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Terms in this set (29)
Parallel and co-planer shafts connected by gears are called spur gears. The arrangement is called spur gearing.
Spur gears have straight teeth and are parallel to the axis of the wheel. Spur gears are the most common type of gears. The advantages of spur gears are their simplicity in design, economy of manufacture and maintenance, and absence of end thrust. They impose only radial loads on the bearings.
Spur gears are known as slow speed gears. If noise is not a serious design problem, spur gears can be used at almost any speed.
Helical gears have their teeth inclined to the axis of the shafts in the form of a helix, hence the name helical gears.
These gears are usually thought of as high speed gears. Helical gears can take higher loads than similarly sized spur gears. The motion of helical gears is smoother and quieter than the motion of spur gears.
Single helical gears impose both radial loads and thrust loads on their bearings and so require the use of thrust bearings. The angle of the helix on both the gear and the must be same in magnitude but opposite in direction, i.e., a right hand pinion meshes with a left hand gear.
Herringbone gears resemble two helical gears that have been placed side by side. They are often referred to as "double helicals". In the double helical gears arrangement, the thrusts are counter-balanced. In such double helical gears there is no thrust loading on the bearings.
Intersecting but coplanar shafts connected by gears are called bevel gears. This arrangement is known as bevel gearing. Straight bevel gears can be used on shafts at any angle, but right angle is the most common. Bevel Gears have conical blanks. The teeth of straight bevel gears are tapered in both thickness and tooth height. In these Spiral Bevel gears, the teeth are oblique.
Spiral Bevel Gear
In these Spiral Bevel gears, the teeth are oblique. Spiral Bevel gears are quieter and can take up more load as compared to straight bevel gears.
Zero Bevel Gear
Zero Bevel gears are similar to straight bevel gears, but their teeth are curved lengthwise. These curved teeth of zero bevel gears are arranged in a manner that the effective spiral angle is zero.
Worm gears are used to transmit power at 90° and where high reductions are required. The axes of worm gears shafts cross in space. The shafts of worm gears lie in parallel planes and may be skewed at any angle between zero and a right angle.In worm gears, one gear has screw threads. Due to this, worm gears are quiet, vibration free and give a smooth output.Worm gears and worm gear shafts are almost invariably at right angles.
Rack and Pinion
A rack is a toothed bar or rod that can be thought of as a sector gear with an infinitely large radius of curvature. Torque can be converted to linear force by meshing a rack with a pinion: the pinion turns; the rack moves in a straight line. Such a mechanism is used in automobiles to convert the rotation of the steering wheel into the left-to-right motion of the tie rod(s). Racks also feature in the theory of gear geometry, where, for instance, the tooth shape of an interchangeable set of gears may be specified for the rack (infinite radius), and the tooth shapes for gears of particular actual radii then derived from that. The rack and pinion gear type is employed in a rack railway.
Internal and External Gear
An external gear is one with the teeth formed on the outer surface of a cylinder or cone. Conversely, an internal gear is one with the teeth formed on the inner surface of a cylinder or cone. For bevel gears, an internal gear is one with the pitch angle exceeding 90 degrees. Internal gears do not cause direction reversal.
Face gears transmit power at (usually) right angles in a circular motion. Face gears are not very common in industrial application.
Sprockets are used to run chains or belts. They are typically used in conveyor systems.
Crown gears or contrate gears are a particular form of bevel gear whose teeth project at right angles to the plane of the wheel; in their orientation the teeth resemble the points on a crown. A crown gear can only mesh accurately with another bevel gear, although crown gears are sometimes seen meshing with spur gears. A crown gear is also sometimes meshed with an escapement such as found in mechanical clocks.
Driven gear/Driving gear
ex. Driven Gear (30 teeth)/Driving Gear (60 teeth)=30/60= 1:2
AKA It means that the DRIVEN gear makes TWO rotations for every ONE rotation of the Driving Gear.
ex Driven 75/ Driving 25=75/25= 3:1
AKA for every 3 rotations of the driving gear, the driven gear makes one rotation.
Parallel axis gears
Gears involving two axis, which are parallel to each other, are called Parallel Axis Gears. For the
transmission of rotation/power by parallel axis, Spur, Helical and Internal Gears are generally used. These are the most commonly used gears, with a wide range of applications, in various industries.
Intersecting axis gears
Gears involving two axis crossing at a point are called Intersecting Axis Gears; general applications include rotation / power transmission of Bevel gears. Bevel Gears with gear ratio of 1, are called Miter gears. Bevel Gears are classified as Straight-Bevel Gears or Spiral-Bevel Gears, depending on the tooth form.
Non parallel/non intersecting gears
Gears involving two axis, which are not intersected or parallel, are called Nonparallel and Nonintersecting Axis Gears. They are generally used as worm gear pairs or screw gears. These gears transmit rotational force/power by the relative slippage between gear-tooth surfaces.
