Introduction to Gears:
What is a gear (or) Explain gears?
Gears are used to transmit motion and power between rotating shafts by means of
progressive engagement of projections called teeth.Gears use no intermediate
link and transmit motion by direct contact.
Gears operate in pairs, the smaller of the pair is called the
“PINION” and the larger is called the “GEAR” usually, the pinion drives the
Gears and when the teeth are meshed, rotation of one shaft and pinion causes
the Gear and its shaft to rotate.
Gears
can be used to
- a. Change the direction
- b. Run angular ways
- c. Avoid slipping, which is a common phenomenon in belts.
Applications of Gears:
Now–a–days, Gears are used in most machinery & they range in size from the smallest gear in watch mechanism to bigger(around 100 feet diameter) Gears in radar antennas. In many machines like machine tools, automobiles, tractors,hoisting, mechanisms, rolling mills, marine engines, etc… Gears play a vital role.
Classification of Gears:
The Gears of toothed wheels may be classified as follows:
According to the position
of the shaft axis:
a)
Parallel shafts
1.Spur Gears
2.helical Gears
3.Herring bone or double helical gears.
b)
Intersecting shafts
1.Bevel Gears
1.1 Straight Bevel Gears
1.2 Spiral Bevel Gears
1.3 Zero Bevel Gears
c)
Non-parallel shafts
1.Hypoid Gears
2.Crossed Helical Gears
3.Worm and Worm wheel
Explain about Spur Gears and classification of spur gears?
What is a SPUR gear (or) Explain Spur gear?
Spur gears have straight teeth cut parallel to the rotational axis. The tooth form is based on the involute curve. Practice has shown that this design accommodates mostly rolling, rather than sliding,contact of the tooth surfaces.The involute curve is generated during gear machining processes using gear cutters with straight sides.Near the root of the tooth, however,the tool traces a trochoidal path, providing a heavier, and stronger, root section. Because of this geometry, contact between the teeth occurs mostly as rolling rather than sliding. Since less heat is produced by this rolling action, mechanical efficiency of spur gears is high, often up to99%. Some sliding does occur,however.
And because contact is simultaneous across the entire width of the meshing teeth, a continuous series of shocks is produced by the gear. These rapid shocks result in some objectionable operating noise and vibration. Moreover, tooth wear results from shock loads at high speeds.
Noise and wear can be minimized with proper lubrication, which reduces tooth surface contact and engagement shock loads.Spur gears are the least expensive to manufacture and the most commonly used, especially for drives with parallel shafts. The three main classes of spur gears are: external tooth, internal tooth, and rack-and-pinion.
External-tooth gears
The most common type of spur gear, has teeth cut on the outside perimeter of mating cylindrical wheels, with the larger wheel called the gear and the smaller wheel the pinion.The simplest arrangement of spur gears is a single pair of gears called a single reduction stage, where output rotation is in a direction opposite that of the input. In other words, one is clockwise while the other is counter-clockwise.
Higher net reduction is producedwith multiple stages inwhich the driven gear is rigidly connectedto a third gear. This third gearthen drives a mating fourth gear that serves as output for the second stage.In this manner, several output speedson different shafts can be produced from a single input rotation.
Internal (ring) gears
Ring gears produce an output rotation that is in the same direction as the input, As the name implies, teeth are cut on the inside surface of a cylindrical ring, inside of which are mounted a single external-tooth spur gear or set of external-tooth spur gears, typically consisting of three or four larger spur gears (planets) usually surrounding a smaller central pinion (sun). Normally, the ring gear is stationary, causing the planets to orbit the sun in the same rotational direction as that of the sun. For this reason, this class of gear is often referred to as a planetary system.
The orbiting motion of the planets is transmitted to the output shaft by a planet carrier.In an alternative planetary arrangement, the planets may be restrained from orbiting the sun and the ring left free to move. This causes the ring gear to rotate in a direction opposite that of the sun. By allowing both the planet carrier and the ring gear to rotate, a differential gear drive is produced, the output speed of one shaft being dependent on the other.
Rack-and-pinion gears
A straight bar with teeth cut straight across it is called a rack.Basically, this rack is considered to be a spur gear unrolled and laid out fiat.Thus, the rack-and pinion is a special case of spur gearing. The rack-and-pinion is useful in converting rotary motion to linear and vice versa. Rotation of the pinion produces linear travel of the rack. Conversely, movement of the rack causes the pinion to rotate.The rack-and-pinion is used extensively in machine tools, lift trucks, power shovels, and other heavy machinery where rotary motion of the pinion drives the straight-line action of a reciprocating part. Generally, the rack is operated without a sealed en-closure in these applications, but some type of cover may be provided to keep dirt and other contaminants from accumulating on the working surfaces.
What are Helical Gears & give their classification?
Helical gearing differs from spur in that helical teeth are cut across the gear face at an angle rather than straight,Thus, the contact line of the meshing teeth progresses across the face from the tip at one end to the root of the other, reducing the noise and vibration characteristic of spur gears. Also, several teeth are in contact at any one time, producing a more gradual loading of the teeth that reduces wear substantially.
