A gear is a toothed machine part, such as a wheel or cylinder, that meshes with another toothed part to transmit motion or to change speed or direction or it can be a complete assembly that performs a specific function.
Types of Gears
There are three types of gear, namely: internal gear, external gear and bevel
An internal gear is one with the teeth formed on the inner surface of the cylinder or cone, which has the advantage of not causing output shaft direction reversal. The internal gear always meshes with the external gear. An example is an annular gear. The features of an internal gear are as follows:
- In the meshing of two gears (internal and external), rotation goes in the same direction.
- Care should be taken as to the number of teeth when meshing a large (internal) gear with a small (external) gear.
- Usually internal gear is driven by external gear.
- It allows compact design of the machine.
An external gear is one with the teeth formed on the outer surface of a cylinder or cone. Examples of external gears are spur gear, helical gear, rack gear, screw gear, worm wheel gear and skew gear.
a) Spur Gear
- It is easy to manufacture.
- There will be no axial force.
- It is relatively easy to produce high quality gears.
- It is the commonest type.
b) Helical Gear
- It has high strength compared with the spur gear.
- It is effective in reducing noise and vibration compared with the spur gear.
- Gear in mesh produce thrust force in the axial direction.
c) Rack Gear
Changes a rotary motion into a rectilinear motion.
d) Screw Gear
- It is used in a speed reducer.
- It tends to wear as the gears come in sliding contact.
- It is not suitable for transmission of horsepower.
e) Worm Gear
- It provides large reduction ratio for a given centre distance.
- It has quiet and smooth action.
- A worm wheel gear is not easy to drive except for special occasions.
A bevel gear is one of a pair of gears used to connect two shafts whose axes intersect and the pitch surfaces are cones. Teeth are cut along the pitch cone. There are different classes of bevel gear depending on tooth trace.
- Straight bevel gear.
- Spiral bevel gear.
- Miter gears.
Each of these types has its features as follows:
a) Straight Bevel Gear
- It is relatively easy to manufacture.
- It provides reduction ratio up to approximately 1:5.
b) Spiral Bevel Gear
- It has a higher contact ratio, higher strength and durability than an equivalent straight bevel gear.
- It allows a higher reduction ratio.
- It has a better efficiency of transmission with reduced gear noise.
- It involves some technical difficulties in manufacturing.
c) Miter Gears
This is a special class of bevel gear in which the shafts intersect at 90? and the gear ratio is 1:1.
Categories of Gears
There are different categories of gears in accordance with the orientation of areas. Examples are:
- Parallel axes: These include: spur gear, helical gear, rack and internal gear.
- Intersecting axes: These include: miter gear, straight bevel gear and spiral bevel gear.
- Non-parallel, non-intersecting axes: These are screw gear and worm.
- Others: They include splineshaft and bushing; gear coupling; and pawl and rachet.
Uses of Gears
Gears are used in three different ways. They are:
- Power transmission.
- Changing direction.
- Selecting speed.
Gears are used for transmitting power from one part of a machine to another. In a bicycle, for example, its gear (with the help of a chain) takes power from the pedals to the back wheel. Similarly, in a car, gears transmit power from the crankshaft (the rotating axle that takes power from the engine) to the driveshaft, running under the car which ultimately powers the wheels.
Each time power passes from one gear wheel to another, one of these three things occurs.
1) Increased speed: If two gears are connected together, the first one has more teeth than the second one (generally that means it has a bigger sized wheel), and the second one has to turn round much faster to keep up. In this arrangement, the second wheel turns faster than the first one but with less force.
2) Increased force: If the second wheel ina pair of gears has more teeth than the first one (that is, if it is a larger wheel), it turns slower than the first one but with more force.
3) Changed direction: When two gears mesh together, the second one always turns in the opposite direction. So if the first one turns clockwise, the second one must turn counter clockwise.
A special shaped gear makes the power of a machine turn through an angle. In a car, for example, the differential (a gear box in the middle of the rear axle of a rear wheel drive car) uses a cone-shaped bevel gear to turn the driveshaft’s power through 90? and turn the back wheels.
Gear Ratios and Speed Rotation
Gear ratio is also known as speed ratio, which is the ratio of the angular velocity of the input gear to the angular velocity of the output gear. The gear ratio can be calculated directly from the number of teeth on the gear.
The gear teeth are distributed along the circumference of the pitch circle so that the thickness of each tooth and the space between neighbouring teeth are the same. The pitch, P of a gear which is the distance between equivalent points on neighbouring teeth along the pitch circle, is equal to twice the thickness of a tooth.
P = 2t
The pitch of a gear GA can be computed from the number of teeth TA and the radius rA of its pitch circle is
In order to mesh smoothly, two gears GA and GB must have the same sized teeth and therefore, they must have the same pitch P, that is:
This equation shows that the ratio of the circumference, the diameters and the radii of the two meshing gears is equal to the ratio of their teeth number:
Gear ratio of two gears rolling without slipping on their pitch circle is given as:
Then, we can say that the gear ratio or speed ratio is inversely proportional to the radius of the pitch circle and the number of teeth of the input gear.
Speed Rotation or Rotational Speed
One of the properties of gears is that they change the rotational speed of the axles that hold them. The relationship between linear speed and rotational speed is:
V = W x r
Where V = linear speed, W = rotational speed and r = radius of the gear.
The radius is half the gear’s diameter. In a situation whereby a 40-teeth spur gear output axle is meshed with an 8-teeth spur gear on the input axle, then V40=- V8, we can substitute the above equation for V and get:
The rotational speed of the 40-teeth gear is 1/5 of the 8-teeth gear, or 5 times slower. The negative sign means the rotational speed is in opposite direction.
Functions of Lubricants in Gears
Maintaining a vehicle or any engine requires the use of lubricants. One of the basic functions of gear lubricants is to reduce friction between the gear teeth under contact and prevent premature contact and wear in order to extend equipment service life. A high quality lubricant must protect gear systems.
The lubricant is also a protection against oxidation, thermal degradation, rust, copper corrosion and foaming. In summary, we can say that lubricants are used for three purposes in gears:
- to reduce friction.
- to prevent wear.
- as a protective cover against corrosion.
Reducing Friction in Gears
The lubricants form a layer or film between the metal surfaces and actually keep the metals from touching (the moving parts literally ride on the lubricant). In the instance of two metal surfaces sliding across each other where space cannot be provided for ball bearings, the lubricant serves as a liquid bearing. In all gear devices, lubrication is necessary to counteract friction as much as possible.
Gear lubricants are a mixture of high-viscosity oils, which are suitable for high gear tooth pressure and moderate speeds.
The lubricant provides an oil film at the contacting surface and serves to remove and dissipate heat from where it is generated. This prevents gearing component temperature from rising to excessive levels. Other lubrication functions include the transfer and or removal of wear particle, as well as the filtration of rust, corrosion and other undesirable contaminant.