A gear is a wheel provided with teeth, which mesh with the teeth on another wheel, or on to a rack, so as to give a positive transmission of motion from one component to another.
Gears constitute the most commonly used device for power transmission or for changing power-speed ratios in a power system. They also afford a convenient way of changing the direction of motion. A number of devices such as differentials, transmission gear boxes, planetary drives etc., used in many construction machines employ gears as basic components. Gears are made from both metals as well as non-metals. "Metals" normally used for making gears include cast iron, cast steel, structural steel, gun metal, brass, etc. The "non-metals" commonly used for gear manufacture include synthetic plastic, fibres and laminated wood. With rapid industrialisation, the gears are called upon to yield noise free operation, ensure high load carrying capacity at a constant velocity ratio. The wear and fatigue strength of the gear tooth are the factors that govern its durability and reliability, and depend to a large extent on the manufacturing technology employed.
Types of Gears
The different types of gears used are enumerated and described below:
1. Spur gear.
2. Helical gear.
3. Bevel gear.
4. Worm gear.
5. Rack and pinion.
1. Spur gear:
A spur gear is a gear wheel or pinion for transmitting motion between two parallel shafts. This is the simplest form of geared drive. The teeth are cast or machined parallel with the axis of rotation of the gear. Normally the teeth are of involute form.
The efficiency of power transmission by these gears is very high and may be as much as 99% in case of high-speed gears with good material and workmanship of construction and good lubrication in operation. Under average conditions, efficiency of 96-98% are commonly attainable. The disadvantages are that they are liable to be more noisy in operation and may wear out and develop backlash more readily than the other types.
2. Helical gear:
Helical gear is one in which teeth instead, of being parallel with shaft as in ordinary spur gears, are inclined. This ensures smooth action and more accurate maintenance of velocity ratio. A disadvantage is that the inclination of the teeth sets up a lateral thrust. A method of neutralising this lateral or axial thrust is to use double-helical gears (also known as Herring bone gears)
3. Bevel gear: A bevel gear transmits motion between two shafts which intersect. If the shafts are at right angles and wheels equal in size, they are called mitre gears. If the shafts are not at right angles, they are sometimes called angle bevel gears. Spiral toothed bevel gears are preferred to straight toothed bevels in certain applications because they will run more smoothly and make less noise at high speeds.
4. Worm gear: Worm gears connect two non-parallel, non-intersecting shafts which are usually at right angle. One of the gears is called the 'worm'. It is essentially part of a screw, meshing with the teeth on a gear wheel, called the "worm wheel". The gear ratio is the ratio of number of teeth on the wheel to the number of threads on the worm.
One of the great advantages of worm gearing in that high gear ratios(i.e., ratio of rotational speed of worm to that of worm wheel) are easily obtained. Worm gearing is smooth and quiet.
5. Rack and pinion:
A rack is a spur gear of infinite diameter, thus it assumes the shape of a straight gear. The rack is generally used with a pinion to convert rotary motion into rectilinear motion.
Gears constitute the most commonly used device for power transmission or for changing power-speed ratios in a power system. They also afford a convenient way of changing the direction of motion. A number of devices such as differentials, transmission gear boxes, planetary drives etc., used in many construction machines employ gears as basic components. Gears are made from both metals as well as non-metals. "Metals" normally used for making gears include cast iron, cast steel, structural steel, gun metal, brass, etc. The "non-metals" commonly used for gear manufacture include synthetic plastic, fibres and laminated wood. With rapid industrialisation, the gears are called upon to yield noise free operation, ensure high load carrying capacity at a constant velocity ratio. The wear and fatigue strength of the gear tooth are the factors that govern its durability and reliability, and depend to a large extent on the manufacturing technology employed.
Types of Gears
The different types of gears used are enumerated and described below:
1. Spur gear.
2. Helical gear.
3. Bevel gear.
4. Worm gear.
5. Rack and pinion.
1. Spur gear:
A spur gear is a gear wheel or pinion for transmitting motion between two parallel shafts. This is the simplest form of geared drive. The teeth are cast or machined parallel with the axis of rotation of the gear. Normally the teeth are of involute form.
The efficiency of power transmission by these gears is very high and may be as much as 99% in case of high-speed gears with good material and workmanship of construction and good lubrication in operation. Under average conditions, efficiency of 96-98% are commonly attainable. The disadvantages are that they are liable to be more noisy in operation and may wear out and develop backlash more readily than the other types.
2. Helical gear:
Helical gear is one in which teeth instead, of being parallel with shaft as in ordinary spur gears, are inclined. This ensures smooth action and more accurate maintenance of velocity ratio. A disadvantage is that the inclination of the teeth sets up a lateral thrust. A method of neutralising this lateral or axial thrust is to use double-helical gears (also known as Herring bone gears)
3. Bevel gear: A bevel gear transmits motion between two shafts which intersect. If the shafts are at right angles and wheels equal in size, they are called mitre gears. If the shafts are not at right angles, they are sometimes called angle bevel gears. Spiral toothed bevel gears are preferred to straight toothed bevels in certain applications because they will run more smoothly and make less noise at high speeds.
4. Worm gear: Worm gears connect two non-parallel, non-intersecting shafts which are usually at right angle. One of the gears is called the 'worm'. It is essentially part of a screw, meshing with the teeth on a gear wheel, called the "worm wheel". The gear ratio is the ratio of number of teeth on the wheel to the number of threads on the worm.
One of the great advantages of worm gearing in that high gear ratios(i.e., ratio of rotational speed of worm to that of worm wheel) are easily obtained. Worm gearing is smooth and quiet.
5. Rack and pinion:
A rack is a spur gear of infinite diameter, thus it assumes the shape of a straight gear. The rack is generally used with a pinion to convert rotary motion into rectilinear motion.
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