Simple Gear Train
The simple gear train is used where there is a large distance to be covered between the input shaft and the output shaft. Each gear in a simple gear train is mounted on its own shaft.
When examining simple gear trains, it is necessaryto decide whether the output gear will turn faster, slower, or the same speed as the input gear. The circumference (distance around the outside edge) of these two gears will determine their relative speeds.
Suppose the input gear's circumference is larger than the output gear's circumference. The output gear will turn faster than the input gear. On the other hand, the input gear's circumference could be smaller than the output gear's circumference. In this case the output gear would turn more slowly than the input gear. If the input and output gears are exactly the same size, they will turn at the same speed.
In many simple gear trains there are several gears between the input gear and the output gear.
These middle gears are called idler gears. Idler gears do not affect the speed of the output gear.
Compound gear train
In a compound gear train at least one of the shafts in the train must hold two gears.
Compound gear trains are used when large changes in speed or power output are needed and there is only a small space between the input and output shafts.
The number of shafts and direction of rotation of the input gear determine the direction of rotation of the output gear in a compound gear train. The train in Figure has two gears in between the input and output gears. These two gears are on one shaft. They rotate in the same direction and act like one gear. There are an odd number of gear shafts in this example. As a result, the input gear and output gear rotate in the same direction.
Since two pairs of gears are involved, their ratios are "compounded", or multiplied together. Example- The input gear, with 12 teeth, drives its mating gear on the counter-shaft, which has 24 teeth. This is a ratio of 2 to 1. This ratio of DRIVEN over DRIVER at the Input - 2 to 1 - is then multiplied by the Output ratio, which has a DRIVEN to DRIVER ratio of 3 to 1.This gives a gear ratio of 6 to 1 between the input and the output, resulting in a speed reduction and a corresponding increase in torque.
Reverted Gear Train
A reverted gear train is very similar to a compound gear train. They are both used when there is only a small space between the input and output shafts and large changes in speed or power are needed. There are two major differences between compound and reverted gear trains. First, the input and output shafts of a reverted train must be on the same axis (in a straight line with one another). Second, the distance between the centers of the two gears in each pair must be the same.
Epicyclyclic (Planetary) Gear Train
Like a compound gear train, planetary trains are used when a large change in speed or power is needed across a small distance. There are four different ways that a planetary train can be hooked up. A planetary gear train is a little more complex than other types of gear trains. In a planetary train at least one of the gears must revolve around another gear in the gear train. A planetary gear train is very much like our own solar system, and that's how it gets its name. In the solar system the planets revolve around the sun. Gravity holds them all together. In a planetary gear train the sun gear is at the center. A planet gear revolves around the sun gear. The system is held together by the planet carrier. In some planetary trains, more than one planet gear rotates around the sun gear. The system is then held together by an arm connecting the planet gears in combination with a ring gear. The planetary gear set is the device that produces different gear ratios through the same set of gears. Any planetary gearset has three main components:
· The sun gear
· The planet gears and the planet gears' carrier
· The ring gear
Each of these three components can be the input, the output or can be held stationary. Choosing which piece plays which role determines the gear ratio for the gearset.
Gearboxes (or gear reducers) are used to increase the output torque or change the speed of a motor. They are usually made with steel materials and typically consist of gears, shafts and bearings which have been mounted in a factory lubricated housing. The gearbox can be damaged if it's maximum input speed is exceeded.Spiral
Spiral Hypoid Gear
This gear is a cross between a bevel gear and a worm gear. It's mating gear axes do not intersect. They typically require more lubrication but are durable and quiet.
The gap or play between the components of a mechanical system. It helps avoid interference and wear, prevents excessive heat generation and ensures proper lubrication. If the clearance is correct, the gap between the teeth of one gear will be slightly larger than the tooth width of the mating gear. Maintaining proper clearance requires that when installing gears, both gears in a set are changed, not just one.
The loose motion resulting from the clearance between the contacting parts of the gears. It is determined by the thickness of the gears teeth. The amount depends on how much the width of a tooth space exceeds the thickness of the engaging tooth on the pitch circles. The backlash also depends on the play between mating tooth surfaces at the tightest point of mesh in a normal direction.
Right angle drive
Transer power from an input shaft to an output shaft at a right angle. They are made up of two meshing gears and, although installation costs are higher, can better tolerate torque.
Double Enveloping Worm Drive
Used for high torque. It has a larger contact area so it can handle torque deviations better. The fact that the worm diameter is smaller in the middle allows the shaft centre lines to be closer.
Double Lead Worm Drive
Has two thread starts. This type of worm drive can have as many as six starts and is most commonly used when gear ratios require smaller differentials.
Mechanical energy lost due to friction between moving parts of a machine. With most gears, the efficiency loss is between 1-2% in each mesh. So, if a gear has six meshes, the efficiency will be between 88-94%.
A differential is a gear train with three shafts that has the property that the angular velocity of one shaft is the average of the angular velocities of the others, or a fixed multiple of that average
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