Gears are classified according to the shape of the tooth pair and disposition into spur, helical, double helical, straight bevel, spiral bevel and hypoid bevel, worm and spiral gears You may study it in Design of Machine Element, Kinematics of Machines, Dynamics of Machinery and Theory of Machines Here we will discuss about definition and types of gears with their uses. There are mainly 5 types of gears we will descuss here however some sub types of gears are there you may check in your books
What is Gear ?
Gear is a cylinder, wheel, or disk on the surface of which are cut parallel teeth. The larger gear is called the gear the smaller, the pinion. Usually when two gears in mesh and one is larger than the other is, the names gear pinion is are replied. Spur gear is used to transmit power from one shaft to another in cases where those shafts have their axis parallel.
Types of Gears
There are five types of Gears. They are use in different different applications. so just check out
- Spur gear
- Helical gear
- Bevel gear
- Worm & Worm gear
- Rack and Pinion
This is a cylindrical shaped gear in which the teeth are parallel to the axis. It has the largest applications and, also, it is the easiest to manufacture. Tooth contact is primarily rolling, with sliding occurring during engagement and disengagement. Some noise is normal, but it may become objectionable at high speeds.
This types of Gears are used in series for large gear reductions. The teeth on spur gears are straight and are mounted in parallel on different shafts. Spur gears are used in washing machines, screwdrivers, windup alarm clocks, and other devices. These are particularly loud, due to the gear tooth engaging and colliding. Each impact makes loud noises and causes vibration, which is why spur gears are not used in machinery like cars. A normal gear ratio range is 1:1 to 6:1.
Applications of Spur Gear In mechanical systems where we’ll require simple design, more efficient power transfer, and low speed applications where noise does not matter, we should go for spur gears. Spur gears have a wide range of applications. They are used in:
- Metal cutting machines Power plants
- Marine engines
- Mechanical clocks and watches
- Fuel pumps
- Washing Machines
- Gear motors and gear pumps
- Rack and pinion mechanisms
- Material handling equipments
- Automobile gear boxes
- Steel mills
- Rolling mills
Helical gears are used for parallel shaft drives. They have teeth inclined to the axis as
shown in Fig. 1.9. Hence for the same width, their teeth are longer than spur gears and
have higher load carrying capacity. Their contact ratio is higher than spur gears and
they operate smoother and quieter than spur gears. Their precision rating is good. They
are recommended for very high speeds and loads. Thus, these gears find wide
applications in automotive gearboxes as illustrated in Fig. 1.10. Their efficiency is
slightly lower than spur gears. The helix angle also introduces axial thrust on the shaft.
Helical gears operate more smoothly and quietly compared to spur gears due to the way the teeth interact. The teeth on a helical gear cut at an angle to the face of the gear. When two of the teeth start to engage, the contact is gradual-starting at one end of the tooth and maintaining contact as the gear rotates into full engagement. The typical range of the helix angle is about 15 to 30 deg. The thrust load varies directly with the magnitude of tangent of helix angle. Helical is the most commonly used gear in transmissions. They also generate large amounts of thrust and use bearings to help support the thrust load. Helical gears can be used to adjust the rotation angle by 90 deg. when mounted on perpendicular shafts. Its normal gear ratio range is 3:2 to 10:1.
Double Helical Gear OR Herringbone Gear
Double helical or Herringbone gears used for transmitting power between two parallel shafts. They have opposing helical teeth with or without a gap depending on the manufacturing method adopted, Fig Two axial thrusts oppose each other and nullify. Hence the shaft is free from any axial force. Though their load capacity is very high, manufacturing difficulty makes them costlier than single helical gear. Their applications are limited to high capacity reduction drives like that of cement mills and crushers
Internal gears are used for transmitting power between two parallel shafts. In these gears, annular wheels are having teeth on the inner periphery. This makes the drive very compact Fig. In these drives, the meshing pinion and annular gear are running in the same direction. Their precision rating is fair. They are useful for high load and high speed application with high reduction ratio. Applications of these gears can be seen in planetary gear drives of automobile automatic transmissions reduction gearboxes of cement mills, step-up drives of wind mills.
