A gear is a power and motion transmitting mechanism in between One shaft to another shaft. And also to transmit motion with out slipping with constant velocity ratio. The gear drives as compared to other drive like as chain drive, belt drive and friction drive are more compact, that can operate high speed and require of precise time. It is also used to large power to be transmitted. When the force are required to holding the gear in position is very less than in equivalent friction drive. So the gear drive to get the result with lower bearing pressure and low wearing on bearing surface.
Advantages of gear drive
- There is no sleeping, when the gear drive is working condition. So that is obtained the exact velocity ratio.
- That can be transmitting The large Power capacity than chain drive and belt drive.
- Require less space of power transmit for one shaft to another shaft as compared to belt and rope drive.
- That should be more efficient upto 99% ( the efficient mean power transmission during the process)
- Possible of transmit motion at very low velocity condition, it is not possible at belt drive.
Disadvantages of gear driving
- The manufacturing of gear drive require for a special tooling and advanced equipment.
- The cost of manufacturing and maintenance is very high.
- error in cutting teeth may be cause noise and vibration during the operation.
Define gear mechanism
The circular body of the cylindrical shape and of uniform small width, teeth provided within uniform formation on the surface of outer circumferential is called toothed wheel or toothed gear.
Classification of gears
The gears are classified based on different manners
(1) classification of gear based on relative position of their shaft axes:
(i) parallel shaft
Examples: spur gear, helical gear, Rack and pinion, herringbone gear and internal gear
(ii) Intersecting Shafts
Examples: bevel gear and spiral gear
(iii) non parallel non intersecting shafts
Examples: worm gear, hypoid gear and spiral gear.
(2) classification based on the relative motion of the shaft rotation
(i) row gears: it is the motion of the shaft relative to each other It is fixed
(ii) planetary and differential gear
(3) Classification based on peripheral speed(v)
- Low velocity gear
- Medium velocity gear
- High velocity gear
(4) Classification based on the position of teeth on wheel
- straight gears
- helical gear .
- herringbone gear
- curved teeth gear
(5) Classification based on type of gearing
- External gearing
- Internal gear
- Rack and pinion
The gears are placed on straight and parallel to axis of the wheel. That gears are called as spur gear. Spur gear are used to transmit the rotary motion between parallel shafts. The gear are may be as internal or external gears. When the external gear are rotated in opposite direction and the internal gear are rotated in same direction.
Spur gear terminology or nomenclature
- The smallest of two matting gear. The largest often called the gears or the wheel.
- It is an imaginary circle, which is pure rolling action. For actual gear to give the same motion.
Pitch circle diameter:
It is diameter of the Pitch Circle. When the size of the gear is specified by pitch circle diameter. It is also known as pitch diameter.
It is a common point of contact between the two pitch circle.
It is a surface of Rolling disc which meshing gears are replaced at pitch circle.
Two mating gear of pitch are same.That should be having Three Types of pitch.
What is distance between circumference of a pitch circle from a point of one tooth to the corresponding point of adjacent tooth.
It is the ratio between the pitch circle diameter to number of teeth. it is called diametral pitch.
It Is ratio between pitch circle diameter to the number of teeth.
Addendum Circle(tip circle):
The circle drawn through the top of the teeth and it is concentric with the Pitch Circle.
it is radial distance between the Pitch circle to bottom of the tooth.
Dedendum circle( root circle)
It is circle draw through bottom of the circle.
The radial distance of teeth in between pitch circle to bottom of the teeth.
The Clarence is a radial distance between the top of teeth to the bottom of the teeth in two meshing gear. A circle is drawn through the top of the meshing gear is called as clearance circle.
It is radial distance between the addendum circle to dedendum circle of gear.
So we can calculate the total death:
Total depth = addendum + dedendum
It is a Radial distance between the addendum circle to clearance circlet.It is also equal to sum of addendum of the two meshing gear.
The Measuring of width the tooth along the Pitch Circle.
It is measurement by space of width between to adjacent teeth measuring along the pitch circle.
It is difference between space of the tooth and thickness of the tooth along the Pitch Circle.
Backlash = tooth space – tooth thickness
It is measurement of gear width by parallel to its axis.
It is your surface of top of the teeth. it is called top land.
It is surface of the bottom of the teeth between adjacent fillets.
It is tooth surface between top Land to Pitch Circle.
The tooth surface between bottom land to pitch circle including fillet.
It is curved shape of the toothed flank at root circle.
Pressure angle( angle of obliquity):
The angle between the common normal to two gearing of teeth at point of contact and a common tangent at a point of pitch Point. It is called pressure angle in the Gearing. The standard pressure angle of the teeth is 14 1/2 degree and 20 degree.
Path of contact:
The point of contact of two teeth from beginning to end of engagement.
Length of path of contact( contact length):
It is measured by length of the common normal cutoff by addendum circle of the pinion and wheel.
Arc of content:
The path is measured by a point on the circle from beginning to end of engagement in pair of teeth. The arc of contact is divided by two types. The are
(i) arc of approach
(ii) Arc of recess
Arc of approach: It is path of contact from the beginning of engagement to end of pitch point.
