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Elastomers are similar to thermoplastics but have sufficient cross-linking between molecules to prevent stretching and creep
RESINS
BASIC PRINCIPLES OF WELDING
GEAR
CONSTRUCTION OF AN AUTOMOBILE
TWO-STROKE AND DIESEL ENGINES
AC MOTORS
AUTOMATION IN INDUSTRY. FIXED AND PROGRAMMABLE AUTOMATION
HISTORY OF ROBOTICS
COMPUTERS
Навигация
GEAR
English language for technical colleges
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7. GEAR
Gear is a toothed wheel or cylinder used to transmit rotary or reciprocating motion from one part of a machine to another. Two or more gears, transmitting motion from one shaft to another, constitute a gear train. At one time various mechanisms were collectively called gearing. Now, however, gearing is used only to describe systems of wheels or cylinders with meshing (постоянное зацепление) teeth. Gearing is chiefly used to transmit rotating motion, but can, with suitably designed gears and flat-toothed sectors, be employed to transform reciprocating motion into rotating motion, and vice versa.
Simple Gears
The simplest gear is the spur (зубчатая) gear, a wheel with teeth cut across its edge parallel to the axis. Spur gears transmit rotating motion between two shafts or other parts with parallel axes. In simple spur gearing, the driven shaft revolves in the opposite direction to the driving shaft. If rotation in the same direction is desired, an idler gear (паразитная) is placed between the driving gear and the driven gear. The idler revolves in the opposite direction to the driving gear and therefore turns the driven gear in the same direction as the driving gear. In any form of gearing the speed of the driven shaft depends on the number of teeth in each gear. A gear with 10 teeth driving a gear with 20 teeth will revolve twice as fast as the gear it is driving, and a 20-tooth gear driving a 10-tooth gear will revolve at half the speed. By using a train of several gears, the ratio of driving to driven speed may be varied within wide limits.
Internal, or annular, gears are variations of the spur gear in which the teeth are cut on the inside of a ring or flanged wheel rather than on the outside. Internal gears usually drive or are driven by a pinion, a small gear with few teeth. A rack, a flat, toothed bar that moves in a straight line, operates like a gear wheel with an infinite radius and can be used to transform the rotation of a pinion to reciprocating motion, or vice versa.
Bevel gears (конические передачи) are employed to transmit rotation between shafts that do not have parallel axes. These gears have cone-shaped bodies and straight teeth. When the angle between the rotating shafts is 90°, the bevel gears used are called mitre gears.
Helical Gears
These gears have teeth that are not parallel to the axis of the shaft but are spiraled around the shaft in the form of a helix. Such gears are suitable for heavy loads because the gear teeth come together at an acute angle rather than at 90° as in spur gearing. Simple helical gearing has the disadvantage of producing a thrust that tends to move the gears along their respective shafts. This thrust can be avoided by using double helical, or herringbone, gears, which have V-shaped teeth composed of half a right-handed helical tooth and half a left-handed helical tooth. Hypoid gears are helical bevel gears employed when the axes of the two shafts are perpendicular but do not intersect. One of the most common uses of hypoid gearing is to connect the drive shaft and the rear axle in motor cars. Helical gearing used to transmit rotation between shafts that are not parallel is often incorrectly called spiral gearing.
Another variation of helical gearing is provided by the worm gear, also called the screw gear. A worm gear is a long, thin cylinder that has one or more continuous helical teeth that mesh with a helical gear. Worm gears differ from helical gears in that the teeth of the worm slide across the teeth of the driven gear instead of exerting a direct rolling pressure. Worm gears are used chiefly to transmit rotation, with a large reduction in speed, from one shaft to another at a 90° angle.
8. BEARINGS
Bearing is a mechanical device for decreasing friction in a machine in which a moving part bears—that is, slides or rolls on another part. Usually in a bearing the support must allow the moving part one type of motion, for example, rotation, while preventing it from moving in any other way, for example, sidewise. The commonest bearings are found at the rigid supports of rotating shafts where friction is the greatest.
Bearings were invented early in history; when the wheel was invented, it was mounted on an axle, and where wheel and axle touched was a bearing. Such early bearings had surfaces of wood or leather lubricated with animal fat.
Modern bearings have been arbitrarily designated as friction bearings and antifriction bearings. The first comprises sleeve or journal bearings; the second, ball and roller bearings. Neither type of bearing is completely frictionless, and both are highly efficient in reducing friction. A large, modern aircraft engine, for example, has more than 100 bearings, including both types; yet the total power consumed in overcoming bearing friction is less than one per cent of the total power output of the engine.
Friction bearings (скольжения) of the sleeve or journal type are simpler than antifriction bearings in construction but more complex in theory and operation. The shaft supported by the bearing is called the journal, and the outer portion, the sleeve. If journal and sleeve are both made of steel, the bearing surfaces, even if well lubricated, may grab or pick up, that is, rip, small pieces of metal from each other. The sleeves of most bearings therefore are lined with brass, bronze, or Babbitt metal. Sleeve bearings are generally pressure-lubricated through a hole in the journal or from the housing that contains the bearing. The sleeve is often grooved to distribute the oil evenly over the bearing surface.
Typical clearance (difference between the diameters of journal and sleeve) is nominally 0.0025 cm for every 2.54 cm of journal diameter. When the journal is rotating, it may be about 0.0000001 cm from the sleeve at the side with the greatest load. The journal is thus supported on an extremely thin film of oil, and the two parts have no actual contact. As the rotational speed increases, other variables remaining constant, the oil film becomes thicker, so that the friction increases in less than direct proportion to the speed. Conversely, at lower speeds the oil film is thinner if other factors are unchanged. At extremely low speeds, however, the film may rupture and the two pieces come into contact. Therefore, friction is high when the machine is started in motion, and the bearing may fail if high stresses are put on it during starting. Ball bearings, on the other hand, have low starting friction.
Jewel bearings are used to mount very little shafts such as those found in fine watches. They are friction-type bearings in which the ends of the shafts are mounted in extremely hard substances. The bearing is lubricated with a microscopic drop of fine oil.
In a ball bearing, a number of balls rotate freely between an inner ring, which is rigidly fixed to a rotating shaft, and an outer ring, which is rigidly fixed to a support. Both balls and rings are made of hardened alloy steel, usually finished to extremely fine tolerances. The balls are generally held in position by a cage or separator that keeps them evenly spaced and prevents them from rubbing against each other. The bearing is lubricated with grease or oil.
A roller bearing is similar to a ball bearing, except that small steel cylinders, or rollers, are substituted for the balls. A needle bearing is a roller bearing in which the rollers are extremely long and thin. An ordinary roller bearing may have 20 rollers — each twice as long as it is wide — whereas a needle bearing may have 100 needles, each 10 times as long as it is wide. Needle bearings are particularly useful when space is limited.
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