They are not recommended for precision meshes because of design, fabrication, and inspection limitations. They should only be used when internal feature is necessary. However, today precision machining capability has led to their usage even in position devices like antenna drives.
Bevel gears are used to change the direction of a shaft’s rotation. Bevel gears have teeth that are available in straight, spiral, or hypoid shape. Straight teeth have similar characteristics to spur gears and also have a large impact when engaged. Like spur gears, the normal gear ratio range for straight bevel gears is 3:2 to 5:1.
The teeth on bevel gears can be straight, spiral or hypoid. Straight bevel gear teeth actually have the same problem as straight spur gear teeth — as each tooth engages, it impacts the corresponding tooth all at once
1. Straight bevel gears
Straight bevel gears are used for transmitting power between intersecting shafts, Fig. They can operate under high speeds and high loads. Their precision rating is fair to good. They are suitable for 1:1 and higher velocity ratios and for right-angle meshes to any other angles. Their good choice is for right angle drive of particularly low ratios. However, complicated both form and fabrication limits achievement of precision. They should be located at one of the less critical meshes of the train. Wide application of the straight bevel drives is in automotive differentials, right angle drives of blenders and conveyors. A typical application of straight bevel used in differential application
2. Spiral bevel gears
Spiral teeth operate the same as helical gears. They produce less vibration and noise when compared to straight teeth. The right hand of the spiral bevel is the outer half of the tooth, inclined to travel in the clockwise direction from the axial plane. The left hand of the spiral bevel travels in the counterclockwise direction. The normal gear ratio range is 3:2 to 4:1.
Spiral bevel gears shown in Fig. are also used for transmitting power between
intersecting shafts. Because of the spiral tooth, the contact length is more and contact
ratio is more. They operate smoother than straight bevel gears and have higher load
capacity. But, their efficiency is slightly lower than straight bevel gear. Usage of spiral
bevel gears in an automobile differential
3. Hypoid Bevel Gear
Hypoid Bevel Gears are also used for right angle drive in which the axes do not intersect. This permits the lowering of the pinion axis which is an added advantage in automobile in avoiding hump inside the automobile drive line power transmission. However, the non – intersection introduces a considerable amount of sliding and the drive requires good lubrication to reduce the friction and wear. Their efficiency is lower than other two types of bevel gears. These gears are widely used in current day automobile drive line power transmission.
Application of Bevel Gear – The bevel gear has many diverse applications such as locomotives, marine applications, automobiles, printing presses, cooling towers, power plants, steel plants, railway track inspection machines, etc.
Worm & Worm Gears
Worm gears are used in large gear reductions. Gear ratio ranges of 5:1 to 300:1 are typical. The setup is designed so that the worm can turn the gear, but the gear cannot turn the worm. The angle of the worm is shallow and as a result the gear is held in place due to the friction between the two. The gear is found in applications such as conveyor systems in which the locking feature can act as a brake or an emergency stop.
They are used in right-angle skew shafts. In these gears, the engagement occurs without any shock. The sliding action prevalent in the system while resulting in quieter operation produces considerable frictional heat. High reduction ratios 8 to 400 are possible.
Efficiency of these gears is low anywhere from 90% to 40 %. Higher speed ratio gears
are non-reversible. Their precision rating is fair to good. They need good lubrication for
heat dissipation and for improving the efficiency. The drives are very compact
Spiral gears are also known as crossed helical gears, Fig. They have high helix
angle and transmit power between two non-intersecting non-parallel shafts. They have
initially point contact under the conditions of considerable sliding velocities finally gears
will have line contact. Hence, they are used for light load and low speed application
such as instruments, sewing machine etc. Their precision rating is poor. Application
of spiral gear used in textile machinery.
Rack and Pinion Gears
A rack and pinion mechanism is used to transform rotary motion in to linear motion and vice versa. A round a spur gear, the pinion, meshes with a spur gear which has teeth set in a straight line, the rack. The rack and pinion can transform rotary motion into linear motion and vice versa in three ways.
- Rotation of the pinion about a fixed center causes the rack to move in a straight line.
- Movement of the rack in a straight line causes the pinion to rotate about a fixed center.
- If the rack is fixed and the pinion rotates, then the pinion’s center moves in a straight line taking the pinion with it.