Arc of recess: it is path of contact from Pitch point to end of engagement of pair of teeth.
It is ratio between speed of driving gear to speed of driven gears.
The ratio between length of Arc of contact to circular pitch.it is called as Contact ratio.
Law of Gearing:
To obtain a constant velocity ratio, the common normal of any instant of teeth at each point of contact should always pass through a pitch point, Situated on line joining center of rotation of the of mating gear.
Mean of Other Words:(The condition made by the tooth profile to maintain a constant angular velocity ratio in between two pair of gear. The fundamental of condition which must be satisfied designing the profile of the teeth of gear wheel.)
General form of gear tooth profile:
The shape of two curve that fulfill the law of gearing, it can be used in teeth profile. if the profile of teeth of one matting gear is arbitrarily and profile of other gear is determined,so to satisfy the law of gearing, it type of teeth is known as conjugate teeth. The gear are having conjugate teeth can be successfully used for power transmission of motion. but there are difficult to manufacturing by using a special device which are costly. So the conjugate teeth are not used in most common method.
So the common form of the profile used in actual practical purpose;
They are (i) involute profile (ii) cycloidal tooth profile
The common difference between involute tooth profile and cycloidal tooth profile.
Standard System of gear tooth profile:
American gear manufacture Association(AGMA) and American National standard Institute(ANSI) for standardized following for form of gear teeth profile depending upon the pressure angle.
1. 14 ½ ⁰ composite system
2. 14 ½ ⁰ full depth involute system
3. 20⁰ full depth envelope system
4. 20⁰ degree stub involute system
Advantages of 14 ½ ⁰ involute system
It type of profile teeth could be having smooth and noiseless operation.
Very strong teeth
Advantages of 14 degree involute system
It will be reduced the risk of undercutting
It will be stronger teeth with a load carrying capacity
It will be having greater length of contact
the modern industrial widely various gear materials are used. The gears material are classified in two types. They are Metallic and non metallic material.
(1) metallic material:
- Steel: it is more common material of manufacturing of gears. Almost all types of Steel have been used in this manufacturing purpose.
- To combine the property of toughness and teeth hardness, steel gears are heat treated.
- The Steel With Brinell hardness number < 350 are used to light and medium duty purpose. But >350 BHN are used to heavy duty and it require to compactness.
- The plain carbon steel used for medium duty application such as 50 C 8,45 C 8, 50 C 4 and 55 C 8.For the heavy duty application alloy Steel are used like as 40 Cr 1 , 30 Ni 3 Cr 65 Mo 55.For planetary gear train the recommended Steel of 35 Ni 1 Cr 60.
The cast iron is a extensive gear material. Because of low cost,good machinability, and moderate Mechanical properties. For general purpose, the large size gear made from grey cast iron (FG 200, FG 260 or FG 350)
Disadvantage: it is a low tensile strength
- It is mainly used to worm gears drive because of ability of highest sliding load.
- It is most suitable for corrosion and wear problem.
Disadvantage: there are costly material.
- The bronze Alloys like as manganese bronze, aluminium bronze, Silicon bronze or
- phosphorus bronze are used in manufacture of gears.
(2) non metallic material:
The non metallic materials like as wood, rawhide, synthetic resin, compressed paper are used for gears.
Advantage:i. Cost very low ii. Noiseless operation iii. Damping of shock and vibration.
Disadvantage: low heat conductivity and low load carrying capacity
Selection of Gears Materials:
Selection of gears material depends on the various reason as follows,
- Service type
- The speed of peripheral
- Manufacturing method
- Degree of accuracy requirement
- Material cost
- Wear and shock resistance
- Weight and space limitation
- Impact load , maximum load and life of gear
- The safety and other consideration
Gear manufacturing method:
The manufacture of gear followed by various process and that can be classified,
Gear milling process:
- Gear milling is a process of cutting a material by feeding a work piece under rotating of multiple tooth cutter. When the cutting action for many teeth around the milling cutter and providing a fast method of machining. So the machined surface maybe followed by flat,angular, or curved.The machining of surface in above require shape of material.
- Spur gear, helical gear and straight bevel gear are milled by this method.
- The machining of surface finish having about 3.2μm.
The gears are formed together in serious of passes by generating tool shape like as mating of gears one to another.The hubs or shapers can be used by this method. The surface finish of machining process can be obtained by 1.6μm.
it is an simply method, which cutting tool shaped in worm. The gear hobbing method can produce almost any Shape of external tooth form expect bevel gears. It also control tooth spacing, lead angle and profile.
The gear shaper is used and the teeth may be generated with either rack cutter or pinion cutter. It can produce internal and external spur gears, helical gears, herringbone gears and face gears.
It is achieved by mass production of gears.
It can produce light weight gears of thermoplastic materials.
The die casting method, the shape of gears depends on the Die shape. It is similar to Melton metal casting. Brass, aluminium, Zince and magnesium gear are made by this process.
It is used to manufacture of small and heavy duty gears for pumps and instrument. Iron and brass material are most common material of sintering method.
It can produce medium duty gears of iron and steel for rough application.