industry and domestic applications.
Used in Electric screwdriver, oscillating sprinkler, windup alarm clock, washing machine and clothes dryer

27. Spur Gears Spur gears are inexpensive to manufacture.
Advantages
Spur gears are inexpensive to manufacture.
They cause no axial thrust between gears.
They offer constant velocity ratio
Spur gears are highly reliable
Spur gears are simplest, hence easiest to design and manufacture
A spur gear is more efficient if you compare it with helical gear of same size
Spur gear teeth are parallel to its axis. Hence, spur gear train does not produce axial thrust. So the gear shafts can be mounted easily using ball bearings.
They can be used to transmit large amount of power (of the order of 50,000 kW)

28. Spur Gears Disadvantages Spur gear are slow-speed gears
Gear teeth experience a large amount of stress
They cannot transfer power between non-parallel shafts
They cannot be used for long distance power transmission.
Spur gears produce a lot of noise when operating at high speeds.
when compared with other types of gears, they are not as strong as them

29. Hypoid Gears

30. Hypoid Gears
Similar to spiral bevel gears, but connect non-parallel shafts that do not intersect. The pitch surface of a hypoid gear is a hyperboloid of revolution (rather than a cone, the pitch surface in bevel gears), hence the name. Hypoid pinions (the smaller driving gear) are stronger than spiral bevel pinions because the helix angle of the pinion is larger than that of the gear. Hypoid gears are stronger, operate more quietly, and can be used for higher reduction ratios than spiral bevel gears. They also have sliding action along the teeth, potentially reducing efficiency. Hypoid gears are found in auto differentials. A hypoid gear is a style of spiral bevel gear whose main variance is that the mating gears' axes do not intersect. The hypoid gear is offset from the gear center, allowing unique configurations and a large diameter shaft. A hypoid gear can be considered a cross between a bevel gear and a worm drive.

31. Hypoid Gears Advantages
Hypoid gearings are used in power transmission products that are more efficient than conventional worm gearing.
They are considerably stronger in that any load is conveyed through multiple teeth simultaneously. By contrast, bevel gears are loaded through one tooth at a time.
The multiple contacts of hypoid gearing, with proper lubrication, can be nearly silent, as well.

32. Hypoid Gears Disadvantages
A disadvantage of the hypoid drive is that it has a higher tendency to seize because of the sliding motion along the length of the tooth contact. However, the seizing possibility is eliminated by the application of antiseizing oil (hypoid oil) and heat treatment of the gear teeth to give high surface hardness