The increased amount of sliding action between helical gear teeth, however, places greater demands on the lubricant to prevent metal-to-metal contact and resulting premature gear failure. Also, since the teeth mesh at an angle, a side thrust load is produced along each gear shaft. Thus, thrust bearings must be used to absorb this load so that the gears are held in proper alignment. The three other principle classes of helical gears are: double-helical, herringbone,and cross-helical.
Double-helical gears
Thrust loading is eliminated by using two pairs of gears with tooth angles opposed to each other. In this way, the side thrust from one gear cancels the thrust from the other gear. These opposed gears are usually manufactured with a space between the opposing sets of teeth.
Herringbone gears
Teeth in these gears resemble the geometry of a herring spine, with ribs extending from opposite sides in rows of parallel, slanting lines. Herringbone gears have opposed teeth to eliminate side thrust loads the same as double helicals, but the opposed teeth are joined in the middle of the gear circumference. This arrangement makes herringbone gears more compact than double-helicals. However, the gear centers must be precisely aligned to avoid interference between the mating helixes.
Cross-helical gears
This type of gear is recommended only for a narrow range of applications where loads are relatively light. Because contact between teeth is a point instead of a line, the resulting high sliding loads between the teeth requires extensive lubrication.Thus, very little power can be transmitted with cross helical gears.
Explain Bevel Gears & their classification?
Unlike spur and helical gears with teeth cut from a cylindrical blank, bevel gears have teeth cut on an angular or conical surface. Bevel gears are used when input and output shaft center lines intersect. Teeth are usually cut at an angle so that the shaft axes intersect at 90 deg, but any other angle may be used. A special class of bevels called miter gears have gears of the same size with their shafts at right angles. Often there is no room to support bevel gears at both ends because the shafts intersect.
Thus, one or both gears overhang their supporting shafts. This overhung load (OHL) may deflect the shaft, misaligning gears, which causes poor tooth contact and accelerates wear. Shaft deflection may be overcome with straddle mounting in which a bearing is placed on each side of the gear where space permits. There are two basic classes of bevels: straight-tooth and spirals.
Straight-tooth bevels
These gears, also known as plain bevels, have teeth cut straight across the face of the gear. They are subject to much of the same operating conditions as spur gears in that straight tooth bevels are efficient but somewhat noisy. They produce thrust loads in a direction that tends to separate the gears.
Spiral-bevels
Curved teeth provide an action somewhat like that of a helical gear, Figure 10. This produces smoother,quieter operation than straight-tooth bevels. Thrust loading depends on the direction of rotation and whether the spiral angle at which the teeth are cut is positive or negative.
Explain Hypoid Gears & their classification?
Hypoid gears resemble spiral bevels, but the shaft axes of the pinion and driven gear do not intersect.This configuration allows both shafts to be supported at both ends. In hypoid gears, the meshing point of the pinion with the driven gear is about midway between the central position of a pinion in a spiral-bevel and the extreme top or bottom position of a worm. This geometry allows the driving and driven shafts to continue past each other so that end-support bearings can be mounted. These bearings provide greater rigidity than the support provided by the cantilever mounting used in some bevel gearing.
Also adding to the high strength and rigidity of the hypoid gear is the fact that the hypoid pinion has a larger diameter and longer base than a bevel or spiral-bevel gear pinion of equal ratio. Although hypoid gears are stronger and more rigid than most other types, they are one of the most difficult to lubricate because of high tooth-contact pressures. Moreover, the high levels of sliding between tooth surfaces reduces efficiency. Infact, the hypoid combines the sliding action of the worm gear with the rolling movement and high tooth pressure associated with the spiral bevel. In addition, both the driven and driving gears are made of steel, which further increases the demands on the lubricant.
As a result, special extreme pressure lubricants with both oiliness and anti-weld properties are required to withstand the high contact pressures and rubbing speeds in hypoids. Despite these demands for special lubrication, hypoid gears are used extensively in rear axles of automobiles with rear-wheel drives. Moreover, they are being used increasingly in industrial machinery.
What is a WORM & WORM WHEEL?
Worm gear sets, consist of a screw-like worm (comparable to a pinion) that meshes with a larger gear, usually called a wheel. The worm acts as a screw, several revolutions of which pull the wheel through a single revolution. In this way, a wide range of speed ratios up to 60:1 and higher can be obtained from a single reduction. Most worms are cylindrical in shape with a uniform pitch diameter. However, a double enveloping worm has a variable pitch diameter that is narrowest in the middle and greatest at the re ends.
This configuration allows the worm to engage more teeth on the wheel, thereby increasing load capacity.In worm-gear sets, the worm is most often the driving member. However, a reversible worm-gear has the worm and wheel pitches so proportioned that movement of the wheel rotates the worm. In most worm gears, the wheel has teeth similar to those of a helical gear, but the tops of the teeth curve inward to envelop the worm. As a result, the worm slides rather than rolls as it drives the wheel. Because of this high level of rubbing between the worm and wheel teeth, the efficiency of worm gearing is lower than other major gear types.
One major advantage of the worm gear is low wear, due mostly to the full-fluid lubricant film that tends to be formed between tooth surfaces by the worm sliding action. A continuous film that separates the tooth surfaces and prevents direct metal-to-metal contact is typically provided by a relatively heavy oil, which is often compounded with fatty or fixed oils such as acid less tallow oil. This adds film strength to the lubricant and further reduces friction by increasing the oiliness of the fluid.
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